Why size is not a bigger is better thing with engines....
05-05-2002, 06:49 PM
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Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by LS1derfull:
<strong>Will race for food, what does your car run? Thats a interesting list in your sig.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">We are still in the building stage. Rollcage being done, as well as 9" rear. Then next 4-link, mini-tubs, parachute, etc. Plus front susp'. Pure race car. My last race car was an 16 point cage with a half chassis (from B pillar back). Looking to run low tens N/A on a good track.
Cheers <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
<strong>Will race for food, what does your car run? Thats a interesting list in your sig.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">We are still in the building stage. Rollcage being done, as well as 9" rear. Then next 4-link, mini-tubs, parachute, etc. Plus front susp'. Pure race car. My last race car was an 16 point cage with a half chassis (from B pillar back). Looking to run low tens N/A on a good track.
Cheers <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />
05-06-2002, 12:47 AM
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Re: Why size is not a bigger is better thing with engines.... You could never compare aftermarket cylinder heads by cfm, cc, etc. Flow benches vary so much and cc doesn't mean too much unless you know where the material was taken out. You could port a intake port to 210cc and never touch the runners. But I do agree, there seems to be an advantage with the smaller intake ports. Afterall, I believe the Stage I ports on my car are only 208cc on the intake side with very little removed from the walls. In fact, the rocker boss is still in place and the swirl vane nearly untouched. Basically the only work that was done was bowl work and valve guide tapering. Now the exaust side is a little different and VERY important in my opinion to make power with the LS1.
Another thing I've been saying all along is that it doesn't take large intake valves, in fact I think the smaller the better on the stock bore. Yes you can increase flow on the flow bench, but will it increase flow with the manifold bolted on, restricted by the bore, in actual operating conditions? The only thing you KNOW you'll get is increased valvetrain weight requiring heavier springs which all affects how quickly the motor will rev etc. But everyone wants you to believe that you need BIGGER parts that requires this that and everything else.
The advantage with aftermarket valves is NOT size, it's shape.
I'm going to do a set of high velocity runner LS6 heads for my car next. I'm going to use stock modified 2002 LS6 valves because they're about 40% lighter than LS1 stock which means over 50% lighter than the aftermarket valves, especially the Ferrea's. The valves will be down on cfm compared to the Ferrea's, but I think the motor will respond quicker which can't be measured on a flow bench or dyno. That little extra flow can be offset with a properly matched cam. If you get it right, the cam will cheat the port and draw extra cfm out of it that won't show on the flow bench either. That's why cam selection is so important and why we've found cams in certain ranges that seem to work well.
The cam I use will be a custom and probably use different ramp rates on the intake/exaust.
My $.02
Another thing I've been saying all along is that it doesn't take large intake valves, in fact I think the smaller the better on the stock bore. Yes you can increase flow on the flow bench, but will it increase flow with the manifold bolted on, restricted by the bore, in actual operating conditions? The only thing you KNOW you'll get is increased valvetrain weight requiring heavier springs which all affects how quickly the motor will rev etc. But everyone wants you to believe that you need BIGGER parts that requires this that and everything else.
The advantage with aftermarket valves is NOT size, it's shape.
I'm going to do a set of high velocity runner LS6 heads for my car next. I'm going to use stock modified 2002 LS6 valves because they're about 40% lighter than LS1 stock which means over 50% lighter than the aftermarket valves, especially the Ferrea's. The valves will be down on cfm compared to the Ferrea's, but I think the motor will respond quicker which can't be measured on a flow bench or dyno. That little extra flow can be offset with a properly matched cam. If you get it right, the cam will cheat the port and draw extra cfm out of it that won't show on the flow bench either. That's why cam selection is so important and why we've found cams in certain ranges that seem to work well.
The cam I use will be a custom and probably use different ramp rates on the intake/exaust.
My $.02
05-06-2002, 02:03 AM
#23
Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by LS1derfull:
<strong>There is no magic when it comes to flow versus rpm and port size. Aftermarket heads choose a volume to flow ratio when they design and machine a head, it either fits your engine size/rpm range or you choose a different casting. A LS1 or 6 head that has more flow to volume(cc's) ratio than a stock head will maintain lower and mid range power capabilities while making more peak power than unported and smaller head. I hope this came out clear, im tired. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">There actually is a Magic, or mathematical approach to this. First let me reference a good article in Drag Racer Magazine (other than the twin engine nailhead railjob) in the May 2002 Issue. http://www.dragracermag.com/Default.htm It's on page 86-7
Well the volume to runner ratio is very important. In fact when looking at a choice of 4-5 different heads, using this ratio will 9 out of 10 times find you the head you need. What I do is take the flow numbers that the cam will run in (for example) with max lift of .550 (not a LS1) I compared flow from 5 different heads between .200lift and .550 lift and port size. I would have gone lower in lift down to .025 lift but the info was not available.
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
With some good computer simulations you can also guess at port velocity, but that has more to do with the article from Drag Racer.
Go to the store and read it, it's about 5 minutes worth.
Jantzer, I agree that the stock valves are a good size and the '02 stock valves are worth their lack of weight in gold. I also think that for a LS1 with a bore of 3.898 we need anywhere from 2.02 to 2.10 valves for max performance, but that's not my though originally, but Reher-Morrison Racing Engines.
As far as the LS1, I really don't like the move from the 4.00 x 3.48 to the 3.898 x 3.622. A larger bore will give you better head flow, lower piston speed and the ability to put a longer rod in there without sacrificing the compression height. There has to be a reason for the longer stroke because BMW, Acura and GM are all moving toward a engine that is closer to Square(bore and stroke being closer together in size) I think it's for emissions.
I'm tired,(3:00am) talk to you guys later. I only started this because I wanted something other than "What Cam should I use" or "What headers to get" thread to talk on. Keep it going.
Bret
<small>[ May 06, 2002, 02:08 AM: Message edited by: SStrokerAce ]</small>
<strong>There is no magic when it comes to flow versus rpm and port size. Aftermarket heads choose a volume to flow ratio when they design and machine a head, it either fits your engine size/rpm range or you choose a different casting. A LS1 or 6 head that has more flow to volume(cc's) ratio than a stock head will maintain lower and mid range power capabilities while making more peak power than unported and smaller head. I hope this came out clear, im tired. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">There actually is a Magic, or mathematical approach to this. First let me reference a good article in Drag Racer Magazine (other than the twin engine nailhead railjob) in the May 2002 Issue. http://www.dragracermag.com/Default.htm It's on page 86-7
Well the volume to runner ratio is very important. In fact when looking at a choice of 4-5 different heads, using this ratio will 9 out of 10 times find you the head you need. What I do is take the flow numbers that the cam will run in (for example) with max lift of .550 (not a LS1) I compared flow from 5 different heads between .200lift and .550 lift and port size. I would have gone lower in lift down to .025 lift but the info was not available.
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
With some good computer simulations you can also guess at port velocity, but that has more to do with the article from Drag Racer.
Go to the store and read it, it's about 5 minutes worth.
Jantzer, I agree that the stock valves are a good size and the '02 stock valves are worth their lack of weight in gold. I also think that for a LS1 with a bore of 3.898 we need anywhere from 2.02 to 2.10 valves for max performance, but that's not my though originally, but Reher-Morrison Racing Engines.
As far as the LS1, I really don't like the move from the 4.00 x 3.48 to the 3.898 x 3.622. A larger bore will give you better head flow, lower piston speed and the ability to put a longer rod in there without sacrificing the compression height. There has to be a reason for the longer stroke because BMW, Acura and GM are all moving toward a engine that is closer to Square(bore and stroke being closer together in size) I think it's for emissions.
I'm tired,(3:00am) talk to you guys later. I only started this because I wanted something other than "What Cam should I use" or "What headers to get" thread to talk on. Keep it going.
Bret
<small>[ May 06, 2002, 02:08 AM: Message edited by: SStrokerAce ]</small>
05-06-2002, 03:31 AM
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Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I am curious as to what the theoretical basiw for this is.
My problem with this technique is that it evaluates a set of heads independent of the bottom end it is going on.
E.G. a set of heads will score "good" on this scale whether they go on a 302 or 502 inch motor. In real life the absurdity of claiming a single set of heads is good for both situations is self evident. If the formula can't discern the proper setup in a situation like that why would it work in others?
My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point. Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies.
This isn't to say it's magic - there are definite answers and solutions, I just think coming up with a theoretically sound one is beyond the scope of what's being discussed here.
Chris
<strong>
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I am curious as to what the theoretical basiw for this is.
My problem with this technique is that it evaluates a set of heads independent of the bottom end it is going on.
E.G. a set of heads will score "good" on this scale whether they go on a 302 or 502 inch motor. In real life the absurdity of claiming a single set of heads is good for both situations is self evident. If the formula can't discern the proper setup in a situation like that why would it work in others?
My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point. Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies.
This isn't to say it's magic - there are definite answers and solutions, I just think coming up with a theoretically sound one is beyond the scope of what's being discussed here.
Chris
05-06-2002, 05:40 AM
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Re: Why size is not a bigger is better thing with engines.... Strokerace, i dont think you understood what im saying here, Chris B is a lot closer.I still stand behind what im saying please reread this whole post and maybe i will make more sense.
05-06-2002, 01:05 PM
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Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">My formula is a easy way for him/her not to just use the flow numbers, but to find to right velocity for the engine.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
My main concern is that the formula still doesn't take displacement into account. This is a HUGE factor imho.
Let's say we had a stock 302" SBC engine. Now let's look at a set of AFR RR 215 heads - an AFR porting.
0.3 220
0.4 276
0.5 306
0.550 312
0.600 315
At 215cc intake runner. That gives us
((220+276+306+312+315) / 5)/ 215cc
This gives us a 1.33 ratio
Now let's look at a set of stock LT1 heads
0.2 113.0
0.3 167.0
0.4 203.0
0.5 212.0
At 170cc's nominal for the intake port.
That gives us a ratio of 1.022
Obviously using your formula the AFR 215rr heads are the way to go on the 302" engine. The ratio is much larger (greater flow per intake cc).
Yet using common sense the converse is true.
if a formula can not discern vs. obvious choices like that what makes it valid for more minute chages.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
But if the formula isn't accurate I would submit that there is no point to using it. We aren't talking about a lower percent confidence in the results (eg. 85% vs. 95%) - we are talking about fundamental inaccuracies in the theoretical model.
Chris Bennight
My main concern is that the formula still doesn't take displacement into account. This is a HUGE factor imho.
Let's say we had a stock 302" SBC engine. Now let's look at a set of AFR RR 215 heads - an AFR porting.
0.3 220
0.4 276
0.5 306
0.550 312
0.600 315
At 215cc intake runner. That gives us
((220+276+306+312+315) / 5)/ 215cc
This gives us a 1.33 ratio
Now let's look at a set of stock LT1 heads
0.2 113.0
0.3 167.0
0.4 203.0
0.5 212.0
At 170cc's nominal for the intake port.
That gives us a ratio of 1.022
Obviously using your formula the AFR 215rr heads are the way to go on the 302" engine. The ratio is much larger (greater flow per intake cc).
Yet using common sense the converse is true.
if a formula can not discern vs. obvious choices like that what makes it valid for more minute chages.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
But if the formula isn't accurate I would submit that there is no point to using it. We aren't talking about a lower percent confidence in the results (eg. 85% vs. 95%) - we are talking about fundamental inaccuracies in the theoretical model.
Chris Bennight
05-06-2002, 03:29 PM
#27
Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ChrisB:
<strong> </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">My formula is a easy way for him/her not to just use the flow numbers, but to find to right velocity for the engine.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
My main concern is that the formula still doesn't take displacement into account. This is a HUGE factor imho.
Let's say we had a stock 302" SBC engine. Now let's look at a set of AFR RR 215 heads - an AFR porting.
0.3 220
0.4 276
0.5 306
0.550 312
0.600 315
At 215cc intake runner. That gives us
((220+276+306+312+315) / 5)/ 215cc
This gives us a 1.33 ratio
Now let's look at a set of stock LT1 heads
0.2 113.0
0.3 167.0
0.4 203.0
0.5 212.0
At 170cc's nominal for the intake port.
That gives us a ratio of 1.022
Obviously using your formula the AFR 215rr heads are the way to go on the 302" engine. The ratio is much larger (greater flow per intake cc).
Yet using common sense the converse is true.
if a formula can not discern vs. obvious choices like that what makes it valid for more minute chages.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
But if the formula isn't accurate I would submit that there is no point to using it. We aren't talking about a lower percent confidence in the results (eg. 85% vs. 95%) - we are talking about fundamental inaccuracies in the theoretical model.
Chris Bennight</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Chris that is really good.
What I would say is that the HP level you will achive with the 215 AFR's and the 170's you put out there are not going to be very close at all.
Yes a 302 is differnet from a 350, but the bore could be (and should be) the same. So now we have take out the problem of increased flow from a larger bore. What I use this for is to compare heads with similar flow results. It's just a small way to gauge what I want to use.
I'd like to look at it this way.
Take
Brodix 215 (WP T1's) CC/CFM Ratio 1.325581395 250 ft/sec Velocity Occurs at 7000rpm
Brodix Smokey Yunick (187 cc's) CC/CFM Ratio 1.361853832 250 ft/sec Velocity Occurs at 6400rpm
GM Vortec Heads(170 cc's)CC/CFM Ratio 1.362745098 250 ft/sec Velocity Occurs at 6200rpm
First you have to use flow values at the same lifts. So what you started off with was not apples to apples. Your point of size for the engine is important, and I wouldn't consider 210cc heads or 215cc heads for a 302, first you have to have some descresion. When you are going to pick heads for a 302 your requirements are much less. Here we see that the Vortec Heads are the Winner, but they are not. You would really benefit from the Smokey Yunick Heads to the tune of 20hp average and 50hp at peak. One thing the ratio does tell us is that the engine will not do as well with the bigger 215 cc heads. It does get you close to where you should be looking though. Using this in conjuction with other values is very important. Knowing that the 7000rpm TQ peak is too High is one of them. You could also use this formula with the one that will give you MAX HP potential out of a set of heads to find where you want to be. Obviously a 600hp 360 cu in Chevy has different needs than a 350hp 302 cu in Chevy.
This is by no means a be all end all. It's most usefull when shopping for heads that are similar in flow numbers. That's preominatly what we have here with the LS1.
Now if you use Visceral's idea and plot the time that the valve will be open at differnet lifts, or know that your limit is X then you can narrow the flow figures down more.
Yes this is very complex, and this is simple but isin't picking a head because it flows the most alot more basic?
Bret
<strong> </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">My formula is a easy way for him/her not to just use the flow numbers, but to find to right velocity for the engine.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
My main concern is that the formula still doesn't take displacement into account. This is a HUGE factor imho.
Let's say we had a stock 302" SBC engine. Now let's look at a set of AFR RR 215 heads - an AFR porting.
0.3 220
0.4 276
0.5 306
0.550 312
0.600 315
At 215cc intake runner. That gives us
((220+276+306+312+315) / 5)/ 215cc
This gives us a 1.33 ratio
Now let's look at a set of stock LT1 heads
0.2 113.0
0.3 167.0
0.4 203.0
0.5 212.0
At 170cc's nominal for the intake port.
That gives us a ratio of 1.022
Obviously using your formula the AFR 215rr heads are the way to go on the 302" engine. The ratio is much larger (greater flow per intake cc).
Yet using common sense the converse is true.
if a formula can not discern vs. obvious choices like that what makes it valid for more minute chages.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
But if the formula isn't accurate I would submit that there is no point to using it. We aren't talking about a lower percent confidence in the results (eg. 85% vs. 95%) - we are talking about fundamental inaccuracies in the theoretical model.
Chris Bennight</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Chris that is really good.
What I would say is that the HP level you will achive with the 215 AFR's and the 170's you put out there are not going to be very close at all.
Yes a 302 is differnet from a 350, but the bore could be (and should be) the same. So now we have take out the problem of increased flow from a larger bore. What I use this for is to compare heads with similar flow results. It's just a small way to gauge what I want to use.
I'd like to look at it this way.
Take
Brodix 215 (WP T1's) CC/CFM Ratio 1.325581395 250 ft/sec Velocity Occurs at 7000rpm
Brodix Smokey Yunick (187 cc's) CC/CFM Ratio 1.361853832 250 ft/sec Velocity Occurs at 6400rpm
GM Vortec Heads(170 cc's)CC/CFM Ratio 1.362745098 250 ft/sec Velocity Occurs at 6200rpm
First you have to use flow values at the same lifts. So what you started off with was not apples to apples. Your point of size for the engine is important, and I wouldn't consider 210cc heads or 215cc heads for a 302, first you have to have some descresion. When you are going to pick heads for a 302 your requirements are much less. Here we see that the Vortec Heads are the Winner, but they are not. You would really benefit from the Smokey Yunick Heads to the tune of 20hp average and 50hp at peak. One thing the ratio does tell us is that the engine will not do as well with the bigger 215 cc heads. It does get you close to where you should be looking though. Using this in conjuction with other values is very important. Knowing that the 7000rpm TQ peak is too High is one of them. You could also use this formula with the one that will give you MAX HP potential out of a set of heads to find where you want to be. Obviously a 600hp 360 cu in Chevy has different needs than a 350hp 302 cu in Chevy.
This is by no means a be all end all. It's most usefull when shopping for heads that are similar in flow numbers. That's preominatly what we have here with the LS1.
Now if you use Visceral's idea and plot the time that the valve will be open at differnet lifts, or know that your limit is X then you can narrow the flow figures down more.
Yes this is very complex, and this is simple but isin't picking a head because it flows the most alot more basic?
Bret
05-07-2002, 12:23 AM
#28
Re: Why size is not a bigger is better thing with engines.... Guys,
"My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point."
I perfectly understand that. This is just a simple way to make selection of a head easier. When you put that formula in conjunction with a good engine simulator and the article in Drag Racer which basically says "flow velocity of 240-260 ft/sec will happen at TQ peak" Once you guys read the article alot of what I am saying will fit in with what you are saying.
"Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies."
Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs. If I had the ability to actaully test and hold every set of heads I wanted to buy, then the process would be much easier. Since most of us pick parts on here by what people say or who got what numbers. I'm just giving a way for people to compare apples to apples. And pick the right part for them. I can do this with heads, I only wish I could do it with Cams.
When you give a guy who drives his car to work a set of heads meant to make peak TQ at 6000rpm you gave him the wrong part. My formula is a easy way for him/her not to just use the flow numbers, but to find to right velocity for the engine. With a smaller port and good flow you will make more average power for a street car because it's operating range is much lower than say a drag car. I still think that this formula is good for drag engines also because it will enhance the TQ below the HP peak which is what gets you down the track faster.
Since this formula is for narrowing down your choices on a set engine, you would never use this to compare a set of heads for a Small Block and Big Block. In fact to compare 18 Deg heads and 23 Deg Heads with this would be stupid since the runner lengths are different.
"A LS1 or 6 head that has more flow to volume(cc's) ratio than a stock head will maintain lower and mid range power capabilities while making more peak power than unported and smaller head. I hope this came out clear, im tired. "
LS1derful, That is exactly right! Add on that it bumps the compression ratio too and it will keep the low end where it is.
PLEASE GO GET THE MAY ISSUE OF DRAG RACER! Just stop by and read the article on page 86. This is a really good addition, and it's sad that it's just now becoming so prominent. Smokey Yunick knew this decades ago.
What I am focusing on here is "high performance street engines" We have a larger RPM range to make power in and a more constraints on the engine. I pick my RPM range by what piston speed my parts can handle. Then I try to maximize the whole rpm band with Heads and Cam. Building a drag race engine means you don't have to worry as much about engine life, but the engine in my Camaro has to be there everyday, and i'm not going to touch it unless i'm changing oil. This is why the smaller port size is important. I can have the HP level i'm looking for and the rpm range is not going to go way above the stock internals limit. God even though the Lunati Crank is strong, the piston speed is worse so you never really gain much liveable RPM range with that, just cubes. As I said I wish they went with more bore and less stroke on the LS1 rather than the other way around.
Bret
"My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point."
I perfectly understand that. This is just a simple way to make selection of a head easier. When you put that formula in conjunction with a good engine simulator and the article in Drag Racer which basically says "flow velocity of 240-260 ft/sec will happen at TQ peak" Once you guys read the article alot of what I am saying will fit in with what you are saying.
"Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies."
Yeah, that is exactly what happens, but with simple math how are you going to find all of this out. Obviously you are going to need some advanced calculations to do this. I am taking what we as consumers are given and then using it to help the average guy find what he needs. If I had the ability to actaully test and hold every set of heads I wanted to buy, then the process would be much easier. Since most of us pick parts on here by what people say or who got what numbers. I'm just giving a way for people to compare apples to apples. And pick the right part for them. I can do this with heads, I only wish I could do it with Cams.
When you give a guy who drives his car to work a set of heads meant to make peak TQ at 6000rpm you gave him the wrong part. My formula is a easy way for him/her not to just use the flow numbers, but to find to right velocity for the engine. With a smaller port and good flow you will make more average power for a street car because it's operating range is much lower than say a drag car. I still think that this formula is good for drag engines also because it will enhance the TQ below the HP peak which is what gets you down the track faster.
Since this formula is for narrowing down your choices on a set engine, you would never use this to compare a set of heads for a Small Block and Big Block. In fact to compare 18 Deg heads and 23 Deg Heads with this would be stupid since the runner lengths are different.
"A LS1 or 6 head that has more flow to volume(cc's) ratio than a stock head will maintain lower and mid range power capabilities while making more peak power than unported and smaller head. I hope this came out clear, im tired. "
LS1derful, That is exactly right! Add on that it bumps the compression ratio too and it will keep the low end where it is.
PLEASE GO GET THE MAY ISSUE OF DRAG RACER! Just stop by and read the article on page 86. This is a really good addition, and it's sad that it's just now becoming so prominent. Smokey Yunick knew this decades ago.
What I am focusing on here is "high performance street engines" We have a larger RPM range to make power in and a more constraints on the engine. I pick my RPM range by what piston speed my parts can handle. Then I try to maximize the whole rpm band with Heads and Cam. Building a drag race engine means you don't have to worry as much about engine life, but the engine in my Camaro has to be there everyday, and i'm not going to touch it unless i'm changing oil. This is why the smaller port size is important. I can have the HP level i'm looking for and the rpm range is not going to go way above the stock internals limit. God even though the Lunati Crank is strong, the piston speed is worse so you never really gain much liveable RPM range with that, just cubes. As I said I wish they went with more bore and less stroke on the LS1 rather than the other way around.
Bret
05-07-2002, 12:58 AM
#29
Re: Why size is not a bigger is better thing with engines.... </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ChrisB:
<strong> </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I am curious as to what the theoretical basiw for this is.
My problem with this technique is that it evaluates a set of heads independent of the bottom end it is going on.
E.G. a set of heads will score "good" on this scale whether they go on a 302 or 502 inch motor. In real life the absurdity of claiming a single set of heads is good for both situations is self evident. If the formula can't discern the proper setup in a situation like that why would it work in others?
My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point. Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies.
This isn't to say it's magic - there are definite answers and solutions, I just think coming up with a theoretically sound one is beyond the scope of what's being discussed here.
Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I agree here, insofar as we are talking about numerically quantifying the affects of various cams and head ports on various bore/stroke configurations.
As ChrisB suggests, there are many other factors that are going ot relate to how the combination outputs power. A few examples:
If you plot a valves position over time, using the cam lobe as a approximation, you can determine a function x=y(theta) where y is a function that describes the lift (x) at a cam angle theta. Mathmatica can do a nice little fuction if you give it a few datapoints. Now you have to create a function out of your flow. Same deal with Mathmatica. give it a few datapoints. Now f=g(x), where flow can be calcutlated from a function g based on lift x. Add the two equations, f=g(y(theta)). Integrate across 360 degrees and you have the REAL area under the curve, flow wise, for a set of heads and a cam. BUT, this is static. This would presume that the piston is a open 3.90 inch bore (or whatever size you used), and the differential of pressure was a constant 28 inches of mercury. Not likely in the real world ;-). Now you have to determine what vacuum a particular bore and stroke and rod length will impart over theta, and it will be a function of RPM, bore, and stroke, and scavenging and maybe a couple other things. This is obviously where "port velocity" comes in. And that vacuum will be between the cylinder and the port, which is still going to depend dynamically on how the intake path performs at various RPMs. This may be different for each cylinder intake port.
Should we give up? no. But I dont think we can use math to really simplify much in this case ;-)
Nor can I use it to afford a new bottom end :-|
\chrisn
<strong> </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong>
Basically you average the flow ((.200CFM + .300CFM + .400CFM + .500CFM) / 4) and then divde by the CC's This will give you Average CFM per cc. That number is very important. BTW I've found that 1.3906 CFM per cc is a good goal.
</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I am curious as to what the theoretical basiw for this is.
My problem with this technique is that it evaluates a set of heads independent of the bottom end it is going on.
E.G. a set of heads will score "good" on this scale whether they go on a 302 or 502 inch motor. In real life the absurdity of claiming a single set of heads is good for both situations is self evident. If the formula can't discern the proper setup in a situation like that why would it work in others?
My other issue would be that it is simply averaging the flow straight across the board. If you look at a lobe profile you do not spend the same amount of time at each lift point. Also the "flow" at a given lift point will not be constant - so using flow numbers derived at 28" H2O or whatever isn't accurate - the depression isn't fixed but is a function of piston velocity and bore size - which varies as the lobe profile varies.
This isn't to say it's magic - there are definite answers and solutions, I just think coming up with a theoretically sound one is beyond the scope of what's being discussed here.
Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I agree here, insofar as we are talking about numerically quantifying the affects of various cams and head ports on various bore/stroke configurations.
As ChrisB suggests, there are many other factors that are going ot relate to how the combination outputs power. A few examples:
If you plot a valves position over time, using the cam lobe as a approximation, you can determine a function x=y(theta) where y is a function that describes the lift (x) at a cam angle theta. Mathmatica can do a nice little fuction if you give it a few datapoints. Now you have to create a function out of your flow. Same deal with Mathmatica. give it a few datapoints. Now f=g(x), where flow can be calcutlated from a function g based on lift x. Add the two equations, f=g(y(theta)). Integrate across 360 degrees and you have the REAL area under the curve, flow wise, for a set of heads and a cam. BUT, this is static. This would presume that the piston is a open 3.90 inch bore (or whatever size you used), and the differential of pressure was a constant 28 inches of mercury. Not likely in the real world ;-). Now you have to determine what vacuum a particular bore and stroke and rod length will impart over theta, and it will be a function of RPM, bore, and stroke, and scavenging and maybe a couple other things. This is obviously where "port velocity" comes in. And that vacuum will be between the cylinder and the port, which is still going to depend dynamically on how the intake path performs at various RPMs. This may be different for each cylinder intake port.
Should we give up? no. But I dont think we can use math to really simplify much in this case ;-)
Nor can I use it to afford a new bottom end :-|
\chrisn
05-07-2002, 05:54 PM
#30
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Re: Why size is not a bigger is better thing with engines.... I think you are still missing my point though. At the most basic level, this formula evaluates a set of heads for an application with out taking into account the displacement, etc. of that application. Without at a minimum that set of data I don't think it's a valid formula.
Chris
Chris
