Advanced Engineering Tech - Head EI vs. Cam Duration

I am in the process of evaluating a set of heads, and there now is a smorgasbord of high quality heads available. I am running a Crane cam with 216/224 .583/.583 115 LSA (Note: lift is with 1.8 rockers) with QTP Tri-Y Corvette headers and a CAI and now have 370 rwhp cam only.

Since my cam is split with a larger duration on the exhaust side, how much does the EI ratio for any particular head play a role in selection? Can I go "too high of a flow" on the exhaust side and hurt my performance as say compared to a head that flows less on exhaust? Is there an optimum EI for a particular cam split?

I am looking for input on how I should weigh the available information on the heads available (flow, port size, EI, etc.). I am trying to increase my low end performance, i.e., I don't want to turn the motor to high RPM to achieve power since this is a street only car (fun factor on the street).

I'm not the forum expert by any means, but you should be aiight on your exhaust, considering your LSA is wider than most aftermarket cams... in fact, I'm looking at getting the same cam for my WS6(That is a Comp grind, right???).

You run into too efficient exhaust scavenging when you run a tighter LSA. That gives you more valve overlap, and while both valves are open, a very efficient exhaust will actually suck part of the intake charge out of the combustion chamber at lower rpm. That same principle is what gives tighter LSA cams that lopey idle that sound all cool and stuff.

Look for heads designed for high intake velocity. Then get the ones that flow the best at your maximum valve lift. Beehive springs are a plus, too( http://www.ls1tech.com/forums/showthread.php?t=394940 ).

Duuuhhhhhh, I feel smart, now. Totally missed that you run a Crane cam.

In my opinion the better an exhaust port flows on the head the later you can open the exhaust valve which allows you to preserve every last bit of the power stroke of the motor (generally speaking means you need a smaller exhaust lobe). I understand that a certain nascar team found power by opening the exhaust valve later (preserving the power stroke like indicated before) and imporving to exhaust port of the head (to avoid pumping losses caused by not being able to remove the exhaust gases). Now it's my understanding that longer rod motors are more sensitive to opening the exhaust valve too early cause they move slower in the first half of the power stroke and faster in the second half of it. An ls1 is kind of in the middle I guess with a r/s ratio of 1.68 (6.098/3.622) so it may not be as affected as a much higher r/s ratio nascar motor. Now on the other side of increasing the overal duration of the exhaust lobe you also close the exhaust valve later which adds overlap to the system. How overlap you need appears to be determined by your wants from the car, or in other words how much of a nasty idle are you willing to put up with for the added power of more overlap. My best advice to you would be to go back and re-read some of the many many great cam topics in this advanced tech section.

You run into too efficient exhaust scavenging when you run a tighter LSA. That gives you more valve overlap, and while both valves are open, a very efficient exhaust will actually suck part of the intake charge out of the combustion chamber at lower rpm. That same principle is what gives tighter LSA cams that lopey idle that sound all cool and stuff.

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I may be off on this but my understanding of why the motor lopes like it does is cuase the exhaust gases actually revert back into the combustion chamber at lower speeds through the later closing of the exhaust valve by a cam with more overlap which polutes the intake charge and causes the missfire/lope sound. At the same token the reason tight lsa cam's don't perform well at extremely extremely high rpms is because the exhaust system is unable to remove all of the gases from the system effectively creating a higher pressure in the exhaust system and causing the exhaust gases to flow back into the intake at extreme rpms.

I would imagine overscavenging would only become an issue if fuel mileage is a concern (i.e. nascar race or endurance race of anykind). I guess there's an instance in which the overscavenging can reduce the effective mean pressure, I don't know???

One of the considerations re cam specs should be the fact that the IVC point is by far the most important, and the EVO one of the least. In other words, if you take a cam which is anywhere in the ball park of what you need, and time it to get the IVC where it needs to be, it ought to work fairly well regardless.

One of the considerations re cam specs should be the fact that the IVC point is by far the most important, and the EVO one of the least. In other words, if you take a cam which is anywhere in the ball park of what you need, and time it to get the IVC where it needs to be, it ought to work fairly well regardless.


Would you say IVC should be picked in attempts to optimize dynamic compression or should it be picked based on attempts to optimize in a given rpm band. I guess they are kind of influence each other???

I'd agree that the IVC is the most important moment in the valvetrain. The intake charge has momentum coming into the chamber, kinda like its own mass helps to "ram" more air in. Whatever rpm you're gonna run the motor in, your intake valve should close at the point that the incoming air has "rammed" all of itself it can into the chamber. FWIW,that point should also be where you get your highest dynamic CR, if I'm not mistaiken.

EVO should actually happen before BDC, because as the exhaust is exiting, it's still under pressure, so it's gonna push on the piston still. In my own little mind, I picture enough overlap of EVC and IVO to allow the exhaust scavenging to suck the intake charge into the chamber, but not out the exhaust port... sort of a head start for the intake charge, if you will.

Would you say IVC should be picked in attempts to optimize dynamic compression or should it be picked based on attempts to optimize in a given rpm band. I guess they are kind of influence each other???

Does anyone want to take a crack at this one???

Sure! (Would have posted sooner, but I haven't figured out yet how to sign up for e-mails advising me of a new post on a thread I'm following.)
I'd say IVC should be picked first for the application as one of the six basic cam specs; IVO, EVO, EVC, IVL & EVL being the others. (Notice I don't list LCA or advance. To my way of thinking, those specs are just another way of identifying the opening and closing events and serve to confuse rather than enlighten.)
Once the cam is properly chosen, then it's time to determine DCR based on fuel, operating conditions, engine characteristics, etc. and then finally you are in a position to calculate the resultant mechanical CR and see if it's practical to achieve it. If you can't reach the required value due to mechanical considerations, etc., you might want to back off a bit on IVC to get a better match.

Does anyone want to take a crack at this one???

Its a little of both actually. Remember, if you want more mid range, then close the valve earlier to build pressure faster. But if you want to carry out the RPM range, then close it later. Its funny because some engine builders actually pick compression ratio after they choose their camshaft. The only real thing the camshaft does is dictate how the motor runs and what power band you want. The rest of the stuff you can add later.

Rick

Thanks for the good replies guys.

Mad Bill, what is EVL and IVL? Did you by chance mean ICL and ECL (exhaust and intake centerlines)? Just curious:)

Rick, i've heard the same thing, which people choose the IVC first and then choose compression.

Sorry, we can get a bit cryptic here, can't we? I meant intake and exhaust valve lifts. Once you know the actual valve events, centerlines, LCA and advance can be calculated.

Sorry, we can get a bit cryptic here, can't we? I meant intake and exhaust valve lifts. Once you know the actual valve events, centerlines, LCA and advance can be calculated.

Ha ha, yeah, definitely a code around here. Thanks!

The only real thing the camshaft does is dictate how the motor runs and what power band you want. The rest of the stuff you can add later.

Rick
Rick,

If the cam shaft dictates how well a motor copes w/given inlet and exhaust characteristics, why does it make sense to add the cam before you know those characteristics? Why wouldn't you want to choose the cam as the last component?

Rick,

If the cam shaft dictates how well a motor copes w/given inlet and exhaust characteristics, why does it make sense to add the cam before you know those characteristics? Why wouldn't you want to choose the cam as the last component?


Because when you pick out your camshaft, its because you are making the choice about where my power band is at, when it starts, when it stops, how it drives, does it pass smog, does it work with my nitrous....ect. Then you need to figure out how much compression to put back into it. So thats one way of looking at it. The cam is your engine.

Rick

If you run the eval version of Performance Trends Engine Analyzer Pro (you need the $500 pro version for this) there are nice dynamic graphs of flow and port velocities vs. crank angle vs. engine speed.

The cam selection really needs to be broken down into 4 parts: the valve opening and closing events. As others have stated, the exhaust opening is a tradeoff of getting the exhaust out at higher engine speeds (where there is less time) vs, losing power by shortening the effective power stroke. Intake closing is a tradeoff of losing dynamic compression at low speeds vs increasing the time for inlet flow hat higher engine speeds. Intake opening is a tradeoff between having it open further sooner on the intake stroke and having a flow established sooner vs. pumping intake out the exhaust port and exhaust back in the intake port. The exhaust closing just has to be after TDC. The exhaust gas really exits at the end of the power stroke when it is at 10 atmospheres.

While a big overlap may causing thumpa-thump at idle, it is actually used as a form of EGR on engines with variable camshaft timing. Your cam is pretty normal for a mild street cam, an upgrade from factory performance cams. It is smaller than many on this forum who have race only cars or ignore emissions requirements. One 402 on the drag tread had something like a 248/250.

From my work with the Dynomation engine simulation program (similar sophistication and $$ to the best Engine Analyzer Pro program), it appears the heads, particularily the mininum cross section area of the intake port, are the primary determinant of the 'natural' peak power RPM. Good or bad head airflow can bias it one way or the other, as can carb/TB sizing, cam timing and exhaust and inlet dimensions, etc. but the engine is usually at it's best when you match everything to the optimum inlet mach number (x-section area), rather than trying to crutch it way off in one direction or the other with the other components.

I am in the process of evaluating a set of heads, and there now is a smorgasbord of high quality heads available. I am running a Crane cam with 216/224 .583/.583 115 LSA (Note: lift is with 1.8 rockers) with QTP Tri-Y Corvette headers and a CAI and now have 370 rwhp cam only.


I have to ask you this because of your cam specs. I'm also hoping you have an A4 with stock rear gearing. Does your engine feel a bit lazy until the RPM reaches a certain point, and then pull very hard similar to a small shot of nitrous?

I have to ask you this because of your cam specs. I'm also hoping you have an A4 with stock rear gearing. Does your engine feel a bit lazy until the RPM reaches a certain point, and then pull very hard similar to a small shot of nitrous?

No it pulls hard from off idle to 6K where power falls off. The cam has 5* advance and I am running a MN6. Mid-range power in higher gears (cruising on the highway in 6th for example) are also much improved. In lower gears, the RPM jumps, but I think that is only the tires breaking loose :)

From my work with the Dynomation engine simulation program (similar sophistication and $$ to the best Engine Analyzer Pro program), it appears the heads, particularily the mininum cross section area of the intake port, are the primary determinant of the 'natural' peak power RPM. Good or bad head airflow can bias it one way or the other, as can carb/TB sizing, cam timing and exhaust and inlet dimensions, etc. but the engine is usually at it's best when you match everything to the optimum inlet mach number (x-section area), rather than trying to crutch it way off in one direction or the other with the other components.

How does one figure this out without these simulations? For instance, I am trying to make all my power under 6K RPM, so should I basically go with the most flow and smallest port I can? Would something like port volume devided by flow provide a good indicator for head selection?

How does one figure this out without these simulations? For instance, I am trying to make all my power under 6K RPM, so should I basically go with the most flow and smallest port I can? Would something like port volume devided by flow provide a good indicator for head selection?

If you could somehow figure out average port cross section this would probably be your best indicator without a flow bench and measuring velocities at different points which none of our sponsors do for their regular heads (too time consuming i'd imagine). The problem with getting yourself in the head volume trap is that volume is three dimensional and there are many different ways about getting similar volumes with drastically different results. Your best bet would probably be to look at what other similar setups there are to yours then choose accordingly. If one is trying to maintain power under 6k rpm with a stock shortblock then the AFR 205 heads come to mind. Good luck!

Unfortunately, usually you can find the port volume and the flow data, but an area number that would be indicative of flow velocity is usually not found.

I agree on the AFR's, they are very high on my list :)

Awesome, I have been looking for this discussion on another board than my own:) I am working on a set of gen 3 3.4 V6 Gm heads right now and have been trying to find the best EI ratio, and why. I have read many different magazine articles online as well as other sources and have come up with nothing conclusive. Some say 70% EI ratio, others 75%, then 80%, then 85%, and 70-80% as a grey area, etc. No where do I see anything on powerband or cam specs other than some interesting information on theoldone.com on cam specs.

The stock ratio for low to mid lift on these heads are well over 80%, as close as 99% for .150 life. 94% for .200 and .250 lift. With the intake side ported/blended in areas for the highest velocity per cfm increase, I have got the ratio down to 95% at the highest and still in the upper 80s low 90s one exhaust. Im afraid to even touch the exhaust ports based on what I have read.

This is 1 quote from a magazine source.

"Single or Dual-Pattern?


A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.

Then there's that 70-80-percent "gray" area in between. The only way to find out is to try both styles of cams, different ratio rockers, or (if the cam is mechanical) lash-loops"

Here is a link to the rest of what I have found interesting so far on my site.

http://www.60degreev6.com/index.php?name=PNphpBB2&file=viewtopic&t=7807

Now, i have found a way to lower the .050 and .100" lift values some on the exhaust about 5%, with the .150 and beyond averaging around 10% increase in flow (yes, I would have 102% EI ratio with this at 1 lift point). Point well taken on the exhaust doing too well of a job and having the intake charge flow out the exhaust, which is why I am thinking that a worse low lift value of exhaust flow could be beneficial.

I disagree on looking for heads with the best flow at the highest lift. This is one of the least important aspecs of the heads as they are only at that lift 1 time, vs the mid lift flow numbers since the valve is there longer.

Any speculation or theory or even documented power vs ratio changes would be excellent.

Side note if someone can, what % margin is considered flow matched? Search engines have failed me in my attempt to see what companies consider flow matched.

How does one figure this out without these simulations? For instance, I am trying to make all my power under 6K RPM, so should I basically go with the most flow and smallest port I can? Would something like port volume divided by flow provide a good indicator for head selection?
There's a lot of discussion possible on this, but per remarks below your post, you often can't get all the data you need without buying the heads and measuring for yourself. In a nutshell though, yes: Especially for your application, you can't go wrong with a high flowing, small port head! The problem comes when trying to decide between say a 300 CFM 210 cc runner piece and a 330 CFM 245 cc one. At this point you need either a ton of experience or a good simulation program and enough experience to interpret what it tells you, to determine which will work best for your application. The large port should certainly work well on a stout 400 +c.i engine, but how about a 327? Well, perhaps, if you're looking to twist it to 9,000 RPM, otherwise the smaller more efficient one will likely be better.
Once you are looking at a specific design, e.g. standard height/port spacing SBC heads, the volume is a pretty good indicator of minimum cross section, which all my experience and research says rivals flow numbers for importance. (Most long time racers can relate at least one case where an engine with a huge, high flow intake port fell spectacularly flat due to miserable gas velocity)

Moving on to E/I ratio, one of the problems is that a different ratio is 'ideal' depending on the application. It's no stretch to see that a supercharger or a heavy shot of nitrous is going to cause a spectacular increase in exhaust gas volume, which can best be dealt with via a big valve, high flow exhaust port, even if it means going to a smaller inlet valve to make room. The alternative of opening the valve early to avoid massive pumping losses on the exhaust stroke doesn't appeal, due to having all that 'lost' energy booming out the header.
Alternatively, as the compression ratio increases, more and more of the gas force is transmitted to the piston early in the expansion/power stroke, so the penalty of biasing valve size and port flow to the intake and crutching things with an early EVO is minimal. Consider a typical 'umpteen to one' CR Pro Stock engine. I have no idea (but perhaps someone will chime in...?) what the E/I flow ratio might be, but with say a 2.45" inlet valve and maybe a 1.6" exhaust, it would probably look spectacularly 'bad' to us peasants.
Last point: I may get some argument here, but in the search for E/I balance (if still needed after heeding above) I can't imagine it ever being a good idea to deliberately make the exhaust worse, unless the changes are directly required to improve the intake. Similarly, given the preceding exhaust flow discussion, it seems to me low lift exhaust flow is far more important than high lift, to get a fast blowdown while allowing a later EVO to capture all available cylinder pressure. As a matter of fact, Dynomation usually shows that using exhaust lift equal or greater than intake actually costs HP. [Allan Lockheed, creator of The Engine Expert simulation program, told me he once advised a Dodge-powered Modified owner that his EVL, (same in this case as IVL) was too high. The guy went from 1.7 to a 1.5 exhaust rocker on the spot and gained so much power he had to re-gear to stay off the limiter. The dyno later revealed a 40 HP gain.]
As far as exhaust flow being 'too good', and letting the intake charge flow through and out, this suggestion would be a case of 'shooting the messenger'. In the overlap period, the port merely communicates the pressure signal from the exhaust tract. If fresh mixture is being over-scavenged, the problem is a cam/header mismatch, not a 'too good' port.
Well, over the years I've found that I rarely learn anything while I'm talking, so enough of my blather and back to listening/reading.....

In my case, its only a couple CFM at .050 and .100 lost in exchange for 10s of CFM past that. Im just afraid of the 100% + exhaust ratio because of what I have read as "ideal" even though I have never read any particular reasons for such ideals. This particular head will be nitrous fed occasionally and NA the rest of the time, using 10:1 compression. Thanks for the clarification on the over scavenging during overlap.

Well, I can't add much to this discussion technically, but the input thus far has been great :)

Maybe just clean up your stock heads if you want to maintain or improve low end.I've built a 96 impalla SS with LT4 ported heads/hot cam and even though the car was faster it didnt feel as quick cause I lost low end.Then I built a 00 camaro ss with ported heads/and the similar spec cam as yours.again lost some street fun factor even though the car was fast.Now I got a magcharged z06 with stock heads/forged pistons Torque is good.If you clean up a set of stock heads and improve your intake manifold,I wonder if the fast manifold improves low end?port match,etc.I am willing to bet that you would like a set of stock LS6 heads,mildly ported,with nearly no change in intake but a little more on the exhaust side for daily driving.also,they breathe at high lift and the light valvetrain stuff will give you more revs wich will complement the cam you have.You get low end as well as more power up top simply due to a broader rpm range.If you gotta have ported or aftermarket heads:I was looking at some of the ported LS heads and it looks like the intake port volume is up to around 243cc on average where the AFR's flow nice and keep a 205cc intake port so figure the AFR more likely better bet for low end.GMHTP did a dyno of these heads on a camaro a few mos back so look for the article. Kickin up compression is also going to help a bit all around.I hate losing any low end torque--If you find a head that can do it all I'd be interested.