The truth about LSA
10-21-2005, 01:36 AM
#61
Ok well lets break this up again....
First off... most times with a LOW LSA you have a LOW ICL, which closes the intake valve earlier, which improves the DCR and low to midrange TQ. You can have a low ICL with either advance or low LSA. There are places for both, I preffer one method over another.
Let's break these things down....
Yes this is true
Yes and no.... you can increase TQ in the standard dip that's seen on a engine dyno (especially in high output street motors like those found in the Engine Masters contest) in the 3000-3500rpm area by decreasing the overlap. This is because the intake and exhaust tuning events aren't working well together here overlap hurts things even more. In the Engine Masters guys started using heavily worked dual plane intakes to take care of this area and it works pretty well.
I would say that most of the timing increasing the LSA is not going to help the average power numbers. So going to a 114LSA from a 112LSA is going to hurt power.
Increasing LSA and keeping the same advance (notice not ICL) will make the IVC point happen later which will move the powerband higher in the RPM range but maybe not up the HP scale.
I like all of that statement (with a but) which includes:
- helps with emissions
- smoother idle (higher idle vacuum)
- Increasing LSA = decreasing overlap (if duration stays the same)
I don't like that the higher LSA will show a higher HP RPM peak... if you have more than enough overlap for the motor, yeah it can move it higher, if you don't have enough overlap, then it will not move the HP peak higher.
Now on to Adrenaline_Z's comments....
Again it's all in the valve events, a earlier IVC is going to make more TQ. If you have too much overlap it can hurt but otherwise it's not going to hurt the TQ. Again if you have too much duration for the setup then you will be SOL in this deparment and it will hurt power.
watch out there.... overlap and DCR are not related at all... DCR deals with the IVC event and overlap with the EVC and IVO events.
BMEP (Brake Mean Effective Pressure) is directly related to VE so we don't need to talk about both of them... compression and cylinder pressure play into BMEP but the curves follow the same trend, more VE means more BMEP. If they diverge then something is causing you to lose ring seal.
Anywho...
The VE will make the effective compression inrease when it increases, but when you don't have high VE's (before the exhaust and intake tuning waves get working for you) you need the DCR from the early IVC point to help your TQ curve out. Balancing those two things out with the octane requirement (expecting that you have the tuning pressures in check) will get you the meanest street motor.
IMHO really mean street motors are as much fun as race motors. You don't have the problems of keeping things from going boom due to RPM, boost or lots of N2O, but you do have a lot of issues all tied in together to make the motor make power AND work as a street car.
Daddy SS,
I need you to rephrase that one..... Make it longer and think what you want to say and I'll try my best to explain what I can.
Bret
BTW thanks for making me think and explain stuff... it helps me as much or more than you guys to have a better idea of motor crap in my head.
First off... most times with a LOW LSA you have a LOW ICL, which closes the intake valve earlier, which improves the DCR and low to midrange TQ. You can have a low ICL with either advance or low LSA. There are places for both, I preffer one method over another.
Let's break these things down....
Long overlap periods work best for high-rpm power. For the street, a long overlap period combined with long-duration profiles combine to kill low-speed torque
Yes this is true
- Reducing overlap on a long-duration cam will often increase midrange torque at the expense of peak power, but if the average torque improves, that’s probably a change worth making.
Yes and no.... you can increase TQ in the standard dip that's seen on a engine dyno (especially in high output street motors like those found in the Engine Masters contest) in the 3000-3500rpm area by decreasing the overlap. This is because the intake and exhaust tuning events aren't working well together here overlap hurts things even more. In the Engine Masters guys started using heavily worked dual plane intakes to take care of this area and it works pretty well.
I would say that most of the timing increasing the LSA is not going to help the average power numbers. So going to a 114LSA from a 112LSA is going to hurt power.
Increasing Lobe Seperation Angle moves your powerband higher in the rpm range, and lowers your low/mid rpm torque output.
Increasing LSA and keeping the same advance (notice not ICL) will make the IVC point happen later which will move the powerband higher in the RPM range but maybe not up the HP scale.
It also helps with emissions testing and makes the idle smoother. "Increasing LSA" means the same thing as "decreasing overlap". So, a 114 LSA cam will idle better, make less emissions, and show peak HP higher in rpm than a 112 LSA cam.
I like all of that statement (with a but) which includes:
- helps with emissions
- smoother idle (higher idle vacuum)
- Increasing LSA = decreasing overlap (if duration stays the same)
I don't like that the higher LSA will show a higher HP RPM peak... if you have more than enough overlap for the motor, yeah it can move it higher, if you don't have enough overlap, then it will not move the HP peak higher.
Now on to Adrenaline_Z's comments....
As for tight LSA making power down low, I tend to believe otherwise.
Again it's all in the valve events, a earlier IVC is going to make more TQ. If you have too much overlap it can hurt but otherwise it's not going to hurt the TQ. Again if you have too much duration for the setup then you will be SOL in this deparment and it will hurt power.
You have long durations and overlap contributing to a loss of dynamic compression (DCR) and cylinder pressure (BMEP).
watch out there.... overlap and DCR are not related at all... DCR deals with the IVC event and overlap with the EVC and IVO events.
BMEP (Brake Mean Effective Pressure) is directly related to VE so we don't need to talk about both of them... compression and cylinder pressure play into BMEP but the curves follow the same trend, more VE means more BMEP. If they diverge then something is causing you to lose ring seal.
Anywho...
If the VE curve peaks later due to the valve timing, the effective compression
and cylinder pressure also rise later in the RPM band.
and cylinder pressure also rise later in the RPM band.
The VE will make the effective compression inrease when it increases, but when you don't have high VE's (before the exhaust and intake tuning waves get working for you) you need the DCR from the early IVC point to help your TQ curve out. Balancing those two things out with the octane requirement (expecting that you have the tuning pressures in check) will get you the meanest street motor.
IMHO really mean street motors are as much fun as race motors. You don't have the problems of keeping things from going boom due to RPM, boost or lots of N2O, but you do have a lot of issues all tied in together to make the motor make power AND work as a street car.
Daddy SS,
Can someone explain how overlap of varying duration affect teh flow at high versus low rpm, and why more overlap works for high or low RPM power and torque?
Bret
BTW thanks for making me think and explain stuff... it helps me as much or more than you guys to have a better idea of motor crap in my head.
10-21-2005, 06:19 AM
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Nice reply SStrokerAce!
IVC is directly related to DCR (as per formula). Does my generalization of
bigger durations not apply to lowering DCR as a rule of thumb (because the
IVC normally occurs later)?
So...we can alter the torque peak with the intake opening and closing points.
Earlier opening IVO coupled with an earlier closing IVC improves torque at lower RPM...IF...the exhaust valve closes early as well.
IE (seat to seat):
IVO - 30* BTDC
IVC - 62* ABDC
In. Duration 272 degrees
EVO - 80* BBDC
EVC - 28* ATDC
Ex. Duration 288 degrees
Overlap is 58 degrees
This above combination would typically yield a high lobe separation which
rolls of the HP curve at higher RPM.
On the other hand, with a later closing IVC and later closing EVC we're dealing
with a camshaft that wants to live in a fast spinning motor. This combination
would move the torque peak slightly higher in the RPM spectrum and also allow
HP to peak higher.
The above being true if the IVO creates a large amount of overlap.
IE (seat to seat):
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
Maybe we can begin to add cam cards and valve events to clarify this topic
from this point forward?
Let's also use a test bench motor to study the theoretical effects of valve timing. How about a showroom stock 2002 LS1?!
IVC is directly related to DCR (as per formula). Does my generalization of
bigger durations not apply to lowering DCR as a rule of thumb (because the
IVC normally occurs later)?
So...we can alter the torque peak with the intake opening and closing points.
Earlier opening IVO coupled with an earlier closing IVC improves torque at lower RPM...IF...the exhaust valve closes early as well.
IE (seat to seat):
IVO - 30* BTDC
IVC - 62* ABDC
In. Duration 272 degrees
EVO - 80* BBDC
EVC - 28* ATDC
Ex. Duration 288 degrees
Overlap is 58 degrees
This above combination would typically yield a high lobe separation which
rolls of the HP curve at higher RPM.
On the other hand, with a later closing IVC and later closing EVC we're dealing
with a camshaft that wants to live in a fast spinning motor. This combination
would move the torque peak slightly higher in the RPM spectrum and also allow
HP to peak higher.
The above being true if the IVO creates a large amount of overlap.
IE (seat to seat):
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
Maybe we can begin to add cam cards and valve events to clarify this topic
from this point forward?
Let's also use a test bench motor to study the theoretical effects of valve timing. How about a showroom stock 2002 LS1?!
Last edited by Adrenaline_Z; 10-21-2005 at 06:29 AM.
10-21-2005, 11:15 AM
#63
If I thought in seat to seat terms only it would help. My brain still deals with .050 numbers and valve events.... only way I could teach it to thing in valve events.
Would be about a 218/230 112LSA +6° Advanced with about zero overlap @ .050
Explain that one again to me Lucy?
234/234 110LSA +4 14° of overlap @ .050
I hate saying move everything up in the RPM range here, because if anything these changes are going to cause a rotation of the TQ curve at the HP and TQ crossover (5252rpm) where the larger cam is going to loose TQ below that point and make more above it.
Those two cams aren't for the same application IMHO. RPM ranges would greatly differ and if you threw converters into the mix the converter choice would be vastly different.
When I'm talking about these things all happening I'm talking about two cams that are very close on specs and we are adjusting them around for the same application to make them work better for the said application.
Bret
IE (seat to seat):
IVO - 30* BTDC
IVC - 62* ABDC
In. Duration 272 degrees
EVO - 80* BBDC
EVC - 28* ATDC
Ex. Duration 288 degrees
Overlap is 58 degrees
IVO - 30* BTDC
IVC - 62* ABDC
In. Duration 272 degrees
EVO - 80* BBDC
EVC - 28* ATDC
Ex. Duration 288 degrees
Overlap is 58 degrees
This above combination would typically yield a high lobe separation which
rolls of the HP curve at higher RPM.
rolls of the HP curve at higher RPM.
IE (seat to seat):
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
I hate saying move everything up in the RPM range here, because if anything these changes are going to cause a rotation of the TQ curve at the HP and TQ crossover (5252rpm) where the larger cam is going to loose TQ below that point and make more above it.
Those two cams aren't for the same application IMHO. RPM ranges would greatly differ and if you threw converters into the mix the converter choice would be vastly different.
When I'm talking about these things all happening I'm talking about two cams that are very close on specs and we are adjusting them around for the same application to make them work better for the said application.
Bret
10-21-2005, 12:56 PM
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This above combination would typically yield a high lobe separation which
rolls off the HP curve at higher RPM.
Explain that one again to me Lucy?
rolls off the HP curve at higher RPM.
Explain that one again to me Lucy?
What I meant to type was; with a modest overlap, would the HP curve roll
off sooner (using the valve timing in my previous post)?
Last edited by Adrenaline_Z; 10-21-2005 at 03:08 PM.
10-21-2005, 06:34 PM
#65
How about we do it this way.... pick a cam and change one thing. Either the Overlap (LSA) or keep the same overlap and change the durations. Changing a few things at a time isin't going to get us anywhere because too many other variables are chaning.
Still would be a good idea to put up all the specs of these popular cams and I can put the overlap in there so we have more room to compare them.
Bret
Still would be a good idea to put up all the specs of these popular cams and I can put the overlap in there so we have more room to compare them.
Bret
10-21-2005, 06:43 PM
#66
Originally Posted by SStrokerAce
When I'm talking about these things all happening I'm talking about two cams that are very close on specs and we are adjusting them around for the same application to make them work better for the said application.
Bret
Bret
. Thank You!Most of the cam discussions (that I've read) do not stick to a narrowed range of specs (variables).....and this leads to confusion, IMHO.
10-22-2005, 10:00 AM
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Alright, if nobody minds, let's use these cam specs with an LS1 platform
(seat to seat):
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
Intake Centerline: 106
Exhaust Centerline: 114
Lobe Center: 110
Since we're discussing the effects of overlap, why don't we play with IVO first?
Any objections? Otherwise, take it away!
(seat to seat):
IVO - 40* BTDC
IVC - 72* ABDC
In. Duration 292 degrees
EVO - 80* BBDC
EVC - 32* ATDC
Ex. Duration 292 degrees
Overlap is 72 degrees
Intake Centerline: 106
Exhaust Centerline: 114
Lobe Center: 110
Since we're discussing the effects of overlap, why don't we play with IVO first?
Any objections? Otherwise, take it away!
Last edited by Adrenaline_Z; 10-22-2005 at 10:06 AM.
10-22-2005, 11:15 AM
#68
Can you do this in .050" numbers because this puppy can be way different if its a flat vs. a roller. Plus that's how guys normally look at cams on here so it will relate ten times better.
Also take the advance crap out of the thing, then we can advance/retard the cam and see what that does.
Just change your example and then I'll rewrite this post and we can change the specs from there.
Bret
Also take the advance crap out of the thing, then we can advance/retard the cam and see what that does.
Just change your example and then I'll rewrite this post and we can change the specs from there.
Bret
10-22-2005, 12:19 PM
#69
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Why are you all so caught up in trying to efficiently fill a +/- 71cc combustion chamber instead of concentrating on trying to fill the 712cc cylinder? AKA: concentrating on overlap instead of lift and duration. Run the overlap that idles good and doesn't make your car sound like a pile of ****. Run max lift for your head/valvespring/valve/retainer. Run apropriate duration for intake runner length and size/exhaust runner length and size and be done with the deal. (including intake manifold and header)
10-22-2005, 01:09 PM
#70
I dont get the jist of even trying to go there with this, But anywho ..BINK mentioned variables and thier are many ,,,are we assuming there is stock manifolds or stepped kooks on that engine..is it ls6 or c5r heads on it...stock intake or sheetmetal.... valve events may change things around but all three above can move torque up or down , hp higher or lower..independently from the cam itself and thats just 3 major factors with many sub factors.... thing is you cant just look at ONE aspect- xxx cam and even begin, for anyone here less than engine savvy to understand ... but i guess if math was your favorite class ( which it was for me to ,but i don't like it that much) go for it...you guys will be typing for weeks... what is being discussed now can be searched out and found,on more technical forums or hell in the back of a comp cam, crane cam etc catalogue ...I say if you are going to discuss lsa discuss it on 1 whole combo ...I mean in all reality a cam doesnt run by itself... but take a stock ls1 complete and go from there , who knows maybe gm will come in and hire some lucky guy to choose thier next camshaft...do a search copy paste and save yourself some time.All this info can be found , its generic when only talking about the cam itself... you make the ivo earlier it does this...BUT what engine combo would it benefit ... anyone see my point here ????
10-22-2005, 02:54 PM
#71
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Daddy SS,
I need you to rephrase that one..... Make it longer and think what you want to say and I'll try my best to explain what I can.
I need you to rephrase that one..... Make it longer and think what you want to say and I'll try my best to explain what I can.
10-22-2005, 03:21 PM
#72
Launching!
Originally Posted by DaddySS
In other words, it has been my understanding that you want more overlap (and longer duration) to make more power at high rpm but in doing so you give up low RPM power and torque. So how does overlap help at high RPM and hurt at low RPM, (especially torque). Conversely, why does more LSA (less overlap) help at low RPM and hurt at high RPM?
Overlap can help an engine make more power when the engine has a tuned exhaust system (long tube headers) that is set up to create negative pressure during the overlap cycle at given RPM ranges (usually higher RPM ranges). The exhaust system actually helps to fill the cylinder under such conditions, increasing power.
Here is a link, which I posted separately as well, to an article which describes this well:
http://www.popularhotrodding.com/eng...exh/index.html
10-22-2005, 11:33 PM
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Why are you all so caught up in trying to efficiently fill a +/- 71cc combustion chamber instead of concentrating on trying to fill the 712cc cylinder?
stroke.
It's also a fact that a tuned exhaust will begin pulling intake charge during
the overlap period before the piston even begins moving downward.
Bret, I'll convert the events to 0.050" tomorrow.
10-22-2005, 11:45 PM
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Joe, we've already set the LS1 engine as a platform (stock).
We're also going to change one aspect of the valve timing and comment on
the effects on engine dynamics.
I doubt most of this could be found on MFG sites, or most tech forums for that
fact.
I'd like to discuss what the effects of +10 milliseconds of overlap will have on
pulse tuning at 6000 RPM. Possibly supporting mods to make the extra overlap
work (IE: Higher SCR, 6 inches of tuned intake/exhaust runner, etc.)
It's all hypothesis, but it beats watching re-runs of American Hot Rod.
We're also going to change one aspect of the valve timing and comment on
the effects on engine dynamics.
I doubt most of this could be found on MFG sites, or most tech forums for that
fact.
I'd like to discuss what the effects of +10 milliseconds of overlap will have on
pulse tuning at 6000 RPM. Possibly supporting mods to make the extra overlap
work (IE: Higher SCR, 6 inches of tuned intake/exhaust runner, etc.)
It's all hypothesis, but it beats watching re-runs of American Hot Rod.
10-23-2005, 11:12 AM
#75
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Bret, try this:
(0.050")
IVO - 3* BTDC
IVC - 51* ABDC
In. Duration 234 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 11 degrees
Intake Centerline: 114
Exhaust Centerline: 114
Lobe Center: 114
If this doesn't fit the discussion, please select something that does.
Let's open the intake valve (IVO) earlier by 2 degrees @ 0.050".
My thoughts are:
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
Take it from here. See how far we can move IVO and what we need to consider
for each step?
(0.050")
IVO - 3* BTDC
IVC - 51* ABDC
In. Duration 234 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 11 degrees
Intake Centerline: 114
Exhaust Centerline: 114
Lobe Center: 114
If this doesn't fit the discussion, please select something that does.
Let's open the intake valve (IVO) earlier by 2 degrees @ 0.050".
My thoughts are:
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
Take it from here. See how far we can move IVO and what we need to consider
for each step?
10-23-2005, 06:29 PM
#76
Originally Posted by Adrenaline_Z
Bret, try this:
(0.050")
IVO - 3* BTDC
IVC - 51* ABDC
In. Duration 234 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 11 degrees
Intake Centerline: 114
Exhaust Centerline: 114
Lobe Center: 114
If this doesn't fit the discussion, please select something that does.
Let's open the intake valve (IVO) earlier by 2 degrees @ 0.050".
My thoughts are:
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
Take it from here. See how far we can move IVO and what we need to consider
for each step?
(0.050")
IVO - 3* BTDC
IVC - 51* ABDC
In. Duration 234 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 11 degrees
Intake Centerline: 114
Exhaust Centerline: 114
Lobe Center: 114
If this doesn't fit the discussion, please select something that does.
Let's open the intake valve (IVO) earlier by 2 degrees @ 0.050".
My thoughts are:
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
Take it from here. See how far we can move IVO and what we need to consider
for each step?
Anywhoo...
IVO - 5* BTDC
IVC - 49* ABDC
In. Duration 236 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 13 degrees
Intake Centerline: 113
Exhaust Centerline: 114
Lobe Center: 113.5
Going to the 236/244 113.5 (113ICL / 114ECL) cam is going to basically just take the TQ curve and bring it down below HP peak. This is because while you are lowing the ICL you are doing so by adding duration to the opening. The extra overlap should help the peak HP number since for that application there is not enough anyways.
My thoughts are:
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
- low RPM torque/VE will increase
- HP will peak a touch higher due to extra overlap (to help scavenge the chamber)
- These numbers are very conservative for high RPM use. More fit for a street
cam with good low to mid range power (1500-5000 RPM)
- Smooth idle, good fuel economy.
- low RPM TQ/VE decrease
- the HP peak might raise in RPM but most likely it will just be slightly higher if the exhaust tuning events like the additional overlap which I think they will here
- the duration and LSA numbers are more fitting to a high RPM range than you listed
- the idle will be rougher now than before due to the overlap and no increase in DCR.
Now lets SUBTRACT the 2 degs to the IVC
IVO - 3* BTDC
IVC - 49* ABDC
In. Duration 232 degrees
EVO - 56* BBDC
EVC - 8* ATDC
Ex. Duration 244 degrees
Overlap is 11 degrees
Intake Centerline: 113
Exhaust Centerline: 114
Lobe Center: 113.5
As you can see we have the same ICL and LSA as the above example of adding the 2° to the IVO. Now we have less duration at .050 on the intake, and the same overlap that we started from.
What should happen...
- low RPM TQ/VE increase below HP peak. Whole curve should shift up.
- the HP peak should be around the same RPM but might be slightly lower in output and RPM
- the duration and LSA numbers are more fitting to a lower RPM range than the cam we started with you listed orginally
- the idle will be better now than before due to the same overlap and increase in DCR.
Bret
Last edited by SStrokerAce; 10-23-2005 at 06:55 PM.
10-23-2005, 09:40 PM
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- low RPM TQ/VE decrease
- the HP peak might raise in RPM but most likely it will just be slightly higher if the exhaust tuning events like the additional overlap which I think they will here
- the duration and LSA numbers are more fitting to a high RPM range than you listed
- the idle will be rougher now than before due to the overlap and no increase in DCR.
- the HP peak might raise in RPM but most likely it will just be slightly higher if the exhaust tuning events like the additional overlap which I think they will here
- the duration and LSA numbers are more fitting to a high RPM range than you listed
- the idle will be rougher now than before due to the overlap and no increase in DCR.
However, when comparing to +2 IVO, I can see why your analysis fits.
#2. Cool with that.
#3. I figured 232 degrees intake would be a little shy for high RPM use. It's
pretty amazing to see the stock duration of the LS1 specs compared to gen1/gen2
small blocks.
#4. Agree. Still thinking carb'd early model small blocks. It's easy to forget
that a higher compression LS1 with fast burn chambers, EFI, etc. is going to
handle cam timing in a totally different method.
Where do we go from here? We're barely scratching the surface!
10-24-2005, 02:00 AM
#78
I'm only comparing the first cam against the changes... not against a stock cam
Why don't we start doing a overlap or LSA change, and that's it....
So lets take the 234/244 114 LSA cam and take 4 degs out of the LSA for ***** and giggles.
IVO - 7* BTDC
IVC - 47* ABDC
In. Duration 234 degrees
EVO - 52* BBDC
EVC - 12* ATDC
Ex. Duration 244 degrees
Overlap is 19 degrees
Intake Centerline: 110
Exhaust Centerline: 110
Lobe Center: 110
So now the intake valve is opened sooner and closes sooner to raise the DCR. The exhaust valve opens and closes later.
The IVO and EVC are both increasing the overlap, which is going to help the top end HP and cause a rougher idle and low speed TQ decreases. The earlier IVC helps increase the DCR and will help out the low and midrange TQ. The later EVO is also going to help the midrange TQ as well.....
The net effect should be a similar or decrease in low end TQ, a increase in midrange TQ and a gain in HP at the high end. All with a rougher idle and about 1.5-2" of vacuum drop at idle. The only issue here is that lowering the LSA like this is basically diminishing returns, the lower you go the less you gain until you come to the best LSA/overlap for the motor. Then you can use the same overlap and find the best duration.
If it was me I would do a bunch more changes to this cam, but this isin't about what I would do for a cam, it's about what changes do what to the power curve
Bret
Why don't we start doing a overlap or LSA change, and that's it....
So lets take the 234/244 114 LSA cam and take 4 degs out of the LSA for ***** and giggles.
IVO - 7* BTDC
IVC - 47* ABDC
In. Duration 234 degrees
EVO - 52* BBDC
EVC - 12* ATDC
Ex. Duration 244 degrees
Overlap is 19 degrees
Intake Centerline: 110
Exhaust Centerline: 110
Lobe Center: 110
So now the intake valve is opened sooner and closes sooner to raise the DCR. The exhaust valve opens and closes later.
The IVO and EVC are both increasing the overlap, which is going to help the top end HP and cause a rougher idle and low speed TQ decreases. The earlier IVC helps increase the DCR and will help out the low and midrange TQ. The later EVO is also going to help the midrange TQ as well.....
The net effect should be a similar or decrease in low end TQ, a increase in midrange TQ and a gain in HP at the high end. All with a rougher idle and about 1.5-2" of vacuum drop at idle. The only issue here is that lowering the LSA like this is basically diminishing returns, the lower you go the less you gain until you come to the best LSA/overlap for the motor. Then you can use the same overlap and find the best duration.
If it was me I would do a bunch more changes to this cam, but this isin't about what I would do for a cam, it's about what changes do what to the power curve
Bret
Last edited by SStrokerAce; 10-24-2005 at 11:59 AM.
10-24-2005, 11:07 AM
#79
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I think I see why this LSA & overlap business is so confusing, even with a bunch of knowledgeable guys trying their best to make it clear!
The problem is that LSA, overlap and even duration are not truly defining characteristics of a camshaft; they are merely a measurable resultant of four independent variables of differing importance, namely IVO, IVC, EVO and EVC. Rather than rattling off all four, plus lifts, etc. in trying to describe a cam, we naturally look for shorthand summaries. For any normal cam design, the I & E durations are reasonable approximations of its general RPM characteristics, but overlap and more so LSA are less clear-cut pointers.
For example, since IVC is by far the most influential event, by advancing a cam we are primarily just closing the intake sooner, thus biasing the engine characteristics towards a lower RPM range. The other 3 events are dragged along for the ride, but any effect from the change in IVO, EVO and EVC is relatively minor. Similarly, if we widen the LCA with no other changes, say by retarding the intake lobe 4 degrees, the exhaust events are unchanged and although we have also reduced the overlap, it is mostly the later IVC to which the engine responds.
In a similar fashion, overlap is not a spec that the cam designer selects directly, it is the resultant of the chosen IVO and EVC points. (How would an engine respond to a cam with 60 degrees of overlap if it was generated by an IVO of 60 and an EVC of zero, rather than 30-30?)
In summary, because overlap and LCA are 'ground-in' resultants which cannot be changed without creating a new cam, it is technically incorrect to talk of 'increasing' the former or 'widening' the latter. We should be speaking directly of the changes to individual events. Human nature being what it is, we will continue to 'shorthand' cam descriptions, but we should at least try to remember the underlying realities.
Thought for the day: Considering the wildly different conditions and requirements, not to mention valve sizes, etc., what are the odds that at any given RPM an engine really needs exactly the same I & E lift, duration and timing, as provided by so many cams?
The problem is that LSA, overlap and even duration are not truly defining characteristics of a camshaft; they are merely a measurable resultant of four independent variables of differing importance, namely IVO, IVC, EVO and EVC. Rather than rattling off all four, plus lifts, etc. in trying to describe a cam, we naturally look for shorthand summaries. For any normal cam design, the I & E durations are reasonable approximations of its general RPM characteristics, but overlap and more so LSA are less clear-cut pointers.
For example, since IVC is by far the most influential event, by advancing a cam we are primarily just closing the intake sooner, thus biasing the engine characteristics towards a lower RPM range. The other 3 events are dragged along for the ride, but any effect from the change in IVO, EVO and EVC is relatively minor. Similarly, if we widen the LCA with no other changes, say by retarding the intake lobe 4 degrees, the exhaust events are unchanged and although we have also reduced the overlap, it is mostly the later IVC to which the engine responds.
In a similar fashion, overlap is not a spec that the cam designer selects directly, it is the resultant of the chosen IVO and EVC points. (How would an engine respond to a cam with 60 degrees of overlap if it was generated by an IVO of 60 and an EVC of zero, rather than 30-30?)
In summary, because overlap and LCA are 'ground-in' resultants which cannot be changed without creating a new cam, it is technically incorrect to talk of 'increasing' the former or 'widening' the latter. We should be speaking directly of the changes to individual events. Human nature being what it is, we will continue to 'shorthand' cam descriptions, but we should at least try to remember the underlying realities.
Thought for the day: Considering the wildly different conditions and requirements, not to mention valve sizes, etc., what are the odds that at any given RPM an engine really needs exactly the same I & E lift, duration and timing, as provided by so many cams?
10-24-2005, 11:59 AM
#80
Good Post.
Throw in there the effectiveness of the intake and exhaust systems to help make power, the flow characteristics of the heads.... etc... and then it makes you wonder what cams really need as you say the same I & E lift, duration and timing.
Bret
Thought for the day: Considering the wildly different conditions and requirements, not to mention valve sizes, etc., what are the odds that at any given RPM an engine really needs exactly the same I & E lift, duration and timing, as provided by so many cams?
Bret