vettenuts

iTrader: (4)

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).

Ric

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( https://ls1tech.com/forums/advanced-engineering-tech/394940-those-who-want-big-nasty-cams-lots-lift-etc.html ).

Ric

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

DAPSUPRSLO

iTrader: (14)

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.

DAPSUPRSLO

iTrader: (14)

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.

DAPSUPRSLO

iTrader: (14)

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???

MadBill

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.

DAPSUPRSLO

iTrader: (14)

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???

Ric

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.

DAPSUPRSLO

iTrader: (14)

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

MadBill

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.

Rick@Synergy

iTrader: (5)

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

DAPSUPRSLO

iTrader: (14)

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.

MadBill

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.

DAPSUPRSLO

iTrader: (14)

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

sn95leen

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@Synergy

iTrader: (5)

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

DavidNJ

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.

MadBill

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.

white2001s10

iTrader: (4)

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?