JS

iTrader: (4)

GM runs wide dur in the LS1/LS2/LS3/LS7
GM has a HOT and ASA that both have between 9 and 10 degree spilts

The LS7 S2/3 cams are even more?

Whats the advantage to running large splits on the int/exh. duration
More interested in the Hot and ASA style cams...

Old SStroker

The answers to this very good question should be interesting.

The basic answer is "the engine likes the split". I think you want to know the "why". I'll defer to others for that answer.


Jon

JS

iTrader: (4)

Wouldnt the longer exhuast dur. help fill the the cyl with more air via the cam timing events...I would also think the longer dur would aid in crutching the heads once the intake was bolted into the equaztion?

3.4camaro

iTrader: (1)

Looking for some people to dig themselves in a nice hole, are we? Should be a good laugh.

Old SStroker

Nah, not really. I don't think many folks who currently post here understand why the big splits work on these engines. Those who do are making horsepressure and money from the knowledge. GM, for one.

I'd never tell, even if I knew why! It isn't about pv clearance, BTW.


Jon


PS, on another topic think "delta p".

1989GTA

I always thought the primary reason had to do with emission controls.

Drew04GTO

iTrader: (1)

Just a guess but the pressure drop across the much smaller exhaust valve seat would require more time (duration) to effectively evacuate all spent gasses. Most people that I see take full advantage of cams that arn't split usually have heads that flow excelent on the exhaust side and have large headers. Infact, most l92 setups usually have huge splits in cam timing due to their much higher intake flow compared to exhaust.

JS

iTrader: (4)

Yes but what does that mean in laymens terms?

Scoggin Dickey

iTrader: (34)

It means that the head's capability to evacuate exhaust gasses is not sufficient in relation to the head's ability to draw fuel/air into the cylinder. The larger duration on the exhaust side just helps to make up for that deficiency.

Basically, the exhaust is too slow, so it needs longer to do the same job. How's that for layman's terms?

5.0stang

iTrader: (1)

If I recall, when the exhaust gases are being evacuated out the exhaust valve, it helps pull in air on the intake side, simulating a vacuum. Correct me if I am wrong...

JS

iTrader: (4)

Okay I got u so far,but what are these cams (ASA/HOT) designed to do threw the rpm range and why are they used for other than showroom stock racing..

EG..
How would they work in a street car,a drag car,power from 2000 to 5500 etc?

Drew04GTO

iTrader: (1)

The headers/exhaust/mufflers actually have more to do with exhaust scavenging than the cam itself does. I guess a cam with too much exhaust duration could cause you to overscavenge if you have an exhaust that's way beyond overkill. But, from what I've seen a well tuned exhaust side (including cam) can influence the intake charge about 8 times more effectively than the pressure differential caused just by the piston moving down in the bore.

JS, from what I can tell cams like the ASA and other aftermarkets that are supposed to work with stock heads or ported stock heads will usually have a split to them to help out the sub-par ls1 exhaust ports. Most of these will be awsome for your application if its ~230 degrees duration or less on agressive lobes. I personally love the crap out of the ASA cam, Show me another cam anywhere near its size that does 440hp/427ft lbs with ported 243 heads.

Gary Z

The engine likes the split because intake and exhaust conditions are very different – pressure, temperature, and valve diameters are all different. I am surprised symmetrical cams work as well as they do.

Old SStroker

By "symmetrical" did you mean equal intake and exhaust duration rather than lobe shape?

Jon

Gary Z

Yes - I meant, because intake and exhaust conditions are different, equal duration cams seem to me to be a less likely optimum than unequal duration.

gametech

iTrader: (1)

I'll take a shot at digging myself into a hole here. The EPA is the reason cams are favoring exhaust flow. Between cats to clean emissions, and mufflers to quiet sound, the exhaust side takes a bigger beating on the overall flow than the intake.

Drew04GTO

iTrader: (1)

Cats maybe, mufflers definately not. Mufflers are nothing more than pressure wave terminators, if placed in the right spot they can make more power than a full length exhaust without mufflers. I think 100% of the cam "split" is because the intake is leaps and bounds better than the exhaust. So much to a point that larger exhaust durations much be used so you don't dilute your intake charge.

Am I to China yet?

Jessica

That right there sir should end this discussion.

It amazes me about the lack of anything else even close in this thread, but you can't expect much, OldSStroker nailed it on the head as to why your not going to get a better answer.

Gary Z

It is not always true that “the engine likes the split”. GM stock cams have the largest splits: LS6 - 204/218, LS7 - 210/230. Stock cams are compromises reached by trading conflicting constraints and goals against one another. Stock cams must accommodate a semi-restrictive exhaust system. A no-compromise cam for maximizing horsepower with no concern for fuel economy, smooth idle, or emissions, assumes a free-flowing exhaust system, and will have a smaller split. A large displacement Gen III engine with longtubes and restrictive heads and intake will make max power with a reverse-split cam.

ArcticZ28

iTrader: (14)

So do you care to share your thoughts?