Explain wide duration cams?
Thread Starter
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From: Delray Beach, Fl.
Explain wide duration cams? 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...
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...
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From: Upstate NY
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
Thread Starter
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From: Delray Beach, Fl.
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?
Last edited by JS; Jan 31, 2008 at 08:35 AM.
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I'd never tell, even if I knew why! It isn't about pv clearance, BTW.
Jon
PS, on another topic think "delta p".
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.
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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?
Basically, the exhaust is too slow, so it needs longer to do the same job. How's that for layman's terms?
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Thread Starter
Joined: Nov 2001
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From: Delray Beach, Fl.
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?
EG..
How would they work in a street car,a drag car,power from 2000 to 5500 etc?
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.
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Jon
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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.
Last edited by Gary Z; Jan 31, 2008 at 06:13 PM.
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.
Am I to China yet?
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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.
Originally Posted by OldSStroker
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.
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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.
