i see some c5&F LS1's are 445rwhp with a 230-227 112 lsa cam.
1'st time i've seen less duration on the exhaust.
Years ago stock heads flowed so bad they needed more exhaust.
Now lots of aftermarket heads have great exhaust flow.
I'm thinking this could help maintain good vacuum/idle with bigger intake duration even on old sb's .
any positive thoughts?


[Modified by Matt Gruber, 10:09 AM 7/21/2002]

 

It goes along with the idea of more lift on the intake than the exhaust that you see on dyno test motors from time to time in the different magazines like Hot Rod.

 

Seems like more and more cams are having the same pattern for both intake and exhuast. Have a 226/226 in my LT-1.

 

I've found that if the head is a well balanced head (i.e. flows 0.75I,1.00E), that the exhaust duration is usually shorter than the intake duation for higher performance engines. This is partly due to the fact that the E and I lobes are selected from the same family of lobes. Ideally the exhaust would have a custom opening profile to create the right timing and strength for the scavenging wave in the header. An intake-like opening on the exhaust just wastes power and "clogs" the header with energy that was supposed to make it to the rear tires.

Another reason that the intake is longer is strengthened ram tuning from the large plenum fuel injection manifold. The bigger the area change from the runner exit to the plenum, the stronger the ram tune. This means that some intense cylinder filling comes late in the intake cycle, requiring later closing for best possible performance. Carbureted manifolds need smaller plenums for good signal to the carb, and this severly limits the strength of the ram tune. The intake valve must be closed earlier with typical carbureted manifolds, thus the shorter duration and reduced ILCA.

 

the problem is all the testing required to verify. like redrocket ,it is easy to get a single pattern cam, but as you suggest, there could be a better cam; why shoot power stroke energy out the exhaust?
i guess one could copy the LS1 cam and hope for the best, with expert advice from the cam company. still sounds dicy without testing.

 

You're right, don't guess at the exhaust lobe. It might need to be shorter or longer than the intake. A quick call to the cam company might lead to a guess, too.

 

Quote:
"why shoot power stroke energy out the exhaust?"

That is not likely to happen.

Matt,
You have posted an interesting topic. I have been there with this type cam you speak of a few times.
The 1000cc Suzuki Wes Cooley rode for the factory was cammed this way by Yoshimura. The result was tremendous midrange that hept him in trouble most of the time. That particular engine had a really nice exhaust port and a rather poor in comparison intake tract. The only way they got by by doing this was the use of a open header,,a large tube one at that and the motor still "walled" and was shifted early to compenstate

I have had some custom ground cams done for a few with aircooled VW engines that needed more in the middle ,again with great success but these motors were short shifted to an extent

On the small Chevy certainly the aftermarket has stepped up and made a much better exhaust port but for the most part imo it still only allows for a single pattern cam if using any type of a streetable exhaust system.

When you see a cam grind from a Company that features a shorter lift on the echaust it is merely to reduce risk of valve to valve collision during the overlap cycle with a canted valve head like the Rat motor.

I have had cams ground with high intensity intake lobes "oval track" and long lazy exhaust lobes before but only to help the valve job survive when large doses of NOS was used every time the engine was used

Would a cam like that work on say a stock 77 vette? I think it might if it had 3.08 gears and automatic and the owner just wanted low and midrange only and the car was shifted at say 4500 rpm for example. The owner would need be prepared carb wise to jet down as this would increase the volumeric efficiency and would need leaned out some

Although a somewhat outdated book,Smokey Unicks "Power Secrets " has some good reading on extraction of exhaust and how a single pattern excells at this if all esle is correct.Even with a GM head but again,a Race exhaust is used

On those LS1's,,they might be having trouble getting enough mixture in is why they are trending towards this,I know little about those engines so cannot comment further

The mazda miata engine,the thing is almost square and still is rev happy,,why? The ram effect of the manifold. A ported exhaust will outflow a ported intake on one of those motors suggesting the need of more cam on the intake there that would really help the midrange and I have thought of doing this but cannot find a way to play with the mixture short of putting a set of Webers on the car which will not be done,,especially with the G Reddy Turbo kit laying in the corner


:)

 

23 degree SBC heads do not respond well to this. The 15 degree LS1 has MUCH better exhaust port and will exhibit this tendency but it doesn't transfer over to the LT1/4 really.