Reason for Exhaust Bias of GM Cams?
 

Is the large exhaust duration bias for the following three GM cams to match up with the poor flowing stock exhaust manifolds (i.e., no headers)?

If not, why is this bias greater than what is typically seen for aftermarket cams?

Hotcam
219 228 0.525 0.525 112

ASA
226 236 0.525 0.525 110

GrandAm
239 251 0.570 0.570 106

I belive it is due to the flow of the intake and exhaust, not limited to the engine. ie intake all the way from the lid and exhaust all the way to the tail pipes. :chug:

smaller intake duration keeps the power lower in terms or RPMs. Maybe its a reliability issue with spinning the motor too high?

Does the ASA and the GrandAm rules allow for headers?

One word will answer your question.......Emissions. Look at LPE's cams..they are all wide split but, they all pass emissions.

The additional exhaust flow is to make up for poor(er) flowing exhaust ports on stock LS1 heads compared to the intake ports.

compared to other heads the intake to exhaust ratio is very good...like I said it was mainly because of emissions.

I dont think its any higher than any other cams.

A popular Lunati cam for the Fords is a 218/226 .498/.510

One of the most popular cams of all time is a 234/244 .488/.510

Most traditional cams are single pattern or split patterns with 10 degrees of split.

Nearly every Edebrock cam made is a either a single pattern or a 10 degree split no matter if its Ford or Chevy.

The hotcam really isn't supposed to be used in vehicles that need to be emissions compliant though. Off-road use only...LOL.

thats why i love florida lol! no emisssions! lol

emissions?...we don't need no stinking emissions!

most engines, heck ALL conventional piston engines, prefer more exhaust duration then intake duration for power.
think about this:
you're sucking in, 43.375ci of air (ideally).. but after the cyl ignites, you have much more then that to push out. the more exhaust you can get out, the more fresh air you can pull in.

most aftermarket cams (for any motor) are either straight (ie 224/224, 232/232, ect..) or split with more exhaust....
i see some guys on here with a "reverse" split.. why, i donno.. seems like they're shooting themselves in the foot, and the results go along with that, but whatever. :)

GM designs their cams to work on an engine with stock manifolds and exhaust. Therefore they hold the valve open longer to allow a decent amount of air to find its way out since its not a good exhaust system.

Not true. All of the GM off-road cams listed above are designed for use with headers. In fact, GM engineers will tell you that the Hot Cam will make less power than stock cam if you don't put headers and free flow exhaust on the car. The reason is these cams all have intake exhuast/overlap for scavenging action that you can only get with headers.

ASA cam and Grand Am cams were designed for RACE cars with Longtubes ;)

Infact, the ASA cam is for the ASA series, that has headers from the get-go ;)

The GM Performance catalog I have specifically says the kit (12480033) is designed to be used on engines with no other modifications. And I don't believe that a GM engineer would tell you the hotcam will make LESS horsepower than the stock cam without headers.

Ok, none of you guys know what the hell you are talking about! :devil:

:werd:

i'm probably wrong, but i always through turbo cars used less exhaust duration to get the gas velocity higher. NA cars with more restrictive exhaust would want an exhaust bias to allow more time for the gases to escape.

I could probably dig up the quote if you're interested. The engineer was the one in charge of the team that designed the LS1 cylinder heads, John Juriga. The interviewer had asked him how the new Corvette with the LS1 would respond to the GM Hot Cam. He said the engine would actually lose power with the stock Vette exhaust system and hot cam. So we know it would even worse with an fbody, since the Vette has a more free flowing exhaust. As I said before, cam overlap and headers work together to provide scaveging action. Without headers, the overlap is just bleeding off cylinder presure with no real benefit other than the cool exhaust sound.