which affects an engine more, intake duration....or exhaust duration? And how do the two combined affect each other? (such as split dur., rev. split dur. and even durations?) I know its pretty basic stuff, but I want to make sure I understand it correctly.

 

Welllllll.....leaving other camshaft variables (lobe separation, centerline, lift, ramp speed, et al) alone for a moment, and without getting too technical, the biggest factor is intake duration, as long as exhaust duration is not so much lower as to cause serious restriction (and this is not the case on any normal cams of which I am aware). The bottom line is the ability to get more exhaust gases out doesn't do anything if you don't get the air in first for compression and combustion.

Intake duration largely determines how much power a cam will make and where in the powerband it will make it (bigger intake duration means more power, and moves powerband higher in RPM range). You can theoretically continue to get more power by going bigger, but you begin to lose torque at lower engine speed.

Total duration (intake + exhaust), or the average has an impact on valve overlap, and thereby on manifold vacuum and idle quality. The bigger you go on total duration, the greater the overlap and lower the vacuum you get, resulting in a rougher idle, again other variables held equal.

A conventional split duration cam has more duration on the exhaust side than on the intake. This is a way of keeping the powerband where you want it via the intake duration, while providing more time for exhaust scavenging. This approach has been used traditionally to compensate for heads with weak exhaust ports (SBC). Some also feel it can make up for a more restrictive exhaust system although I'm not sure this has really been adequately demonstrated.

LS1/6 head design is so good that conventional splits have fallen from favor somewhat. Better results seem to come from single patterns (intake and exhaust duration equal) at any level of total or average duration. Going bigger on the exhaust side appears to be a waste, even with stock manifolds, although GM still does on its factory grinds.

Some guys are using the new "reverse splits" in our engines, with good results. These have intake durations slightly higher than on the exhaust side. The attempt is to take advantage of the head flow capacity and compensate for the relatively restrictive intakes that are available for our cars. These cams come into play at higher durations and power levels where the intake restrictions start to be a factor.

You can search and get more on this subject. I believe some of the cam manufacturers like Competition Cams and Crane also offer valve timing tutorials on their websites. Hope all of this helps. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

<small>[ April 15, 2003, 02:40 PM: Message edited by: SSLink ]</small>

 

That's some great info for green horns like myself. So the norm is (example) 222/222 and the out of norm (example) is 224/222 but the later seems like what is widely used because of the restrictive intakes LS1's have.

Great info.

 

Don't quote me on this, but I believe some testing MTI did way back in the days when they developed the famous B1 and T1 cams showed that going over 224 degrees duration on the exhaust side showed rapidly diminishing returns on stock cubes. Therefore a 227/224 might be a good starting point for reverse splits. As you can tell from my sig, I'm a cam conservative and would probably never have need for a reverse split, unless I get a stroker for my new GTO (dream on).
<img border="0" alt="[driving]" title="" src="graemlins/gr_driving3.gif" />

<small>[ April 15, 2003, 10:03 AM: Message edited by: SSLink ]</small>

 

There is generally agreement on the subject of the effects of duration. Bring up the impact of LSA and you always seem to stir up a fight.

It's amazing though how the other camshaft variables can effect performance. For example, on the '02 LS6 cam, the TR "old man" grind and the LPE bumpstick that are really hot right now. They all feature relatively low intake duration, but still make good top end power due to high lift, and a wide LSA to extend the powerband. What I think they trade off is the strong combination of better midrange torque and top end horsepower you get with the bigger duration/tighter LSA approach. The latter gives you more "area under the curve" IMHO. <img border="0" alt="[USA]" title="" src="graemlins/patriot.gif" />

<small>[ April 15, 2003, 03:04 PM: Message edited by: SSLink ]</small>

 

Funny thing, i made a post very very similar to this one but asked my questions slightly diffrent. This one got the answer i was looking for. Thanks for taking the time to enlighten us. I have friends running any number of diffrent combinations on their LS1s and im getting close to ready for a cam myself so any comparisons i can find are great.