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Discussion Starter · #1 ·
This is one area of camshaft tech that I don't quite get yet.

I've noticed that most LT1 cams are installed so that the intake centerline is about 4* advanced. For example, a cam has an LSA of 112, but the IC is set at 108. With an old school SBC this can be done by installing the cam a certain way in the block, bu this won't work with the LT1 due to the Camshaft position sensor and the Opti-Spark, right? However, it can be ground into the cam and it'll work abotu the same, right?

The guy on the CompCams Tech Line said this is to help torque. It would make more sense to me to just choose a lobe that's got lower duration if you want torque. :confused:

I kinda understand the whole reversion thing, but not in exactly where this works with different durations if that even matters.

This paragraph from this page helps, but I'm still not understanding it totally.
<BLOCKQUOTE>quote:</font><HR>I mentioned that the cam’s Lobe Centerline is often confused with LSA/LCA. I’ll try to explain LC now. Remember when I was talking about cam timing and the intake valve’s closing point? This is the cam’s Lobe Centerline. It is the intake lobe’s center (at its’ highest point) position in relation to the position of the piston at TDC of the intake stroke. The LC is expressed in a measurement of degrees like LSA is. It is usually with 4 degrees of the LSA designation, so it is often confused. When the piston is at TDC intake stroke, the intake lobe will be pushing the lifter up, opening the intake valve. The center of the intake lobe will be around 106 degrees before the piston is at TDC, or the piston’s position of 0 degrees. I’ll try to clarify that last sentence a little. For every two revolutions that the crankshaft makes, the cam will rotate once. All measurements of degrees are actually "crank degrees". One full crank revolution is 360 degrees. When the piston is at TDC, piston position is 0 crank degree and when it is at BDC, piston position is 180 crank degrees. When the piston is at approximately 106 degrees past TDC intake stroke, the intake lobe will be straight up and the intake valve will be fully open. Cams will come with a recommended centerline position from the manufacture. The one in this example is installed on a 106 Lobe Centerline. When a cam is advanced or retarded, the Lobe Centerline is changed. If we were to advance this cam 4 degrees, we would install it at 102 degree Lobe Centerline and 110 degree Lobe Centerline if we retarded the cam 4 degrees. I mentioned earlier that advancing the cam will increase cylinder pressure. It will to a point. When the cam is advanced, the intake valve will open earlier during the exhaust stroke and the exhaust valve will shut earlier during the intake stroke. If the cam is advanced too far, reversion will occur and the exhaust gasses will not be adequately scavenged. Four degrees advance is usually the most that you can safely advance a cam beyond the manufacture’s recommended LC. When the cam is retarded, cylinder pressure will be reduced but the scavenging process is increased. If you are experiencing pre-detonation, retarding the cam will help. It also has a tendency to move peak hp to a higher rpm. Again, care should be taken when changing cam timing. Another consideration when playing with cam timing is piston to valve clearance. When you change the valve events (timing), the clearances will change and should be checked.<HR></BLOCKQUOTE>

Here is another article by the same guy that is VERY GOOD: http://www.chevytalk.com/tech/engine/Cam_Selection.html

After reading that article I now understand better why George went with 5* advanced. He increased the exhaust duration, but he didn't want the exhaust closing point to be moved too much, so he advanced the cam so that the extra exhaust opens earlier. it's still confusing, but I'm gettnig a better idea at least.

I think it would be MUCH easier if somebody could/would make a graphical way of showing where the valves are at different piston locations. It would be REALLY AWESOME if we could plug in our own specs (duration at different lifts, valve lift with different rockers, etc) and have the timing stuff changed around on the graphic. Hey George, wanna give this a shot? :D
 
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Discussion Starter · #3 ·
Hey Cheston,

The reason I advanced the cam a bit is to relieve cylinder pressure with the juice early, in order to cut down on pumping losses, which can get astronomical with Nitrous. It also makes the car sound tougher!!

As far as the program, I could do it, but it would take a long time. I'll just stick with Desktop Dyno for now


By the way, those were excellent articles. However I do disagree with him when he says not to consider a cam because of the sound :D
 
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Discussion Starter · #4 ·
The reason I ask this is simple. I've put in some cam specs for how I hope my engine will turn out and I like the results with the LSA and IC both at 112.

Check these out from Crane.
.05 lobe" adv .200 1.5 - 1.6
226/3467 282 149 .520 .555
238/3667 294 161 .550 .587

With both the LSA and IC at 112 it puts the peak flat from 6000-6500 which is about where I want to shift at and it give a good power curve above 4000RPM. The only way I could come too close to that curve would be to do the LSA @ 111.5 and set the IC at 111. The HP peak would be a bit lower, but there would be more torque everywhere. PLUS the sound would be awesome


 
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