Intake Closing

[Purple440]

Hey all,

I'm trying to find a cam profile that will help me lower my cranking compression.  My understanding is that a wider LSA will close the intake valve later and therefore help bleed off some pressure.  Well my cam has a 109 LSA and the intake closes at 42*.  I checked out the Crower d271 at a 112 LSA and the intake closes at 39*.  Finally I checked the Voodoo 60302 at a 112 LSA and the intake closes at 38*.  So it looks like the wider the LSA, the sooner the intake closes....

Cam Cards:
lunati voodoo: https://www.holley.com/data/Products/Technical/60302.pdf
crower: http://www.crower.com/misc/cam_spec/cam_finder.php?part_num=32242&x=32&y=10
current cam (lunati): http://www.holley.com/data/Products/Technical/00032.pdf

I also checked using this calculator: http://www.rbracing-rsr.com/comprAdvHD.htm and the cranking pressure shows my current cam on a 109 LSA producing the LEAST cranking compression, showing 172ps, about the same reading I got from my gauge.  It went up with the other two cam timings.

So what's up with that, anyone?  So confused 

- Doug

[aifilaw]

I think you've got something backwards... the smaller LCA is one way for you to operate with a higher static compression ratio.
Lobe Center Angle is the distance between the peak of the intake and exhaust valve in degree's. Your current cam has a 109 degree LCA, whereas the other two have 112 degree's. That is just one way to do it, and probably the most common method... but I think either of those two choices will fail you if you are looking to bleed off some pressure to prevent detonation... although the voodoo definitely has some good torque potential.

The number really has little to do with when the valves open and close, it solely has to do with the distance between the two... you could have the exhaust and intake opening and closing 20 degree's too soon each which would make the engine run terribly, and yet have the same LCA.

Having a lower IVC degree, or intake duration is another way to do it, but usually results in less power and efficiency. I normally opt for the smaller LCA to give more overlap which if done correctly helps the entire process, but that usually requires more work to the heads and valves. But all of this really depends on the rest of the combination, larger duration does not necessarily... but in regards to off the shelf cams... typically ends up giving you a higher functional rpm bandwidth that the cam is good for, and throws off everything else you may have done for the drivetrain, headwork, et cetera.

[firefighter3931]

Doug, the intake closing point is the critical factor. LSA will influence where the intake valve closes and duration also plays a part. Here's a quick & easy Chart from Comp Cams comparing wide vs narrow LSA's and their effects :


http://compcams.com/Technical/FAQ/LSAproperties.asp



Ron