cam ideas

[plovett]

Please let us know what you end doing. 

paulie

[c9zx]

After a lot of re-thinking almost everything, let me approach this a second time. 2500 lb. Cobra, no vacuum accessories, TKO 600 trans., 3.55 gear, Genesis 4.250 bore, 4.125 stroke, 6.700 rods.The Diamond pistons PN 42510 (468 CID) yield 10.87 with the piston .005 in the hole. The first priority is that the engine must run on 91 octane pump premium with no high speed detonation. The heads are still Edelbrock 72cc, Stage-X (Barry R.), intake is still BT dual plane, 850 HP or "true" 950. Please look at the following cam specs on a solid flat tappet and give me your opinions especially regarding the detonation issue. I'd rather leave power on the table than fail the cam and/or the engine. All input is appreciated. 294/300 Adv. Dur., 248/252 Dur. @ .050, 109 LCA, install on 105 .620/.630 lift. Is there something to be gained by using slower ramps on the exhaust lobe?  Thanks, Chuck

[plovett]

I come up with a dynamic compression ratio of 8.4:1 with that cam installed at a 105 ICL with 10.87 static compression.  I think that would work fine with your vehicle weight, cubic inches , overall gearing, and 91 octane pump gas.  Even so, given the little need for low end power with such a combination, I would play it even a bit safer.   I would install the cam at a 107 or 108 degree intake centerline.  That would drop the DCR to 8.25:1 or less.  That will help just in case you happen to have two 200 pound guys in the car, 100 pounds of luggage in the back, and are going uphill in 5th gear at 30 mph in 105 degree heat, and you romp on the throttle, you should still be okay.

Your ignition timing curve will play a factor as well.

JMO,

paulie

[plovett]

I think I made a mistake in my previous post.  Sorry.  I recalculated the DCR and came up with 8.16:1.  That's with 10.87 static compression and the 294/300 cam installed at a 105 degree intake centerline.  That should be fine with 91 octane gas with your combo.  Bring the ICL back to 107 degrees and the DCR falls to 8.01:1.

It is pretty common for the exhaust lobe on a cam to have milder/slower ramps than the intake lobe.  I'm not positive about the details, but I don't think it's as critical for the exhaust lobe to open and close so quickly.  The exhaust has residual cylinder pressure and a rising piston to help force it out.    The intake side is a different story.  It relys mostly on the pressure drop caused by the piston dropping and the cylinder volume increasing.?    So in general you want to get the intake valve open wide, where it flows more, as fast as you can, then shut it fast when the intake flow stops, so as not to lose any intake charge back out the manifold. 

paulie

[jayb]

+1 on what Paulie said, except that in my opinion you can get any engine to knock if you lug it in a low enough gear or put a high enough load on it.  But you should certainly be safe at higher RPM under normal driving conditions with that combination.