Cam + Compression

[440Charger]

Could someone explain the relationship between the camshaft lift/duration and the compression ratio?  In other words, how does one match them up.

thx,
- Doug

[Chryco Psycho]

Cam / lobe lift has nothing to do with compression other than at higher rpm you will fill the cylinder more completely
Duration & overlap control compression , short duration cams tend to trap more air @ lower Rpm causing low rpm detonation , more duration & overlap tend to build more pressure at higher RPM & at high rpm the velocity of the exhaust leaving can really help pull air into the cylinder with the potential to actually mildy super charge the engine so the cylinder can actually fill by more then 100% capacity meaning the A/F charge si pressurized slightly in the cylinder
 overlap is the period of time where both valves are open at once

[440Charger]

Thanks Neil - we're considering camming down our 440 to get more low end performance, and since I'm already detonating I'm wondering if taking the intake duration from 230 to 224 or 226 will increase my problem. 

[Chryco Psycho]

not nessisarily , you are detonating at high rpm , a small cam will reduce compression at high rpm

[PlumCraZRT]

It doesn't matter so much what the entire duration is either... it really depends more on what your intake closing angle (ICL) is.  Of course you do get better scavenging with long duration cams, but the cylinder pressures (at least at low RPM) is more determined by your ICL.  Basically this determines how much of the cylinder volume is waiting to be compressed when the intake valve closes.  A lot of different cam profiles will have the same ICL despite the fact that their durations are different.  There's a good chance changing your cam will keep the same ICL and your problems won't be fixed or worsened.

[moper]

not nessisarily , you are detonating at high rpm , a small cam will reduce compression at high rpm

I dont think that's true as I understand things. The combination of intake lobe closing event, and physical make up of the engien will determine a cylinder's pressure. Too high, and the air/fuel mix may be ignited prematurely. You hear that as "pinging". Detonation is not quite the same thing, but is just as destructive. Once an engine hits the detonation threshhold for the fuel and pressure, it never goes back down. The only thing you can control is the rate that the pressures build. And that is done with that closing event, and the cam's "overlap". Which is what Neil described. A smaller cam very generally, closes the intake valve sooner in terms of crankshaft degrees, during the piston's up stroke on the compression stroke. It makes sense really, the sooner the cylinder is sealed, the more pressure will build up in it while the piston is rising. As rpms increase, the amount of pressure will rise, as the physics and dynamics of a running engine help fill the cylinder. Things like exhuast scavenging, an intake pressure wave tuning can add more air into a cylinder than it theoretically can draw in itself. So the pressure do rise as rpms increase. But they wont ever decrease, unless/until the valves float. If you lose control of the intake valve clsoing (it bounces off the seat) then the cylinder will lose pressure. If you are pinging or detonating now, then you need to figure out why. If it isnt all the time, like only at light throttle cruise, or only on 87 octane, then it may be possible to re-tune and remove the detonation. But if it's caused by excessive pressure, the only ways to fix it are retard the cam, or replace it with a bigger one, or replace the fuel with one more suited to that pressure.

[440Charger]

But if it's caused by excessive pressure, the only ways to fix it are retard the cam, or replace it with a bigger one, or replace the fuel with one more suited to that pressure.

Engle told me that I may need a bigger cam to release the pressure.  My current cam is 230@.050 (in/ex) and 480 lift.  The compression is about 9.8:1 on a set of '78 452's, 3:55 gears, 727 auto.  I don't know if he meant more lift or duration.  I use 92 octane.

My LSA is 109, and ICL is 109, cam card here http://www.holley.com/data/Products/Technical/00032.pdf

Any advice?

thx,
- Doug

[Chryco Psycho]

a Bigger cam will have more duration / overlap

[moper]

a Bigger cam will have more duration / overlap

yeah, that's where Engle was going. You will "bleed off" the pressure at lower rpms, raising the threshold (hopefully) beyond where you have it now. 452 heads have that huge chamber. Have you tried any decarbonizing stuff? Carbon build up in the chambers creates hot spots that could be working against you too. Water in a mist-it bottle works great for that...So does ATF (dosed carefully). Plugging in guestimate values on teh dynamic calculator, I'm coming up with a touch over 8:1 dynamic ratio. That should be OK on pump fuel. So, you may have oil leaking into the chambers from rings or guides, or and intake leak (lean and/or oil) or carbon buildup, or lean jetting, or too fast an ignition curve for your car/fuel type. I think mechanically speaking you should be fine. So I would first do three things. pull the plugs when it's hot(all , and get a cranking compression reading from various cylinders. Look at teh plugs...are any wet with oil, or have large amounts of soot or carbon on them? Last, put them back in, get it hot again, and while holding the throttle open and keeping the revs around 1800, spray a full mist it bottle of water down teh carb...It may sputter and it should pump a lot of steam out. Then take it for a couple full throttle under load (like up a hill on the highway) balsts. You should see a big cloud of soot come out at first. if the cylinder psi reading is higher than 180, you need to either replace or retard the cam, or remove compression. If it's under 180, you should be able to tune out the ping on high test.