Originally Posted by fosterdl
Once again I turn to the vast knowledge base here. I've just installed a new Comp Thumper cam in my 65 SS Impala which has a 78 350 in it. I has talking to a mechanic the other day and he said I would have to advance my timing by as much as 15 to 35 degrees and that i'd find that information on my cam line sheet. Maybe I'm dumb, but all I see are specs for the cam in relation to the valves. My question is, does one change the spark timing when installing a new cam and if so how do you know where it should be. Sounds like the old "go get me a mud valve" line. Any help is appreciated.
I'll second what 454C10 has said. A hot cam reduces low speed mixture density inside the cylinders because of the effects from the late closing intake valve and to an extent the increased overlap where the in and out valves are both open as exhaust ends and intake begins. In the former case the late closing intake lets the piston push mixture back out the intake until the engine is turning enough RPM to develop port velocities fast enough to overcome this characteristic. In the latter case, when both the intake and exhaust are open at low RPMs, the engine will variously trap some exhaust gases in the cylinder or leak incoming charge out the exhaust till the revs get high enough for the intake charge to completely blow the exhaust out and provide enough overcharge in the cylinder that losses past the exhaust valve are compensated for. These are the principle causes of what’s called reversion.
These conditions cause the engine to loose bottom end power and are the reason the engine lopes on idle. Lope you can't do much about, but the low density mixture causing a loss of power at low revs is improved by increasing compression which forces the available mixture into less space which increases its density in the cylinder which increases its burn rate while, also, increasing early spark advance to give the lower density mixture more burn time will improve bottom end performance with a hot cam. Keep in mind that neither increasing compression or increasing the base timing will restore bottom end power compared to a milder cam; it just improves the situation compared to doing nothing.
Now at the other end of the world, the hot cam combined with the mixture ram effect at higher RPMs tends to super fill the cylinder so density goes up. This means that you don't need the additional spark advance that is used at low RPMs. With me? OK so what you have to do is create a situation where you're adding base timing thru advancing the distributor's housing position. But the degrees added to the base advance have to come off the variable advance or the engine will be over advanced at high RPMs. In this condition at a minimum it will surge at the worst it will go into destructive detonation.
So if the engine with a mild cam was set up with 5 degrees base advance and had 30 degrees of centrifugal advance for a total of 35 degrees, the total doesn't necessarily need to change with a hot cam but the relationship of base advance may become 15 degrees and the centrifugal has to be reduced to 20 degrees. The total is still 35 degrees. This is done either with a new short cam plate in the distributor or by filling the slot in the advance plate you have with weld or braze to create a new and shorter positive stop. The springs that react against the counterweights of the centrifugal mechanism do not control the amount of total advance; they control the rate at which the advance comes in. Stiffer springs slow the rate; lighter springs allow it to come in faster. Mixing springs allows adjustment between very slow with two stiff springs and very fast with two light springs.
The vacuum advance also needs to be played with if you're using one. Since hot cams reduce manifold vacuum, you will need to move to a vacuum advance that comes on at lower vacuums. The best solution is an adjustable unit that gives control over when it comes in and out with the available vacuum and adjustment as to how much. This needs to be synchronized with the centrifugal so that you're not getting both at higher RPMs or the engine will again become over advanced. This is more important to a street driven engine because the vacuum can be used to get a lot of advance at cruise where the RPMs of and load against the engine are fairly low and the throttle mostly closed causing the mixture to be of low density. Here again, with a low density mixture, you want considerable timing advance. For a competition engine where cruise economy is of no consequence, you can skip having a vacuum advance all together.