Originally Posted by v8hed
383 with GM HEI distributor (modded with aftermarket module, curve kit, coil and shimmed for end play). Before going to the chassis dyno, I'd set my timing up (with a good timing light) for 34* total. Checked it up to about 4,000rpm and I could see the advance curve adding more timing until around 3,000rpm then it remained static. As I said, I only went up to about 3,500-4,000rpm with the timing light... just enough to see that the advance curve had finished adding timing. On the chassis dyno, we were messing around with the timing (just adding/subtracting a couple of degrees to do some fine tuning). I asked the dyno operator to take it up to 6,000rpm and keep the timing light on the balancer. He said total timing went up to something like 50* by 5,500rpm! We checked with another light and same thing. Couldn't hear any pinging (although the car is v.loud). What I don't understand is how could this possibly be happening? I still don't believe everything is at it seems, since taking timing out resulted in less power. If is was really running up around 50* total timing, there's no way the motor would be making power. I've previously tried experimenting locking the mech advance out and didn't see over 36* (with about 14* initial). Since the holes in the base plate physically limit max advance, what's going on with this wacky timing reading? It had occurred to me that perhaps the ProStreet balancer I'm using could be slipping, but it's pretty new and looks perfectly fine to the naked eye.
Anyone got any other ideas or explanations? If not, my next step will be to try a brand new MSD HEI just to eliminate the distributor from the equation. What about spark scatter or some other kind of effect?
If you're running vacuum advance, which you are, you have to go further than just pulling the vacuum can hose and plugging the port the advance plate also needs to be screwed down so it cannot move, otherwise its position is simply dependent upon the return spring in the vacuum can which can be excited by all the vibrations going on when the engine is running at high RPMs.
Another common event is the natural looseness in the timing chain and how it reacts to high RPM excitement. Then there are a number of clearances such as the clearance between the cam's distributor gear and the distributor's gear. The thrust clearance of the cam, typically the flat tappet chevy is unrestrained in forward thrust except by the distributor gear and there is a clearance of the distributor shaft and the housing allowing it some freedom of movement. Even roller cams have a little thrust freedom that works on the distributor gear mesh. All cams have a lot of rotational shake from the slowing when valves are opened against their springs to accelerations on the closing side. Nothing in this system is truly stable. All these clearances can and do gang up to change the timing. This is the reason why competition engines use the crankshaft as the timing reference to trigger the spark rather than a distributor at the end of a group of gears, chains and shafts. There are also other gadgets sold that apply tension to the timing chain for a more consistent holding of set position by not allowing any slack in the chain. Not as good as a crank trigger for ignition timing but very helpful in keeping the camshaft to crankshaft alignment steady. A thrust button on a flat tappet cam is also helpful to limit its fore and aft travel so that doesn't disturb the distributor gear mesh with the cam. Gear drives can be useful in eliminating the gear chain issues but you buy something of the quality and design of the Shaver-Wesmar to really get the needed effects the popular inexpensive drives don't solve this problem and make a lot of noise not doing it.