Newbie question.

I see in my CSM, to set the initial timing on my '66 Toro, they want you to set the engine speed up to 850 RPM while dialing it in rather than do it at the curb idle speed of 575 RPM. Just wondering, why we do this? I mean, initial timing is the same at 575 or 850 right because the centrifugal advance isn't kicking in yet correct?

Thanks!

 

It all depends upon the point where the advance starts. The Olds engineers had the timing curve figured out and specified the degrees at a specific RPM that matched their defined curves. Any deviation from the factory advance curves means the settings need to be adjusted appropriately.

 

All of that was set when gas was still good. With the fuel of today all of those factory settings are just starting points for what will work for your application. Don’t get too hung up on factory settings.

 

It was the same way back when. I think the reason to raise the idle speed is to get a rock solid engine speed with no fluctuations.

 

In almost all cases, the mechanical advance IS operating at those low engine speeds. You can test this yourself. Check the timing at idle and again at the higher speed.

 

Ahh, I see. I guess they just want a stable, fairly low, starting point to make the adjustment consistant and right where they want it.

 

Could be other things such as alternator cut-in point, water pump rpm & fan, vacuum for accessories, maybe even torque converter rpm on a mild incline. I'm not an engineer just a knuckle-dragging mechanic and my brain hurts now.

 

Frankly, I've never paid too much attn to that, I set timing at whatever my curb idle speed is.

 

idle smoothness for steady timing light indication is why. If you idle smoothly at curb, do it there.

 

In case it helps, my big cars (not hot rods, not really) have a 18° at idle around 700-750 rpm when warmed up. What I’ve found with latter day gas and a lot of fiddling is that a lot advance at idle makes them idle smoothly, but the amount of advance built into HEI’s caused my cars to ping (knock) on alcohol-gas we get in CA. I played with different vacuum advance cans, weights and all that for years and years and always wound up with smooth idle + knock, or no knock + no performance from dialing the timing back.

Not a shill but in the end what solved it was a Progression Ignition Bluetooth HEI. Neat device - no advance <900 rpm for easy starts, then you build your own timing maps (easily!) to suit your engine, gas and performance goals. In my case 18° at idle and all in at 28° degrees total at around 3,000 RPM.

I’d love to go for more advance, but then I’d have to buy AVGas at airports for big-bucks per gallon. It’s a someday dream of mine to get a tankful at Sears Point Raceway or an airport just to see what high octane was really like all those years ago.

Chris

 

the mechanical advance in your distributor does not start until 1,200 engine rpm so it will not have any affect on your initial timing at 850..

you’re right in the higher idle gives a more stable rpm than the lower 575. you can see that yourself if you have a digital readout timing light that has rpm..at 575 it’s actually bouncing all over the place by 25 or so rpm.

 

That makes sense, although not all curves are the same. All the non CCC Canadian 307 I have seen were anywhere from 13 to 22 degrees at 1100 rpm for base timing. I did have one HEI distributor add timing at 900 rpm, all in by 2500 rpm with lighter springs. I have come across 4 different HEI curves, a couple the complete opposite plus the aftermarket Chinese HEI. I like 900 to 1000 rpm in park, about 750 to 800 rpm in gear, much more stable.

 

I found similar with the Moroso HEI Advance Curve kit. The lightest springs were allowing mechanical advance at idle speeds (around 700 RPM).

 

And some of these may have been aftermarket. The one with the points like curve had an Autolite reman tag. One I had to run 25 base to get mid 30's total timing. The one with the quick advance I ran 22 base to get 38 total. I found multiple distributors with similar mechanical advance but wouldn't come all in before 3000 rpm no matter the advance springs we're used. The progression ignition is very cool, makes multiple things not possible with conventional mechanical distributors. Mark on his custom tune for Holley Terminator X Max has more timing at cold idle and it decreases as the motor warms up.

 

It’s a 66 toro