Thanks Mike I'll shot for 33* - 35* mechanical advance then. I am curious thou why all the mechanical advance has to stop at 3000 rpm. wouldnt it be better to have it span the length of the whole RPM range?
datzenmike wrote:Mechanical advance is about 20 plus the 10-12 static timing at idle for the L18 or L20B.
The only way to have both vacuum and mechanical advance at the same time is at cruising speed, (when all the mechanical advance is present) and with the throttle closed maybe coasting down hill, giving maximum manifold vacuum say 20-25 degrees. The two add together to over 50 degrees.
These numbers aren't absolute. All engines are different. The mechanical advance could be 18 or even 24 degrees. Vacuum advance could be more or less. This is just for argument's sake. Most engines do however, seem to like a total mechanical advance in the 33-35 degree range.
None of this applies to a race motor which seldom goes below 3,000 RPM and is at full throttle with very low intake vacuum anyway. A street car should always have vacuum advance even if it is the smog ported vacuum. Otherwise you only have the mechanical which is not load (throttle) sensitive. Using vacuum to vary the timing in relation to load, (throttle) increases the engine efficiency. You get better mileage and the car is much easier to drive.
TranzAm wrote:The distributor is a Cardone # 31-620 for 75-78 L20 engines. Is 36* total advance the max. At what rpms does your vacuum and mechanical advance take effect? also whats your engine combo? Anything like this one?
James wrote:This has a lot to do with what distributor you are using, and how much mechanical advance it has.
Also - you should be checking the timing at idle with the vacuum advance disconnected. I would start at 10 deg. BTDC (stock 510 setting) and go from there. My L-18 likes more advance at idle, so I shortened up the slots in the advance mechanism to allow me to run about 18 deg BTDC at idle, and still maintain about 36 deg total advance.
A little late back to the party - (couldn't get on the site for some reason...)
My engine combo is this: L-18, w53 closed chambered head, 478, 280 cam, su's, sss distributor.
Mike had some really good info there - perfect description.
I'll shot for 33* - 35* mechanical advance then. I am curious thou why all the mechanical advance has to stop at 3000 rpm. wouldnt it be better to have it span the length of the whole RPM range?