Timing Question, Highway cruising Options

[Dr. Roger]

I have a mechanical and vacuum advance mallory distributor.

I understand that when highway cruising the throttle is slightly cracked and the rev's are at about 2500RPM's. (high vacuum state and moderate mechanical advanced timing) FYI, I have the flipped H gear and a V8.

In order to maximize mileage, power, and reduce chances of overheating due to retarded timing, shouldn't timing be advanced as much as possible?

Then, if the throttle is mashed down, the vacuum shoud retard quickly until RPM's increase ?

The reason I ask all of this is because i noted o a few racing websites they offer agressive advance curves. Now I interpret this as timing advancing more quickly but does this also allow more total timing? And isn't there a limit to total timing?

Thanks,
Roger

[Aaron Cox]

little blurb of type 4's.. apply to v8's as necessary..

with a mallory dizzy, centrifugal advance ends before 3k at around 28 degreed btdc...
with the vacuum can, at partload, part throttle cruising, some people run up to 42 degrees.....

[airsix]

QUOTE

In order to maximize mileage, power, and reduce chances of overheating due to retarded timing, shouldn't timing be advanced as much as possible?

Well, yes... sort of. Assuming all conditions are right.

QUOTE

Then, if the throttle is mashed down, the vacuum shoud retard quickly until RPM's increase ?

Again... sort of. High manifold pressure (low vacuum) advance will never be as high as low manifold pressure (high vacuum) advance, even at high rpm. For example, my engine has a max low manifold pressure (high vacuum) advance of 40 degrees. At high manifold pressure (WOT) max advance is only 27 degrees.

QUOTE

The reason I ask all of this is because i noted o a few racing websites they offer agressive advance curves.  Now I interpret this as timing advancing more quickly but does this also allow more total timing?  And isn't there a limit to total timing?

I think what these are doing is for the most part squeezing out the safety margin. The stock advance curve isn't the OPTIMAL curve from a performance standpoint. It's the optimal curve plus a SAFETY MARGIN to account for things like the ocassional tank of bad gasoline, pulling a trailer up a grade on a hot day, etc. On a very modern engine with ECU controlled advance, knock sensors, engine/air temp sensors, etc. the advance can be maintained very close to optimal but on a carbuerated engine with a mechanical distributor you need to build in a safety margin. This is just my take on things. Others may have differing oppinions. As an aside, when tuning my TypeIV with PEFI/ignition I found that the stock advance curve was all but impossible to improve upon. The only real gains I was able to make were with regard to the elimination of timing float, which was as much as 3 degrees with the mechanical distributor. After going to crank-fire the timing doesn't bounce around at all, but the "works best" advance curve and absolute settings are virually identical to stock. Go figure. The engineers actually knew what they were doing. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)
-Ben M.