The Jalopy Journal
Discussion in 'The Hokey Ass Message Board' started by HD74, Jun 3, 2010.
What's the advantage of fast advance curve vs. slow advance curve?
Depends on cam timing, induction and compression. Some motors are happier when its all in by a low rpm, some aren't.
Ina perfect world , (industrial engine/race car engine ) where the rpm is a pretty known fact the advance would just be set at a given degrees etc and left there.
The problem is a driveability issue when we go from idle to 3-4000 rpm etc and back to idle.
Most factory advance curves are all in at 3000-3500 rpm, when we modify or put these engines in a lighter hot rod style car the curve needs to match the engine rpm at cruise or part throttle.
Most are then curved to be all in at 2500 rpm which usually equals cruise rpm.
Just a factor of "tuning " the engine to the car......
As far as "seat of the pants" tuning goes, years ago I put lightweight springs and weights in the stock distributor in my '67 Firebird in an attempt to get the 326 to behave more like a 400. The good part was that it was noticeably livelier off the line; the bad part was it made it spark knock bad enough to necessitate going from regular to hi-test.
The reason I ask is I'm trying to get my FH in F1 to perform a bit better. There is a MSD dizzy on it and found that the builder of the motor used the lightest springs and the next to the smallest bushing for advance, which put me @ 30 deg total advance. The truck was a bit of a dog to say the least. Corrected that by changing to the black bushing and with intial 5 deg. give me a total advance of 23 deg.. That said with six spring combo's I just have to find the one that gives me the best overall performance.
Put it on a dyno, and see what spark advance the engine likes best at various rpm full throttle.
Then put your distributor on a distributor machine and play around with both weights and springs to match that advance curve as best you can.
A lot of variables from compression to bore size and the camshaft grind, automatic or stick, gears etc. A good rule of thumb on most mild cams is a total of 36-38 degrees in by 22-2500. I like to run 12-14 at the crank with a auto and 14-18 with a stick, 20-24 degrees in the dizzy, makes for a snappy ride. Then you have the Vortec style combustion chambers which make best power at 32-34 total. Most mild cams respond well to ignition timing mods, a dyno makes short work but it is a weekend thing of joy to have to drive the car to your local "test" area to launch it to find the sweet spot!
The old racers say on a really wild flathead no more than six degrees initial, and 27 max.
If your engine is a mild one, back those numbers down about 2.
Yes, I assumed an OHV mill, my bad for not reading of a flattie. What compression are those, 6.5:1 or ?
Up-date, I've tried a few spring combinations with the black bushing. So with 5 deg. initial advance and 23 deg. total advance the engine seems to like the light silver/blue spring coming on @ 2000 rpm. It's pulling through the gears much stronger.
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