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Old 06-01-2010, 12:26 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by bubbahotep
Quickly: I have a 383 stroker weekend driver with 38' total distributor locked full advance and using an MSD timing computer for 20' retard on start and adjustable idle timing (I can idle anywhere under 38'). Been like this for at least 10 years, maybe 15?? The engine calls for 38' total timing.

No weights, no vac advance used for years.

Even with the MSD computer retarding my start timing 20' (goes down to 18' when the starter is cranking) I still get some hot days where the engine wants to spin in reverse and the starter is crying a little. I have a switch to kill the ignition which works. Usually its after driving, getting gas, and then going to restart the car (all warmed up and hot and gas flowing too well)

Today I tried this setup which appears to work well so I ask opinions:

I have a vacuum canister (for brakes) that holds somewhere around 18hg after I rev the engine. Need that for my brakes with the cam I have. My hg never goes below 10 when I press on the brakes and stays there when I hold the pedal down. At idle the engine wont get below 10hg but cant get higher because of the cam, thats just the average hg at idle give or take. But the brakes hold well and stops fine

I just hooked my vac advance on the dizzy, adjusted to kick in instantly, to the vacuum canister for the brakes. THEN, retarded my timing something like 20' (just a guess). The point is that when I first start the car I am NOT locked at 38', my distributor is probably locked at 18' now because I turned it back. After about 2 seconds as the vacuum canister fills (empties, you get the point), the vac advance kicks in at 5hg, raises my total timing back to 38' (which I checked to confirm). The timing holds because my vac advance never releases.

I ran the car around and never stumbled because the vacuum canister always had well over 10hg...enough to hold the vac advance canister in total advance. And, at WOT where I have no vacuum, it doesnt matter because the vac is coming from the canister which is still holding enough to pull the vacuum advance in.

Long winded but I wanted to cover the story. My idea is when I start the car I have maybe 18' on the distributor because its locked, the MSD timing computer throws in additional retarding for a better start, and then the vacuum advance pulls in once the car is started to get me back to 38' total locked timing. The MSD computer is still dropping my idle timing down and doing what it should.

YES, I know ideally I should have weights and vac advance canister hooked up. But aside from fuel economy, does anyone see this as a really dumb idea? Its nothing different than any other radical cam riding at total timing locked out except that my total locked out timing isnt in effect until the car runs.
If you're running locked in timing, having a vacuum advance is inconsequential. Vacuums intent is to pick up the low to mid RPM timing to overcome the loss of mixture density under partial throttle conditions. This does more than improve gas mileage in this condition, it greatly improves power in this stage. The problem with this is that long duration cams knock down induction vacuum regardless of throttle position. Lock in mechanical advance is used to overcome this deficiency in vacuum, but has a dark side which makes the engine difficult to start.

In your case the set up would probably be better with putting in 6-8 degrees mechanical to come up say starting from 2000 or 2500RPM and all in by 3000 RPM. Set the base at 10-12 degrees. With 30-32 locked in, the electronic retard dropping 20 will leave 10-12 degrees for starting. The final 6-8 degrees would come in on the top end of the mid RPM rev range where it can be used. Then run no vacuum advance.

Keep in mind that locked in timing is a feature of race engines, it's old school technology to overcome timing problems with big cams before the age of computer driven timing. It was also done to remove another failure prone part on race engines, especially those that see a lot of G loading in frequent and sharp corners, beyond that it serves no useful purpose.

In your case, running a total of 30-32 degrees up to 2000-2500 RPM is plenty. Frankly with 9.5 compression and 38 degrees of timing from the git-go, I'm surprised this will stay below the detonation threshold at moderate RPMs on 93. I'd guess that it does because the vehicle is light and perhaps "low geared" against it's weight and rolling/aero drag factors. Low geared in this case is a relative term variable with the work the engine has to perform against the load of the vehicle to be moved.

Bogie
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