Quote:
Originally Posted by 1funride
Really 20 at idle?
What is the timing curve, initial, full, and all in by?
I'm not running EFI timing yet, using an MSD distributor.
Is it hard to start with 20 degrees initial or do they start at a lower timing then switch to 20? Interesting.
I guess this is the age-old question port or full vacuum, and I admittedly have not set up my car for full vacuum timing nor have I driven it that way. I went port since I wanted the timing to pick up right off idle rather than drop, then pick up. My idea being this is easier to tune and offers a smoother timing curve and maybe a little more timing at part throttle cruising where the mixture is lean and needs the extra time to burn.
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It's easier to do all this with the EFI timing control for sure. With EFI timing control you can have as much initial timing at idle as you wish, but it doesn't have to crank there. In order to accomplish that with a mechanical distributor you have to run the vacuum advance to a manifold source. So it will crank on the initial timing and then once running the additional timing is added via vacuum advance on the manifold.
However this takes a modified vacuum advance and other modifications to work well.
The biggest difference between the mechanical way and the EFI controlled way is that the EFI is much more stable and is not affected by altitude. A mechanical vacuum advance is only going to be as good as the engine is at making vacuum and the elevation you drive the car at.
So yes to your first question. What I do with the mechanical setups.....
First I modify the breaker plates and/or weights to limit advance to about 16 or 18 degrees at the crank. Then I set my initial to 16 or 18 depending on the total I'm looking for. Total depends on where it made best power on the dyno.
Some engines I run 34 degrees, some 36 or 38 degrees. Either way I modify it to end up with about 16-18 initial timing and yes all the cars crank perfectly fine this way.
From there I modify the vacuum advance. I start with an adjustable unit and set the spring tension to start working around 6 inches and all done by 8-10 inches. So when it's on manifold vacuum and idling, all the vacuum advance is in and stable at idle as most engines should idle with at least 8-10 inches of vacuum even with a pretty rowdy camshaft. This also helps with idle performance as far as vacuum generated, how smooth it is, and how it idles in and out of gear with very little rpm drop.
Second thing I'll do is weld the slot on the vacuum advance to limit it's travel. I usually shoot for .200" to .250" depending on the breaker plate, and this gives me about an additional 10 degrees of vacuum advance.
So at idle, with 16 or 18 initial, and vacuum advance added in, it actually idles around 26-28 degrees. Provides good idle manors with big 260-ish @ .050 camshafts on tight LSA's and actually makes them very drivable.
At light throttle cruise I'm usually seeing 46-48 degrees of timing with vacuum advance added. This returns good mpg.
But as I said, all of this is much easier to accomplish with EFI timing control.