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mark houghton mark houghton is offline
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Join Date: Jul 2008
Location: Central Washington State
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Quote:
Originally Posted by equality72521 View Post
How much centrifugal advance is there? If set to 26* @ 4000 rpm what should I see at idle if the pot is disconnected?
It all depends on your distributor curve. I think there are several versions as Porsche continued to tweak them for different years, and for different reasons. Your best bet is to look up the timing curve charts in your manual if you have one. Unless you're trying to pass some emissions testing, who really cares what the timing is at idle....it's the timing at 4000 rpms that you want to be accurate.

This crap can get real confusing even for the most seasoned tuners amongst us. I THINK I have a pretty good grasp on how a dual-pot distributor works but still not totally sure on the single pot. Boost retarding is important....26* on boost will cause knocky-knocky noises and the sound of money rattling around in a tin can. Deadly. What follows is what I know relative to a dual pot system:

When you ask "how much centrifugal advance is there"...well, the total advance is really a combination between the vacuum retard going away when the throttle plate opens enough to drop the vacuum off - and the mechanical centrifugal advance coming in. Combined is what gets you the 26* at 4000 rpms. That should be where you set your timing by twisting the distrubutor, then check it at idle with hoses connected. Depending on individual car specs (some may have you checking with hoses disconnected), your timing at idle may be anywhere from 0 to 6*retarded (hoses connected, but do check the specs for your year).



Here's a written picture of what most dual pot distributors have for a curve. Should help to confuse the picture for you:

Vacuum is retarded 10* at idle due to vacuum being present. WVA means "with vacuum advance". In other words, in the absence of a vacuum signal when you step on the throttle the 10* retard goes away.

1460 RPM 2*= 12* WVA

2260 RPM 6*= 16* WVA

3000 RPM 9*= 19* WVA

3600 RPM 12*= 22* WVA

4000 RPM 16*= 26* WVA

This info was "borrowed" from a friend who actually had his distributor spun and the curve plotted. What should happen after 4000 rpms would be for additional retard to kick in (about 8 to 10 degrees), so you end up with 16 to 18 advance on boost.

That's about the extent of things as I know it.

But wait!! Here's my theory on how the singe pot may work: Perhaps the port is actually vacuum advance at idle. As the vacuum signal gets progressively weaker with the throttle opening, all the initial advance falls off leaving you with just what's left of the mechanical. Then, as the boost builds, the pot is moved in the opposite direction and begins to retard things. Thus, your timing at idle is probably advanced as opposed to the later cars' retarded. Net-net, your 26* at 4000 is all centrifugal advance, and as boost builds the pot moves the distibutor plate in the opposite direction and begins to retard things.

Man, that's a mouthfull. No more....
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 03-29-2010, 03:13 PM
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