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fredmeister fredmeister is offline
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Join Date: Oct 2002
Location: Baltimore, MD, USA
Posts: 2,235
Originally Posted by JFairman View Post
That bottom left port on the 930 throttle body is the distributor vacuum retard port and looking at the counterclockwise rotation of your distributor and the vacuum pot connection position and shape of the vacuum pot and internal linkage rod it is going to pull the plate inside the distibutor in the same direction it is rotating and that will retard timing when intake vacuum is applied.

That TB port only has usable vacuum up until around 1500-1700rpms of throttle position when there is no load on the motor so it's not good for anything else other than retarding ignition timing at idle, deceleration, and the first few hundred rpms above idle to lower emissions.
If there is a catalytic converter the higher exhaust temps from retarded timing will heat it up more.

The top left TB vacuum port is for operating a blow off or compressor bypass valve and deceleration vacuum limiter valve if using one. It gets intake manifold vacuum combined with some TB venturi effect between the top edge of the throttle burrerfly and TB wall and vacuum port while it is partially opened because it's on the top side of the throttle body where the butterfly is opening downwards. Top right TB port is for the control pressure regulator boost signal.

If you've rebuilt carburators and studied the idle mixture hole and multiple idle to main jet progression holes (especially in Webers) next to it in the throttle body wall next to the downstream opening edge of the throttle butterfly they work on the same principal and it's easy to understand.
Wouldn't the car accelerate better then off idle if you changed to tapping into the top left port then (#III) instead of the #V port as indicated in the chart?
I just think you want to use a stronger vacuum signal to get you going off the line since the timing needs to be set so retarded as it is without the addition of adjustable ignition system boxes (meaning a stock CD box).
I am timed to 0 degrees at idle right now for this reason but I would like ot be able to run more idle advance without being over advanced under boost.
1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 09-08-2011, 09:15 AM
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