Re: what is this distributor mod i been hearing about - Saab Performance Bulletin Board - Saabnet.com


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Re: what is this distributor mod i been hearing about

Posted by binky (more from binky) on Sat, 19 Nov 2005 23:51:13 Share Post by Email
In Reply to: what is this distributor mod i been hearing about, john, Sat, 19 Nov 2005 22:48:16
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For Saab Turbos with LH 2.2 or 2.4, the distributor has a pressure/vacuum-actuated lever that advances timing (vacuum) and retards it (boost). This lever is attached to the hall sensor in the distributor. During boost or vacuum, the lever moves the sensor back and forth with respect to its shutter wheel and changes the timing, that is, it causes the spark to fire earlier or later. By today's standards, it's a crude system, but, eh, it works pretty good I guess. Saab replaced it with something better back in 88 (DI+APC, then DI/APC, integrated).

The idea is to modify the lever, aka vacuum/boost capsule, so you can run more static advance and more high-vacuum advance while preserving the SAME amount of retard under boost (11* BTDC). More advance makes the car a bit snappier when you're not in the boost. (Unlike non-turbo cars, the old "twisting the distributor for more advance" trick does NOT WORK because, if you did this on a Saab Turbo, you would simultaneously DECREASE the amount of retard under boost, making the engine KNOCK.)

16 degrees BTDC is stock static timing. In stock form, the boost/vacuum capsule will advance timing about 5 degrees - advanced to 21 degrees BTDC - and retard timing about 5 degrees - retarded to 11 degrees BTDC - for a TOTAL timing change of 10 degrees. This range is determined by the capsule's physical dimensions. You are going to modify the capsule to increase this 10 degree range while maintaining the same amount of boost retard.

To modify the capsule, remove it from the distributor. But first, to see how it works, tug down on the lever, feel the spring tension in the capsule, and note how the lever travel is limited by a small round stopper piece. This is what happens during boost. Try pushing up, too. Again feel the spring tension. This is what happens during high vacuum (there are actually two springs in the capsule - one for retard and one for advance).

Now, onto modifying. Bend the stop down just a little, away from the lever, to allow more travel. Maybe double the gap between the stop and the lever. This extra travel allows a WIDER total spread between the lever's static and full retard positions, so if you bent the stop - if you doubled the gap between it and the lever - you now have 15 degrees total spread (the same stock 5 degrees of advance plus the now doubled retard travel of 10 degrees).

Refit the capsule and time the car to a higher static advance. You want to KEEP the same 11 degrees BTDC of retard under boost, so set the static timing to 21 degrees BTDC. I assume you are using high octane fuel.

To reiterate, now when you drive the car, the amount of total advance remains unchanged - 5 degrees - but the amount of retard change is now up to 10 degrees. So under full boost, the car still runs at 11 degrees BTDC retard. Note that if your car doesn't like that much advance, you'll need to bend the stop back a bit. I can run 19 degrees static timing on 87 octane with no problem.

Here's the stock spread, static timing at 16 degrees, with 5 degrees of vacuum-induced advance and 5 degrees of boost-induced retard:

21* . . . . 16* . . . . 11*

Now here's the modified spread, static timing at 21 degrees. Note the new static timing setting of 21*, the same 5 total degrees of vacuum-induced advance and the SAME boost retard of 11*:

26* . . . . 21* . . . . . . . . . 11*


So, to recap: you bend the stop to allow more static timing for better off-boost performance. You maintain the SAME 5 degrees of vacuum advance but INCREASE the amount of retard. HOWEVER, you are still retarding to the SAME 11*.

posted by 70.230.7...

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