Hey Everyone,

I just posted this on T-bricks and I thought I'd post it here as well to see what some of the local experts think about this.

Here's the situation. As most of you know I'm about to do a AW72 to M46 swap on my '95 940T. I wanted to lighten up the manual flywheel before I installed it but I couldn't find a machine shop willing to lighten the stepped LH 2.4 manual flywheel for me. I then decided it would be easier and cheaper to take a flat B21/23 flywheel (9 lbs lighter) and have a machine shop drill the crank position sensor (CPS) holes on it so it could be used with LH 2.4. I told the machinist that the holes were for a CPS assuming he would know to make sure the sensor holes were lined up properly relative to the crank bolt holes so as to keep the TDC gap at the exact right place on the flywheel. That was a mistake. I should've been more specific about that.

Long story short it turns out he didn't line everything up properly before he started drilling so the the TDS gap on the modified flywheel is out of synch with the bolt holes so the timing would be off. It's only 2.5 degrees off, however, and looking at the flywheel from the tranny side the TDC gap on the modified flywheel is shifted 2.5 degrees CCW relative to the stock flywheel. I'm pretty sure that means the system would be sensing TDC 2.5 degrees earlier with the modified flywheel which means the crank, ignition, etc. timing would be advanced 2.5 degrees relative to the standard set up wouldn't it ?

I know it often helps performance to advance your ignition timing a little but what about this situation, where everything would be slightly advanced ? My question is would this be a bad thing ? Would this likely help or hurt performance ? Would it have any detrimental effects on the engine to run like this ? Keep in mind this is a Turbo we're talking, not N/A. I have a boost controller and have it set so the max boost I get is about 12 psi so not running tons of boost.

So, what do all you experts think ?

Thanks,

Roj

Yeah, you're right on the money. Timing would be 2.5* further advanced, which won't be much of a problem. Apparently the LH2.4 ignition computer is of the "learning" kind too, but at the very least there is an article on Turbobricks about jumpering certain pins on the ignition computer to modify the overall timing bias. i.e., you should be able to easily take out 2 or 3 degrees to counteract the flywheel shift.

Hey that's good news ! With any luck the computer will be able to compensate for the timing shift and there won't be much of a difference when all is said and done. I kind of wonder how much error there is built into that system anyways. At high revs that flywheel is spinning pretty damn fast so one would have to wonder if there might be close to 2.5 degrees of error inherent within the system to begin with. Who knows, maybe advancing the TDC mark on the flywheel will take some of the error out and it will work better ! (now there's an optimist for you ).

I will have to search around on T-bricks to find that article. Is this the one you were referring to ?

http://www.turbobricks.com/forums/showt ... hp?t=75838

Question: What is the CPS tied to ? Does it just effect ignition timing or is injector timing tied into it as well ? If it's just ignition timing then it's probably no big deal. 2.5 deg advance on the spark is no big deal. But if the advance also effects the injector timing will that cause any potential issues/problems ?

Thanks,

Roj

2.5 degrees is small beans. There is enough slop in the bolt holes for a degree or two and you can jumper various pins to shift the timing 2-3 degrees at a time. If that is all you are out by, party on.

I hate LH2.4. With all its gadgetry and replaced parts, both my ancient k-jet cars run better with no idle valves or anything. That and bosch breakerless k-jet ignition as used on the k-jet cars is GOD.

Injector timing is no issue at all. They all fire at the same time. 2.5 ignition advance might be a good thing at our altitude.