M117 advance curve
 

Wondering if the advance curve for the preceding M116 3.5 is "better" than was used on the M117 which had to be designed to meet more stringent US smog regs that went into effect in 1972. If so, can the M116 curve be achieved merely by switching advance curve springs?

The engines have different combustion chambers and compression ratios. I doubt changing the distributor will make a difference on a low compression engine. I'm sure you could change the weights/springs. I have several 3.5/4.5 distributors from JY pulls and they all look the same inside.

My 4.5 with 3.5 heads and cams sure pulled better at high RPMs on the ass-dyno, same when using the 3.5 distributor, but without an actual dyno sheet or knowledge that my 4.5 distributor wasn't crap to begin with, it's just ass-dyno.

I had a 74 BMW bavaria with the 3 liter six. The original distributor had an advance and a retard feature which kicked in when cruising on highway. The performance was clearly choked off. I had a couple of older distributors around off mercedes and VW and mixed and matched weights and springs so that it performed like a non smogger. The difference was truly amazing. I also had to open the holes in the distributor plate to allow more movement. It was well worth the effort.

I also bought a euro only four barrel intake manifold for the car and installed it with a quadrajet which was jetted like it had been for the pontiac sprint ohc motor. With the all mechanical secondaries you had to be careful not to open the throttle until about 3000 rpm or it would bog down.

Fitting the manifold required some juggling of motor mounts and grinding material off the manifold to clear the power brake booster.

It was a lot of work but I was young and full of energy.

I imagine you can find a distributor which will interchange without too much trouble which has pre smog performance.

The distributor for the w116/117 engine is somewhat unique in that it includes a secondary chamber below the ignition points that contains the trigger points that control the eight Bosch D Jetronic injectors, so there's no ready substitution.

123ignition has distributors for old cars with D-jetronic. They might offer some different advance curves.

why not just megasquirt the thing and use modern ignition system to boot? -CTH

If you do decide on Mega-squirt or another electronic ignition option I have the 36-1 trigger wheels for the M116/M117's.

DIY Autotune and Digikey have the sensors in stock.

Yes, but their site says only for D-Jet equipped SIX CYLINDER W108/109 cars, which makes no sense since there are NO six cylinder W108/109 with D-Jetronic. Only the 1971-72 W108s and W109s with the 3.5 and then 4.5 V8s had D-Jetronic.
Something is wrong here!

b:

Have you determined the shape of your existing M117 advance curve? If not, perhaps you can locate a shop near you that has an old-style Sun distributor testing machine. Those machines mechanically drive a distributor, and are capable of determining degrees of centrifugal advance vs. RPM. The vacuum units (if installed) can also be checked for the amount of advance and/or retard that they provide.