m117 vacuum advance idea

[vandor]

OK, so the m117 (4.5 liter V8) engine in my MB has vacuum advance, but it is only used to kick up the idle speed when the AC is engaged. So with the AC off it there is no vacuum advance at all. On my other car a working vacuum advance can give close to 10% improvement in highway fuel mileage. Would connecting the vacuum advance to constant vacuum have any ill effects on the m117?
Thanks,

[Graham]

Originally Posted by vandor

OK, so the m117 (4.5 liter V8) engine in my MB has vacuum advance, but it is only used to kick up the idle speed when the AC is engaged. So with the AC off it there is no vacuum advance at all. On my other car a working vacuum advance can give close to 10% improvement in highway fuel mileage. Would connecting the vacuum advance to constant vacuum have any ill effects on the m117?
Thanks,

Vandor, that is not totally correct. The engine actually has vacuum retard. The timing normally gets retarded at idle speed. The valve on the vacuum line gets energized when the AC is turned on or when the coolant temp is 100+ C. This eliminates vacuum to the distributor and in effect does advance the timing (from it's retarded state at idle).

Connecting to constant vacuum will keep your distributor retarded. The distributor has mechanical advance.

Best thing to do for timing, is to set it to provide as much advance as you can get at 3000rpm. It should be about 27-30 BTDC. The distributor will be almost fully turned CCW in the clamping slot. I found I could do this by setting idle at 750rpm, hooking up a vacuum gauge to either line to MPS or connection at back of intake manifold and then adjusting timing to give maximum vacuum. Backing off just slightly seemed to be best. I would them check timing at 3000rpm and it would come out at about 27 BTDC. If you can't get that much advance, the centrifugal weights in your distributor may be gummed up.

Don't know if adjusting timing helps mileage much. For that, setting the mixture using MPS and a wideband AFR gauge did the trick for me. You can then choose between economy and performance!

[vandor]

Hi Graham,

Thanks for the explanation of how the system works. Are you sure the switch-on temp is 100C, not 100F? The engine would normally never reach 100C :-)

I had trouble with advance weights and ended up replacing my distributor, so everything works fine in that department. Advancing the timing at part-throttle is a well known way to reduce fuel consumption. With only the centrifugal advance the timing only changes with RPM, and you have to set it so it does not ping at full throttle. But the engine could use a lot more advance at part throttle for optimum economy, that is why many cars have vacuum advance.
bye,