correct pully size for a 68

[preS]

What is the correct diameter for a MY 68 alu case engine?

Thanks

Richard

[304065]

The early ones are 116mm. 68 may have a second groove for smog drive. 911.102.017.50

later one is 131mm in 1976 (911.102.017.04)

[JT912]

Single groove for '68. Smog belt is run off pulley on driver's side camshaft.

[ibmiked]

The early ones are 116mm. 68 may have a second groove for smog drive. 911.102.017.50

later one is 131mm in 1976 (911.102.017.04)

John is the 131mm the pulley commonly referred to as the 'overdrive' modification pulley for these motors? I seem to recall you once advised me that the pulley enclosure needs to be modified if a larger pulley is needed for more cooling. I'm planning ahead for possibly 2.2S pistons on my 2.0L and trying to build in extra cooling without running an external cooler.

Sorry for the thread hijack, please return to your regularly scheduled programming.

[preS]

Mike I appreciate your question. John can you reply in this thread?

BTW the engine had a second groove but I removed the smog stuff.

Richard

[JT912]

912 had a second groove for the smog pump drive. Early 911s had one for the A/C it believe. Wasn't me who Mike talked with re: the size of the pulley or modifications needed for the larger pulley. I do remember reading something about an oversized pulley that sped up the fan. My '68 is off being massaged so I can't measure its' pulley.

[304065]

Yes, the 132mm pulley can be used to speed up the fan. You would have to figure out how to use the later engine mount, as the larger pulley comes so close to the SWB engine mount that you can't get the belt on or off. And I'm not doing partial engine drops to change the belt, that is not my idea of fun.

Here is the big pulley with SWB engine mount:



And here is the stock one:



At the moment I am using the 116, works fine, and I can't get the oil temps above 170, but that is with a brand-new engine, 32 degrees of timing and a mixture in the 11-12 range.

The real question, having now thought about it for half a decade, is whether you can make more power by keeping everything cooler with the larger fan. Theoretically this should be so. Practically, the only time you care about this is for race purposes, in which context you are spinning the motor at 5000-9000 RPM, which may put you beyond the point of diminishing returns in the tradeoff between horsepower consumed by the fan and the cooling effect of higher fan speed.

[ibmiked]

Cleared it. FYI, I will be at Cars and Coffee in New Canaan Sunday morning if you want to try out the 'R' Seats from Vintage seats.