In looking at some 66 information regarding SS cars, it appears that gm used different size water pump pulleys on AC cars. The diameter of the AC pulley being about 1" less; thus increasing the flow of water about 20-30%. They also degreased the diameter of the alternator pulley to speed it up.

Has anyone tried this to get more flow and thus increase the cooling?
Thanks.

First Guess:

The smaller-diameter water pump and alternator pulley were used on Taxi Cabs and other LOW-RPM engines.

Any engine that was expected to REV got pulleys that SLOWED DOWN the accessories; to prevent over-revving them.

In looking at some 66 information regarding SS cars, it appears that gm used different size water pump pulleys on AC cars. The diameter of the AC pulley being about 1" less; thus increasing the flow of water about 20-30%. They also degreased the diameter of the alternator pulley to speed it up.

Has anyone tried this to get more flow and thus increase the cooling?
Thanks.
James that pulley diameter reduction also increases the fans speed,which would seemingly help at idle and in low speed traffic. I think it would be a worthwhile experiment.
Mike

Don't think the small-diameter water pump pulley were used on only LOW-RPM engines. Maybe the taxi cars used an even smaller pulley.

My 66 AIM shows pulley # 3890419 where the OD is 6.3" was used on AC cars with L35 or L34; it shows pulley # 3788472 for the NON-AC L35 & L34 cars which has a diameter of 7.1".

The liteature that I have from a guy by the name of Greg Cockerill on 1966 engine pulleys, states that the standard pulley would make .95 revolutions for every one revolution of the crank; whereas with optional AC pulley, it would make 1.16 revolutions for every revolution of the crank. 1.16/.95 = 1.22. pr a 22% increase, if his fact sheet is correct. My thinkink would be that you just divide inside diameter of where the belt rides of the larger pulley by the inside diameter of where the belt rides of the smaller pulley. I would guess the increase to be more around 14 %.

I am in the process of swapping out my pulleys to get the water pump spinning faster. I used to have 4.10 gears, but I switched to 2.73s and now the engine runs hotter. I hope to regain some of my cooling by switching. The set that is on it now has the crank and water pump pulleys about the same diameter - the replacement set has a significantly smaller water pump pulley than crank pulley.

The AC pulley would work; but would look a little funky with the double grove.

I wonder, if ther would be another other water pump pulley that would fit the 64-68 short gm pumps and have the correct depth for the alternator belt and have a smaller OD, like aroudn 6 1/4"? Anyone know of such an animal or how I could find one? Thanks.

James,on the A/C pulley which groove drives the water pump?
Mike

Worked for me. I have a 71 SS454 Elco with a late model serpetine belt system off a BB 98 GM truck. Was running hot(about 200) when a local GM mechanic sold me on the smaller pully that would have come with the same system with A/C being the only differance. Dropped running temp by 10-12 degrees. That's checked via a thermometer in radiator. Factory guage with sending unit in intake(should be in head) only showd a 5-7 degree drop.

Not positive which groove; but think it would be the one closest to the engine. Thats the way it looks in the AIM for 66.

Just checked on the sumitt catalog and it looks like they have a couple of the aluminum pulleys for a 66 BB with the short pump with a small outside diameter. One has a 6.1" diameter.

Would like an original looking style pulley.

Don't think the small-diameter water pump pulley were used on only LOW-RPM engines. Maybe the taxi cars used an even smaller pulley.

My 66 AIM shows pulley # 3890419 where the OD is 6.3" was used on AC cars with L35 or L34; it shows pulley # 3788472 for the NON-AC L35 & L34 cars which has a diameter of 7.1".

The liteature that I have from a guy by the name of Greg Cockerill on 1966 engine pulleys, states that the standard pulley would make .95 revolutions for every one revolution of the crank; whereas with optional AC pulley, it would make 1.16 revolutions for every revolution of the crank. 1.16/.95 = 1.22. pr a 22% increase, if his fact sheet is correct. My thinkink would be that you just divide inside diameter of where the belt rides of the larger pulley by the inside diameter of where the belt rides of the smaller pulley. I would guess the increase to be more around 14 %.
I have NO TROUBLE believing that A/C equipped vehicles came with pulley ratios that speed up the water pump (and therfore the cooling fan.)

Let's keep in mind that the highest-performance engines--the ones that GM knew were going to be revved to the moon--were NOT OFFERED WITH A/C.

A water pump--or a fan for that matter--is not an unlimited-rpm device. Think of both as pumps: one for engine coolant, one for air. Turn them slowly, they don't pump much. Turn them too fast, and they also lose efficiency. The speed of the pump has to be kept within certain limits; and it's going to be VERY possible to over-speed either kind of pump.

Biasing the pulley ratios to increase the pump speed means that the overall engine speed better be kept within tighter limits to prevent water pump/fan overspeed. But, yeah, it'll move more coolant and air when the engine is at low speed.

Thanks, that all make sence. I don't think that AC was offered on the L78 cars in 66.