22 replies to this topic

[CAU998]

Probably been asked before, but i've noticed that my old 1275 had a smaller pulley on the water pump than the 998 i had and looking at other cars its the same. Does this mean the fan spins faster meaning better cooling? And is it a worthwhile modification on a 998?

[Udo]

On higher reving engines use the bigger pulley as this slows down the pump (will prob need longer belt)

I think I recall reading that the pump is most effective at around 2000rpm so start winding it up to 6-7000rpm and it just thrashes the water and doesnt actually pump very well (more tech term for thrahsing is cavitation)

[CAU998]

Thanks thats great, i've just always wondered. So i take it if i build 1150cc small block unit i may awell use the 998 pulley as it'll still be quite a high reving engine.

[Udo]

Yes the bigger one, usually come off metros I believe

[Ethel]

If you're using the latest water pump, it's been around that long you almost certainly are, you shouldn't have any problems that way.

Only be worth fitting a large pulley to save HP on a track racer really.

[CAU998]

Thanks for your help!!!

[Dan]

There's a lot of half truths there so I'll clear a couple up.

The size of the pulley has nothing to do with the size of the engine or the car it came from, and the speed of the fan is only the secondary effect of the pulley size. The most important thing, as stated above it the speed of the water pump.

The original pulley was 3 7/8" diameter and was fitted to all A series engines, regardless of size. When the A+ design was being developed it was discovered that this pulley size runs the pump too fast. So fast in fact that the coolant cavitates around the impeller at higher engine speeds. That means that the pump is just thrashing the water about and chopping it up, producing voids in the coolant. that massively reduces the coolant flow. So to slow down the pump slightly the A+ engines used a 4 3/4" pulley. This does slow the fan down too obviously but to say that running the fan faster would improve rad cooling isn't strictly true. Fans work in a certain speed range for the same reason the pump impeller does. One blade causes turbulence to the blade following it and if they follow too rapidly they work less efficiently.

Generally, the larger pulley works better especially with the large impeller pump and that's why the small one has been out of production for so long. AFAIK nobody even makes an aftermarket small pulley. Which is a shame because the large pulley won't fit a small bore engine using an A series head (such as the 12G202, 206 or 295). When the large pulley was introduced to the A+ the thermostat housing section of the head was relieved a little to allow clearance for the pulley. A series heads don't have this feature so the large pulley doesn't fit beneath them on the small bore engine. So if the engine you are building is a small block A series or a small block with an A series head, use the small pulley. In all other cases use the large. And always use a large impeller pump.

[CAU998]

wow thanks thats a great help, this bored a small block is getting really tempting now just gotta wait for next pay packet so i can get a good 998 unit and 12g940 head.

[midwestmini]

There's a lot of half truths there so I'll clear a couple up.

The size of the pulley has nothing to do with the size of the engine or the car it came from, and the speed of the fan is only the secondary effect of the pulley size. The most important thing, as stated above it the speed of the water pump.

The original pulley was 3 7/8" diameter and was fitted to all A series engines, regardless of size. When the A+ design was being developed it was discovered that this pulley size runs the pump too fast. So fast in fact that the coolant cavitates around the impeller at higher engine speeds. That means that the pump is just thrashing the water about and chopping it up, producing voids in the coolant. that massively reduces the coolant flow. So to slow down the pump slightly the A+ engines used a 4 3/4" pulley. This does slow the fan down too obviously but to say that running the fan faster would improve rad cooling isn't strictly true. Fans work in a certain speed range for the same reason the pump impeller does. One blade causes turbulence to the blade following it and if they follow too rapidly they work less efficiently.

Generally, the larger pulley works better especially with the large impeller pump and that's why the small one has been out of production for so long. AFAIK nobody even makes an aftermarket small pulley. Which is a shame because the large pulley won't fit a small bore engine using an A series head (such as the 12G202, 206 or 295). When the large pulley was introduced to the A+ the thermostat housing section of the head was relieved a little to allow clearance for the pulley. A series heads don't have this feature so the large pulley doesn't fit beneath them on the small bore engine. So if the engine you are building is a small block A series or a small block with an A series head, use the small pulley. In all other cases use the large. And always use a large impeller pump.

Can visual inspection of a water pump (impellor and body) determine if it pumps okay. I have a 4 year old remfg pump that I suspect is causing some overheating problems. If the body or impellor have lost a little "new look smoothness" can the same cavitation effect occur?

[PACINO]

There's a lot of half truths there so I'll clear a couple up.

The size of the pulley has nothing to do with the size of the engine or the car it came from, and the speed of the fan is only the secondary effect of the pulley size. The most important thing, as stated above it the speed of the water pump.

The original pulley was 3 7/8" diameter and was fitted to all A series engines, regardless of size. When the A+ design was being developed it was discovered that this pulley size runs the pump too fast. So fast in fact that the coolant cavitates around the impeller at higher engine speeds. That means that the pump is just thrashing the water about and chopping it up, producing voids in the coolant. that massively reduces the coolant flow. So to slow down the pump slightly the A+ engines used a 4 3/4" pulley. This does slow the fan down too obviously but to say that running the fan faster would improve rad cooling isn't strictly true. Fans work in a certain speed range for the same reason the pump impeller does. One blade causes turbulence to the blade following it and if they follow too rapidly they work less efficiently.

Generally, the larger pulley works better especially with the large impeller pump and that's why the small one has been out of production for so long. AFAIK nobody even makes an aftermarket small pulley. Which is a shame because the large pulley won't fit a small bore engine using an A series head (such as the 12G202, 206 or 295). When the large pulley was introduced to the A+ the thermostat housing section of the head was relieved a little to allow clearance for the pulley. A series heads don't have this feature so the large pulley doesn't fit beneath them on the small bore engine. So if the engine you are building is a small block A series or a small block with an A series head, use the small pulley. In all other cases use the large. And always use a large impeller pump.

Great answer. Congrats!

[nicklouse]

New small pulleys are available now.

for example DSN do one.

 

https://www.dsnclass...trosport-small/

[kit352]

Is it worth upgrading to an aluminum one. I came across these the other day and now im wondering.

https://www.speciali...ump-pulley.html

Edited by kit352, 27 September 2020 - 06:18 AM.

[PACINO]

New small pulleys are available now.

for example DSN do one.
 
https://www.dsnclass...trosport-small/

The new ones are beautiful. Advantage? Lighter and they will never rust.

[PACINO]

In conclusion, after having looked for information about the pulleys, and which one is best. Could we say that for an engine that runs at high revs, and to avoid the 'cavitation', the large pulley is appropriate? A 1275 engine with double Hs2 carb, large valves .. is it convenient to fit the big (diameter) pulley?

Regards

[imack]

I think the large diameter pulleys can be an issue on a 1275 if you've had the head and block skimmed significantly. You may be able to fit the pulley but you may also have to remove the pulley to replace the belt.
I run the MED 4.2" pulley and can only just get the belt in between the pulley and head.

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Edited by imack, 27 September 2020 - 10:11 AM.