Underdrive pulley recommendations?

[blue2000s]

Originally Posted by JFP in PA

While it may seem counter intuitive, slowing down the water pump can actually produce lower coolant temperatures (within limits). In any cooling system, there is an “ideal” coolant flow rate, at which the heat source is exposed to the coolant for the optimum amount of time for heat transfer to be accomplished. The radiators also have an “ideal” flow rate to get rid of the heat as well. In an Ideal application, the pump would run at a constant speed to provide the best possible cooling (this is one of the several reasons why a lot of race cars use electrically driven water pumps). Unfortunately, because most street automotive cooling systems are mechanically driven (by the belt), once the thermostat is open, the coolant is typically either moving too fast or too slow for optimum transfer due to the range of RPM’s them pump is seeing, resulting in suboptimum heat transfer.

In years gone by, a lot of racers used to pull the thermostats out of engines using belt driven water pumps, assuming “the more flow, the better”, only to find the engine actually ran hotter. But when they replaced the thermostat with a restrictor plate, the engine ran cooler. It had too much flow without the restrictor or the thermostat………..

Faster flow rates keep temperatures more uniform. There's less temperature difference between the inlet and outlet of the engine if coolant is moving faster. This will translate into component temperatures directly. Flow a fluid over a surface and if you don't change the fluid temperature or the surface heat output, the surface will be cooler with faster fluid flow. But the fluid temperature rise decreases as it's speed increases.

An ideal flow rate will keep the most sensitive component below it's critical temperature for any given load/speed/environment condition. This may be faster or slower flow depending on where the component is or even if it's always the same component.

The important point here is that the temperature of the coolant at the radiator outlet isn't telling what the component temperatures are inside the engine. The temperatures have to be directly measured or at least modeled with some very expensive software.

Is there a reliability impact from a smaller diameter pulley? I suspect there is, but I don't know. All I know is what's happening when the smaller pulley is installed and if I personally am comfortable with the risk.

[chaudanova]

Has anyone paired one of these underdrive pulleys along with an oversized power steering pulley as well?

Would that be too much of an underdrive, or would that be a good pairing?

[The Radium King]

i was wondering the same thing after seeing both on the cheetah website.

[jhandy]

I have the raby underdrive. It is the smallest pulley made that still allows you to drive the car daily.

i am assuming you ment to say - put an underdrive and a small pulley for the power steering?

I think that this would be too much. one or the other.

if i am at idle, my alt puts out 12.9-13.0 volts. my power steering works great, but i can tell any less pressure would be too little.

Also if i turn all electrical things on the car, the lights, ac radio, radar dectector, sat reciever, heated seats....eveything... my output at idle reads 11.9-12 volts.

so you can see any slower would not let the car recharge the battery.

Also the advantage of the underdrive over the steering pulley is that it slows the steering pump, and it lowers rotational mass of the crank at the same time. increasing torque and a little HP. just a steering pulley should not do that.

i recommend the raby pulley 110%, for around 300$ and a little work, it is the best bang for the buck hands down.

DONT FORGET THE NEW BOLT AND WASHER!!! dont cut corners or you risk sittng on the side of the road.

[Gary H]

I've also got Jakes UD Pulley - 2K miles, 2 track days (Spa and Nurburgring) and no issues. Interestingly I came back with an '03 986s (which I believe has 5bhp more?) Anyway, in a straight line, 50-130, I would pull maybe a car length every time - just about noticeable, but consistently noticeable.
The real difference is now in the in-gear pick-up - it's noticeably more brisk.

[chaudanova]

Originally Posted by Gary H

I've also got Jakes UD Pulley - 2K miles, 2 track days (Spa and Nurburgring) and no issues. Interestingly I came back with an '03 986s (which I believe has 5bhp more?) Anyway, in a straight line, 50-130, I would pull maybe a car length every time - just about noticeable, but consistently noticeable.
The real difference is now in the in-gear pick-up - it's noticeably more brisk.

You mentioned comparison to an '03 S...what model is your car?

Also, have you noticed any (even if slight) side effects during street driving with all accessories on?

[jhandy]

ok, here is what i have seen with my car. It may be different with someone elses...

The battery does not get a good charge with the 4inch pulley on short rides and at idle. I have accidently run my battery down by playing my radio while in the garage, and the car can drive fine, start fine, but if the battery is already crap, then the alt does not seem to recharge it quickly.
Now if I am not stupid and keep my battery to start the car and nothing else, it does just great.
There is no bad side when driving with all electrics on. I have measured the output while driving and at 1000 RPM and greater the charge is 14 volts.

So there is not a downside to a 4 inch pulley that i can see. Just dont run your battery down with the car off.

So... you meant an underdrive and a larger steering pump? why would you do that? I have not noticed any downside to the steering.

[chaudanova]

Originally Posted by jhandy

I have the raby underdrive. It is the smallest pulley made that still allows you to drive the car daily.

i am assuming you ment to say - put an underdrive and a small pulley for the power steering?

I think that this would be too much. one or the other.

if i am at idle, my alt puts out 12.9-13.0 volts. my power steering works great, but i can tell any less pressure would be too little.

Also if i turn all electrical things on the car, the lights, ac radio, radar dectector, sat reciever, heated seats....eveything... my output at idle reads 11.9-12 volts.

so you can see any slower would not let the car recharge the battery.

Also the advantage of the underdrive over the steering pulley is that it slows the steering pump, and it lowers rotational mass of the crank at the same time. increasing torque and a little HP. just a steering pulley should not do that.

i recommend the raby pulley 110%, for around 300$ and a little work, it is the best bang for the buck hands down.

DONT FORGET THE NEW BOLT AND WASHER!!! dont cut corners or you risk sittng on the side of the road.

Thanks for the feedback JHandy...Seems like a great mod still, just wondering whether it's better for a daily driven car to go with this smaller 4" pulley, or to go with the slightly larger, but still smaller than stock underdrive pullies from Mantis/BBI/RSS, etc. Since those are a size in between comparing the stock pulley and the small 4" pulley, I'd imagine some hp free'd up, but not as much as the 4" pulley, yet less of the accessory/charging issues of the smaller pulley.

And I did indeed mean, smaller crank pulley, and larger power steering pulley for the underdriving system..

Seeing as you experienced some, albeit slight side effects of the underdriven system when you have all of your accessories on, if you were to have an ideal situation or do it all over again, would you consider the other 'slightly less' underdrive pulley offerings, or would you still stick with the 4" pulley? (when all accessories are on, ie, lights, AC, radio, etc, is it enough of a difference to be annoying at all?)

Thanks in advance for the feedback again...