Looking at adding a UDP, has anyone had any issues with engine temp from spinning the pump more slowly? I live in hot a$$ texas.

I saw Jake's post about cooler temps but either that is counter-intuitive or I'm missing something, and I'm comfortable admitting I may be missing more than just something here :) . Shouldn't less spinning = less cooling???

I live in hot ass Texas also. No problems here. Where are you at?

I guess I'm amongst the folks trying to keep it weird.

Well if you are near southlake and want to come check one out let me know.

Chris

I found this read on Underdrive Pulleys interesting: http://www.streettostrip.com/content/view/82/40/

davemon

I also live in Texas and I have installed Chris' (amondc) underdrive pulley and I can tell you first hand that my temps are cooler. It is also a daily driver for me and I have had no ill effects.

One of the best bangs for your buck in my opinion.

Why the lower temps?
 

I presume you are reading the temps off the guages.

Driving the accessories at a slower rate would spin the water pump slower which would evacuate heat slower which would mean that the water getting to the radiators would be hotter wouldn't it?

What am I missing here? Make any other changes?

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………..

You have to look at this as a temperature difference situation. If the engine is dissipating a set amount of heat, it is true that the slower the coolant flows, the higher it's temperature rise will be. However, the heat then runs through the radiators, and the slower the coolant runs through the radiators, the more heat they can drop into the air. The radiators can bring the coolant temperature closer to the outside air temperature at lower flow rates.

The balancing act is controlled by the flow rate of the coolant. It is conceivable that the coolant temperature is actually reading lower if the sensor is at the inlet to the engine. Chances are that the temperature at the outlet of the engine and into the radiators is running higher with the underdrive pulley.

Where you measure anything is always important to consider.

Thanks for the responses.

Suspected that running through the radiators too fast wouldn't create the exposure long enough to cool effectively... but then also suspected that these engines were designed to cool optimally at 4500-6500 rpms, when the cooling is needed more. Or maybe they were designed with more umm "average driving conditions" in mind with the rpms typically in the 2k-4k range and not catering to the enthusiast-type crowd who wants to feel their backs against the seat all the time, but again this is counter intuitive to a lot of other things that are designed to run a hard. Then I suspected I should just ask the experts :rolleyes: Well whatever the ideal range, nobody seems to have any issue with hot engines, even those of us that that live in the heat.

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.

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?

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

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.

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.

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...

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?

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.

Hmmm, interesting feedback. So for short rides, do you find the battery doesn't get sufficient charge to the point where you might have problems starting the car afterwards, and how long do you mean when you say that the alternator doesn't seem to recharge it quickly?

And with the car off, how long does it take for you run your battery down now? Say you're waiting around, and have the car off, with the radio on. Are we talking like 15-20 minutes of that and the battery gets depleted, or much longer periods?

Yeah, from my understanding, the underdrive system... for the crank pulley, underdriving is accomplished with a smaller diameter (than stock) pulley. On the other hand, for accessory pullies, further underdriving of the power steering pump, for example... is actually accomplished with a larger than stock diameter pulley, rather than a smaller diameter. It would take more linear belt movement for each single rotation, and thus each engine revolution it turns less times than it would compared to stock. Hope that makes sense...

I installed the Raby pulley 6 months ago and have not encountered a single charging problem. The pulley unit fitted easily, gives more power at the rear wheels and runs the accessories quieter than the original !!
You have to ask yourself how often do you run the electical system @ 100% + capacity, at low engine revolutions (less than 1000 rpm) for long periods with a questionable battery.
If you answer NO to 3 of the above questions, you won't have a problem with the Raby pulley in real world driving......