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My vehicle is an 07 GMC Sierra with the 4.3L engine and a 5 sp. trans. I added a crank and alternator underdrive pulleys almost 10,000 miles ago. The truck now has 16,700 miles on it. With all that information said I am thinking of adding an underdrive water pump pulley to further increase my power (i.e. economy). Contemporary advice at the time I installed the pulleys stated the vehicle may run hot. Subsequently I have been advised, but have not been able to confirm, that the radiator in my truck is the same as used for the V8 engines. If this is truly the case, it would seem an underdrive water pump pulley would not cause the engine to overheat. Any information and real time experiences will be most useful. And thanks in advance.

What does a Scangauge tell you about water temperatures? That way you'll know for sure.

I have an underdrive pulley, however I don't own a truck. I have a stock radiator, and without a grill block, I run cool at 166F.

Do you know what the ratio is for the UDP compared to stock?


That said, unless it's a drastic change (1/2dia), you should be fine. 2/3rd reduction will reduce flow, it will run warmer than usual, but not enough to cause it to overheat unless you use a grill block.

A friend of mine owns an 04 silverado with the 4.3 but he has an AT trans. I'll check with him and should have an answer in the am.

I wouldn't be concerned about the radiator, I think the water pump speed is the main concern. If the pump moves the coolant too slowly, it may not cool the engine adequately. If you use the truck to commute, the UD pulley should work well. If you use the truck to haul heavy loads or in other situations demanding sustained high engine output, I would leave the current pulley in place.

Don't forget, the time the water isn't in the motor, it's in the radiator cooling off. So you might not have the flow you once did if you put in that pully, you are also cooling the water more. I'd bet it'd be somewhere near break even.

Ok, here are the results from my questioning. You can get a high output alternator and replace the water pump with one that's electronic. You'll see an immediate increase in power and economy. He estimates that you'll probably see a 10hp increase with the removal of the impeller water pump.

I think you're on to something, the new Prius uses an electric water pump. Hopefully all new hybrids will use them.

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Electric pumps are expensive, and an underdrive pulley is a lot cheaper. Plus it's often less rotating mass.

10 hp at the crank doesn't translate to much at the wheels. It's barely enough to feel.

That depends on where you're starting. 100 hp or 250 hp?

Thanks for all the information. It gives me a good place to start. I haven't played with temp reading on my scanguage because I have on board guages. The stock water temp guage normal reading is approx. 190-200 degrees. I use a radiator block during the winter.

Electric pumps are expensive, and an underdrive pulley is a lot cheaper. Plus it's often less rotating mass.

10 hp at the crank doesn't translate to much at the wheels. It's barely enough to feel.

Over the life of the vehicle, you will definitely see a noticeable increase in savings vs a mechanical water pump. There are more losses than just mechanical for using an electronic water pump.

They make electronic water pumps for a host of vehicles though.