Underdrive pulleys...convince me that they work.

Maybe 10 FWHP but not to the wheels. Mine made my car idle smoother and it seemed to rev faster..

they cost about as much as a cheap CAI and the CAI might gain you about 2hp.....I would think the U/D pully's would do more than that

tehy work my friend. What you are doing is lessening the parisitic drag on the engine from the accesories. another good way to help lessen this drag is with a electric fan.

Maybe I am the lone person here but I think they are absolutely worthless junk. They are not worth the money. They gain what, 2hp on the wheels? At the cost of slowing you acc. Save your money. You will see more gain by eliminating the clutch fan like mentioned above.

http://en.wikipedia.org/wiki/Underdrive_pulleys

An Underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such as an alternator pulley) that is designed to drive a vehicle's accessories at a slower rate than stock. Originally they were invented for race cars in the 1950s and 60s, whose engines had been modified and ran at higher RPMs for longer periods of time than the manufacturers had intended. It was therefore necessary to underdrive the belt-driven accessories (among other things like the water pump) to keep the accessories from burning out and the engines running reliably.

Presently, these pulleys are marketed as performance enhancers that actually produce additional torque and horsepower by reducing parasitic drag caused by belt-driven accessories, or by being lighter and reducing rotational mass, or both. However, the gain is usually minimal, on the order of 1-6 wheel horsepower depending on the engine. There are trade-offs for this "free power," however.

Changing the original crankshaft pulley can have negative effects on the harmonics of the crankshaft: a pulley that is too light or not properly weighted cannot reliably balance natural frequencies that occur along the length of the crankshaft at certain RPMs. The original pulley may have been carefully designed, balanced and weighted to counteract (harmonically dampen) this naturally occurring harmonic frequency, and without it, eventual engine damage may result. See: Engine Balancing

A less severe side effect is somewhat intended by the design of these pulleys. Since the alternator, power steering, and/or air conditioning units are not spinning as fast, corresponding dips in alternator voltage, power assist, and A/C effectiveness may be noticeable, especially at idle. The headlights may dim when coming to a stop, or the stereo may lower in volume, for instance. For a race car, this is not of concern, but for a daily driven vehicle it can be an annoyance.

I have not personally done it but I remember hearing someone flat out taking the belt off for a quick dyno pull and the gain was something like 2hp at the wheels, but back to back dyno pulls often vary by that much.
Conclusion is that if no belt is negligable, underdrives aren't going to make you power, they'll only save your accessories if you turn alot of rpms like mentioned above.

You can feel a difference. I put mine on (March Pulley's) and it wasnt the power difference that I felt but the car revved quicker. it feels good and I may have gained a few ponies but your really not gonna feal 5-10 hp in your ass so who knows. For the $50 I paid and the hour it took me to install I guess it was worth it.

Being that you are in Canada, you might get away with underdrive pulleys. The negative effects are low output from the alternator at idle/low speed and a tendency to overheat in slow traffic. Just don't get caught in a traffic jam at night, your battery may take a beating.

Canada has much cooler summers than Florida, Arizona, or New Mexico, so the overheat problem may never surface. For those of us whose summertime temps hit 95 F or more and live in big cities, the overheating problem can be very real. The hotter it is outside, the harder it is for the radiator to get rid of the engine heat into the air. The massive concrete roadways heat up the air even more, making for a 140 F pavement and air temp next to the pavement.

jrichker said:

Being that you are in Canada, you might get away with underdrive pulleys. The negative effects are low output from the alternator at idle/low speed and a tendency to overheat in slow traffic. Just don't get caught in a traffic jam at night, your battery may take a beating.

Canada has much cooler summers than Florida, Arizona, or New Mexico, so the overheat problem may never surface. For those of us whose summertime temps hit 95 F or more and live in big cities, the overheating problem can be very real. The hotter it is outside, the harder it is for the radiator to get rid of the engine heat into the air. The massive concrete roadways heat up the air even more, making for a 140 F pavement and air temp next to the pavement.

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Excellent info...but remember if you perform 10 mods consisting of 3-5 hp your going to end up with a substantial gain. Example would be my Efan upgrade...on the dyno the returns were excellent as RPM climbed, biggest gains found at 5800 but really there were gains all through the graph after 3000 rpm.

yes there are negitive effects such as the stop and go traffic issue but that isn't so much the underdrive pullies fault. Odds are you have modded your mustang quite a bit and you have not done much to the cooling system. It was designed for 225hp after all. If you are going to get 350 hp out of the 302 then it stands to reason that because of the extra heat you will need to upgrade the cooling system. A high flow water pump (perhaps even a electric one to negate the effects as metioned above of the UD pully) a 3 or 4 core rad and a electric fan that moves a lot of air ( not a black magic fan).

SO wait i have a question on all of this ... i want to put unnderirve pullies and an electri fan on my carr ... well i wanted to but now im not so sure ... its sounds to me like i HAVE to put a fan in if i want pullies which i was going to do anyways .. what kind of fan though?? and how do the fans hook up?? do i just connect it to the seloniod??

bubba-dough said:

Maybe I am the lone person here but I think they are absolutely worthless junk.

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You are not alone. The only REAL dyno information I've seen on this came from two locations:

The first was an 8 cylinder but not a mustang. With the addition of JUST underdrive pullies, the dyno showed a whopping 3 HP to the wheels. There was no real way to know for sure whether this was from the pullies itself or just a +/- error for the dyno.

The second isntance was on a car where the serpantive belt was removed all together. The total gain was between 11 and 15 HP. Now consider that a set of underdrive pullies give a 20% reduction. 15 RWHP (benefit of the doubt) x .20=3 HP.


Pretty consistent with what was done on the 8 cylinder above.

I would LOVE to see an engine dyno with before and after graphs for JUST pullies vs. the normal accessories.


As for the individual who said he "felt" a 10 HP increase with his seat-of-the-pants dyno.... well.... I just gotta call Sorry bro.

jfran430 said:

SO wait i have a question on all of this ... i want to put unnderirve pullies and an electri fan on my carr ... well i wanted to but now im not so sure ... its sounds to me like i HAVE to put a fan in if i want pullies which i was going to do anyways .. what kind of fan though?? and how do the fans hook up?? do i just connect it to the seloniod??

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Your mechanical fan will be the largest part of your accessory rotational loss. I REALLY see no need of a 20% under drive if you've taken the largest rotational resistance out of the acessory package.

If you want to get pullies because they look nice... Go For It! I recommend that you keep your accessories spinning at the rates they belong though.

Mustang Enthusiast ran an article where they claimed through dyno testing that underdrive pullies added 13 hp and 8 lbs. torque on a mildly modified 5 liter.If anyone is interested the article called Budget Boltons Part 2 is available on the Comp. Cams website.

bubba-dough said:

Changing the original crankshaft pulley can have negative effects on the harmonics of the crankshaft: a pulley that is too light or not properly weighted cannot reliably balance natural frequencies that occur along the length of the crankshaft at certain RPMs. The original pulley may have been carefully designed, balanced and weighted to counteract (harmonically dampen) this naturally occurring harmonic frequency, and without it, eventual engine damage may result. See: Engine Balancing

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Nobody else cought this BS?? Come on! (WHAT IS THE JOB OF THE HARMONIC BALANCER?)
Seriusly guys...

Dean85GT said:

Ive been looking towards the final little cheap mods to do to my car and underdrive pulleys are on the list. I always thought these things were a gimmic and I dont believe they do anything. Basically Im asking if anyone has any solid info to prove me wrong. If they work then great, its like 100 bucks for up to 10hp but for some reason I dont think its that easy.

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its no gimmick, they do work. depending on your combo it may not help much. but they do reduce drag so they are actually helping. just might not be much.

if u want them and dont want to spend 100$, get a cheap set off ebay. i bought my pullies off ebay for 30 bucks new. look great and work perfect. its a pulley, there isnt anything to mess up. so there isnt any "ebay= junk" arguement for them.

Not really worth it IMO. If you really want them, like said above. Get them cheap and enjoy the 2-3hp gain.