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Old 10-15-2007, 10:06 PM   #1
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Underdrive pulleys

So, I was showing off my new truck to my brother-in-law this evening. He's a gearhead and has a souped up Camaro. I asked him about the best bang for the buck performance mod he'd suggest for me. He said an underdrive pulley would cost about $30-$40 and get 20+ HP and low-end torque by putting less of a load on the engine. He did say that when you're idling, you are pulling partially off the battery, so it'd be a problem in regular bumper to bumper traffic. Since I have the towing package, which includes a beefier alternator and battery, he said it's not a big risk.

This is the first time I've heard of this. Anybody tried it on their Tacoma? Too good to be true?
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Old 10-15-2007, 10:52 PM   #2
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First I've heard of it too...
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Old 10-16-2007, 04:16 AM   #3
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I am a highway hog - if I had heard about this before, I would have done it. Sounds interesting...
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Old 10-16-2007, 07:30 AM   #4
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first i've heard but if anyone knows anything about it id be interested in something like that
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Old 10-16-2007, 07:46 AM   #5
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It's not worth it, (IMO) I'm not sure about doing it on a Tacoma but on other cars such as camaros or mustangs you will only gain about 5 HP (yes they do advertise 15HP gains but you will not get it) if any at all. They can work great if you have a heavily modified motor to squeeze out a little more power. Oh and if your actually able to find a them for $20-40 don't buy them, they will be weak and most likely just junk, when I was looking for some for my mustang the ran about 120-150 for a billet Al set. Don't forget that you will also need a shorter belt to put on.
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Old 10-16-2007, 08:10 AM   #6
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Well, if this is new to folks, let me explain a bit more of what he told me. The pulley in question is the one at the front end of the alternator that the belt turns. A larger pulley would spin the alternator more slowly at the same engine RPM, so basically the engine spends less effort charging the battery. It doesn't look easy getting to the alternator, but is just one bolt off, swap the pulley, one bolt back on job. Depending on how big a difference in the pulley size, and any adjustments available, you might need a longer belt.

Seems to me this'll require some research. It'll be important to know how much current the engine uses at idle, and at your average speed/RPM (ScanGauge data?). Plus you need to know the output current vs. RPM for the alternator (which would be different with a towing package vs. without). This should be available from the OEM of the alternator. We also need the crankshaft pulley diameter that is belted to the alternator. With that raw data we can do the math to find an optimal alternator pulley diameter.

I'll try to find some time to do some more research and post back here. If anyone else wants to do some too, don't be shy. It may be a while before I get a chance.
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Old 10-16-2007, 08:18 AM   #7
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Ah so your looking at alt. pulleys, I was talking about a crank pulley. Same concept just different areas. Underdrive from the crankshaft pulley means the pulley is smaller than the original, from the accessory pulley means the pulley is larger than the original pulley it replaces.
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Old 10-16-2007, 08:22 AM   #8
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Read the part where they did the actual HP gain test (3 HP with zero drag )

http://www.teamscr.com/pulleys.htm
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Old 10-16-2007, 09:33 AM   #9
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Quote:
Originally Posted by TRDeity View Post
Read the part where they did the actual HP gain test (3 HP with zero drag )

http://www.teamscr.com/pulleys.htm
Thanks for the link. That's good info. However, it's Saturn specific. Maybe Toyota has tuned their alternator pulley as well. I don't know. I wonder if it is tuned to the alternator that comes with the towing package separately. Maybe if folks with and without the towing package measured the diameter of their alternator pulleys, we'd know if they're different. If they're different, then Toyota's probably already done this analysis.
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Old 10-18-2007, 08:33 AM   #10
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Don't under drive the Alternator. That can always lead to issues. If you want to under drive anything under drive the water pump and power steering. Once I find a set out of aluminum I will get them. Anything less is a waste of $$ and reliability.
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Old 10-18-2007, 11:36 AM   #11
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Quote:
Originally Posted by m3dragon View Post
Don't under drive the Alternator. That can always lead to issues. If you want to under drive anything under drive the water pump and power steering. Once I find a set out of aluminum I will get them. Anything less is a waste of $$ and reliability.
I'm glad to hear that someone else knows this technique. I'll be interested in hearing any results you have after doing it.
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Old 10-18-2007, 11:45 AM   #12
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Quote:
Originally Posted by sawdust View Post
I'm glad to hear that someone else knows this technique. I'll be interested in hearing any results you have after doing it.
I run under drive pullies on all my cars. I want to free up HP that other wise is lost. Even Toyota over build their cars.
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Old 10-18-2007, 01:06 PM   #13
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Ok, m3, you seem to understand whats going on. What exactly is under driving a pully? How does this work? What does it mean for someone to "over build" a car? All this lingo is new to me
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Old 10-18-2007, 01:24 PM   #14
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Here you go.

What exactly is under driving a pully?
- You increase the diameter of the pulley thus making it spin slower. Hence "under Driving". This also requires less energy to spin which is where the gains come from. You HP does not increase but lets say it takes 20HP to spin all the parts. Under driving them can make it so only 10HP is needed. That means you car is still X amount of HP but more of that HP is going to the ground. A McClaren SLR take 150HP to spin it s supercharger.

How does this work?
- See above

What does it mean for someone to "over build" a car?
- All builder have to meet federal and safety guidelines. So when they make a water pump, alternator, power steering pump they have to meet specific tolerances. However these tolerances are more then what the car really needs.

Under driving also in theory makes the parts last longer as they are not working as hard as them spinning at max speed.

I have not found anyone out there that makes them for Gen 2 Tacoma's yet. I might just need to have contact one of the BMW performance shops to make one for me.

Hope that helps
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Old 10-18-2007, 08:30 PM   #15
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Thanks for sharing your experience. I'm curious though. Seems to me that fabricating an aluminum pulley is no big deal. The hard part is figuring out what size they should be. How do you figure that out? Is it just trial and error?
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Old 10-18-2007, 08:52 PM   #16
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Quote:
Originally Posted by TRDeity View Post
Read the part where they did the actual HP gain test (3 HP with zero drag )

http://www.teamscr.com/pulleys.htm


Thanks man. Always wondered.
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Old 10-18-2007, 09:13 PM   #17
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Quote:
Originally Posted by sawdust View Post
Thanks for sharing your experience. I'm curious though. Seems to me that fabricating an aluminum pulley is no big deal. The hard part is figuring out what size they should be. How do you figure that out? Is it just trial and error?
Trial and error. The pulley only needs to maybe be 3mm bigger to have a effect. If anyone has any leads on pulleys speak up. If not I need to get the time to find a standard ratio for pulleys underdriving.
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Old 10-19-2007, 09:06 AM   #18
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Thanks for the clarification m3, makes sense now
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Old 11-01-2007, 08:08 AM   #19
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Just a update. Was on the customtaco boards and the pulley questions was there too. URD offer a Cam pulley which is a NO NO. While it might work that is the one pulley you should never touch.

I emailed URD and Coast2Coast to see if they would make them.

If anyone can get their hands on a used set of pulleys and send them my way let me know. That way I can take them to a machinist and see how much it is to make them if we have to go that route.

PS the 4 Cyl guys will see more power and HP out of their little motors.
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Old 06-15-2008, 12:53 PM   #20
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Underdrive pulleys

Under drive pulleys have been around for years in the muscle car stage....

I used them on my Mustang, they felt like they worked.....


I know that if you take the belt off and run the thing down the road and back <1miles....She will scream like hell!!!!!!!!!!! Free horses, but it will over heat in a few minutes.......So make underdrive pulleys and get the best of both worlds.....

The dealers OVER build the vehicles due to economics, tooling and such.......So there could be extra horses found if you are willing to accept less of something else, in this case, alt charging is a little less, AC may be a lttle lesss............But you get a little extra of what we all WANT and no it doens't start with a P.......it is well it may...say performance!!!

Now where the hell can I get pulleys for the Taco?
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