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post #1 of 19 Old 06-25-2019, 07:49 PM Thread Starter
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Under drive pulley

Does anyone know the diameter of an underdrive pulley vs. a stock one for a 89TII motor.
Thanks
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post #2 of 19 Old 06-26-2019, 12:00 AM
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Re: Under drive pulley

If you use your car a lot at night, spend a lot of time in stop and go traffic, or rain a lot, I don't recommend an underdrive pulley for you.
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post #3 of 19 Old 06-26-2019, 12:04 AM Thread Starter
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Re: Under drive pulley

I agree. I'm trying to figure out if mine is stock.
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post #4 of 19 Old 06-26-2019, 12:20 AM
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Re: Under drive pulley

Stock one is steel, underdrive pulley is aluminum.

Take a magnet to it.
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post #5 of 19 Old 06-26-2019, 05:27 AM Thread Starter
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Re: Under drive pulley

Thanks

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post #6 of 19 Old 06-26-2019, 10:20 AM
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Re: Under drive pulley

question re underdrive pulley

what happens with the groove in the back for the power steering belt
is it also underdriven or still the same ??

would the 120 alt still be an advantage over the 90a alt with the u-d-p on the 120 a one?
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post #7 of 19 Old 06-26-2019, 10:24 AM
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Re: Under drive pulley

bigger alternator . heavier mass to turn...
if you are looking for power. i would go with the lighter alternator.
:-)

us miata racers hunt down the lightest alternator of the bunch and the lightest starter option of the bunch.

we are weight weenies though
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post #8 of 19 Old 06-26-2019, 12:33 PM
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Re: Under drive pulley

ok something to consider -I'm building a street /road course project , not a drag car

would the underdrive pulley vs lighting issue be better / worse or still same, with LED projector headlamps and their associated hardware ??

should add , I have am electric water pump already so the pulley might be redundant anyways

I'm most curious as to how much the voltage supply actually drops in real time

I've already planed constant voltage supply /fuel pump controler and constant voltage for the ECU add on's so I'm really only interested in knowing if the pulley would screw things up needlessly

(my truck alternator started failing and the ABS light started coming on
replaced alt .. ABS light went out and stayed out
only difference .. voltage became constant again ..)
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post #9 of 19 Old 06-26-2019, 06:43 PM
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Re: Under drive pulley

I don't recommend underdrive pulley for street cars, especially with a/c.
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post #10 of 19 Old 06-27-2019, 04:57 PM
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Re: Under drive pulley

tryingbe doesn't, I never had an issue with a 120amp alt at night. Though I had my headlights relayed as well. Also porting the water pump and shaving every other fin off helps the motor spin up faster, and reduces cavitation in the cooling system.


1st thing I do is throw on a UDP, port and mod the waterpump and an aluminum flywheel. Makes the motor way more fun and rev happy without losing torque.
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post #11 of 19 Old 06-28-2019, 09:52 PM
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Re: Under drive pulley

I have to say not all underdrives are Aluminum . i have a steel version , although it is a vintage very early version .
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post #12 of 19 Old 06-28-2019, 10:23 PM
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Re: Under drive pulley

find an aluminum one and get the weight advantages too. put the steel one on a shelf.
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post #13 of 19 Old 06-28-2019, 10:40 PM
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Re: Under drive pulley

you are missing the point . i get what your saying but they aren't all aluminum . do your home work ,for the parts you plan to use . Here are the pulley dimensions you seek . from outer to inner-107 mm , 111 mm , 72 mm the steel version is most likely a FM part as we used alot of their turbo parts in the 90's .

Last edited by Ythorse; Today at 07:50 AM.
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post #14 of 19 Old Yesterday, 07:56 AM
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Re: Under drive pulley

Quote:
Originally Posted by Ythorse View Post
I have to say not all underdrives are Aluminum . i have a steel version , although it is a vintage very early version .
Pictures please.
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post #15 of 19 Old Yesterday, 03:42 PM
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Re: Under drive pulley

I believe FM had a steel version from years ago. Looked identical to aluminum underdrive pulley. The one I've seen was for early 4 bolt crank.
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