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Old 05-27-2016, 09:41 AM   #1
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Underdrive Pulley Questions

Considering Pedro's Underdrive Pulley Kit but read that IMS Bearing Replacement should be done before because of the need to shave down the "boss" or Top Dead Center bolt receiver/holder/whatever (not sure of technical term).

Can someone elaborate on why shaving down the "boss" may make this more difficult?

Can enough of the "hole" be left intact to use it as a TDC guide?

Any other problems experienced as a result of an Underdrive Pulley Kit?


Recommend any other kit over Pedro's and why?

Thanks in advance.
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Last edited by DRGETZ; 05-27-2016 at 09:45 AM. Reason: TMI
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Old 05-27-2016, 10:03 AM   #2
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Quote:
Originally Posted by DRGETZ View Post
Considering Pedro's Underdrive Pulley Kit but read that IMS Bearing Replacement should be done before because of the need to shave down the "boss" or Top Dead Center bolt receiver/holder/whatever (not sure of technical term).

Can someone elaborate on why shaving down the "boss" may make this more difficult?

Can enough of the "hole" be left intact to use it as a TDC guide?

Any other problems experienced as a result of an Underdrive Pulley Kit?


Recommend any other kit over Pedro's and why?

Thanks in advance.
the 4" diameter pulley blocks theTDC locking hole making it difficult to lock drivetrain @ TDC which is critical for successful IMSB retrofit.
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Old 05-27-2016, 10:21 AM   #3
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Thanks for the Reply...

Can't one just install the OEM pulley prior to an IMS Retrofit?

Is there an alternative to using the boss for TDC work and is it a substantially more difficult task?
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Old 05-27-2016, 10:36 AM   #4
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Can't one just install the OEM pulley prior to an IMS Retrofit?

Is there an alternative to using the boss for TDC work and is it a substantially more difficult task?
Problem isn't the pulley on many models, it is the amount of the boss you need to machine away.
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Old 05-27-2016, 10:51 AM   #5
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Can you recommend one requiring less shaving than others?
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Old 05-27-2016, 11:55 AM   #6
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The majority of pulleys I,ve seen don't have the TDC reference hole thru the pulley. Even if you only remove some of the boss and not all you cant lock the crank without a hole in the pulley. I purchased the Mantis pulley last week strictly because it has the TDC hole in it. I haven,t installed it yet as it is still in the mail.
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Old 05-27-2016, 12:49 PM   #7
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Can you recommend one requiring less shaving than others?
We do not recommend any of them.
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Old 05-27-2016, 07:24 PM   #8
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My 986 had an early RSS underdrive pulley installed in 2009 while shaving off just enough of the timing boss for clearance. When we had the IMS retrofit done a few years later we simply re-installed the stock pulley and everything held together as it should. I had very good reliability with my accessories never overheating and the PS pump never leaked with over 100 track days.

Is it a perfect solution? No. Will it work if you are careful with your installation and save your stock pulley for future engine work? Yes. Shade-tree mechanics shaving down the timing boss smooth is the bane of every engine builder.

Never had a problem with overheating the motor and some have suggested that slowing down coolant velocity may actually improve cooling with longer contact time to absorb/release heat. Ideal heat rise through a heat exchanger or motor seems to be about 40F to get maximum heat transfer. Too fast or too slow and some efficiency is lost. I have not tested this in a 986 with any objective measurement tools, just familiar with heat exchangers and flow rates in my day job. The concept is plausible and my own experience was good.
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Old 05-27-2016, 07:36 PM   #9
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We do not recommend any of them.
Why?
Except the old lightweight ones that broke, everyone seems to be very happy with the results.

For the cooling system, the thermostat is what regulates the flow, the pump is always spinning fast enough.
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Old 05-28-2016, 04:59 AM   #10
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Why?
Except the old lightweight ones that broke, everyone seems to be very happy with the results.

For the cooling system, the thermostat is what regulates the flow, the pump is always spinning fast enough.
Two reasons: Quality and the lack of a TDC boss on many of them.

A couple of the alloy versions have been prone to cracking, which makes me weak. The lack of a usable TDC boss becomes an issue in the shop when we have to add a substantial upcharge to a repair because we now have to pull the new pulley, reinstall an OEM unit, and hope to Hell there is still enough boss left to hold the TDC pin during repair procedures. And when the DIY install chewed away too much of the boss, we simply have to charge the car's owner a bunch of $ only to tell him we cannot fix his or her car.
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Old 05-28-2016, 06:35 AM   #11
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Ok, so less of a concern for a DYIer.

Thanks!
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Old 05-28-2016, 07:11 AM   #12
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Ok, so less of a concern for a DYIer.

Thanks!
Not really, if you decide to do an IMS retrofit after putting one in, you could be either screwed, or forced to spend time taking it back out in order to do the IMS. And if somewhere down the road you need to pull your cam covers to fix an oil leak, or reset your cam timing after repairing your VarioCam system, you better hope you left enough of the boss to allow you to pin the engine at TDC after you also remove the new pulley and reinstall the OEM unit.

These pulley's are not without their merit on the performance side, but they can have very significant time and money consequence's when it comes to future work.
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Old 05-28-2016, 08:04 AM   #13
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Underdrive Pulley might be okay for a track car, but for a DD....meh, not worth the hassle.
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Old 05-29-2016, 10:46 PM   #14
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I've had the one from Pedro for 2 years now and it works as advertised with NO downsides in day to day use, including the 115 degree Phoenix days.
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Old 05-30-2016, 12:43 AM   #15
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A good machines pulley should be fine. I sell underdrives for old Volvo's @ Nordic Car Parts and never had any complaints

I don't know much about the M96 engine yet but usually you don't need to shave anything to install a underdrive pully.
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Old 05-30-2016, 08:09 AM   #16
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on the m96 you do have to shave part of the cylinder 1 TDC boss!
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Old 06-04-2016, 02:00 PM   #17
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Benefits of higher performance underdrive pulley

like everyone else, I am always looking for that next performance upgrade for my 1999 986 Boxster: have already done L&N IMS retrofit, FVD performance software, K&N air filter, FabSpeed Sport Headers, Fabspeed MaxFlo Exhaust system, replaced OEM cable throttle body with 1999 996 74mm cable throttle body so I could install 996 IPD Plenum, put Victor Rims and Yokohama tires on it, adding NT Auto rear wing and diffuser, drives great and sounds even better.......... pardon my ignorance, will replacing the OEM underdrive pulley with higher performance system add HP and/or torque, if so, is there consensus on which manufacturer is best?
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Old 06-04-2016, 03:19 PM   #18
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An underdrive pulley doesn't create horsepower but it spins the belt driven accessories slower leaving 5_7 hp extra for driving the wheels. It also might lengthen the life of the power steering pump
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Old 06-04-2016, 04:38 PM   #19
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On the track, we are running 4k to redline constantly. The peripherals, especially the power steering pump doesn't like this. I intalled the RSS pulley and shaved off just enough of the boss so that it would clear. Extra HP? Not sure but if I can reduce equipment failures when I'm at the track, it's an easy mod. If you don't track it, don't bother.
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Old 06-05-2016, 05:10 AM   #20
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Quote:
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Can't one just install the OEM pulley prior to an IMS Retrofit?

Is there an alternative to using the boss for TDC work and is it a substantially more difficult task?
Thats what I did. Mantis did the ims and installed one of their pulleys.
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