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Underdriven Pulleys

AYW550

By AYW550,
in Engines & Forced Induction

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SKiT_R31 Experienced

SKiT_R31

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Posted (edited)
have you experinced cases where light weight pullies made no difference?? reducing the rotating mass of a motor would decrease its work load, requiring less power to turn its self, they say its a similar effect to that of a light weight flywheel, im not saying i believe their claims hands down, but it makes sense to me...

This is correct, and fine if your removing the weight of accessory pulelys etc, but the crank pulley is needed as a balencer to help harmonics issues the straight 6's have like Chrispy said. Some people actually go heavier...

Yes underdrive pulley's will give some effect, but not much, they are more necesary if your engine spends most of its time in the higher rev range (instead of its designed lower rpm operating range) since for example an alternator might peak at ~ 3000rpm engine speed, after that its just waste, or a water pump might spin too fast to have enough cooling time in the radiator (and thermostats being non-intelligent will just be fully open, when they need to be restricting to slow down waterflow so the radiator can cool it effectively).

underdriving power steering, will do nothing, as there is next to no load when not steering (if you cut your PS belt on a dyno, i'd be suprised if it made 0.1kw [100w] difference). Water pump, you should go electric, for best temperature control, otherwise when not revving the ballbag out of your engine (i.e. long idling, low speed traffic) your going to cook your engine, wheres an electric water pump will have alot more control, and less drain + no cavitation. Ditto for thermo fan.

I'd say leave your power steering alone, and then all thats left is alternator, which depends on your current draw, if you turn on all your accessory's, lights etc, and the alternator is still producing more power than there is being drawn (when underdriven at idle) then thats fine, but if its not, it will just be more hastle then its worth, for the sake of a kW or 2...

You might be able to get a high amperage alternator and underdrive that, but $ per kW, possibly not the best investment.

remove your clutch fan for sure, water pump if your keen (and go electric), but everything else i would leave as is. Best bang for buck though is to save for some cams, $500-$600 will get you some regrinds, $900 will get you some billet brand names (which can be more wild) if your on stock management (bound by stock rev limit + vct), try and keep the duration low and the lift high. If you go the CAM route, you might want to let it breath a little easier also, with a larger or twin TB, and apppropriate sized intake.

Edited by SKiT_R31
AYW550 Collaborator

AYW550

Posted
  • Author
Posted
This is correct, and fine if your removing the weight of accessory pulelys etc, but the crank pulley is needed as a balencer to help harmonics issues the straight 6's have like Chrispy said. Some people actually go heavier...

Yes underdrive pulley's will give some effect, but not much, they are more necesary if your engine spends most of its time in the higher rev range (instead of its designed lower rpm operating range) since for example an alternator might peak at ~ 3000rpm engine speed, after that its just waste, or a water pump might spin too fast to have enough cooling time in the radiator (and thermostats being non-intelligent will just be fully open, when they need to be restricting to slow down waterflow so the radiator can cool it effectively).

underdriving power steering, will do nothing, as there is next to no load when not steering (if you cut your PS belt on a dyno, i'd be suprised if it made 0.1kw [100w] difference). Water pump, you should go electric, for best temperature control, otherwise when not revving the ballbag out of your engine (i.e. long idling, low speed traffic) your going to cook your engine, wheres an electric water pump will have alot more control, and less drain + no cavitation. Ditto for thermo fan.

I'd say leave your power steering alone, and then all thats left is alternator, which depends on your current draw, if you turn on all your accessory's, lights etc, and the alternator is still producing more power than there is being drawn (when underdriven at idle) then thats fine, but if its not, it will just be more hastle then its worth, for the sake of a kW or 2...

You might be able to get a high amperage alternator and underdrive that, but $ per kW, possibly not the best investment.

remove your clutch fan for sure, water pump if your keen (and go electric), but everything else i would leave as is. Best bang for buck though is to save for some cams, $500-$600 will get you some regrinds, $900 will get you some billet brand names (which can be more wild) if your on stock management (bound by stock rev limit + vct), try and keep the duration low and the lift high. If you go the CAM route, you might want to let it breath a little easier also, with a larger or twin TB, and apppropriate sized intake.

awsome answer man, alot of good points, looks like cams will be next, thanks!

  • 3 weeks later...
warps Mentor

warps

Posted
Posted

People don't underdrive pulleys for the power gains (although there is a very slight gain, since power = force x speed, and you're reducing the line speed of the belt). The main reaso they do it is if the engine lives at high RPM, to protect the alt and keep the water pump in the efficient range.

Excessive RPM will caviatate a water pump - not a good thing (means it's trying to pump air instead of water)!! If you plan to race the car, by all means use an underdrive crank pulley for those reasons. You WILL get some free horsepower, but I doubt anyone this side of Michael Schumaker would notice it. If it's just a street engine - probably not worth the effort.

Do NOT change your multi vee belt for a single vee. You might as well swap your RB engine for a pushrod 202 if that's the case - big step back in technology. (less slip and far more resistant to being turned inside out at high RPM). If you've ever raced a rotary, you'll know the heartache of throwing fan belts at high RPM. Multi Vee belts do not do this.

Lastly - reduced rotating mass (eg lighter pulleys) do NOT give more power. That is a fallacy!! At best, they will make the engine free rev quicker.

I'm not a fan of EWP's (have heard horror stories from people who have spent many years in racing circles). Each to their own through, as I know a lot of people use them happily in racing. I won't try and force my opinion on that topic onto others.

iseekool Veteran

iseekool

Posted
Posted

^ spot on to the post above - no power will be gained - the engine will still be the same. You will be taking load of the engine - which will help response - but I imagine the response gains will be tiny for the money spent.

But if budget is not a concern definately go for it. :P

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