florent75

have you seen this new pulley (I think it's not an ununorth pulley) Quicker throttle response, increased horsepower, and improved drivability are just some of the characteristics the under drive pulley kit offers for the Mazda RX8. The pulley is anodized in Brilliant Blue or Silver Bright Dip. The pulley is roughly 20% reduced from the factory component. By having a lightweight and under driven pulley, it will offer much needed throttle response and increase of about 9HP. Each pulley includes new hardware for a bolt on installation and brand new belts for a perfect fit. price : 146 $ do you think it's true or false ??

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vectorwolf

Curious... What, exactly, is an underdrive pulley? Not sure I've ever understood the concept.

s13lover

I installed a light-weight, underdrive crank pulley on my 240sx and didn't notice a thing. It was over 6 lb less than stock. I hear mixed things about these pulleys from different people. My friend swore his 3-piece pulley kit helped his Civic.

smrx8

Good find

Jabberwock

It looks a lot like the agency power pulley, but I could be mistaken.

bgreene

This one is a smaller than stock pulley that replaces the main pulley attached to the front end of the eccentric shaft (analogous to a crankshaft in a piston engine). The power "gains" come about from 2 sources: there is a reduction in the rotational inertia of the powertrain systems, which means more of the engine torque is available for speeding up the wheels (this type of gain can also be achieved with lighter flywheels, driveshafts, brake rotors, rims/tires, pretty much anything that spins up or down with engine RPMs), and there is less work driven into the alternator, A/C compressor and water pump which means more of the engine power is available for moving the car.

All this can come with side effects, though. Underdriving the alternator means less power is available for accessories and recharging the battery (most stock setups have surplus power generated by the alternator, so this isn't neccessarily a problem, just something to be aware of). Underdriving the water pump can affect the efficiency of the engine cooling system, since the pump will be moving less water (however, if the water pump impeller causes cavitation at higher RPMS, underdriving the pump could improve cooling efficiency at those rev rates). I'm not familiar enough with the design of the alternators and water pumps used on the Renesis engine to get into specifics about what amount of underdriving could be safe or at what point use of underdrive systems can cause real problems (this would also likely depend on how the car is driven, since a stop-and-go commuter's engine will see a much different operating profile than a track racer's engine)

I put gains in quotes because these types of pulleys don't really change the total amount of power produced by the engine the way many other mods can, they just reduce the amount of that power which is consumed by the internal workings of the engine, thus increasing the amount of power which can be output to the rest of the car's drivetrain.

Brice-RX8

^^ Also remember that the A/C will be spinning less and the cooling that it produced will be less. So, as many people complain about the A/C on a hot day now, just imagine with that compressor spinning slower.

Red Devil

To add to this discussion, on older RX-7's cavitation was often claimed to be an issue by many of the companies providing underdrive pullies. Was it true? I'm not really sure.

TeamRX8

I would advise you to search "pulley" before departing with your cash

v300

Does not having a harmonic dampner on the underdrive pulley have any adverse effects on a rotary engine?

olddragger

rotary does not have a harmonic damper.--- I dont think.
olddragger

TeamRX8

looks like OBX duplicated it

Jaguar_MBA

Underdrive Pulleys on this car MIGHT get you 1HP. They are worthless. The stock crank pulley is so light already that it is basically no weight performance gain. I have the SR motorsport pulleys. If you must get these then go with the unorthodox ones and save yourself a couple hundred dollars.

Underdrive pulleys work great on cars with a large heavy harmonic balancer built into the crank pulley...the RX-8 does not have a harmonic balancer in the crank pulley. Replacing the crank pulley on the 300ZX reduced the weight off of the crank shaft by ~15 lbs (my guess).......the RX-8 underdrive pulley weighs next to nothing stock.

Nemesis8

I'd like to have the same diameter pulleys, (no underdriving), only in anodized aluminum instead. Anybody have a link to what I need?

Looking for bling for the engine

rotarygod

A physically lighter pulley is going to do nothing for you. All of it's weight is so close to the rotational centerline that it doesn't really do much to affect it. We don't have harmonic balancers so we don't have all the weight associated with them.

Don't be too concerned with the alternator not putting out enough power. That's what a voltage regulator is used for. The alternator hits max capacity pretty quickly as rpm's rise. It doesn't keep getting higher. The voltage regulator prevents this from happening. You won't notice a thing on the voltage side of things.

You may very well currently be hitting water pump cavitation speeds with the stock crank pulley. This will be up near or at redline though. If you aren't running at redline all day like a race car, you shouldn't have any reason to let this bother you. A quick tap of this point isn't going to hurt anything from a coolant standpoint and rarely is a street driven engine going to be at redline rpm's any more than a quick second or so.

Underdriving the crank does give the engine more leverage on the alternator and water pump and shoudl free up a little power. You won't make any more power. You are just wasting less and will get more usable power. How much is the real question. I doubt highly you'll get 9 hp out of it unless this is at the extreme upper end of the powerband and only assuming the water pump and alternator take 9 hp to spin at those speeds. I would suspect more along the lines of 3-4 hp freed up. When I installed an underdrive pulley on my RX-7, I couldn't feel a thing. I still have the pulley but it isn't installed on my current RX-7. It's not enough of a benefit for me to go through the effort of installing it. I might one day if I ever have the engine out.

TeamRX8

Mazdaspeed Motorsports sells a set, but I'm not sure if eliminating the A/C function accounts for their power claim, appears to be made by SpeedSource which may add some credibility to the claim since they probably run them on their RX-8's, it does say racing only though

Part No.
0000-01-8201
Description
PULLEY SET, DRIVE AND ALTERNATOR
Notes
Main drive pulley and alternator pulley set for RX-8. Made of billet aluminum. These pulleys add 8-10 horsepower on the dyno, due to reduced horsepower loss driving the alternator. No provision for A/C. For racing only.

4 years to Supercharge

When it says no provision for A/C doesn't that mean there is not a pulley replacement for the A/C?

Jaguar_MBA

So MazdaSpeed is saying that these underdrive pulleys are adding 8-10HP?

FlyingLeggs

I took my 8 in today to get a few maintenance issues resolved so I stopped by and picked up a Mazda/Mazdaspeed accessory brochure. They do not have the part listed yet. Also, I agree w/ 4yr., who has been educating me on pulleys lately. That sure sounds like they are omitting the a/c altogether. Not good, I live in Florida. And as far as the hp gains go, I can't imagine Mazdaspeed saying anything about gains.( Mazda won't let them)

TeamRX8

it's not a MazdaSpeed dealer accessory part, it is sold through the MazdaSpeed Motorsports racer program, two entirely different programs

the picture and text was copied directly from the MazdaSpeed Motorsport literature, I don't know about power claims, but the parts SpeedSource provides to the Motorsport program are only sold there. SpeedSource will not sell them directly to the public.

toca

the way i see if it gives u 5 to 10 whp u not going to feel it the only way u feel a intake is because of the better responce