Underdrive Crank Pulley Results.

[29OUTLAW]

There’s not much to be found here regarding UDP’s (especially for SRT’s) so I figured I’d add my comments in hopes of helping someone.

I installed a UDP crank pulley on my 2005 Thursday. It’s the aluminum crank pulley made by Mopar Performance. The catalog claims that it’s the same crank pulley used on the Comp Coupe. I have no charging or cooling issues so far and the install took about 30 minutes. To give you an idea how easy the install is, if it hadn’t been for removing the air filters and shroud, the job would have taken about 10 minutes. Stock pulley is about 6” in diameter and the Mopar pulley is 5” in diameter.

It’s common knowledge that with performance pulleys, power is gained not only from the underdrive ratio but also from lowering the rotating mass on the crank. The aluminum Mopar pulley weighs a svelte 12 ounces. The stock steel pulley comes in at a hefty 2.25 pounds.

The problem with the Mopar pulley is that you’ll be hit with Viper tax. Dan Lesser was able to help by getting me the pulley and belt for about $70.00 less than the dealer would have charged.

I have no idea if the pulley gets me any power and I’d certainly never feel any difference with just a pulley, especially since Jason Heffner installed 3.55’s for me the day before, however, I’m just trying to cover all bases.

 

[Skip White]

John, your are absolutly correct in the many bennifts of the lightweight smaller crank pulley.

I'll be getting on of these myself.

Kinda wish it were a bit smaller, but it's still a great mod.

The factory sets these cars up for the worst case senario, such as extended idling, with the A/C and lights on, etc.

I never sit with this car idling for long periods of time, especially with the A/C on.

The idea of not spinning the acc. at full speed is another benifit.

Rotational weight factors are multiplied when using this, as the belt is turning slower, as well as a lighter belt, and all the pulleys are turning slower. Plus the smaller, lighter crank pulley is very much a factor, being that much lighter is worthy of a few free HP through efficiancy.

 

[jk]

Did the pulley swap require a shorter belt or did the tensioner take up the slack?

 

[29OUTLAW]

Did the pulley swap require a shorter belt or did the tensioner take up the slack?

The pulley did require a shorter belt. Mopar catalog lists the part number for the belt required when using that particular pulley. Dan was able to get me the belt as well. Tensioner took up all slack on the new belt.

 

[acrdakota]

I'd hit up March as they are the ones making the pulleys for Mopar, might find them a bit cheaper that way.

I also wouldn't be surprised if you find the car runs cooler on track days with the UD pulley. Reason being, (and i've noticed this with other mopars as well) Is that by slowing the waterpump a bit your keeping it operating in a more efficient range, and out of cavitation.

I doubt you'll have problems running the UD alternator and PS pulley as well.

 

[Skip White]

Underdriving the alt and PS is not really that advantagous. Those pulleys would have to have a larger diameter, and this violates the laws of rotational mass efficaincy. Just do what you are going to do, by way of the cank pulley. The only advantage to slowing down the recieving pulley, is if you have to slow down the alt or ps, beond what the crank pulley can.

 

[29OUTLAW]

I'd hit up March as they are the ones making the pulleys for Mopar, might find them a bit cheaper that way.

I looked into March and although they make pulleys for the SRT-10, they're 0% underdrive. They're the same size as the stock. They are however, lighter and look cool

 

[Viper X]

I wonder what affect this will have on the cooling system while idling in the summer heat with the A/C on? As stated above, this will slow the water pump a bit.

There are kits for Gen II's that speed up the water pump to help with cooling at the track. Not sure about the Gen III's but suspect they are similar.

Please let us know if you notice any difference in the summer heat.

 

[acrdakota]

Underdriving the alt and PS is not really that advantagous. Those pulleys would have to have a larger diameter, and this violates the laws of rotational mass efficaincy.

In theory yes this is true, the HP saved slowing the alternator/PS pump FAR outweighs the slight loss of efficiency by having a larger dia pulley. My march pulleys are quite a bit larger than stock, but lighter, and with all 3 (Crank, Alt, waterpump pulley) have been shown to free up around 10-12rwhp in the upper rpm ranges.

 

[jk]

I wonder what affect this will have on the cooling system while idling in the summer heat with the A/C on? As stated above, this will slow the water pump a bit.

There are kits for Gen II's that speed up the water pump to help with cooling at the track. Not sure about the Gen III's but suspect they are similar.

Please let us know if you notice any difference in the summer heat.

Underdrive pulleys should not have any effect on the SRT10 water pump since it is driven by hydraulics from the power steering pump - that is unless PS pump pressure drops too low.

 

[John Myrick]

I wonder what affect this will have on the cooling system while idling in the summer heat with the A/C on? As stated above, this will slow the water pump a bit.

There are kits for Gen II's that speed up the water pump to help with cooling at the track. Not sure about the Gen III's but suspect they are similar.

Please let us know if you notice any difference in the summer heat.

Underdrive pulleys should not have any effect on the SRT10 water pump since it is driven by hydraulics from the power steering pump - that is unless PS pump pressure drops too low.

The SRT-10 water pump is not driven off of the power steering hydraulic system.

 

[Viper X]

Hey jk,

Where did you get this info? Open the hood - latch just right of center, saftey latch under hood in small cutout and in about 3 seconds, you will see that the water pump is not driven off of the power steering hydraulic system. The radiator fan is driven by the power steering pump.

So, again, I wonder what this mod will do to his cooling system's efficiency in hot weather?

 

[Viper Specialty]

Hey jk,

Where did you get this info? Open the hood - latch just right of center, saftey latch under hood in small cutout and in about 3 seconds, you will see that the water pump is not driven off of the power steering hydraulic system. The radiator fan is driven by the power steering pump.

So, again, I wonder what this mod will do to his cooling system's efficiency in hot weather?

As long as the percent UD does not put the PS pump speed below the pressure limit of its pressure relief valve under fan high speed and wheel steering, it will have no ill effects on Fan Speed. The water pump... is another story. Time and testing will tell...

 

[Mamba man steve]

the heck with the pulley how much did the gear change cost.

 

[29OUTLAW]

the heck with the pulley how much did the gear change cost.

$1096.00 out the door parts and labor.

 

[TCKTPLZ]

I remember a while back West Coast Viper was offering an Underdrive Pulley Kit for GEN II Vipers (I think for GEN III too). Basically it was around $500 for 3 small pulleys. Someone on here purchased a set, put their GTS on a dyno, and noted about a 5 HP increase. Not much for $500 as well as the time and added cost for installation.

It’s common knowledge that with performance pulleys, power is gained not only from the underdrive ratio but also from lowering the rotating mass on the crank. The aluminum Mopar pulley weighs a svelte 12 ounces. The stock steel pulley comes in at a hefty 2.25 pounds.

I find it hard to believe that anyone could notice a savings of less than 2 pounds (24 ounces) while driving the car. Maybe you could be able to see it on the dyno, but even that difference, if any, would be infinitesimal. Just drive your snake over to a buffet, fill your gullet, and there goes the 2 pounds that you saved.

If the reduction of rotational mass is a concern, then money would be much better spent on an aluminum flywheel. On GEN II snakes, the flywheel weighs around 42 pounds. Aftermarket aluminum flywheels weigh either 17.5 (Fidanza) or 18 (RPS), which is more than half the weight of stock. On GEN III SRT snakes, the stock flywheel weighs around 32 pounds. While this is definitely less, you are still saving around 15 pounds. Not only does this weight reduction help in increasing performance, but it also reduces stress on the engine as well. You can get the Fidanzas as cheap as $400 and the RPS's as cheap $465. Therefore, I would venture to say that this modification is a much better value than the Underdrive Crank Pulleys as well as it yields a much more noticeable gain without having to take power away from the critical components of your car.

 

[29OUTLAW]

I remember a while back West Coast Viper was offering an Underdrive Pulley Kit for GEN II Vipers (I think for GEN III too). Basically it was around $500 for 3 small pulleys. Someone on here purchased a set, put their GTS on a dyno, and noted about a 5 HP increase. Not much for $500 as well as the time and added cost for installation.

It’s common knowledge that with performance pulleys, power is gained not only from the underdrive ratio but also from lowering the rotating mass on the crank. The aluminum Mopar pulley weighs a svelte 12 ounces. The stock steel pulley comes in at a hefty 2.25 pounds.

I find it hard to believe that anyone could notice a savings of less than 2 pounds (24 ounces) while driving the car. Maybe you could be able to see it on the dyno, but even that difference, if any, would be infinitesimal. Just drive your snake over to a buffet, fill your gullet, and there goes the 2 pounds that you saved.

If the reduction of rotational mass is a concern, then money would be much better spent on an aluminum flywheel. On GEN II snakes, the flywheel weighs around 42 pounds. Aftermarket aluminum flywheels weigh either 17.5 (Fidanza) or 18 (RPS), which is more than half the weight of stock. On GEN III SRT snakes, the stock flywheel weighs around 32 pounds. While this is definitely less, you are still saving around 15 pounds. Not only does this weight reduction help in increasing performance, but it also reduces stress on the engine as well. You can get the Fidanzas as cheap as $400 and the RPS's as cheap $465. Therefore, I would venture to say that this modification is a much better value than the Underdrive Crank Pulleys as well as it yields a much more noticeable gain without having to take power away from the critical components of your car.

Yea – but I can’t replace my flywheel in 10 minutes like I can with a crank pulley.

Anyway – some geniuses at Unorthodox Racing somehow determined that on average 2.7HP is gained for every pound you remove from the crank’s rotating mass.

 

[GR8_ASP]

Anyway – some geniuses at Unorthodox Racing somehow determined that on average 2.7HP is gained for every pound you remove from the crank’s rotating mass.

Ah, yep. That must be up with Einstein's theory of relativity that mass equals power in some manner.

 

[29OUTLAW]

Richard,

If bang-for-buck is your concern then consider…

…a performance crank pulley that gains 5HP for $300.00 is the same as a $30,000 TT upgrade that gains 500HP. It all works out to $60 per HP yet you’ll never see anyone questioning the wisdom of the latter.

 

[TCKTPLZ]

Anyway – some geniuses at Unorthodox Racing somehow determined that on average 2.7HP is gained for every pound you remove from the crank’s rotating mass.

Well if that's the case, then I should have gained around 67.5 HP when I saved 25 pounds by switching to an aluminum flywheel. And to think they only advertised a 20 HP gain. What a deal!

…a performance crank pulley that gains 5HP for $300.00 is the same as a $30,000 TT upgrade that gains 500HP. It all works out to $60 per HP yet you’ll never see anyone questioning the wisdom of the latter.

Yes, but you could actually feel the difference that a TT upgrade yields in areas other than your wallet, as well as the fact that it doesn't need to take away power to make power. Not to forget that it looks cool as well.

 

[Viper X]

TCKTPLZ,

FYI, the underdrive pulley kit that WCV offers puts smaller pulleys on several accessories, but specifically, it speeds the water pump up by about 400 rpm on average, moving more water. This helps the GTS run cooler. I know a couple guys running it and I am having this set up put on my GTS. It works.

 

[Skip White]

The HP gain is certainly not what some people think. When we lighten and reduce the diameter on the crank pulley, we have multiplied the benifits, and not to forget all the other pulleys are turning slower. So we have a triple benifit, as well as shorter and lighter belt. They all add up. The oem pulley is large and very heavy, just going to aluminum is of benifit, not to mention reducing it's diameter.

For someone to quote a number for HP gain when reducing x amount of pounds is very difficult. The formula to do this is somewhat complex, and when you add in other factors such as slowing down the other pullys, shorter lighter belt, and diameter and poundage of the crank pulley, it's somewhat complex. Also to come up with a number, the rpm the pulleys are going to experiance is part of the formula. the same reduction on an item turning slower will not yeild the same benifit. Acceleration is what's really increased.

I say it's a must to do, for no more than it cost, and easy as it is to change out.

Oh, TCk, that two pounds saving you compared to eating,(stuffing) two pounds at the buffet, is not the same as two pounds spinning around at up to 6000 rpm, not to mention the other pulleys on the engine. Diameter reduction alone is of benifit.