Modern Performance Underdrive Crankshaft Pulley by F 500 Sport

By diyauto
( 1 )

5 minute read

Modern Performance Underdrive Crankshaft Pulley

Compliments of F 500 Sport @


I found out these underdrive pulleys were going to be offered for sale and was able to order one early. I received it Thursday night and wasting no time got the drop lights out and put it in. Installation is straight forward and should take no more than an hour tops. Tools needed are a Phillips screw driver, 10 mm socket (1/4), 13 mm Socket (3/8), 15 mm socket (3/8), 15 mm wrench or preferably a 15 mm belt tensioner tool, 1/4 and 3/8 ratchets. I wont go to far into the instillation except to say the hardest part of the whole job was the trim screw behind the brake line. It is awkward to get at. I choose to do the job on ramps. It would probably be easier removing the R front wheel, just keep in mind 90% of the job is done under the R front side of the car so it has to be high enough and supported. I removed the vertical brace and small splash shield that covers the crank and belt, and the lower horizontal splash shield for more room to work. The wheel well does not have to be removed. Once there you will see the belt tensioner. A 15 mm wrench will work but not all that well as the space is tight and it is tough to get leverage. Loosen and drop the belt. I don't want to offend anyone by saying you cant handle a belt change, but take note on the belt routing, it will go on, and work 2 different ways, but with much less tension. At this point remove the 3 crank pulley bolts, line up the new pulley with the tone wheel and line up both with the crank. IT ONLY GOES ONE WAY, you will see the pin on the crank. The kit comes with a new shorter belt, reinstall and button everything back up. 

My take on the product it self is this; very well made. Nice machine work. the pulley is one piece billet and anodized black, comes with a new tone ring as the original is pressed on the the original pulley, 3 bolts, the new belt, and directions. I weighed the original and new pulleys with a hanging bike/parts scale and the weights I got were 3.32 lbs. for the original and 1.84 for the new pulley and tone ring. (see pics below) Difference of 1.48 lbs. of rotating weight along with the smaller diameter of the pulley. My scale was not certified by the department of weights and measures, so I cant say it is dead balls accurate, but it is all I had.

Performance wise I am happy with the pulley. Keep in mind if you are modding, we are all in different stages and I have no piggy back box, but almost everything else. The way my car is set up right now it "turns on" about 3000 rpm. As part of my test I installed the pulley and put about 25 miles on my car with combined regular and aggressive driving using both sport and standard mode. Regular tooling around light to light I didn't feel a notable difference, however, for my set up I feel a definite difference from about 3000 RPM up to red line. You can tell the motor spins above 3000 rpm with less effort. I then switched back to the stock set up and drove the same course in the same manner and above 3 grand I feel a definite advantage in the underdriven lighter pulley. I switched back and went for a third ride, same course, same driving style and I have to say I am sold. All this was done without the air on. 

This piece with a light weight flywheel would be the faschizzel IMHO. 

You will notice the factory pulley has a damper between the 2 pieces. The guys at Modern tell me the factory piece is not a balancer and the materiel between the 2 pieces is there to absorb resonance that may be generated by the accessories running. Also the engine is internally balanced on this design. I ran my car up to red line on all my test drives and felt no vibration what so ever.

The only science I can offer is the weights of the 2 pulleys and the obvious difference in the diameters between the two that give you a smaller rotating circumference, lighter rotating weight, and 20% underdrive of the accessories. I also used my Innovate data logger and with the A/C, lights, rear defrost, and fog lights on, the voltage difference at idle dropped .6 volts between the stock and underdrive pulley. The underdrive recovered voltage immediately off idle. Last thing is the A/C. It gets hot and humid here however we are not into our really hot weather. We had warm and humid weather over the weekend and my air performed fine. I mention this because obviously you will be underdriving the A/C 20%. As the temp creeps into the 90's with the 99% humidity we get here I may have to make adjustments to my A/C such as turn the fan up a click or two, or use recirculate rather than draw air from the out side. As of right now everything is working out good. 

If you are considering this mod hopefully this will answer some questions and give you some helpful information.


Weight of new pulley and tone ring.

Weight of original pulley and tone ring.

Side by side.

New over original.