I have been asked about running the UR crank pulley with the SC kit and finally decided to run the numbers. This is just for the UR under drive crank pulley which is smaller than the stock crank pulley.

When the UR crank pulley is used with the SC kit, the blower would be turned only slightly faster than the engine even with the HBP. So you would only be making about 1.6 PSI boost. This would give up about 30 - 35 WHP at this low of boost; or the majority of the gain the SC provides.

Here are the numbers for the Accessory Belt (in millimeters):

Stock Crank / Alternator Side
175 / 67 = 2.612 Ratio

UR Crank / Alternator Side
141 / 67 = 2.104 Ratio

Here are the numbers for the SC Belt:

Base Pulley / SC Pulley
76 / 100 = 0.76 Ratio

HBP / SC Pulley
83 / 100 = 0.83 Ratio

To determine the final ratio, multiply the two values. For example with the standard SC kit:

2.612 * 0.76 = 1.985

In this case the blower is turning 1.985 times faster than the engine. This can be directly correlated to boost since it is a positive displacement blower.

For another example, using the HBP and the stock crank:

2.612 * 0.83 = 2.168

Using the UR crank pulley and the HBP gives the following ratio:

2.104 * 0.83 = 1.746

Obviously slower than even the base SC pulley with the stock crank. Here is how you use these ratios to determine boost. Since it is a 62ci blower it moves that amount every revolution. But since this is a four stroke engine, a cylinder only ingests every other revolution, we have to take half of our displacement. Half of our 3.2L in cubic inches is about 97.5ci.

To calculate the theoretical boost you use the following formula:

62 * Ratio / 97.5 * 14.7 - 14.7 = Boost

Where 62 = the blowers displacement
Ratio = the drive ratio
97.5 = half of the engine’s displacement
14.7 = the assumed atmospheric absolute pressure

For example with the base SC kit

62 * 1.985 / 97.5 * 14.7 - 14.7 = 3.86 PSI

That 3.86 PSI boost is about correct with the base kit. Here it is with the HBP and stock crank:

62 * 2.168 / 97.5 * 14.7 - 14.7 = 5.57 PSI

Which, once again, is about right.

Now for UR pulley with the HBP:

62 * 1.746 / 97.5 * 14.7 - 14.7 = 1.62 PSI

IMO, it is better to stick with the stock crank pulley.

Just for fun, here are the numbers with the base SC pulley and the UR crank pulley:

62 * 1.599 / 97.5 * 14.7 - 14.7 = 0.25 PSI :o

 

what about the UR crank pulley that doesnt under drive made for the SC application just 6 lbs lighter weight

 

If it it the same size then the stock numbers would be used. I mainly spoke of boost which is independant of weight, just size.

 

I always wondered, and was recently asked buy someone interested in the blower that has the UR underdrive pulley. By your numbers the UR underdrive crank pulley is 20% smaller, but the final boost difference is 93% less.

 

Yep, it is that multiple ratios and the heavy depepandance on the crank which play a part. But most importantly it is the aspect that the blower speed has to be greater than the engine speed by a good margin in order to make boost.

You need a minimum of 1.57 times the engine speed to make boost. If you are running the blower twice the engine speed and reduce this by 20%; you end up with a 1.6 ratio. That has changed the boost making potential, as you stated, 93%.

What a huge impact and it goes to show that small overall changes have a dramatic affect on boost.

 

Calculations well done Steve..

 

im sorry i thought from the context of what you wrote that you were just putting out the data on the UR pulley to show the negatives of what will happen when using the 2 together...i meant what i said as if to suggest that you werent aware of a non under driven version...

 

I understand and do appreciate the help and insight.

 

since we're on the subject...what do you think about the non under driven ur pulley for sc'd or non sc'd cars for that matter?

 

I can't say for sure. The SC would put an additional load on the belt drive system which was never accounted for. I don't think it would be an issue though.

 

scalbert, thank you for clearing this topic up. i called comptech and UR, both said the boost would be so low that it would defeat the purpose of the SC

 

UR makes a crank pulley if u have the blower

 

This thread has been brought to my attention so I thought I would do the same for everyone by bumping it

 

ADHD just kicked in yo!

 

Bringing back this old thread because I have a question about the supercharger and crank pulley. I have the UR underdrive crank pulley and am getting the supercharger installed tomorrow. Can you change the crank pulley without having to remove the supercharger? I was going to order the UR stock diameter pulley and get it put in later this week.

 

I think you should be able to do it, because the pulley is accessed via the bottom of the car.

 

Thanks Moodist. Anyone know for sure? If I have to, I'll just have them put a stock pulley back on. But I'd rather wait and get the UR pulley if I can.

 

Yes you can install the crank pulley separate from the supercharger.

 

Took the liberity and calculated some boost levels for each pulley should someone upgrade to the MP90 unit Pretty interesting stuff.

UR Crank pulley/ Alternator Side
141 / 67 = 2.104 Ratio

Planning on running these ratios:
Stock Crank / Alternator Side
175 / 67 = 2.612 Ratio

3.0V6 base pulley (2.83")
71.9/100=0.719 Ratio

Base Pulley(3.0") / SC Pulley
76.2 / 100 = 0.762 Ratio

HBP (3.28") / SC Pulley
83.3 / 100 = 0.833 Ratio

Second final ratio is using the UR S crank pulley with identical alt. pulleys

2.83" pulley 2.612 * 0.719 = 1.878/1.51 = 2.85psi / MP90:10.78/ MP90+UR: 5.79

3.00" pulley 2.612 * 0.762 = 1.990/1.60 = 3.90psi / MP90 12.30 / MP90+UR 7.01

3.28" pulley 2.612 * 0.833 = 2.176/1.75 = 5.64psi / MP90 14.83 / MP90+UR 9.08

 

interesting...

 

Is upgrading to the MP90 even possible without serious fabrication?

 

I compared the specs on the magnison site and it would require some serious fabrication

 

I've been going around to some blower shops and asking around, imaginegetting another half liter out of each revolution of the rotors
PLus Doug from Hondata always thought the MP62 was small for our cars, im split on it, would be nice to work out and perfect however

 

If it could be done, you could get a lot cooler charge with less boost and similar flow. I haven't seen a map of a MP90 to see where it's efficiency range is compared to the MP62.

 

Exactly thats the point





MP62


MP90


MP62


MP90

 

Whipple Supercharger is in my town. I should go visit them and see what they think. They tuned the Ford GT of a guy I know and it's putting down over 1000HP on 91 octane and still meets CA Smog.

 

I would love to see this motor with a MP90.

 

haha go ask
Thats pretty impressive, I bet the GT is rated at EV (Emission Vehicle)