Does anyone know if the ALTA Overdrive Crank Pulley is dynamically balanced?

Most centrifugal supercharger have a dynamic balancer for pullies, similar to a tire balancer but they remove metal at a pin point to achieve balance.

 

I don't know but, Randy Webb has just posted on his site www.webbmotorsports.com his approval of the Alta Crank Pulley of up to +2% on stock or 15% SC pulley equiped Minis. He has done over 25,000 miles of testing with no problems. He DOES NOT recommend +2% on minis that have over a 15% SC pulley.

 

Its funny because El Diablito said months ago the same thing, 17% max, and nobody listened.... We have seen it a ton of times on the dyno where the cars with more than 17% total arent making power like you would think they should.

I have examples of cars that added more than 17% and lost power, and more than once !

 

oh yeah I should add that the crank pulleys are actually a small % larger than the stated 2%, 3% etc so a 3% + a 15% = 18.xx %

The actuall math was posted here a while back.

 

Great write up! But, I wish he had published numbers for the more-modded cars that he ran. I think that would be an even more telling bit of data: Do you get more return? Less?

 

less, at least from what I have seen on cars with more than 17%

What I mean is the closer to the limit the car already is the less % gain you will see from a given mod. If you are asking about 0% pulley I have no Idea at this point if the gains are bigger or smaller than on a less modded car.

 

No, I was specifically referring to the overdrive. I have a 2% here and I'm waiting for some free time.

 

Is the power loss a case of not compensating for the extra fuel needed in order to keep an appropriate air-fuel mix? Should we be putting on larger injectors, fuel pump, ect...

dan

 

here is some food for thought:


the Mini does not use a harmonic dampener (I don't think), yet it has a weaker (cast steel) crank and less vibration absorbing stuff on the belt, and of course, less hp.

the S has the dampener (added by BMW, according to Webb), yet it has a stronger crank (forged steel) and more vibration dampening stuff on the (longer) belt, and of course more hp.

If the reason to add the dampener was to handle the 80 more hp (what the pistons put out, but not the net after the SC loss), wouldn't adding even more hp (to get to 200 wheelhp) be even more reason to keep the dampener?

remember that it is the torsional pulses that need to be damped out and they are of considerably more amplitude in the S, more so a 200hp S

 

I am seriously thinking about doing this,

a question to anybody who has done a 2% crank
are the differences like going from stock to 15% sc pulley?
also a question to Randy at Webb any chance you could do one of your priceless How-to's for this mod?

 

I can do a how-to.

The change is no where near a supercharger pulley, just a bit better response. The gain on the dyno for the 2% crank pulley is about half of what you get with a 15% SC pulley.

Hope that helps!
Randy

 

The write-up on Randy's site quotes horsepower gains but doesn't say much about torque.

What is the effect of an oversized crank pulley on the torque curve?

 

After reading this thread and the other thread, two words keep jumping to the forefront of my thoughts: Risk -- Reward.

Reward - Maybe 4 - 5 HP
Risk -- Your engine.

Uhhh, no thanks. I'll figure some other way to get HP>

 

Would the 19%+ not increasing hp due to the stock head flow limitations

 

nope, I have a ported head and cam. I believe it has to do more with the effeciency of the s/c and how fast its spinning with a 19% +

 

I did show what the torque gains were. You don't lose any torque or horsepower any where on the curve, and the max gains are posted for the stock size. The 2% was a little better obviously.

Hope that helps!
Randy

 

And even better: That's like taking an MCS from 175 whp to 200 whp ... without changing anything the size of the pulley.

 

Baahhh!!!

 

Please see my response to this on the Webb Motorsports site - I'd like to see the factors in the equation you used to show this, seriously - not trying to be confrontational at all.

Thanks,
Randy

 

Apologies ... I read the product info and not the review page ... found the Works torque figures on the latter

As you stated in the review, the dyno sheets are hard to read. It would be nice to see overlays of the Works torque curves

 

KE = 0.5 * m * V^2

That part doesn't really matter since your test car presumably had the same mass and velocity both before and after the test. It doesn't matter what your elevation is.

Power = dKE/dt

So, the change in kinetic energy over the change in time is power. You are saying that you performed a change the 3500-6000 rpm change in kinetic energy in 7.5s and then swapped on the same-size crank pulley and did it in 6.6s. So:

Power stock = KE/7.5 = KE * 0.133
Power pulley = KE/6.6 = KE * 0.152

To compare the two, we can cancel out KE and just have the ratio of:

0.133 stock and 0.152 pulley

0.152 - 0.133 = 0.0185
0.0185 / 0.133 = 13.9 %

I guess I was being conservative with the 175 whp MINI. Your 210 whp MINI would have about 239 whp after installing the pulley. Pretty amazing, and WAY better gains than replacing the stock supercharger pulley with a 15%!

 

BTW, I also don't understand how replacing the crank pulley with a same-size lighter one can make the torque curve a radically different shape and how it can change the A/F ratio significantly as well.

 

The runs were at different temps - remember I used worst of against best of for the comparison. That's what would change the A/F ratio.

The torque curve was a problem we had with one of the plug wires used for the dyno - if you look at the spike on the first run, it corresponds roughly with the curve of the second run if you extended the line from that spike. I have many other runs that look very similar, but again, I thought it would be tough to argue the worst condition for the mod against the best condition for the baseline.

That may also have something to do with the difference in acceleration - I'm running a 2% lightweight pulley and I used conservative numbers. There are certainly variables not taken into consideration on your equation. I did the runs, so I know they are right - we averaged them. There are several things going on - the tachometer, the stopwatch, the reaction time, the dyno runs being SAE corrected, the 2% pulley being used, etc. Like I said, I appreciate the equation, and math is never wrong, we just sometimes don't give math all of the inputs.

That was an average of 4 runs I believe over time, each - I am at home again with no data though.

Hope that helps!
Randy

 

There are no variables in the equations other than time. If you did the runs to show the change before and after installing the crank pulley, then it is assumed that you kept everything else the same. If one 3500-6000 run took 7.5s and then another took 6.6s, there is a 13.9 % difference in the amount of power that you have. It doesn't matter what brand of stopwatch you used, what your elevation is, what other mods you have, etc. IMHO, gaining 13.9% in power under the curve (which means your AVERAGE power gains are 13.9% from 3500-6000, some areas gain even more) is not realistic with a crank pulley being the only change.

Fun fact: The above equations are how Dynojet dynos work too. The rollers have a known mass. The Dynojet monitors how fast the rollers are spinning and calculates the kinetic energy at any point in time. The change in kinetic energy over the change in time is power. Less time, more power and the change in power is directly proportional to the reciprocal of time.

 

A key principle of the Dynojet "inertial" dyno is the drums' inertia acts as a sort of passive power absorption device. "Mass equivalent" is a term used to quantify the difference in inertia of a mass in linear or, more properly, "translational" motion and one in rotating motion. The mass equivalent of a rotating drum is quite different than its mass for translational motion so the weight simulated by the drums when rotating is different than their actual weight.

During manufacturing, Dynojet figures the mass equivalent of each pair of drums to four places and bearing drag to five places.

Those proprietary figures are figured into the computation the dyno computer makes. If the mass equivalent of the drums is known and the rate at which a vehicle's drive wheels accelerate the drums is accurately measured, then the "thrust force," in pounds, at the wheels can be computed with a high degree of accuracy.

A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work (in pound-feet) the wheels are doing, "m" is mass equivalent (the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (drum circumference). Once we have the work, we can find horsepower. One horsepower is 550 pounds-feet of work done in one second, so we divide the work number by the length of time measured, then divide the number we get from that by 550. To simplify: we get horsepower by multiplying the mass, acceleration and the distance, then dividing that product by time multiplied by 550. This can be expressed by: hp=(m X a X d) / (t X 550).

Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. Generally with wheel numbers, axle ratio is not considered in the torque computation. For comparison purposes, this makes more sense. The computer factors out the axle ratio by using engine speed data in the torque derivation.

In the real world, the measurements and computations are not quite that simple, but they are a good indication of how an inertia type dyno works.

I just know we must be missing something, because we did several runs, and the least amount of time gained was .6 seconds. There was a significant change. Why do I mention altitude? Usually it throws a wrench into the equations - here because of the measurement of power we are using and the correction factor for it, then going to the real world in acceleration.

Hope that helps!
Randy

PS - Some of the above info is from Hib Halverson's book, the DynoJet Revolution.