I get the pinging question often and it makes me think that very few people in America realize the necessity of reducing compression, especially with those who run a 17% pulley or larger. I do the modifiacation with all the heads I sell and install.
I also don't think people realize that a 15% under-drive s/c pulley and a 2% over-drive crank pulley do not equal a 17% pulley. It actually has an effective rate of about 20%.

 

wow..
so it produces more power than the regular 17??

 

And before anyone asks the obvious question:

No, increasing the head gasket thickness to decrease the static compression ratio does *not* cancel out the benefits of the higher boost.

This is why most of the cars out there with crazy-high boost (and horsepower) numbers have static compression ratios down in the 7:1 - 8:1 range. If you're limited to a particular overall dynamic cylinder pressure level in order to prevent pinging, you'll get more power by using low static compression and high boost than if you used high static compression and low boost.

Scott

 

And how did you get that?

-Brian

 

LOL That's the new dmh math Brian !

 

The crank pulley diameter is 2% larger than stock, and the supercharger pulley diameter is 15% smaller than stock. To see the resulting effect on supercharger pulley speed, divide 1.02 by .85. This gives you 1.2, or a 20% increase over stock.


The actual numbers are a little different, because the belt itself changes the effective diameter of the pullies. Ted Leist published this table to make things a little easier:

<begin Quote>
Stock crank pulley dia. is 5.46 inch.
Stock SC pulley is 2.58 inch.
Correction for belt is 0.126 inch. (thanks Andy)
Effective stock crank dia. is 5.586 inch.
Effective stock SC pulley is 2.706 inch.

Crank _______SC___________% SC
Pulley________Pulley _______Overdrive
% over _____% under
0 ___________0 _____________0
0 ___________15 ___________16.69
0 ___________16 ___________18
0 ___________17 ___________19.34
0 ___________19 ___________22.12

2 ___________15 ___________18.97
2 ___________16 ___________20.31
2 ___________17 ___________21.68
2 ___________19 ___________24.51

3 ___________15 ___________20.11
3 ___________16 ___________21.46
3 ___________17 ___________22.84
3 ___________19 ___________25.70

4 ___________15 ___________21.25
4 ___________16 ___________22.61
4 ___________17 ___________24.01
4 ___________19 ___________26.90



That is, a 17% undersize SC pulley spins the SC 19.34% faster than stock.
A 15% undersize SC pulley + 2% oversize crank pulley spins the SC 18.97% faster than stock.
<end Quote>

Scott

 

With that math, you're saying that conventional knowledge is correct and that a 15/2 combo is similar to a 17% pulley. Both the 17% and the 15/2 spin the SC close to 19% faster.

I'm still waiting on dmh to provide their rationalization, maybe they have some other table.

 

Good point - I mis-read DMH's post. I thought he was saying that a 15/2 combo provides more than a 17% overall increase in supercharger pulley speed (which is true).

It's not correct, however, to say that a 15/2 combo provides a bigger increase than a 17% pulley and a stock crank pulley.

Scott

 

This may come to a shock to some but reduction pulleys of the same noted size are in reality not the same size. I've measured three different manufactures pulleys -- go figure. And I did use the word about! But with any s/c pulley of greater size than a 17% you will run into a few problems -- I run into unhappy Mini owners almost daily with engine/pulley related issues. And not to run a crank damper, well that's another story all together.

 

To continue with dmh's findings...Is it possible to collect some data for the various 15% pulleys on the market? I'll measure my Mini Madness pulley this afternoon and post back here later. Others would be welcome.

 

Well, I've got a 15% Alta pulley out in the garage waiting to be installed, but I think the problematic part is going to be deciding exactly *where* to measure the pullies.

But if you're collecting the data, the Alta 15% pulley measures 2.19" in diameter across the tops of the grooves.

From Ted Leist's post, he measured the stock pulley at 2.58". That means that the Alta 15% pulley is 15.12% smaller than stock, so I'm pretty sure I measured in the right place.

It's worth noting that the 15.12% reduction in pulley size means that the supercharger will be turning 17.81% faster than with the stock pulley (1 / (1 - 0.1512) -1). I think this is what DMH was getting at - an X% reduction pulley does NOT mean that the supercharger will be spinning X% faster than stock. The increase is actually significantly greater than X%.

Scott

 

I have never seen anyone claim that a SC pulley will ROTATE the SC a certain percentage faster, they have always been advertised as a certain percentage smaller. So why would dmh make such a claim?

 

These two comments have nothing to do with each other. I think something is being lost across the internet, but the first statement seems to go against the face of standard knowledge in the mini world. I don't dispute:

since it is about right, give or take a percentage.

However, this statement is what goes against standard knowledge.

I have a 15% helix if anybody wants to compare size.

 

do you know what the compression of the Cooper S is? its 8.3:1! That is stupid low! Especially for a measly 14-16psi!

If you are pinging... on an 8:1 motor on 14-16psi... you have a tunning issue! Supras, vipers, mustangs, that are making 1000+ whp usually run 9:-9.5:1 compression! (don't bring up the fact that they use race gas because vipers make 1000whp on pump)

 

MINIotple-
You have quoted me out of context. These are response to two different posts.
These ideas are actually interlaced regardless of "standard" knowledge in the Mini community. Without going into much detail a larger than 17% s/c reduction pulley produces diminishing returns. You don't even have to take my word on it; check it out on your own. All you need is an OBDII sensor logging device. Then track the knock sensor against MAP and IAT. And for those in CA with only 91 octane it is even worse.

 

This is a perfect example of "standard" rather than factual knowledge. The original poster in this thread even realizes the problems boost create and rectifies it the same way I do.

 

Yes, why has this been brought up as a issue? We all know you can encounter trouble with the belt when decreasing the S/C pulley size lower than 16%. Tight belt rap... Increased heat... and in the 19%'s case, cavitation in the water pump. This is a well known issue that people have delt with by not running above 6k for extended periods.

The percent in which the S/C is being driven faster has not been discussed because most have decided to talk about it in terms of reduction in pulley size. We have seen time and time again, that the M45 will stand a 15/2 combo on track cars running up and past redline for extended periods. I have now been running this combo for about 5k track miles, and around 20k street miles. My redline is 7,500 RPM, with most shifts(at the track) happen at 7000-7200 RPM.

I think most agree you wont be making gobbs more power above 7000, but it is safe to run a bit above this if needed.

I just want to make it clear people should not be scared of running this combo because the actually overdrive % has now been calculated. It has been tested, and proven to last.

-Brian

 

You have got to be friggin kidding me! Not factual knowledge? I live in the one of the HOTTEST climates in the US! And we have the crappiest gas to boot! YET NO PINGING! and thats WITHOUT ANY SOFTWARE!!!!!!!!!!! Now a 19% pulley... that is a differnt story...

Also, as far as factual, i think its proven that I have factual knowledge... I don't bench race big guy

 

Are these cars using Roots-type positive displacement superchargers, or are they running centrifugal superchargers and turbos?

Roots blowers are notorious for not doing much more than beating the crap out of the air at higher boost levels ("higher" in this case meaning more than about 15psig). Because of this, it's not uncommon for a positive-displacement supercharger to heat up the air by a hundred degrees or more, which doesn't help with the pinging either.

I doubt you'll find any car making 1000+ WHP with a positive-displacement supercharger and a static compression ratio greater than 9.0:1, unless you're talking about a 572-inch Hemi or something similar, that could make 500-600 whp *without* the supercharger.

Scott

 

thank you, i do understand how roots blowers work and that they do produce alot of heat... which is why i only make a 15% pulley. And i 100% agree with you, and your point. I don't mean for this last statement to come off as rude...i just can't think of any other way to write the post... I apologize if i am rubbing you the wrong way, it is unintended.

Yes the cars that are making 1000 are usually centrifigal superchargers (turbos on a stick i like to call em )

anywho, stock viper motors (488ci) with a roe supercharger (a positive displacement supercharger) tend with an 8psi -10psi make anywhere from 650rwhp to just about 800rwhp (the torque is nutty at about the 900+ mark) Now with more boost or nitrous they will make over 1000... but hey thats cheating right?

 

isellem-
In this entire thread my comments have been geared towards pulleys larger than 17% -- really about those that equate to around 19% (and how to stop the car from pinging). A more thorough comprehension of my comments would have been more appreciated. After all, you did mention a 19% is a different story.

 

I apologize for the misunderstanding. I had posted ealier and figured that your post must have been directed toward me since you had not addressed my question about your comment about the 15/2 combo not being similar to a 17% pulley and there had been no other posts previous to your reply regarding pulley sizes. Let me refresh your memory.



Which brings us back to the off-topic topic.

 

Hey guys,

We really have more than a couple of topics going on here at once...all of which are very interesting.

About compression ratio vs. pinging...yes the quality of the fuel used is very important toward maintaining cooler combustion chamber temps, but all things being equal, our compression ratio of 8.3:1 does seem rather mild for a boost cooled engine running 14 or 15 psi. to have any serious pinging issue. I have recently read in the 11/06 Motor Trend, that the turbo boosted Mazdaspeed3, 2.2 inline four, is developing 263 (SAE NET) hp, with 9.5:1 compression at 15.6 max psi.!

So...what should this lead us to believe?

 

Ah, we need a turbo?
or not

 

"The belt thickness is irrelevant."
not true. consider a flat belt on pulleys that is .200 thick. the ratio is: (dia1+.100)/(dia2+.100). This is a different number than (dia1)/(dia2). same argument for Vee pulleys/belt. Intuitively, the length of belt that travels for any amount of rotation is measured along the centerline of the belt, which lies at a larger radius than just the pulley radius.