Hey all,

#1
Little confused here. Many posters here, as well as vendors, mention the pressure drop across the intercooler as being a bad thing; that or I mis-interpreted. Isn't the purpose of the intercooler to reduce the IAT, which then means (by the ideal gas law) that as the temperature goes down, pressure necessarily goes down (holding the molar amount of gas constant, with no leakage).

Can someone please explain why pressure drop across the core is a bad thing, and not an integral byproduct of the function of the intercooler?

http://www.thrashercharged.com/L67_htm/intercooler.shtm
http://en.wikipedia.org/wiki/Gas_constant

#2
Also been wondering what the stock supercharger RPM's are at, as well as the RPM's after a 15-16% reduction. Just trying to match them up to some temperature maps.

http://www.rbracing-rsr.com/turbotech.html#eaton

#3
Lastly can anyone show me, by math/physics, why a 15% reduction pulley, as oft quoted, moves peak boost from 10.5 to 15 PSI? Anything I figure comes nowhere close to those figures (i.e. I am dumb):

d0 = "100" = stock size
d1 = " 85" = 15% reduction

(diameter size is irrelevant as far as I figure as long as they have proportional reduction)

C = pi * d
C0 = 314.159...
C1 = 267.035...
C0 / C1 = 1.17 = smaller pulley runs ~117% faster

~1.17 * (10.5psi max, stock?) = ~12.285psi

Please show me where I am definitely wrong (and I am definitely wrong; dyno charts and so forth prove as such).

- Matt

 

This sounds like a question for Dr Obnxs . Give him a PM and get him to chime in if he can. hes done a good bit of testing in this area.

 

I’m not qualified to provide the formulas needed, but I can see that density as function of the efficiency equation has been omitted from your consideration. For a perspective of what most matters, give this a read http://www.bellintercoolers.com/pages/techFAQ.html .

There are other factors related to boost beyond the rpm of the supercharger, volumetric efficiency plays a major role and there are too many varying tolerances (and it is a dynamic system) to begin with to make accurate comparisons across MINIs; temperature maps will only get you in the ball park for that reason.

I’ve performed intercooler experiments and arrived at a few conclusions. I’ve also had some dialogue with an engineer at Bell Intercoolers who is very familiar with the MINI. The bottom lines are: increasing TMIC surface area will only provide very small gains over the stock TMIC (not worth it), greater gains can be had through improved air capture and routing (better cost/benefit/ratio, efficiency increase may exceed 5% in the TM location), the stock design (the standard IC cover and duct not included) is tops for street use given its packaging limitations, the only way to improve on the stock A/A core unit is to go front mount (my next challenge).

I have believed in the “anything is better than nothing” approach, and I certainly have put my money there, but my perspective has changed after a sobering conversion with Gerhard.

 

I agree that the density of the gas (more O2) is also improved in lowering the IAT and also lessens the chances of retarding the ignition timing (i.e lowering the HP) due to detonation that can be aggrevated by hot air charge into the combustion chamber. One of the advantages of direct injection is the ability to advance the timing as it can have a better cooling effect in directing fuel into the chamber.

 

trackster is correct. You need to ask the good Dr.

You can find him at mininuts.com allot these days.

Longboard

 

Thanks.

And khuevo, that site made sense of pressure drop to a certain degree to me.

- Matt

 

I might be able to help you out on this one. The pressure drop referred to is just the inlet-outlet pressure for a give air flowrate thru the IC, WITHOUT any cooling of the air taking place. You are correct that if cooling occurs, the outlet pressure will be lower than if no cooling occurs. The point of discussing pressure drop, though, is just to quantify the resistance to flow thru the IC. Desirably, this is as low as possible, while maintaining the heat transfer. If both heat transfer and flow resistance are combined, as occurs in real life operation of the IC, the pressure drop is not so meaningful, i.e., it is partly good (due to cooling), and partly bad (due to flow resistance).

 

Iniminni,

Beautiful, clear response. Wish more people singled out the portions they were answering to any questions on these boards.

Now just to get the remaining two answered....

- Matt

 

The new issue of MC2 that showed up yesterday has an article by Dr. O about intercoolers - specifically, larger ones. If you can get a copy (or have one coming in the mail), it may answer some of your questions.