Thanks for the kind words, menace. Unfortunately, I do not have any data on the point at where cranking up the RPMs further causes a loss in power. But it would not be parasitic losses that caused the negative power gain, it would be heating of the air. Parasitic losses are relatively linear. Efficiency losses are more exponential.

Since power is about density, not pressure, at some point the air becomes so overheated that adding more pressure actually results in less density (molecules of oxygen per volume).

And there would not be a simple answer to that question anyway. Are we talking negative peak power? Negative average power? Negative area under the curve? Each of these points would be different.

Further, it would not be fair to keep all other things constant in such a test. With lower boost, you could run more timing. So, tuned to the ragged edge of detonation, the differences in power per PISG would likely be even less as boost increases (and timing must be retarded).

I'm just a bench racer (for now). Someone with real experience and hard data will have to answer this question.

 

"Shot" is a relative term. I agree that I could have picked a better term.

But you are simply wrong about overdriving the blower to get peak efficiency. I hate to be rude, but there is just no other way to state it.

Just look at the delta T chart above. That roughly correlates to ("adiabatic") efficiency. The low points on the chart represent better efficiency. Spinning the supercharger faster just makes the curve get steeper, meaning efficiency is decreasing at an increasing rate.

Putting anything bigger than the stock pulley is decreasing the blower's efficiency (i.e., increasing the delta T) relative to engine speed. That is not an opinion, nor is it subject to debate -- the chart above is all that is needed to establish this point conclusively.

 

I know of someone that has a very fast truck while running lower boost than most guys that show up at the track.

As a matter of fact, he ran 12.01 with 11lbs of boost the same night another guy ran 11.97 with 16lbs of boost.

Same guy has been in the 11's with 12lbs of boost and low 11's after a KB and 12lbs of boost

Maybe there is something to this whole "less boost, more power" thing huh?

NEW VERSION

 

Compressor maps can help one understand the relationship between flow, pressure, compressor speed, and efficiency. The one above is from a turbo, but similar ones can be generated for superchargers (SAE standard J1723 for centrifugal superchargers).

The horizontal curving lines are blower speed. The islands are islands of efficiency. The innermost island represents the island of peak efficiency -- this is the sweet spot. Spinning the blower faster may produce more power, but at the expense of efficiency.

The ideal is to select a compressor that, for a given pressure ratio (i.e., boost) and airflow (determined by engine size), produces the maximum efficiency. Just spinning a given undersized blower faster will not get you there. You will make more power, but less power than you would with a larger compressor.

 

When you say overdrive do you mean blower rpm > then with a stock pulley or > then the RPM of the crank
i think this is were we are disigreeing

im under the impresion that to overdrive a supercharger u need a bigger lower pulley then the top

i agree 100% that the Efficency goes down with anything greater then stock pulley
if we disregard heat then a higher blower RPM will yeild more HP untill a point where to spin any faster would take more power to do then what the added boost added

when i used to race a rail, my buddy had a 14:71 on his BBC and we underdrove it I.E blower RPM less then Engine RPM

 

I feel like i'm in fizix class. This is a damn good thread. Keep it comin.

 

i totally agree

 

better efficency,more power is what i would say

 

This isn't going to be as eloquent as Tim's stuff but here it goes...

So, theoretically, one could gain more or the same hp from taking boost down from say, 14# to 12# and bump up timing?

Agreed, density is a primary concern. Cooling that air is #1 on the
hp list.
I can't understand why we are not hearing anything about the Lightining concept's additional boost gains from
the oem air-conditioner cooling a tank of anit-freez and blasting our S\C with cold air, gaining approx. 40hp. Reverse engineering that shouldn't be that difficult for the guru's here. With this method running 12# of boost would surely be more efficient and get you better times!!


-0light1
Hey, pass the popcorn please.

 

I think you are right. You were using the word "overdriving" from one of those old roots blowers, which spin at roughly crank speed, to mean something like a overdrive transmission, where the blower spins SLOWER than the crank.

I thought you were using "overdriving" to mean spinning the compressor faster. The Eaton spins two to three times faster than the crank. If you reduced that speed to anywhere near crank speed, it would trun the blower into just a pretty intake casting.

So we were both right. Feel the love in this room.

 

I doubt if you could bump the timing enough to compensate for two pounds of boost, but you have the right idea.

I too am baffled by why no one has come up with an A/C solution to intercooling. It just makes so much sense. All of the required parts are there -- it's just a matter of a little extra plumbing. If nothing else, the A/C compressor could be used to help idecrease the amount of time that the intercooler fluid takes to settle back down after a long run under boost.

 

HMMMM ... Wonder who that could be?(JJ)

I wonder who ran that 11.97. BTW- I backed it up the next pass with an 11.92.
Yes I was running an 8# lower, but also I am not the smallest cat around and I had "Heavy *** 20s on the front".
With Skinnies it would have been 11.8s.
My L was deweighted though.

I do have to say that my L seemed to run the best with the 4# on.

But I LOVE that bottomed end torque of the 8#.

VINNIE

 

Vinnie, do you run the 8#er daily?

 

I had the same thought in my attempts to cool down my Allen/Magna M90 with the "Bazooka" prototype 12 pound boost pulley (could only hit 12 psi with 30 degree temps) ,until I found this...........

www.coolflow.com

I spoke to the owner. He as been making A/C compressior based intercoolers for over 4 years. He can make them very simple or more complex than the "Concept Lightning" version. He makes air to air and liquid to air A/C based intercooler systems.

 

Yep.

I have now for about 6000 miles. Been down the 1/4 many times and have driven to Indiana and back (1400 miles round trip) , To Virginia and back (700 miles round trip) and I am going to Ohio this weekend (1000 mile round trip).

I do not recommend it , I do have a built block on order , but I just LOVE the bottom end torque.


VINNIE