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Discussion Starter · #1 ·
on the eaton website they have this graph that shows blower rpm vs m90 inlet flow volume in cfm. I decided to expand on that graph to get an idea of how much air the m90s are pushing with some common gtp pulley sizes. Its almost a perfectly linear graph so the numbers shouldn't be too far off. I calculated supercharger rpm by taking the 7" crank and dividing it by the sc pulley size and multiplying that by about 5900 rpms, i did 6200 rpms for the 2.55" pulley. The red line shows what it might be at around 15psi, if my graph is off it gonna be off in the direction that would actually show a little better performance that you would see in the real world.



You can convert cfm to lb/s per minute of air by multiplying it by .069, then move the decimal place over one spot to get estimated max crank horsepower at that level of airflow. As far as I know a 2.55" or 2.6" is the smallest pulley anyone is running, that yields around 730 cfm or 50.3 lbs per minute of air so maybe you could get just over 500 horsepower with a 2.55" pulley and all the mods to be able to run a pulley that small.

Assuming a 20% driveline loss that is just over 400 fwhp. Is there anyone that has made over 400 at the wheels without nitrous or another power-adder? IIRC Zooomer's made like 440?????


BTW, this graph is gonna be for engine around 90-95% volumetric efficiency, thats why it says a 3.8" pulley should make 310 horsepower and they usually make 240, a stock engine just isnt that efficient.
 

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Actually Tom, I don't think Zooomer ever claimed to make 440 at the wheels. IIRC, he claimed 410. Basically your logic is sound, but you also have to consider the sensitivity of your relationships. In other words, how much does the result change if you change one of your input parameters slightly? One example is the 20% derating that we've accepted to reflect drivetrain loss. We believe 20% to be an average, but it can vary from car to car. As you point out, a car that makes 500 hp at the crank with a 20% loss will show 400 at the wheels. However a 19% loss will yield 405 at the wheels, etc. Just some food for thought. Another thing to consider is the accuracy of a dyno. A mere 2% error in the test case can show up as 8 hp.

You are right though that this can be a good way to ballpark hp. I've often done that myself. :)

Good post!

John
 

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Discussion Starter · #3 ·
Thanks for mentioning that John... Its also important to note that the difference between a 90% VE 3800 at 13psi and a 95% VE 3800 at 13 psi is about 35 cfm or as much as 25 horsepower, I think thats the biggest variable in the equation, especially when we are still working with modified stock engine components like heads and intake manifolds, I can't imagine anyone has even hit 95% VE yet, my guess is in the 92-93% range for engines like scotts and zooomers, and pimpdaddyhouses, etc.
 

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Delta T

John, Tom:

Im not sure I follow this correctly....

Flow is one thing, but the Delta T is another side of the story. I would never expect to see near real world results from our eaton heat pumps, or our 2V pushrod.

If GM decided to intercool / twin screw / turbo ($!), then real world figures would be more accurate. Not to mention, this is all "ideal", I know you both have seen the QA on our cars...on an average...I would say theres some ugly stuff in there!

You two probably know all of this....sorry
 
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