Blower boost formula ?

Is there a formula to figure out how much boost a blower will build by blower size the amount of overdrive and engine displacement ? The engine is a AJ 481X 4.5" bore with a 3.5" stroke. Going with a 12-71 at 29% over. We ran a 6-71 at 29% over, am I right in thinking that a 12-71 will make more boost than the 6-71 we had on it ?

 

Theoretically the 12-71 should make more boost than the 6-71. But it isn't a gaurantee. There are many variables- what kind of shape is the 6-71 in compared to the 12? Condition of the teflon and things like that? Another maybe even more important issue is the term 'boost'. We all would like to be able to compared a blower's function simply based upon a number like boost. Unfortunately boost is affected by density altitude, cam specs, even valve lash and many other things. When we measure boost we are measuring the resistance of air flow through the engine. Theoretically the less resistance to air flow the more efficient the motor. So in some respects high boost numbers isn't always a good thing. On the other hand different blowers on the same motor without changing other variables should be a fairly honest comparison of boost numbers. Dave Germain

 

As Dave mentioned, there are a lot of variables. There is a way to calculate a ballpark number and it's generally within a pound or two.

(((Blower Output * 2 * ((% OD/100) + 1))/MOTOR CID) -1) * ((14.8 * Barometer)/29.92)

Using your stuff (411 is a good number for a standard, tight 6-71) and a barometer of 28.70" as an example:

(((411 * 2 * 1.29)/481) -1) * ((14.8 * 28.70)/29.92)

(1.20453) * (14.1965)

17.1 PSI

If you crunch it again and use 494 for the blower (decent, tight 12-71), you'll get 23.4 PSI.

The only time I've seen this be drastically off was for a guy running a flat tappet cam and stock heads! His boost was 12 PSI higher than calculated and understandably so.

Hopefully, the stuff above doesn't get too chopped up and disjointed during posting!

The supercharged tuneup calculator on my website will figure the above for you (among other things).

Spud
www.fuelinjectionent.com

 

Spud, do you have any info for a PSI 200H ? ( CID ? )

 

here's one,blower sizes at bottom of page: http://fuelinjectionenterprises.com/supercharged.htm

 

Hi Mike,

Yup, output would be the blower displacement per revolution in cubic inches.

Sorry, I don't have any info on a 200H. You could call PSI and ask them, maybe they'll have a useful number for you. If you've run it and have a decent tuneup with the atmospheric conditions at the time and the boost it made, we could solve the equation for the blower and figure out what it makes...theoretically.

Spud
www.fuelinjectionent.com

 

Norm told me years ago all 200 series PSI blowers were 375 cubic inches per revolution. When they came out with the A,B,C, and D series they were just a little bigger per revolution. Somewhere around 426 cubic inches per revolution as I recall. Hope this helps.

 

I've now called PSI 9 times ?
4 times, I talked to Michelle, but she wasn't sure ( I know it's old ) . The rest of the calls I missed both Michelle and Roger ?

I sent an email about 3 weeks ago, and when I called Michelle, she had sent the email onto Roger, but still no answer ?

Do you think they have no idea ??????

I wish I would have known to ask Norm , when I had talked to him (((((((