if i wanted to calculate boost from the RPM of the engine and the blower, how would i do that? or atleast be close?


say i got a M90 blower. and i have a 305 engine.

if i understand right, a M90 pushes 90Ci of air for one revolution.
a 305 needs 152.5 Ci of air for one revolution ( im just guessing, but i assume that since only half the Cyls would be firing per rev.)

so that means that for just 100% NA power, i would need to turn the blower 1.694 times for one engine rev.

am i correct so far?


how can i convert this into PSI? or can i?




im starting to think of putting somthing like this on my car after i convert to multipoint efi.....
http://cgi.ebay.com/ebaymotors/ws/eB...category=33741

just trying to learn ahead of time what i need to know.
also any good books on the subject that can directly relate to all of this? prefeably on SBCs too? i dont have alot of cash to waste on crap books, and you cant browse thru the book when you buy them online...

 

100% NA power is a bad way to put it... how about for the same pressure as the air outside the motor... IE, no vac...

 

THere was a post about this a while back. In short, you're on the right track for getting a "rough estimate". Obviously, heads, cam, exhaust, etc all will alther this "theoretical" calcuation, sometimes quite dramatically.

Let me give you a few little tips to get you going:

Atmosperic pressure is roughly 14PSI. So if you were running 14PSI of boost above atmosperic you'd be shoving roughly DOUBLE the air into the motor than it's displacement could normally handle. 10lbs of crap in a 5lb bag, so to speak. See what I mean?

If you calculate in a "fudge factor" of about 75-80% to what the blower is theoretically shoving into the motor you'll get a lot closer to reality in terms of actual observed boost (assuming streetable cam specs, commonly available performance heads, etc.). A big heavy-breather engine and this fudge factor will be way out to lunch.

Again, this will still only get you a ROUGH idea. Under no circumstances should ANYONE reading this post expect to calculate their boost to a fine decimal point using this information. This is just to get you in the ballpark of sanity, nothing more. All pulley changes from here should be based on actual observed boost levels, not from some mathematical formula.

 

A single "stock" M90 running under its "upper RPM" limit will generate approx 4 psi on a 305.

However overdriving, and modifiying the case can generate more psi. One setup I am aware of is ~ 10 psi on a 5.0L LTD.

83 Crossfire TA can give more insite into this as he has on hand experience with that LTD.

My twin setup generates ~ 12 psi on my otherwise stock 305. Using stock M90's, and 2.68 to 1 ratio.


BW

 

yea... ive already read every thread you guys made about the M90, along with everyone mentioning a whipple and the M112.


is there another boost board with a bit more in depth tech info? id rather search and read then ask every question here as i think of it.

if i can get a M112 cheap enough, i may go that route for power since its cheap for crazy fast without runing my daily driver status... id do somthing like you did, but i dont have the room you do to run 2 M90s, and i want to run on a 406 SBC with a roller conversion OR a later model block 383...


alot of this is more dependant upon what parts i can get first, cheap.. i just have to get my stuff together before i start.. that way i know everything will work together with each other....



my goal here is a streetable daily driver that gets decinet gas miliage thru the T56, and still can whomp LS1 cars....

id be really happy if i can whomp some of the cammed or sprayed LS1 cars.... but thats kinda dreamin.... hehe

 

I'm currently going the other way...

Building a 302 with a forged 283 crank, 6" rods, TRW 2441's and a 327 block.

I'll port the stock 350 heads and do more porting of the intake and up the injectors to 36lbers.

 

sounds very fun.

im just a big engine nut. given the choice of a high reving smaller engine or a slower bigger engine..... im going to figure out how to make the bigger engine turn as fast. lol.....

ideally id be working with a 427 or even that new 454 motown block....

but realisticly, i dont have that much money.

going motor -> multipoint EFI -> boost over a time peroid is alot more realistic for me money wise.