How to figure out pulley size for boost..

CHADD

Registered User
Installing a 94-95 M90 on a 2000 4.2L v6.. direct driven with all the other crap... so 1 serpentine belt for everything...

What is the formula to figure out what size pulley to go with for say about 7psi?

was using the search button and everything I read confused me more.... whats the jackshaft? (m90 pulley shaft?)
 
Boost pressure is another way to measure back pressure. So to say that pulley size "x" will be a 7 psi pulley is not really an acurate statement. There are too many variables...head flow, exhaust, etc. The better way to do it, would be to determine what the flow requirements are for your engine in n/a form...how many cfm does it require at WOT & whatever you peak rpm is...then for say 7 psi, you need to multiply the required cfm by 1.5~...That's how many CFM the sc needs to produce...then, assuming a stock (not ported) sc you find out how fast it needs to turn. Then you can figure the pulley size.

On the SC we have 3 belts...one to drive the accessories, one that drive the jackshaft pulley from the crank, and then the jackshaft drives the sc...

Sounds like you've got a good custom project there...good luck with it.
 
007_SuperCoupe said:
flow requirements are for your engine in n/a form....
What is the best way to figure this part out?

007_SuperCoupe said:
how many cfm does it require at WOT
I know there is a formula for this one... does anyone have it laying around?

007_SuperCoupe said:
& whatever you peak rpm is...
peak as in max? or peak hp rpm? right now max rpm is like 6200rpm... and stock engine peak hp is at like 4200rpm...

007_SuperCoupe said:
then for say 7 psi, you need to multiply the required cfm by 1.5~...
why 1.5~... can you please explain?

007_SuperCoupe said:
That's how many CFM the sc needs to produce...then, assuming a stock (not ported) sc you find out how fast it needs to turn.
How would I find out how fast a stock sc needs to turn?

007_SuperCoupe said:
Then you can figure the pulley size.
I see.. but I dont see...

007_SuperCoupe said:
On the SC we have 3 belts...one to drive the accessories, one that drive the jackshaft pulley from the crank, and then the jackshaft drives the sc...
Is the jackshaft pulley the pulley on the harmonic balancer? or is it a pulley attached to that pulley?

007_SuperCoupe said:
Sounds like you've got a good custom project there...good luck with it.
Yeah I do... but will be well worth it in the long run...

show me the ways of the force master.... I will be the young grasshoper...

right now the engine is totally stock... other than P&P throttle body, half shafted throttle body, true dual 2.5" exhaust with factory cats, and a CAI...

but planning on building a forged 4.2L with everything ported as soon as I find a 4.2L spare engine...
 
CHADD said:
What is the best way to figure this part out?
See below:
CHADD said:
I know there is a formula for this one... does anyone have it laying around?
Yes there is, but I don't know where to find it right now...it has to do with displacement, duration, lift, etc...
CHADD said:
peak as in max? or peak hp rpm? right now max rpm is like 6200rpm... and stock engine peak hp is at like 4200rpm...
I'd go with peak rpm to start with. That way the sc will spin slower preventing detonation.
CHADD said:
why 1.5~... can you please explain?
Because at 7 psi you are roughly 1.5 atmospheric pressure. I don't know if this is the best way to do it, but it is better than nothing.
CHADD said:
How would I find out how fast a stock sc needs to turn?
I see.. but I dont see...
That is based upon pulley ratios...there's a bit more involved than I know off the top of my head right this instant, but you want your rotors (inside the sc) to turn at a rate that is greater than your crank, but not too much as it will generate more boost. More boost is not always a good thing.
CHADD said:
Is the jackshaft pulley the pulley on the harmonic balancer? or is it a pulley attached to that pulley?
The jackshaft pulley is an intermediate pulley combination that uses 2 belts. One goes to the crank, the other to the sc. It basically multiplies the drive ratio of the sc so it'll spin faster than off the crank pulley.
CHADD said:
Yeah I do... but will be well worth it in the long run...

show me the ways of the force master.... I will be the young grasshoper...

right now the engine is totally stock... other than P&P throttle body, half shafted throttle body, true dual 2.5" exhaust with factory cats, and a CAI...

but planning on building a forged 4.2L with everything ported as soon as I find a 4.2L spare engine...

I am by no means the "force master"...there are some on this site who are much more knowledgable than I. But since you are from St. Charles, there's a few guys from there who really know their stuff and may be able to help you out as well. David Neibert should be able to show you what the SC engine looks like so you can see how the jackshaft works. He'd probably be able to getting you a good starting point for the speed the sc should be spinning as well.
 
so jackshaft pulleys can attach to the original pulleys? can you get different size jackshaft pulley? Can someone post up the pulley configuration of the supercoupe?
 
if you want 7psi, dont use the jackshaft pulley, just run the supercharger off of the serpentine belt. With the stock m90 pulley it'll probably make 7-8psi that way.
-Thomas
 
Back
Top