Patsy57

iTrader: (37)

Hey guys I was wondering what the formula is for figuring out boost with the different pulley sizes?

Please let me know

Bob@BruteSpeed

iTrader: (16)

How much boost the blower can produce depends on many variables.
* Cubic inch. The larger the engine the less boost it will make with a supercharger.
* Headers and exhaust. A better flowing system will produce less boost, requires a smaller pulley to regain the loss in boost. Ultimately though, more power will be gained.
* Camshaft and heads. A camshaft with some overlap will leak out some boost through the valves. Better flowing heads react a lot like a better flowing exhaust system.
* Altitude. Elevation plays a big part in how much boost is produced. Those people living in the higher elevations have to spin the blower harder to see boost.
* Condition of engine. An engine with real good sealing rings will ultimately produce more boost than one with worn rings.
* Rpm. An engine that is spun up higher in the rpm range will normally produce more boost.
* Head unit used. Blower pulley sizing is affected by which head unit is being used.

Bob

conan

iTrader: (5)

Boost #'s will be differant with every engine depending on it's efficiency. Same blower and same pulley combo might make 15#'s of boost on a 347ci put it on a 408ci and you might see 10#'s. There is a math formula to figure out blower RPM and CFM getting pushed in to the motor.

What Blower are you referring to ?

Patsy57

iTrader: (37)

Thanks guys I can post info on the motor like cam size etc if that will help.

The blower is an F1R

Motor is 430 lsx iron block, afr235 heads, forged bottom end, -28cc pistons (9:1) compression

Headers are dynatech headers 1 3/4 headers

Cam is 238/247. @.50
.637/.629 @.50

Lobe sep 112 degrees +4 adv

Just looking to understand how I get a general idea of what pulley (crank and blower) will be me around what boost

If that makes sense

conan

iTrader: (5)

The math far the procharger is this...

Crank pulley size / Blower pulley * Internal step up ratio * Engine RPM's = blower speed

The Internal step up ratio for the F1A is 5.4 and the max blower RPM is 74,000. I don't know F1 pulley sizes so hopefully Bob will chime back in but here is my D1SC math for an example for you.

7.25''(crank pulley) / 3.7" (Blower pulley) * 4.1 (internal step up ratio) * 6,500 (engine RPM) = 52,219 Blower RPM's

This makes 9#'s of boost on my 383ci and I still have 10,000 blower RPM's left in the D1 before I hit it's max.

So now you can change pulley sizes to get your desired power level. I would get a recommendation from Bob on a good starting point and pulley it from there.

Patsy57

iTrader: (37)

So where is the boost number coming from in the equation?

conan

iTrader: (5)

Your not going to get a boost # from any equation. The boost # really doesn't mean anything anyway.

I had the same pulley combo on my 347ci motor and would see 11#'s. Now with the extra 36ci the boost is reading 9#'s but makes more power. (nothing else in my combo changed)

Boost #'s are a measurement of restriction, take your engine for example... The 1-3/4" headers are a choking point on that big of a engine. So say you make 10#'s of boost with X pulley combo. Throw a set 2'' headers on it and dont change any thing else you will see your boost # drop and power go up. Hope this makes sense.

Patsy57

iTrader: (37)

Makes sense. I am just trying to map this out the best I can as I cannot go over 1000 rwhp and would like to be around 800 rwhp

The motor NA now with 9:1 compression runs 432 rwhp.

I go above that 800 number and I start risking the rear etc.

This is an M6 car as well

Bob@BruteSpeed

iTrader: (16)

I'd go with a 4.25 blower pulley, which should get you in the range you're looking for. Bob

Patsy57

iTrader: (37)

What would you do on crank side Bob?

Patsy57

iTrader: (37)

Last question what would the 4.125 calculate to in a sprag pulley? How many teeth?

Patsy57

iTrader: (37)

Apologies meant 4.25 Thanks again bob! I'm leaning towards the ATI super sprag pulley which is 75mm on the blower side.

conan

iTrader: (5)

Whats a "sprang pulley" ? Is this going to a cog belt setup?

Patsy57

iTrader: (37)

Correct, this is for a COG setup

soundengineer

iTrader: (16)

he meant SPRAG... not "Sprang"


http://www.atiracing.com/superpulley/




dont know where you got that # on the ATI Sprag... but you are WAY off...

the ATI Sprag is a 45 tooth and supports up to a 70mm wide belt(pulley is actually 75mm wide).....and its an 8mm HTD Belt (8mm from center of one tooth to center of the next tooth
45 teeth *8mm = 360 mm Circumference or 114.6mm Diameter or roughly a 4.5" diameter


When doing a Cog Pulley, the same math applies as the formula listed above... Just do both Cog pulleys in Teeth (its way easier for the math to figure out blower RPM)

Example...my setup
81 tooth Crank Pulley
45 Tooth ATI Sprag Pulley
F1c = 5.4 Ratio
I will spin 7400 RPM ~ 7800 RPM
81 / 45 * 5.4 * 7400 = 71928 Blower RPM
or at 7800 = 75816

overspinning the F1 blower by a small amount is not an issue typically, but it will require more frequent rebuilds

Patsy57

iTrader: (37)

Ahh typo

Ok cool thanks for the info!

conan

iTrader: (5)

I got ya, thanks for the link. Do you run one of these with a 6 speed ? Always like to hear how cog setups do on stick cars.

soundengineer

iTrader: (16)

no.... I have an Automatic...
TCI 6x, 6speed AUTOMATIC

I usually highly advise against a stick car with a supercharger....loose too much boost every time you let off the gas to shift...pointless
ok if you keep your foot to the wood while shifting..