Spade

iTrader: (26)

I thought I had a 4.1" pulley but was mistaken of coarse this was after I already ordered the 3.9". I was making around 6.5psi @5500rpms with the larger. What can I expect with the smaller pulley?

ChevyChad

iTrader: (50)

what size crank pulley, and what RPM do you plan to spin the engine to?

Spade

iTrader: (26)

Stock ATI crank 7.25". I plan on just going to 6000prms.

ChevyChad

iTrader: (50)

forgot to ask, is it a P1SC or D1SC????

ChevyChad

iTrader: (50)

Assuming it’s the P1:

The p1sc-1 has a max of 1200 cfm at an impeller speed of 65000. If you currently have a 7.25 crank pulley and a 4.38 blower pulley, spinning the engine to 5500 RPM, the impeller speed is at 37326RPM, which is only 57% of its max, and 57% of max cfm is 689 cfm. By changing your pulley to a 3.9 and spinning to 6000 engine RPM, impeller speed will be 45731 RPM, which equates to 844cfm or 70.355%.

Now, using CFM as a scaling factor, if 689 CFM gives you 6.5#, then 844 CFM should be about 8psi.

Of coarse, this is all just an educated ‘best guess’ but it will get you in the ballpark

Spade

iTrader: (26)

I forgot to say. It is a D1 actually. I was actually wrong about the crank size. Upon some searching the stock ATI crank is 7.65".

Thanks for the help. I found the equation that I was looking for and figured out the ballpark psi. Of coarse without belt slip factored in.

ChevyChad

iTrader: (50)

Same calculations, just using max impeller speed of 62000 RPM, and max CFM of 1400:

Crank Pulley___7.25_____7.25
Blower Pulley__4.38_____3.90
Engine RPM___5500____6000
Impeller RPM__37326___45731
Imp %________60.2%___73.8%
CFM_________843_____1033
est boost______6.5______8.0

so about the same...

ChevyChad

iTrader: (50)

ok, guess you edited your last post while i was typing... But yyeah, just follow the same lines as what I have done above...

eLTwerker

What formulas did you use go get those #'s? Excellent info man!

Spade

iTrader: (26)

Thanks for the help my man.

Spade

iTrader: (26)

max RPM x crank pulley diameter x blower step up ratio/ blower pulley diameter= blower impeller speed

ChevyChad

iTrader: (50)

CFM=CP/BP*ERPM*SR/MI*MC

Where:
CP = crank pulley diameter
BP = blower pulley diameter
ERPM = engine RPM
SR = step up ratio in head unit
MI = max impeller speed
MC = max CFM

You can find SR, MI, and MC here:

http://www.exoticperformanceplus.com...er%20specs.jpg

Then just use CFM to scale the estimated boost.

Forced370GTO

chevychad, your equations are assuming that the boost is relative to the blower speed based on a linear projection. the actual numbers could vary more then 2-3psi. it is my personal experience that an exponential relationship is far more accurate. a more appropriate way of examining boost would be to work backwards using engine displacement and rpm to calculate blower efficiency and then use those numbers to interpolate blower cfm and therefore boost.

ChevyChad

iTrader: (50)

Yes, you are correct in my calculations. The reason for this is simple- look at any dyno sheet that plots boost. The boost line is for the most part a straight (linear) ramp. I know, this is just an estimation, but I do not believe at all that blowers are exponential. Turbos yes, but not superchargers...

BTW- you are not interpolating here, you are extrapolating...

You are correct about the CID being a factor for the equation, but if CID does not change from one pulley, to the next, that can be ignored. Basically, if everything stays the same in a given setup, and the pulley size is the only thing varying is the only time my formula should be used.

eLTwerker

Thanks, but can't read the link. Also, how did you find impeller rpm at given engine rpm? Also, how do you estimate boost from CFM?

ChevyChad

iTrader: (50)

enlarge the picture after you click on the link

impeller RPM is the engine RPM multiplied by the pulley ratios multiplied by the step up ratio of the blower.

you just scale it from an existing given pulley and boost number... for example, if you are making 6.5# at 833 CFM, you want to estimate (linearly) what boost 1033 CFM will make, set up the equation:

6.5/833 = x/1033 then solve for x

just basic 5th grade algebra, nothing fancy here...

eLTwerker

awesome info my man. Thank you very much!