Need help on figuring out what size pulley for boost.

dj65falcon

dj65falcon

Well-known member
Well,this weekend my Paxton supercharger crap on me was whining out to 6200 rpm. I heard a whining noise under the hood. And then my belt split in have. Shut the car off did not hurt my motor. my paxton will no turn over. I think I welded the bearings. Well good news is my friend got a voretch v3 so to give me to use. Now need to figure out what size pulley to get 9 to 10lbs of boost. My crank dia. Is 6.75 outside. Inside the rib is 6 50. So any help would be appreciated. Thank you
 
djfalcon10":2rsq2xmk said:
Sorry I forgot that part. It is a voretch v3 si
Click to expand...
If we figure a 300 six at 6200 rpm without any considerations to temperature gains or pressure drops at a 100% VE.
(300/2 x 6200)/1728 = 538 cfm x .07 = 37 lb/min.
At 10 lbs of boost the pressure ratio at sea level is (14.7 + 10)/14.7 = 1.68

Airflow at 10 lbs of boost = 37 lb/min x 1.68 = 62 lbs/min.

If we use Matchbot to fine tune.
Figuring an engine VE of 90% and an intercooler efficiency of 60% with a .5 psi pressure drop we get a corrected airflow of 55 lb/min and a corrected pressure ratio of 1.71.
See column #6
http://www.turbos.bwauto.com/aftermarke ... sin=92044&

If we look at the compressor map for the V3 Si at a pressure ratio of 1.7 and an airflow of 55 lbs/min the compressor rpm looks to be around 37,000 rpm.
http://www.capa.com.au/pics/supercharge ... map_si.pdf

The V3 step up ratio is 3.61
37,000/3.61 = 10,250 rpm at the supercharger pulley

10,250/6200 engine rpm = 1.65 drive ratio

Depending on what pulley sizes are available something around a 1.65 ratio would be a good starting place.
 
Thank you. My friend found the pulley that was on there. It is. 4.10 pulley. So what would the boost be on this one?
 
djfalcon10":w758oynj said:
Thank you. My friend found the pulley that was on there. It is. 4.10 pulley. So what would the boost be on this one?
Click to expand...
What is the pulley size on the crankshaft?
 
6.75/4.1 = 1.65 ratio
The boost pressure should be around 10 lbs.
The actual boost may be different but the 1.65 drive ratio will be a good starting place.
 
Awesome. Can't wait to put it on and hopefully it will be close to 10# of boost. I will let you know what the boost is actually is.
 
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