Currently building a lq4 with stock 317 head and a f1a blower. Anybody here with that combo or know what pully size to run to make around 12lbs? We have a 4.0 and a 3.85 pully but dont know how much each will make around 6800-7k rpm. What combo are you guys running?

 

Bob at Brute Speed is perhaps your best source.

 

I would start of with a 4.50 pulley. The F series of blowers have a 5.40/1 gear ratio while the P & D series have a 4.10/1 ratio. Besides that the F series push more air, the 5.40/1 gear ratio means you use a larger pulley. On my '99 Z28 which had a 408 with AFR 225 heads and an F-1A, we had a 3.90 pulley on it and it saw 18 to 20 psi at 6600-6800 rpm. Bob

 

Thanks DuWayne! Bob

 

 

To the OP - you need to compare apples to apples as suggested by Bob.

The pulley combo used by D1 or D1X owners won't apply to F series users. You also have to take into account the diameter of your crank pulley whether it is a 7.65 or 8.65 in order to calculate max impeller speed. Also, the smaller the SC pulley the more likely for belt slip.

The calculation:
Impeller RPM = (Crank Pulley diameter / Supercharger Pulley diameter * Gear ratio * Engine RPM)

F1 ----- Max Impeller Speed = 70,000
(7.65/4.5) * 5.4 * 7000 = 64,260
(8.65/4.5) * 5.4 * 7000 = 72,659 (Exceeding Max)

D1X ---- Max Impeller Speed = 65,000
(7.65/4.5) * 4.1 * 7000 = 48,790
(8.65/4.5) * 4.1 * 7000 = 55,167

Note the difference between the two above. The internal gear ratio makes a difference as does the crank pulley combination.

If OP uses 3.85 pulley with a 7.65 crank pulley.
F1 ----- Max Impeller Speed = 70,000
(7.65/3.85) * 5.4 * 7000 = 75,109 (Exceeds max impeller speed)

My Combo (TSP LQ9 370) with a 3.9 pulley
D1 ---- Max Impeller Speed = 62,000
(7.65/3.9) * 4.1 * 6250 = 50,264 (4l60E doesn't like more that 6200-6400 RPMS - shift points)

But if I went back to original 4.5 pulley.
D1 ---- Max Impeller Speed = 62,000
(7.65/4.5) * 4.1 * 6250 = 43,562

My novice unscientific observation to consider also: There's a considerable difference between the rpm level the 4.5 pulley made boost and my current 4.0 or 3.9 pulley.

 

OP:
I'm running F1-A with 4.5" upper & 7.65" lower. My build in the sig.
I'm at a hair under 11 lbs. @ 6K rpm.

Note: that is on a 416, with good flowing IC, large truck hat air filter, 6* overlap on cam
My 4." pulley had me at about 15lbs

 

****, didnt see all the replies since my email is screwed up. Thanks for all the replies guys and it all makes sense. Bob, it's for my buddies car (Kyle Ross) who ordered the water pump from you. Right now it has a 4.00 pully on it but looks like we may need a 4.50 possibly. I'll have to calculate what we got right now.

 

Sounds good, thanks. Bob