Nissan 370Z Forum - Stillen SC Kit Impeller Speed Calcuations

Okay, I've tried searching for this, but it seems the usual reply is 'use a calculator!', and no one seems to come back and actually state what results they got. I don't know about them specifically but even if I did use the calculator I'd still like some feedback to my results with others that have calculated to see if they match, or in the same ballpark.

So, decided to throw this out there and see if anyone can point out an issue with my math.

From what I understand there will be 3 pulleys at play, Engine Crank, Jack Shaft & SC Input. I've found these are the diameters of said pulleys:

Engine Crank Pulley - 5.75"
Jack Shaft Pulley (both sides) - 3.2"
SC Input Shaft Pulley (my own, also known as the '9lb pulley') - 2.75"

Also needed is the Vortech V3 Step-Up ratio, which from googling I seem to get 3.6:1 (one place stated 3.45, but let's stick with 3.6 for now).

I'll be assuming an engine RPM of 7500, where our red-line begins.

1. Calculate ratio of Crank Pulley to Jack Shaft Pulley to find it's RPM @ 7500 Engine RPM.

5.75 / 3.2 = 1.796875 -> 1.8:1

Therefore @ 7500 Engine RPM the Jack Shaft will be spinning at 13500 RPM (7500 x 1.8)

2. Calculate ratio of Jack Shaft Pulley to SC Input Pulley to find it's speed @ 7500 Engine RPM. This is where you would substitute your own Pulley's diameter if not 2.75 inch.

3.2 / 2.75 = 1.163636363636364 -> 1.16:1

Therefore @ 13500 RPM at the Jack Shaft we expect to see 15660 RPM (13500 x 1.16) at the SC Input Pulley

3. Calculate Impeller speed.

Easiest step, which is just to multiply SC Input RPM speed by the SC Step-Up ratio.

15660 x 3.6 = 56376 RPM

The Vortech V3 lists 53000 RPM as a max Impeller speed. You can have bearings that spin to ~70000 RPM as one reputable supplier of rebuild kits offers but it does not negate the actual Impeller's max speed, which I can only guess may be due to just being balanced to a certain RPM? Any further information here appreciated, but as it stands at redline we are over spinning the Impeller by about 6%. 7050 Engine RPM would see the Impeller at it's exact 53000 RPM maximum for the 2.75 inch SC Input Shaft Pulley.

For reference, I believe the stock pulley Stillen supplied with the original kit was 3 inches, which works out to be 51759 Impeller RPM @ 7500 Engine RPM, using my calculation from above.

Main reason for this thread was to just point out in simple terms for those of us with a Vortech just how close to the limit of these units we are if dropping in smaller pulleys for higher boost, and also for some feedback to see if my maths is accurate.

If my pulley diameters are wrong, please let me know and I will revise the post as obviously they'll throw all my figures out.

Cheers! :tup: