I was doing some blower calc's and found a possible problem.

I have a T-trim with a 6.66" crank and 2.75" blower pulley. According to Vortech's website impeller speed calculator, at 6,800 rpm, my impeller speed is 57,000 rpm. This is 4% overspeed, but probably OK.


Here's the problem: when I use Vortech's T-trim compressor map, the numbers don't match.

On the Y-axis, you have pressure ratio. This is P2/P1, in absolute pressure. So, at 13 psi boost, I get

(13+14.7)/14.7 = 1.88

On the X-axis, you have mass flow rate. My MAF max's out at 521 gms/sec. I assumed that the engine may reach 575 gm/s at peak. Convert that to lb/min:

575/454*60 = 76 lb/min

Now, find the point on the chart that coincides with a pressure ratio of 1.88 and 76 lb/min, and you find an efficiency of 70.X% and 43,000 impeller rpm .

So, if the belt/gear step-up says it should spin 57,000 rpm, but the compressor map says it's only performing at 43,000 rpm, then the belt must be slipping 23%???

Anybody else run through this exercise?

Does this amount of slippage sound right?

Mike

 

TTT

 

At 6,800 crank rpm, the 6.66 x 2.75 pulley set up should spin the blower pulley at +/- 16,500 rpm.

23% slippage would make for +/- 3,900 rpm worth of slip ... I think you would see definite evidence, like dust or smoke if the belt was really slipping that much.

More likely IMO is that the internal tolerances are out a bit and the compressor unit is not as efficient as Vortech claims it to be due to the manufacturing tolerances. I've seen that occurance on a T-trim blower car in the area, chased all sorts of things down looking for lost boost, finally they sent back the "brand new T-trim" to Vortech ... blower came back ... and all of a sudden they were making 6 psi more boost.

Hope this helps

LWM

 

Nice thing to know because rebuilding these units its not cheap...

I do think belt will slip quite a bit with a 6.66" crank pulley... step up to a 7"?

I would love to know what have you gotten out of your setup lately... in termos of power and mph.