Pulley Math

[Holden]

OK folks. Since I have ringer telling me he gets 8psi from a CRV pulley and rjaffey and TOO saying it's good for 10psi and I'm seeing 12psi on my boost gauge, I decided to work out some math to make sense of all this.

I think we could all do ourselves a favor to start referring to pulley combinations by the ratio of blower turns to engine turns.

The end result of my math is that the CRV pulley combined with a 4" blower pulley will boost slightly more than a 3.6"/10psi pulley.

I'm taking it off.

Greg


1) GSR Crank Pulley with 4" blower pulley (Stock JRSC):
GSR Crank (Alt. segment) = 6.00" Diameter
Circumference = 2pir = 2 * 3.14 * 3.000 = 18.84"
4" blower pulley: r=2", c=12.56"
1 rev of GSR turns 18.84" of belt
18.84" / 12.56" = 1.5 turns of blower

>>> Factor = 1.5 <<<


2) GSR Crank Pulley with 3.8" blower pulley (8psi setup):
GSR Crank: r=3", d=18.84
3.8" blower pulley: r=1.9", c=11.93"
1 rev of GSR turns 18.84" of belt
18.84" / 11.93" = 1.58 turns of blower

>>> Factor = 1.58 <<<


3) GSR Crank Pulley with 3.6" blower pulley (10psi setup):
GSR Crank: r=3", d=18.84
Using a 3.6" blower pulley: r=1.8", c=11.3"
1 rev of GSR turns 18.84" of belt
18.84" / 11.3" = 1.67 turns of blower

>>> Factor = 1.67 <<<


4) CRV Crank Pulley with 4" blower pulley (My Setup ):
CRV Crank (Alt. segment) = 6.75" Diameter
Circumference = 2pir = 2 * 3.14 * 3.375 = 21.20"
4" blower pulley: r=2", c=12.56"
1 rev of CRV turns 21.20" of belt
21.20" / 12.56" = 1.69 turns of blower

>>> Factor = 1.69 <<<


5) CRV Crank Pulley with 3.8" blower pulley (Insanity!):
CRV Crank (Alt. segment) = 6.75" Diameter
Circumference = 2pir = 2 * 3.14 * 3.375 = 21.20"
3.8" blower pulley: r=1.9", c=11.93"
1 rev of CRV turns 21.20" of belt
21.20" / 11.93" = 1.78 turns of blower

>>> Factor = 1.78 <<<


------------------
Greg Holden
96 GSR, JSRC, I/H/E, yada yada yada...
My JRSC Install Site

[Lip]

but immediatly after you see results with the crv pulley..you get this. Strange that the solution popped up so quickly

http://www.b16a.com/cgi-bin/w3t/show...collapsed&sb=5



------------------
Lip

UT OH...GOIN HYBRID (CAN YOU SAY 11's? cake

[rjaffey]

TOO doesn't say you'll get 10psi with either the CRV or the Endyn stepper pulley, rather the pressure increase, not entirely indicative of airflow increase, is dependant on a number of variables. I'm seeing 10psi now most likely due to the restrictive stock exhaust. I'll be taking care of that situation in the next few weeks and hopefully should be able to confirm that the CRV and stepper pulleys each increase the boost by about 3psi. I'm expecting to see between 9 and 9.5 psi at most after I uncork the exhaust side of the motor.

[This message has been edited by rjaffey (edited August 21, 2000).]

[rjaffey]

Greg,

For the record, your calculations yield different results from mine. I don't have the exact figures in front of me, but I think that on my '94 GSR and MTRS's '95 GSR the ratio between the stock crank pulley and the blower pulley was about 1.4:1. I obtained this measurement by marking the blower pulley, using the master cylinder as a fixed point of reference (a sight if you will), then rotating the the crank 360 degrees and marking the blower pulley again. I then measured the diameter of the blower pulley at the lip and measured the cord between the two marks and applied a little math to arrive at that number.

Now here's the kicker: The same gauge that reads 10psi on my car with stock exhaust and a 12% increase in blower to crank ratio over the 6psi system read over 10psi on MTRS's car after I installed a 3.8" pulley on his blower. I compared the ratios and his had increased just over 5%! It should have read 8psi or maybe a tad over...

[This message has been edited by rjaffey (edited August 21, 2000).]

[Holden]

Rafe: I don't doubt your numbers. The 6" and 6.75" figures I used are actually the outer diamter of the lip (equivalent to the outside surface of the belt). It makes sense that the stock factor/ratio is a bit less than the 1.5 I calculated. By staying with the numbers that are both wrong in the same way, the overall comparisons between options can still be trusted.

But regardless of the exact size/ratio, how can you argue with the fact that the CRV pulley + a 4" blower pulley will SPIN the blower FASTER than a GSR pulley with a 3.6" (a.k.a. 10psi) blower pulley? Regardless of any other factor, my current setup is spinning the blower faster than even the nuts who have put a 3.6" pulley on.

I do however, bow down and say, "Rafe, you were right in your private email warnings to me." I don't know how I managed to convince myself the CRV pulley was equivalent to a 3.8" blower pulley. I listen more next time. Hopefully!

(Take that last part literally. It's not meant to be facetious. I really mean it! I should have listened to Rafe. He said the injectors couldn't handle it, and they were definitely at the ragged edge.)

The pulley will be removed tonight. Anyone want to buy it? At least I only hit WOT maybe 10 times. Probably no damage.

(10PM - It's off. I'm gonna beat the crap out of my car tomorrow )

BTW, the CRV pulley actually UNDERDRIVES the PS pump. Go figure.

Greg "Leap before you Look" Holden

[This message has been edited by Holden (edited August 21, 2000).]

[ringer]

So the consensus is that the crv pulley should give 10psi? I guess mine now seems low at 8. Hmmmmm.......maybe my valves are adjusted loosely or something?

[Holden]

Well, it's not easy to pinpoint the psi inrease (real air mass increase, not just the number on the gauge that is affected by so many other factors) a pulley will provide, I think we're all in agreement that the CRV pulley will spin the blower slightly FASTER than a 3.6" blower pulley (which most people call a 10psi pulley).

I think your experience, ringer, was one of the reasons I thought the CRV pulley would boost around 8psi. IT'S YOUR FAULT!!! jk

Maybe your valves are out of whack.

I'm back to 6psi this morning. It feels slow now that I've tasted the "forbidden fruit".

Greg

[ringer]

Sorry it didn't work out for ya Holden...now I kinda feel bad...

The only reason I did this whole crv pulley thing, was because my original gsr pulley was chipped up pretty bad during a timing belt change. I knew I should get a new pulley and also wanted to up the boost for soon to be installed SC. It seemed a good solution for both.

[badboy]

What about the Endyn "stepper" pulley? Does this pulley replace the one on the sc? I already have the CR-V pulley and was thinking of getting the "stepper" pulley too. What kind of boost would that setup be at? Thanks.

[Holden]

I didn't even bother figuring that combo out, because it is absolutely insane.

The blower would be spinning at more than twice engine speed. I think the blower is only supposed to go to 14K rpms.

Greg