Pulley Swap, Sizes, and Info...

[slt]

Being the most prevelant and worthwile first mod, it would be great if someone who knows what they are talking about could make an endall sticky expaining the gains/requirements/limitations of the three popular pulley sizes and their integration with the bypass mod.
Something like:

Instructions on how to swap you pulley can be found here courtesy of Sp00ner
Pulley removal and Install

Recomended Max Continuous Blower Speed = 16,000 rpms

Stock = 3.35"

• 12 psi
• bypass mod pointless
• you need to upgrade
• Blower speed 14,000 rpms at 7,500+rpms

3" Pulley

• 14+ psi
• 10 hp and 15 ft/lb gains**
• Stock belt/idler pulley OK
• no engine management required
• bypass mod OK - good for additional 10 hp
• Blower speed 14,000 rpms at 6,700 rpms

2.8" Pulley

• 16+ psi
• 15 hp and 25 ft/lbs gains**
• belt or idler pulley modification required
• engine management recomended
• bypass mod OK with engine management, good for additional 10 hp
• Blower speed 14,000 rpms at 6,250 rpms

2.6" Pulley

• 18+ psi
• gains depend on tune**
• belt or idler pulley modification required
• engine management required
• fuel injectors required

**rough guess. This post will be updated when more accurate info is posted

Boost Bypass Mod
Originally Posted by ptparker
The boost bypass system on our superchargers has three parts: the solinoid, the actuator, and the butterfly valve (normally closed). The actuator is a diaphram/piston that has two chambers and a small metal shaft that connects to the buterfly valve. When the ECU triggers boost bypass, it opens a soliniod allowing charged air (in the range on 12+ psi stock to about 18+ with a very small pulley) to fill one of the chambers of the actuator. When this happens the piston in the actuator pulls on the shaft which opens the butterfly valve allowing charged air back into the inlet side of the supercharger instead of forcing that air into the engine.

The ECU uses boost bypass to bleed off some of the boost just before you hit the rev limiter to prevent damage from the engine getting fuel (or spark) cut off at maximum boost. The problem with this is that if you change the pulley, the computer detects that you are hitting the "maximum boost" (near 13 psi) and opens the bypass while you still have a lot of RPM's left to climb.

The idea of boost bypass is that if you allow the computer to open the solinoid, but do not allow the charged air to reach the piston chamber on the actuator, the butterfly (bypass) valve will never open. Many people on this board have this mod done, and it is considered a 'safe' mod, but remember that you are disabling an engine protection feature.

The following statement is true for a stock or near stock setup, but with signifantly smaller pulleys the boost bypass opens earlier:

The boost bypass valve opens up at 6,000rpm and bleeds boost off untill redline. The bypass mod disables it keeping it closed all the way up untill redline. You will see no difference in power under 6,000 rpms if you disable it. They only increase in power comes after 6000 rpms where you will see an additional 10hp gain in peak hp. See the attached dyno graph (courtesy of Sp00ner) The green line is with the bypass valve disabled.
http://i18.photobucket.com/albums/b1.../SpoonDyno.jpg
Instructions on how to do this mod can be found here courtesy of AlphaJaguar5
How To: Boost Bypass Mod

[rlinbatonrouge]

Remember that the belt starts to slip with the stock tensioner with a pulley below 3.0 inches. Alpha knows all about having something below three inches

[ptparker]

Let me know if this helps

[ptparker]

Originally Posted by Sp00ner

How certain of you are those numbers ptparker?

I inferred all of these numbers from alpha's two figures like so:

A 2.8 pulley spins 15657 at 7000 engine rpm. Its multipler is 2.23. So the circumfrence of the 2.8 pulley is 8.796. So using that multipler and the circumfrence of the s/c pulley, the crank pulley is 19.68. So that makes the crank pulley a 6.262.

By doing all of the same calculations above for the 2.6 pulley, the crank pulley comes to a 6.37. I think that .1 inches is perfectaly acceptable margin of error, so I split the difference and called the crank a 6.3 inch pulley. These are geometric calculations based on the two facts alpha stated.

Crank S/C Multiplier ([crank]*PI)/([SC]*PI)
6.3 3.5 1.8
6.3 3.4 1.852941176
6.3 3.3 1.909090909
6.3 3.2 1.96875
6.3 3.1 2.032258065
6.3 3 2.1
6.3 2.9 2.172413793
6.3 2.8 2.25
6.3 2.7 2.333333333
6.3 2.6 2.423076923

[ptparker]

Originally Posted by Sp00ner

Not gonna get a 1lb increase by using a 3.40 pulley that is larger than stock...

That is streight off of SFPH's website. I think that that post should stay, but it needs to be said that this is off of SFPH's site. All I can speak for is the 3.0 SFPH, is about a 4 lbs over stock. I can hit 17 right before the rev limiter with my manual boost gauge. The interceptor reports a litle higher (17.7-18.3).



also do we know for a fact that the crank pulley is a 6.3? I would rather it be observed in some sort of GM documantation or emperically measured instead of calculated, but (as you can tell from my last post) I think that this isn't an exact science.

[jmc007]

Here are my comments :

-I DID measure the stock SC pulley size too; it's 3.35" in effective diameter (top of the v where the belt sit) and it's 3.5" overall outside diameter.

-Someone MUST measure the TRUE diamter of the crank pulley before making any pulley speed calculation.

- PSI numbers achieved with replaced SC pulleys will be UP TO xx PSI for each pulley, since the weather and other conditions have some influence.

- The 14 000 RPM maximum blower speed is the maximum CONTINUOUS operation. I seriously doubt pepole run their engine at a CONTINUOUS speed of 6000 + RPM's.

Thanks to all for the great info's.

[ptparker]

Originally Posted by clevelander2002

The best I've seen is 15 PSI but thats on a mechanical boost gauge not an interceptor. Thats also with the boost mod.

2 things:
1) I don't know how pulley tech measures their pulleys, but I have a SFPH 3.0 and it reads 17 consistantly, and depending on temp (engine, intercooler, and exterior air) can get higher then that. And that is on both a mechanical and the interceptor (a.k.a. ECU sensor values).

2) You cannot come up with a simple size X yeilds psi Y. There are many factors such as the tempreture(s) and the barometric pressure/altitude, and in your case the calibration of the boost gauge. Did you do a before/after comparison of the stock pulley vs. the 3.0 you have on?

[ptparker]

Originally Posted by Project_Redline

I know it says it is OK, but will a system be fine using a 3.0 inch pulley and stock idler. Or will you eventually have problems like slippage?

There is one person out there who had any issues with slipping on a 3.0, there are dozens of us with them on and none of us have had a problem. it is ok.

[Sp00ner]

Originally Posted by Project_Redline

I know it says it is OK, but will a system be fine using a 3.0 inch pulley and stock idler. Or will you eventually have problems like slippage?

I dont think ANYONE has been running one for more than 6-12 months, so any real long term testing is guessing, but I can't imagine it would be too bad. I mean, if your belt streches, get a new one...

[ptparker]

Originally Posted by BlueMonster

So how is it alpha is running a 2.9 pulley with no idler tensioner, all stock. How is this possible and he claims it hasn't slipped once????

3.0 is the smallest you can consistantly run. Not every car is exact. If everyone on this board put a 2.9 on tonight, i promise you a bunch would start slipping. It will work fine for some.

Also, have you ever put on a 3.0? The tensioner doesn't even need to be pressed all the way down to slip on the belt. We are lucky that our tensioner has as much play as it does. The stock size is a 3.4 or 3.5 if I remember. Half an inch is a good bit of travel and the belt still be secure. Any more and you are going to start pushing your luck.