Roush By-Pass Valve for S-II M90 Blower: Should I/Can I Disable?

I see that somme companies off some sort of plug for the by-pass valve of some of the roots blowers.

Should I look into one for my Series II and is it safe? Wy do I ask? Well, when looking at my dyno results, I see a dip in HP between 5500 and 5700 (about a 75 HP drop)with a sudden surge in HP at around 5800 up (+100HP). We though it might be my blower belt slipping, but I was thinking it could be the by-pass valve.

Has anyone experienced this type of problem? If not, any ideas what the problem may be? I should mention that this problem occured before any custom tuning and is still occuring after the tuning

Thanks for all of the help!!!

ag lightning said:

Don't buy the "kit". All the necessary parts can be purchased at your local Autozone for $7.

The "by-pass" mod will simply allow your blower to do consecutive pulls without the loss of boost. For example, if you did a full run the first time, you'd get full boost. However, if you were to goose the car right after the first, you'd get partial boost. The by-pass mod takes care of this.

There are drawbacks to the mod though. The obvious is that you put a lot of heat and wear on your "Heaton" blower, and second, you can get some cavitation.

Hope this helps!

Click to expand...

Thanks for the info!!!

I would like to give the mod a try. Can you step me through the process and identify the parts needed?

One more additional question, is it possible to back-out of the mod once performed?

Thanks in advance for your time and help!!!

ag lightning said:

I will be more then happy to help you through it. I'm going to go and take a look at what I need to do to get this done on my blower today and I'll get back to you. You only need a plastic PCV tee and about 2 rubber nipples. The rest is just plugging the right lines and connecting the others. It's fully reversible!

Click to expand...

I think I found the parts (and isntructions) needed for the by-pass valve:

Parts needed to bypass boost:

*Vacuum T-fitting connecter - 3/16-inch diameter
(not required on Harley Truck)

*Vacuum elbow fitting - 1/8 x 5/32-inch diameter

* Vacuum boot cap - 1/4-inch diameter

Instructions at: https://www.rpmoutlet.com/boost%20bypass.htm

AG Lightning,

Do those instructions look accurate??

Thanks!!!