No boost

[tappet]

Finally finished my m90 project today, started first hit, looks great with the belt and pulleys running perfectly. With the pulley change I estimated 5 to 6lb boost but sadly nothing, even with the bypass valve removed and the butterfly manually operated by hand. I pulled the hose off the manifold for the boost gauge and put my finger over the barb and at no time is there any pressure only vacuum. There is no boost. The charger is a gen111 Buick and is brand new with the outlet ported. Before I tear it down I thought I had better ask the question here.
Have I missed something, could it be the bypass butterfly, or is there a hole I missed blocking in the charger e.g. pcv valve etc?
I am stumped.

 

[JustenGT8]

No pressure with large throttle opening? You won't see boost with the roots SC at any Revs unless it's a large throttle plate angle

 

[tappet]

No pressure with large throttle opening? You won't see boost with the roots SC at any Revs unless it's a large throttle plate angle

Sorry Justen I am not sure what you mean.

 

[JustenGT8]

A roots SC is very very sensitive to any restriction upstream ie if the throttle isn't pretty well full open it won't make boost. If you have just been revving gently at idle its normal not to make boost...it needs lots of throttle

 

[tappet]

That makes sense now I think about it.
When you try and research information for this sort of thing you read things like
"The M90 will make boost at 2000rpm" so with my zero experience at supercharging I expected a decent reading on the gauge at low revs.
Now this bypass has me a little confused, looking down into the charger through the vacuum tree as I have it removed, I can see with the motor not running the valve/butterfly closed. I can see by the action of the actuator that with vacuum it will open the valve. Once again I assumed that the valve would be open when the engine is not running and closed with increased vacuum to create boost.
Am I wrong again?

 

[JustenGT8]

Yup sorry wrong again the bypass is vac opened to avoid the SC stretching the air in high vac ie low throttle open positions. You obviously aren't needing boost in those conditions so you bypass the SC to 'unload' it and provide an alternative air path to the engine. Reduces intake temps A LOT so pretty much a must to have a bypass unless a track only engine.

The SC will make boost from 2000 rpm and even lower, but only at nearly full throttle. I could see 6psi from my M90 free revving aggressively.

 

[tappet]

My car is strictly burnouts but in saying that you spend a lot of time being dicked around before you get onto the pad, therefore it spends a fair bit of time in idle. So you have already answered my next question about whether or not to delete. Cheers.
I forgot to mention that on trying it again I am getting about 3lb boost while punching the throttle to about 3/4 acceleration, so 5 to 6lb looks like its going to be on the mark.
Phew, what a relief!

 

[JustenGT8]

All good then

 

[tappet]

More than good mate! Supercharger stays cold, has a nice whine on full throttle, and you can see in the vid the actuator working. With no bypass operating the supercharger was getting hot instantly.
https://www.youtube.com/watch?v=OHh_BX8abx0

 

[JustenGT8]

Looks tidy, nice work.

 

[tappet]

So today I run the car for about an hour on idle, and revved the shit out of it for a while setting the water injection to come in at 3psi, half of its 6psi boost.
I wanted to check temps , belt/pulleys and pressures etc. so I really gave it a workout. Now when I switched the engine off I put my hand on the supercharger and it was near freezing covered in water from condensation.
I have read over and over the problem is going to be heat so is what's happening here ok?
I am only asking this as everything I have assumed so far has been arse about.

 

[tappet]

I figured out what is happening, I disconnected the water injection and run the car again this time the supercharger just got warm. The water tank is in the cab between the seats and when it is full the water level is just below the injector nozzle. When the car is idling it is in vacuum which in turn siphons water into the manifold thus near freezing the charger. I have since lowered the water tank and ordered an anti siphon valve.
I must say I am shocked at how cold the water makes the intake. By the way this was also on one of our hottest days this summer in Nth Qld.