servo with a supercharger

[ColinChandler]

Hello everyone,
A question for the modifiers amongst you. I have a brake servo fitted to my Morris and as normal, the hose from it is connected to the inlet manifold to provide the necessary suck it requires.
Now, I've acquired a Shorrock's supercharger and just wonder where the servo should be connected? Is it still from the inlet manifold or should it go upstream between the carb and the supercharger? I just wonder if there will be less vacuum in the inlet manifold if the supercharger is supplying boost. I presume the same will apply for those of you who have turbos too. Any help would be appreciated.
Thanks, Colin.

[ColinChandler]

Thats sounds pretty fine to me. I have enough trouble making sure my feet are in the right places anyway without trying to complicate it all.
Thanks for the reply.
Cheers, Colin.

[lowedb]

Please not this is based on the theory of what's happening, rather than advice having done something like this.
The vacuum only exists after the throttle plate when it's closed. The charger will tend to reduce this when boosting, and with a WOT you will get big prerssure rather than any vacuum. The same is true of systems with Turbos, and generally there is either a mechanical pump (for example Rover Diesels have it on the Alternator) or sometimes an Electric vac pump with a switch.
Diesels need it more than petrols as with no throttle plate there is never a decent vacuum. So long as you have a throttle plate you will usually get enough vacuum for a servo, but you might need to think about a vacuum reservoir and non return valves so it doesn't dissapear! On a petrol turbo engine I'm familar with, there is a pump, butb this is only really needed under cold start conditions when the throttle is left wide open even on overrun in order to heat up the Cats quicker. Once the engine is warm, there is plenty of vac after the throttles (except when on boost).

HTH

[rayofleamington]

I'm not sure too much about superchargers, but I fully agree with statements above regarding vaccuum - the vaccuum is there when the engine sucks against a closed (or partially closed) throttle.
As long as the throttle is still in place, then when you let go of the accelerator you will have a decent vaccuum.
so unless you're looking for some heavy left foot braking with full throttle (pretty unlikely ;-) ) then it 'ought' to work ok.
This is how it works on most turbo cars, but as for superchargers... you could try a vaccuum guage to see what is happening at the manifold pressure during driving.

[RobThomas]

Ford Transit Diesel alternator. Has a Vacuum pump on the back of it

Might be worth a try? Diesels don't create a useable vac so they need an extra drive to create it.

[dp]

I asked this question on 'uk.rec.cars.modifications' last year. If you google groups "supercharger vacuum brake servo" you will get the full thread. Basically you should still have vacuum as the engine is effectively idling or returning to idle when you brake. A non-return valve in the pipe from the manifold to the servo would be a good idea. ABS and diesel is a different story.

[lowedb]

ABS and diesel is a different story.

I know this is not Minor related, but why do you say that?

[dp]

Diesels don't have a butterfly valve, thier throttle is always wide open with the acceleration controlled by the amount of fuel being squirted in. So no closed butterfly to cause a vacuum hence the need for a vacuum pump. This is why you should never squirt easy-start into a diesel intake; you will have no means of controlling revs.

The ABS is due to me mis-reading something about accumulators replacing servos on ABS cars. I don't think this is quite correct as there are ABS cars with servos.

[lowedb]

This is why you should never squirt easy-start into a diesel intake; you will have no means of controlling revs.

And you have to be careful when topping up the oil in case it gets sucked through the breather.

The ABS is due to me mis-reading something about accumulators replacing servos on ABS cars. I don't think this is quite correct as there are ABS cars with servos

There certainly are. I'm on my sixth.

[Cam]

In fact I would say that most cars (if not all) with ABS also have a servo.

[lowedb]

You are right, most, but definitely not all. Some cars have a hydraulic booster, but this works like a sevo just using oil instead of vacuum, so could be consdered a servo. Others have an electric pressure pump and hydraulic reservoir so work in a totally different way.