Supercharger Install - Bypass/recirc Queries

Yes and yes. There's no flow in the vacuum line, just pressure.


Bypass plumbing should be like this "Blow through" set up below:

when the manifold sees vacuum the bypass valve allows air to bypass the supercharger, so it isn't compressed and heated etc.

The bypass should negate the requirement for a recirc valve.

It doesn't matter because you don't need to support any kind of flow rate. Just use what ever fits on your components.

The bypass valve solves the issues the second throttle tries to prevent. With the bypass you should be able to run with one throttle body in the standard position.

This too

I built my first dual throttle supercharger system almost forty years ago as part of a twincharging setup.
So I have been at this for quite a long time, have helped many people, and have first hand experience of all the various alternatives and the problems associated with various different approaches.

Schercheeroo has summed it all up very well, and I might be able to add just a little bit more detail to his excellent explanation.

1) Throttle pre charger.
Tip in/throttle response is worse, but by far the nastiest effect occurs during snap throttle closing. When you stamp on the throttle the supercharger has to fill all the dead pipe volume, which takes only a few engine revolutions, but its actually not that bad.
But when you snap the throttle shut (as in gear changing) it seems to take forever for the rpm to drop. Remember you are de-clutched to change gear, and there is no load on the engine, and the revs can take SEVERAL SECONDS to fall which makes gear changing painfully slow.
Unless you drive an automatic transmission that is <grin>.
So pre supercharger throttling really only works with a cold side system.
Its a total disaster with a hot side supercharger and a large air/air intercooler.

2) Single throttle post charger.
This only works with a turbo or centrifugal supercharger, in conjunction with a blow off valve.
It obviously cannot work with a positive displacement supercharger.....

3) Dual throttle.
This is capable of excellent throttle response, with all the original factory drivability and cold startup response, provided there is somewhere for the air coming out of the supercharger to go when the throttle is closed or only partly open.
One thing that surprises many people is that the second throttle mounted over the supercharger intake is not there to control the airflow !
Its the original throttle still mounted on the intake plenum that controls all of the air going into the engine.

If an air bypass system around the supercharger is set up and working properly, the engine will run perfectly well without any throttle on the blower intake.
In fact, if you were totally deaf, the car would drive perfectly well, and you would never know the difference !!

But the air coming out of the supercharger is very noisy angry air, and if there is an acoustic path open between the blower discharge and the blower intake, connected to a pod filter (or whatever) its going to produce a very nasty blower rotor drone, whenever the bypass is open. Its going to be open all the time, except when under boost.
It only goes quiet when you floor it and the blower bypass shuts the noise path off completely.

So the way around that is to fit a second throttle body over the blower intake, and bypass the air after this second throttle body back into the blower.
The second throttle body is there ONLY to reduce noise.
It closes right down at small throttle openings to block the noise without significantly throttling the incoming air.

Its not in the least bit critical to set up, and exact synchronization is not a requirement.

Basically you jam the blower intake throttle open with a pencil or something similar, and warm up the engine so its idling normally, all controlled by the main throttle on the plenum, and the factory idle speed control system.
Just like the stock engine.
You will hear some nasty supercharger noises even at idle because the air bypass will be fully open.
Remove your pencil, allowing the supercharger intake throttle body to close against its idling set screw.
If it closes down too far, the engine will stumble through lack of air.
If its open too wide, there will still be some rotor noises....

Its not to difficult to find a sweet spot where it idles fine, without any nasty noises.
The supercharger intake throttle body should be made suitably large so that it opens with the main throttle body without being restrictive. That is a pretty easy requirement to meet.

Getting both throttles working together is the first requirement, and the easy way is to just use two throttle cables to do it.

The second requirement is the actual air bypass and its control.

The air bypass does not need to be huge. If you have ever chased a boost leak, you will know that even a small hole can lose an awful lot of boost. And that is all the bypass needs to do, drain away excess boost ahead of the main throttle, so the bypass and bypass pipework can be made much smaller than the main intercooler pipework. Inch and a half should bypass enough air to totally unload the supercharger under closed throttle.

The air bypass should open whenever the main throttle is closed or only slightly open, by feeding manifold vacuum into the actuator on your bypass.

If you are just starting out, a lot of this is not immediately obvious. But hopefully once the general idea is pointed out, and you have a system on your car, and think it all through, its not difficult to get it working once you know what its all supposed to do.

Sorting out the fine details.....

Getting two separate throttle bodies to open and close together, is conceptually fairly simple.
But it all becomes a lot easier with a little planning.
The first issue is throttle cable travel, and the least painful solution to that, is to fit an identical MX5 snail cam to your new (larger) supercharger intake throttle body. Not only will the total cable travel then be the same, but so will the opening angle.

Each throttle body (by law) must have two return springs.
If you have ever had a throttle stick wide open you will understand why!
But now we have two throttle bodies and four return springs, and the pedal weight becomes just too much.
Its perfectly reasonable to remove one or both outer return springs to get back to a more sane pedal effort.
Both throttles are in flow series, so as long as one can still close, the engine cannot run away.

Other common issues have to do with the vacuum actuator for the air bypass.
The commercial actuators are usually a cheap and nasty sealed metal can, and whatever spring is in there is what you are pretty much stuck with.
While they are designed as a vacuum actuator, direct manifold pressure is going to also see full boost, and provided boost is low, there may not be any problems.
But high supercharger boost pressures can really give the diaphragm a tough workout, and it may eventually tear or leak and may have a fairly short life.

A much better solution is a two port actuator, where the actuator has two sensing lines that are connected to before and after the main throttle body on the plenum. The actuator diaphragm only ever sees the pressure drop directly across the throttle.
It still sees engine vacuum at idling and part throttle. But at wide open throttle, the pressure drop ACROSS the throttle body drops to zero, regardless of how high final boost pressure goes. So its much more gentle on the actuator when running high boost.

The ultimate air bypass actuator would probably be something removed from a low cost Chinese turbo wasegate.
These come apart, so you can change the spring or replace a diaphragm, and they all have two ports.
There is a lot more diaphragm area too, so fitting a very light spring is possible.

A very light actuator spring means that only a very small vacuum is required to hold the supercharger bypass open.
Its then possible to drive around off boost up to perhaps three quarter throttle.
You can keep up with the traffic off boost and use less fuel.
But that last bit of throttle opening really brings on serious power!

This is much nicer to drive for an every day road car, especially in the wet.

If you use a sealed commercial vacuum actuator can, whatever vacuum range it works over is what you are stuck with.
It will work fine for you.
But if you want to play around with drivability and make something really nice to drive, set up exactly how you want, a home brew dual port actuator fitted with the perfect spring of your own choice is the way to go.

I've been reading this thread for a couple weeks, keep coming back to schercheeroo and warpspeed posts, both interesting and very knowledgeable.

Hadn't quite grasped that the second throttle body is only there as a silencer when the diverter valve is open. My first step is to have a charge tube and the stock supercharger pulley (4psi), running a mini diverter valve. I had thought about just using the inlet mani throttle body only, but this has me considering running two from the get go, once I have it running like this it would then be smaller pulley and OTP intercooler.

My pipework that I've bought pre fab'd has everything for running the mini by-pass and second throttle body.

I had been reading of a few that run an intercooler with only the stock throttle body and a dump valve post intercooler.