Supercharging CIH

[moodoo]

Some numbers and thoughts...corrections and opinions welcome!

The Eaton M45 has a capacity of 0.737l/rev, say .75L per rev. and maximum a speed of 14000rpm.

2.0L engine has an intake capacity of 1000cc per rpm. Maximum rpm is say 6000rpm. Gear the supercharger to rev twice as fast as the crank.

Supercharger now gives 1.5L of air per revolution of the crank. Which is 1.5 times the air needed, 1000cc

1.5 X atmospheric pressure = 14.7 X 1.5 = 22.05 PSI, Less atmospheric pressure of 14.7 = 7.35PSI of boost.

So should give 6Psi ish, allowing for losses?

In terms of setup, it would look something like this.

 

There seems to be some disagreement out there on whether a 'recirculation valve (like a blowoff valve basically, just not venting) is needed.

High throttle openings: TB1 open, TB2 open, Bypass closed, Recirc (if fitted) closed.
Low throttle openings/idle: TB1 closed, TB2 closed, Bypass open - this allows the engine to draw in air without it having to go through the supercharger.

Now lets say you have wide open throttle, and you snap lift off to change gear. TB1 shuts - but you have lots of pipework, intercooler etc full of 'high pressure' air (I realise it's not very high pressure, but still)...is a recirc valve needed to relieve that pressure to somewhere back before the supercharger? If not, will it not be like putting the brakes on the SC, as it suddenly has nowhere to push the air to? I would think a single suitably sized bypass/recirc line will be sufficient. Having a suitably big recirc line as shown below would be a lot of extra pipework to bring from inlet manifold side all the way over to before the supercharger. Good enough to keep the Bypass line short, just from one side of supercharger to the other, but still fairly large diameter?

Any thoughts?

[Jessopia74]

Achievable boost is definitely a reasonable value as I previously stated. You can gain more at the cost of gearing, but it can bog down the engine a little at lower rpm as it saps so much torque out of the engine as they are both trying to develop rpm ( inertia ) , so 6-8psi is a fair calculation.
 

Bypass and Recirculation valve in a Supercharger 😬  They do the same task effectively. A supercharger does not stall like a turbine wheel (turbo), but will just no longer compress as it’s efficiency drops off. So Unlike a Turbine in a turbo that benefits from a recirculation to stop the back pressure stalling it, the supercharger is to the purposes of your engine, not affected. It’s just adding complexity for no gain.

The only other point is to keep pipewirk as short as possible, hence why supercharged engines usually go with a charge cooler over intercooler (and better efficiency).

If you are keeping the the LE Jetronic AFM, you might need to okay with the spring tension a little, and consider going for a larger bmw? One, but that would work as it is with only a FPR change needed.

Might need a brake booster vacuum pump off a Saab (electric self switching) factoring in too.

[moodoo]

Great, thanks for the help Jess....sounds like you know what you're at...unlike me!

Brake booster pump...why so? Because the 'pressurised' inlet plenum has less vacuum to supply to servo?

[Jessopia74]

22 minutes ago, moodoo said:

Great, thanks for the help Jess....sounds like you know what you're at...unlike me!

Brake booster pump...why so? Because the 'pressurised' inlet plenum has less vacuum to supply to servo?

The amount of time your inlet will be in a vacuum is limited if you SC your car, trust me 😀

The pumps are straight forward and are from the 95 turbo autos, they basically just need a fused ignition feed and will keep your brake booster vacuum correct all the time. You can go without initially but as soon you start to hear the SC wine, you WILL be driving it lol

[moodoo]

...on the AFM, the idea would be to ‘stiffen’ the spring on the standard unit, so the metering flap doesn’t max out to full open too soon? Basically to make sure there is always flap range available over the full throttle range?

Does the BMW (e30???) one just basically achieve the same thing as standard, I’m guessing because it needs to come from something with a 2.5/3.0 engine?

[Jessopia74]

18 minutes ago, moodoo said:

...on the AFM, the idea would be to ‘stiffen’ the spring on the standard unit, so the metering flap doesn’t max out to full open too soon? Basically to make sure there is always flap range available over the full throttle range?

Does the BMW (e30???) one just basically achieve the same thing as standard, I’m guessing because it needs to come from something with a 2.5/3.0 engine?

Yes, to compensate the fuel rail pressure is raise . As fuel inj is based on time/pressure it will need to be a bit trial an error but it will mean that you will keep better feeling over whole rpm range when boost builds up. The rising rate also increases the rail pressure to make sure that is a constant psi over manifold. So at 7psi boost, it needs to be as per your calls above of Atmospheric + boost. 
 

The reason for the larger AFM is to help the SC airflow. As you can already see the airflow requirements you would benefit to hold pressure as the demand for air increases. Although think of it as a nice to have. I can’t remember on the size, but it’s not the big lumps, the 320 too. The 12v Monza/senator A should have a larger you can use too. And less messing with spring tension I expect tbh. This is just theory here, not done it myself but the principle is correct.