Function of the plenum
Turbo Mini Forums

As far as I can discover from the searches the plenum causes a restriction to enable presurisation of the fuel bowl.

Does a fuel pressure regulator not do the same job?

nopes :) and its not the plenum that restricrts but the brass ring thing inside it, turbodave posted atleast one topic on this from memory :)

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Yes I read that one and the ideal size for the brass ring is 36mm and the plenum should be 1.3 x the cc of the engine.

So what is its function?

does it just hold boost?

I was wondering if you have an intercooler do yo need a plenum or would it just work with a pipe me and dan couldnt think why

Previous Engine: 1040cc Morespeed engine, 1275 turbo head T2 Turbo Mirage Manifolds Megajolt
Previous engine: STD metro turbo, megajolt
Drives: 399bananahp (flywheel) Honda Civic Vtec b18c4 T28 Turbo 1968 Mini

Megajolt maps to download: http://www.jamesfawcett.co.uk/cms/index.ph...&gid=3&Itemid=3

read teh post thats described above if you can find it mate. it explains the purpouse of the restrictor. as for the purpouse of the plenum, your guess is as good as mine

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

I will try and explain it as I understand it but bear in mind I am NOT an expert.
The function of the plenum chamber is primarily to provide a chamber of air for the carb to draw from. The restrictor ring is nothing to do with the plenum chamber it just so happens that in the case of the Metro/Mini set up it is fitted into the plenum. It is fitted into the bellmouth of the carb in most other instances using a blow through carb set up.
Its purpose is to restrict the flow of air into the carb which creates a pressure difference between the main jet in the carb and the pressurised float chamber. The reduced (restricted) pressure over the jet can then draw fuel from the float bowl at a higher pressure. Had the inlet not been restricted the pressure difference would be so low that insufficient fuel would flow into the carb/engine
The plenum can also provide a convenient location for a blow off valve, a pressure take off for the various actuators and an air inlet temperature sensor should the engine be controlled by an engine management system.
The Fuel pressure regulator is there to provide an increasing fuel pressure just greater, by 2-3 Ps, that boost pressure so that fuel will continue to flow into the pressurised float chamber as the boost pressure increases.
The 1.3 x engine capacity is more applicable to normally aspirated engines than turbo engines (I think) and it is supposed to steady the airflow down prior to entering the carb/s.
I hope this is correct and is of help, no doubt some of the more experience guys will put it right if I am wrong.
Writing it down has certainly got it all a little clearer in my mind if nothing else.
Regards
Dave

Edited by danboy on 29th Jan, 2007.

Thanks for the reply!

Is the float chamber pressurised from the plenum?

Yes it is
Regards
Dave

what sort of psi drop exists between the turbo snail and the plenum on a decent intercooled set up?

what if you pressurised the carb from the turbo snail?
you may never get off the line, but if you did manage to get on boost, they restrictor may not be needed?

i know im chatting shit, but just thinking aloud..

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Ideally, you want as little restricion as possible. Every setup will be different

What benefit do you think you would see, by pressurising the float bowl with higher pressure ??

As mentioned above, most blow through turbo carbs will reccomend some form of restrictor. It just helps get the fuelling right, throughout the rpm range.

And IMO, big plenums are good.

They usually offer a better supply of air to the engine, as there is a reservoir of air to draw from, rather than simply relying on the turbo performing instantly.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will