Thinking of fabricating a plenum? Important
Turbo Mini Forums

I made up my own plenum, and did not fit the restrictor.
As already mentioned, this caused major fuelling issues. At the time, I didnt understand why ( back in early 94 )

I worked around this by making the carb able to supply more fuel.

Im running a 0.125" jet and needle, filed as needs be to make it run well Not very scientific, or anything, but it was all I could do at the time to get it to work.

I actually had to drill the 0.100" jet to accept the 0.125" needle.
I dont know my AFR's, but it runs fairly well, without the restrictor in place. Although it would make sense to always use a restrictor, knowing why its there.

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

BTW, has anyone real life data of how much more power you got from homemade plenum compared to factory item ?

The std plenum looks crap, that´s why I chose to fiddle with it years ago with limited success but the bloody thing works despite the looks. When I fitted Dellorto carb I used restrictor rings (like stub stacks inside the plenum) inside the homemade plenum. I have tried rings of various diameters but have not found the ideal size yet. If you think that SU is touchy relative to restriction size you should try a fixed choke carb. It is pain in the A**. Anyway, what I will try next is a factory style restrictions tubes insted of rings.

For those how don´t have a faintest idea what I am talking about have a look at http://www.geocities.com/jharkola/1380turbo4.html

One suggestion: when you fabricate a plenum use a material that you can easily modify later. I used aluminium since I had access to AC TIG at that time. Now I don´t and it is very complicated to try and locate a welding equipment to use...

Dont think people go for homemade plenums for power advantage, more for better intercooler/dump valve piping.

Dave,

Are you saying that instead of having a 50.8 mm hole at the carb entrance, you have a restictor plate with a 37mm hole? not forgetting to allow for the 3 pressure holes.
This would have to have a gasket either side to seal everything..
This would be equivalent to the std plenum chamber with restictor plate.
Am I understaning you correctly..
I have binned my std plenum and have made up a new much simpler affair, but have yet to run it..
I will be rolling roading it at minispeed once I have made these changes..

Cheers

Nigel

yep...

BUT...

A plenum without the restrictor 'tube' is still not (as Akirch has found out) exactly the same as a stock plenum. I now realise it is 'close' to being right, but seems to be much more critical of changes in boost than the standard affair. Consequently, a needle takes a lot longer to perfect - and it is pretty unhappy at 3500rpm on light cruise if im honest (this is compounded because i've got a silly little turbo that makes so much potential boost at low revs, that i've set the needle so it'll run perfect 'foot to the floor' but clearly it can't take the fuel out under cruise).

Once i've got my welder, i'm considering getting plenums templates water-jet cut out of ally and sold on. For the restrictor - I'll be using an identically dimensioned tube to that fitted to the metro.
If you want it perfect (like the rest of your gorgeous mini ) then fit a 'restrictor tube that follows the standard dimensions - or do what many have done - just use the bottom half of the stock plenum and make a whole- new top half...
To summise; The restrictor ring works well with patience. The standard restrictor tube works perfectly.

Edited by TurboDave16V on 1st Dec, 2005.

Good day guys,

Picking up on old topics, I'm trying to figure out my own plenum to make since finding one in Venezuela will be rather imposible.

Can anyone (please) post some detailed pictures of a standard plenum, and maybe some sizing?

Thanks in advance.

Cheers

OAP

.

Edited by hario on 3rd Jul, 2008.

hmm.. if we use a restrictor ring in the plenum to create a slight delay in pressure felt by the venturi allowing the pressure pipe going to the float bowl to keep the float several psi above boost pressureslightly richening the mixture as the boost rises then the correct A/F ratio can be caken care of by the carburettor. This ovcourse stops the fuel and boost pressure becoming even at which point nothing would come out the needle..
So.. this signal line can be taken from a point slightly 'upriver' of the plenum/carb to have the same effect without the need for any kind of restriction (BAD)
i think taking it from before the intercooler would inevitably mean there was too much of a delay and pre-boost richening would be too extreme and poor fuel economy would be felt despite you pistons loving the cooling effect.. therefore why not take the signal from just after the intercooler because the couple of right-angles in the pipework after it would ensure that pressure would always lag slightly behind carb pressure..
any thoughts.......

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