Thinking of fabricating a plenum? Important
Turbo Mini Forums

I've at last got round to flow testing a plenum (with the brass restrictor in place) and comparing it to a simple diameter flat restrictor, so you keep the same kind of resrtiction at the carb mouth on your fabricated plenum.

I've always used 40mm on my plenum, but have realised in the last few months that this must be incorrect, as the needle profile was extreme to say the least - it was far too sensitive at low rpm and needed to be wafer thin at the tip.

So, after boring a few different pieces of alloy sheet out to different sizes, we did tests on the plenum, then on the samples. The answer was then extrapolated as the change in area of the various diameters. To cut to the chase, 37mm is what you need.
It might not sound like a significant difference, but using (pi*D^2)/4 to calculate the area, 40mm ends up around 16% larger area than 37mm. I will be modifying my plenum to suit, then changing my needle after the winter rebuild...

Hope this helps you guys - i just wish i had access to a flowbench years ago!!!!

oh - if you do make a plenum, don't forget to add the holes just like in the std plenum to presurise the float chamber and piston assy!!!

D

Edited by TurboDave16V on 10th Nov, 2004.

Take it this restriction is jsut flat sheet, not flowed in any way?

cheers for the info guna be makeing my own plenum. anotehr question though, what happens if you leave ou tthe restriction all together? not enough fuel due to a lack of pressure difference between float chamber(plenum) and venturi?

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Dave would to be able to find out how mine flows? Although my plenum used the std restrictor ring, I'm still confused to why my car is not running lean on the std BDD needle.

Edited by Jimster on 26th Dec, 2003.

Team www.sheepspeed.com Racing

I was thinking that myself STU, great idea!

Edited by minimark on 26th Dec, 2003.

Everyone knows that instructions only have to be read if the thing doesn't work....

This was a totally flat sheet, no corner radius or anything.

Jim, the standard plenum measures 120.25cfm @ 15". The restrictor ring is the restrictor, no other part of the plenum will affect this... I don't know why yours is like that - it just is!!!

I did try removing the brass ring from the plenum. The result was that once the engine saw boost it refused to rev any further due to lack of fuel. Felt the same as a leak in the system. In short, it needs the ring !

I did cut the openings on the brass ring a bit larger because it looked like a major restriction to the flow. Well, basically that?s what it is !

Dave, have you got the "Turbomania" book ? It deals with the restriction rings or "modulators" for Dellorto carbs. Judging the data given in the book the ID size of the ring is roughly the same as choke diameter.

Jukka

SOrry to be a pain

but what is this brass ring you all speak of ????


GRIN

Steve

TurboLessTosser

I realy want to loos the restrictor. Is it possible to plug the pipe going from the plenum to the fuel bowl, remove the restrictor and put in a t-piece on the hose to the sense signal for full pressure regulator an connect this directly to the turbo.
I assume i would get an higer pressure in the fuel bowl as there is some pressure drop over the intercooler an thru the pipes.


Hope this is undeerstandbal and make sense.

Dag Roar
Norway

Dars, You could try and modify the restrictor, and if you pressurised your float chamber using the pipe just forward of the fuel inlet, the best place to take the pressure from would be before the intercooler.
Remember though, in a static sense this is great. Dynamically, it might not be ideal... It's up to you. Personally, i'm sticking with the restrictor...

where in the plenum is the restrictor?
i have a brass ring i mine where the plenum mounts to the back of the carb, it seems to be exactly the same size as the hole, it does not have a lipif you put your finger in to it and it seems as though it would not restrict anything.
is this the ring or is it somewhere else?
DAN :)

i'll get a photo i can't explain it very well

That's it Dan - the brass ring...

It's a restrictor - look at the base of it - a restricted opening...

i see it now, cheers dave
:)

here's a pic if anyone else is confused about the brass ring like i was
http://atlantic.photoisland.com/sessions/1.../25269051lg.jpg

Edited by clubman_dan on 22nd Jun, 2004.

does it mean that i can make my own plenum in any wich way as long as I use the standard restrictor and copy the holes?

What it means is you can make your own plenum in any wich way as long as you use a 37mm hole instead of a restrictor, and copy the location of the holes.

Edited by TurboDave16V on 10th Nov, 2004.

Hey!

I think this is the problem with my own plenum! On high boost the mixture is getting lean. The T-piece-trick is not helpful. The hole is too big:



Dave, does it have to be a ring with a certain thickness, or is a hole with 37mm sufficient?



Andreas

http://www.minifreunde.at

At this time the diameter is 46mm!!!

http://www.minifreunde.at

Now i made my own restrictor-plate with a 37mm-hole. So the cross sectional area is about 11cm?. I want to fit it between plenum and carb:



The restrictor of the original plenum makes a cross sectional area of about 18cm? (with a 48mm-hole). And when i add two-times the area of the rectangles which make the brass inside the plenum, its also about 18cm?. This makes no cross-section constriction:



So for me it's not logical, that i need a hole with 37mm. But i will try...
Is it compatible to the BCA-needle?

Regards, Andreas

http://www.minifreunde.at

I looked at the plenum yesterday again, and saw that the brass ring makes a cross section constriction inside the cast part, because of the special casting mould, which I did not realised the first time. So you are right, Dave!

Hopefully my self-made restrictor-plate makes no turbulence of air inside the carb, which maybe has influence for the fuel-mixture generation?!?

http://www.minifreunde.at

Now it looks like that:

http://www.minifreunde.at

The results i got was from a flow bench test between a stock plenum, and a sheet of ally with lots of different sized holes...
I found the exact match out, and then tried it on the car! Easy Peasy!

Forget the crappy self-made plenum...
I have problems with it. Now i make a new construction:





I will post pic's when its ready...

http://www.minifreunde.at

http://www.minifreunde.at

So akrich, did your homemade plenum never work?

What was the problem?

Wil

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk