Runs extreme lean on boost, new install... Pitot tube vs. Restrictor
Turbo Mini Forums

Hi,

I have just installed Turbo to my DKW Munga, and as I have plenty of Minis, HIF44 was natural choise for giving some fuel to the engine. But naturally again, nothing went like in the movies.

When I'am in boost (only 0,2bar/3psi), engine runs fine on low rpms. On medium/high rpms it runs so lean that engine just stalls and dont take any more revolutions. It does not run to lean slowly, in just parts of a second I run from 12AFR so lean that engine does not work. Modifying needle or spring does not work, have tried different combinations and problem stays. With soft spring and insane rich (really, insane) needle it runs best, but in top end it suddendly runs lean. Inlet and plenum is home made and carb is HIF44 and engine is 1200cc Opel OHV. Fuel pressure is fixed to 0,5bar/7,3psi.

Now when I have been tuning, I have not sealed dashpot. Could it really make this kind of thing. As I have thought it could not have nothing to do with it, but I have been wrong before in my life... I'am just now in the edge to fit Vems and throttle body from my stock.

Edited by MikaJ on 16th Mar, 2009.

sounds liek a problem with your fuel pressure regulator! Fuel pressure needs to rise with the boost not stay at a set level.

are you using the metro turbo plenum? if your not have you put a 'restriction' on the mouth of the carb? (it needs one to work on boost!)

I know that my fuel side is not how it should be on turbo applications, it is fixed 7psi, no regulator to increase it with boost pressure. But I'am running only 3psi boost, so it should work fine (4psi over maximum boost). I have pressure gauges installed on both boost+fuel, and they are fine during drive.

But for restrictor, I dont have orginal plenum or any restrictor. What is size and shape of orginal restrictor?

Picture from install, plenum has modified after this picture (and even this version, there was milled route for air to to fuel chamber and dashpot)
http://mikaj.1g.fi/kuvat/Munga/IMG_3365.jpg/full

[edit]
Found aswer to restrictor size question...
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=513
[/edit]

Edited by MikaJ on 16th Feb, 2009.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I think you're all missing the point. The fuel pressure is largley irrelavent, all that it does is allow fuel to enter the float bowl when on boost, it dosen't pressurise it.

The fuel pressure differential is created by the float bowl being pressurised from the plenum (through the extra hole in the gasket that goes to the float bowl). Then the restrictor ring reduces the pressure across the jet by 3psi.

So to make this work you need to pressurise the float bowl at 3 or 4pis above boost pressure. A conventional plenum and restrictor creating the differential is one way. Or you need a feed to the float bowl 3psi or 4psi above the boost pressure. If you has a lot of pipework and an IC this may be possible, but with short pipes you won't ever be producing 7psi anywhere. So plenum it is.

You are right that you don't need a reg for 3psi though. Although in my experience 7psi is too much for the float needle to hold off.

Edited by wil_h on 16th Feb, 2009.

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

www.twin-turbo.co.uk

Only point is with fuel pressure, is that it is always over air pressure what is in fuel chamber. There is no need pressure regulator on low boost applications, if needle valve can handle enough fuel pressure. On my case, 8psi fuel pressure did not create leak. Example that 4psi fuel pressure on boost is well enough, atleast for me. Pressure differential is only important in fuel injection systems (well not even in there if that is taken care in injection program).

I will end of project have also supercharger and couple coolers, so maybe then I can get that little higher pressure from somewhere else in piping.

I will try to today get restrictor made, if my better half does not have any "better" plans for me...

How is your float bowl being pressurised at the moment? Does it have a pressurising feed from the turbo?

Edited by wil_h on 16th Feb, 2009.

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

www.twin-turbo.co.uk

Float bowl is now pressured straight from plenum, so turbo pressure is routed there.

So that makes sense then, the pressure over the jet is the same as the fuel pressure, meaning no fuel will be drawn under boost.

Good luck avoiding whatever your missus has planned!

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

www.twin-turbo.co.uk

Its not just the fuel here, the SU carb needs a differential pressure above and below the piston and the restrictor provides that.. The restriction causes a slightly higher pressure in the plenum than in the carb throat, this higher pressure is fed to the float bowl to ensure fuel can flow into the carb (no pressure difference, no fuel-- thats why you lean out) and underneath the piston to lift it, It even scales the differential according to how much boost. Without it the carb can not work once the engine comes under any boost at all,

You definitely need a restrictor... tell her!

Edited by Turbo Tel on 16th Feb, 2009.

website:- http://www.terryhunt.co.uk

Well I trusted that venturi effect would work with overpressure same way than naturally aspirated. But now when I try to work it out in my head, makes reason that it does not work so well. With more progressive needle/jet combination it might work, but tuning might be nightmare. At this time feels stupid, not to realise this first place. Naturally SU cant measure air mass, with overpressure volume flow with WOT will be the same (well not really, but in small scale), but more air will travel trough it and it feeds same amount of fuel.

I went to garage today, looked and thinked... Realised that with my corrent setup there is no possible to put restrictor. As you can see, I dont really have any plenum, just straight pipe. That is now 40mm inside diameter, so I would need to make quite small restrictor, and even then working would be questonable. Problem is that I dont have space for real plenum, what would be necesary. So now situtation is that I will swap it to throttle body with injectors, time to stole some parts from my Mini, damn...

Edited by MikaJ on 16th Feb, 2009.

Would one of these fit?

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

www.twin-turbo.co.uk

then from scratch, I would try a pitot tube, that's waht you need

Ok, I don't know that do you fab have some experience about pitot tube for this kind of issue. But it sounded so crazy and in theory it mighted be working solution, that I decided to make one more try before swapping to EFI.

Here is setup I quickly made, Pitot pipe is just 6mm steelpipe with 4mm inside diameter, no fancy stuff.
http://mikaj.1g.fi/kuvat/Munga/17022009_00...pg/_smaller.jpg
http://mikaj.1g.fi/kuvat/Munga/17022009_00...pg/_smaller.jpg
http://mikaj.1g.fi/kuvat/Munga/17022009_00...pg/_smaller.jpg

I was amazed that it really worked, I drove small test and no more lean on top end with boost. My testing was not so long, now when it works clutch is slipping like hell. With quite much modified turbo needle it runs really lean, but with extreme rich needle it runs even litlle rich. So making new needle should work things out, hopefully.

So big thanks to you guys!

try to use this one but inclined 45° in airflow with no bends, not 90° , and try it with first needle,then start to enlarge tube ID, it should point you near balls, then if needed file a little more your needle.

Little update. There was some strange problem in first SU carburator itself in first test with Pitot tube. Also choke was leaking on boost, so I decided to remodify my new SU so it could be run with Pitot tube modification (basically back to original).

Now it runs great. Needle profile is basically quite close to original turbo Mini profile. So it seems like it is really good substition to orginal restrictor.

I dont get at all how this works.

Google here i come.

Witchcraft i tell the!!

When air flow impacts pitot tube, it generates more pressure inside pitot tube. So example when you have 1bar boost, inside pitot tube there is 1,2bar boost. Pitot tube is routed to SU fuel chamber and forces fuel out of fuel jet.

impeccable!

So where is the pitot positioned, & how is it connected to the float bowl ? Any pictures ?

WWW.TURBO-MINI.COM

Schrödinger's cat - so which one am I ???

Oh yeah ! Bugger. Lol. Must be half asleep. :)

WWW.TURBO-MINI.COM

i put mine in front of the intercooler ,so i got more pressure the higher the revs.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Hi Wil
to anwer your questions

1 totally standard metro turbo installation no mods

2 Will have to check the needle car at tuners

3 will check if restrictor is fitted

4 not sure how the float chamber valve should be fitted

5 I have been advised that the regulator and return are correct

6 not a series he said they seem to be a lot different that other turbo installations

any suggestions for the stem oil seals?
cheers
Roy