Exhaust Gas Back Pressure
Turbo Mini Forums

This subject came up last week whilst discussing Rimflow Valves.

I was under the impression that it would be possible to achieve maximum boost without EGBP rising above boost pressure. Apparantly this is not the case and this has led me to consider the science in a bit more detail.

It started the other week when i was trying to read some turbine maps:



Now the energy in the exhaust gas is nearly three times that at the inlet, based on 850 deg C temperature.

If the turbine and compressor efficiencies are both 70%, you have an overall efficiency of 50%. Hence you should only need twice the energy in the exhaust gas to spin up the turbo, without the EGBP exceeding boost.

So why is this not the case?

My theory is that the A/Rs in use are a bit on the low side so that the boost comes in early.

The flow through the turbine nozzle is limited by the speed of sound. Once it reaches Mach it cannot go any faster and this will cause the EGBP to rise without any further benefit to the engine.

So, I was thinking that maybe we should fit a second wastegate, activated by EGBP that will prevent it going too high.

Any thoughts????

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."

This is exactly the thinking behind the VNT25 I am fitting on the 7 porter,

I am deliberatly using an exhaust turbine that is oversize to minimise choking at the topend and getting back the low end boost with the variable geometry exhaust.

Thats the Idea anyway

also I'm working on the assuption that (on a conventional turbo) at full chat the wastegate is on the verge of opening then this will be transfering the most torque/power to the compressor if that makes sense....

or to put it another way a smaller turbine bleeding 50% of the exhaust flow off will not support as much compressor power as a larger turbine bleeding 10% of the exhaust flow.

Edited by Joe C on 19th Nov, 2007.

This was one of pete/mini1071/magazines favorite subjects. iirc, he reckoned he achieved it with a T3 using a fancy head and manifold.

I think F1 cars of the '80 were suposededly achieving it too.

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

I can remember him talking about this, I found it very interesting and asked how one would go about achieving it, never heard anything back.

It is mentioned in corky bells book too, doesent give any details though,

sounds a bit perpetual motion to me.

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

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."

yes i agree,, i have one in the post as we speak !!

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

Perhaps just aim for larger A/R's, and larger compressors.

Its surprising how well some large compressors spool.

Either way, its always a compromise one way or another. if you can get VNT to work, then it is probably the best compromise.

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

really hate the use of acronyms. The standard method of use is to use the real words first then the acronym later. Really helps learners like myself.
But I really love turbos, especially now I know where the pipes go on the SU carb.

or use google/search finction/ or jsut ask to find out what the acronyms mean, no-one will bite! and theres certainly no shame in asking. in paulS's post above for example he would have to type "exhaust gas back pressure" three times in full and "Variable Nozzle Turbine" out twice. when typing a post this saves alot of time and meens the discusstion can progress at a faster rate. aside form the fact typing it out in full is jsut annoying. :)

Edited by evolotion on 9th Dec, 2007.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

damn you beat me to it

Robert, what turbo are you going for now? and can you let me know before the masses so I can buy up some before the eBay prices go the way of the GT17.

Edited by Rob H on 10th Dec, 2007.

Ignore me!

Edited by Bat on 10th Dec, 2007.

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My Mini build diary

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

I apologise if my use of acronyms has caused grief.

Have a read of this and it will all become clear as mud.

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."

Back on topic, As no one seems certain as to the relationship between Boost pressure and Exhaust back pressure I thought of a simple way to find out.
I have just aquired the guage in the attached picture. It is a 0-25 psi dual needle jobbie.
If the inputs are boost and egbp it will be a doddle to see quickly and easily the relationship between the two.
The guage may be a little over damped but I cant be sure until it is tried.
I don't have a turbo motor running at present so I am offering a loan of the guage to anyone who would care to try it. All I want in return is some decent feedback of the results.
Anyone up for it in a reaonable time scale
Dave

Attachments:

26.15kb

cool gauge!

local milatary breakers?

it's a fuel pressure guage from a twin engined Dakota.
Dave

Edited by danboy on 29th Dec, 2007.

That's why staged turbo are used (Garrett). To avoid back pressure and to not loose the boost.
That's also the reason why race cars often use external wastegates. They flow better and more. And the exhaust can be done as required and not as Garrett decided for you.

However I'm still convinced we have a lot to gain by optimizing the intake. If our NA engine has a 70% VE then at 300mbar boost we'll still be trying to get it work as a modern ~100% VE engine. Going MPI should help to increase the VE, I'll measure how much when I'll finish mine.

BTW, GT15 exists now in the VNT version. It's used on recent Renault 1.9 engines.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

How will MPI improve VE? VE is really how much air you manage to pump relative to the engine displacement. Can't honestly see the fuel injector phasing making much difference in that.

1275 with Back Door Turbo

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm