Turbo Back pressure?

[jasonsansfleece]

Is there an ideal back pressure for any given turbo application?
I have head it said that some is desirable. Why might that be?
Did I see a post where there was a boost gauge mounted just before the turbo so a comparison could be made, pre and post combustion, and conclusions made?

I have an eco k14 and a aaz header, and the header is restrictive. I also have a straight through 2 1/4" exhaust pipe.
I plan to replace the 2 1/4" with 3". I think thats a good choice regardless of header size.

But as far as replacing the header, I am not sure that bigger is better but I dont know why.

[theman53]

I think it is Exhaust Manifold Pressure. 53willy's did it on his. I think the desired ratio is 1:1

Bigger the ex. mani the longer the spool. You will have the potential for more flow with a bigger ex mani, but the more volume the longer the spool.

I don't think backpressure is desirable, but more of what happens when the turbo is in the way.

[AudiVWguy]

Is the down pipe 2 1/4"?
I'm running AAZ manifold with K-14 (not eco) exhaust side ported to increase flow and a 3" down pipe to 3" exhaust. The larger down pipe really helps lower the back pressure and results in lower EGT's. Hyway speeds of 70 mph is 6 to 9 lbs of boost with an EGT of 540 to 600 F. I happy with the way it's working for me.
Cheers,
-JB

[rabbitman]

I'm not an expert but it seems to me like 1:1 is the worst it should ever be in a steady cruise, beyond that is terribly inefficient.

I think that if you can get 20lbs of boost with 10lbs of EMP then good deal. I don't think it's possible though.

The ratio probably changes slightly depending on whether you have a intercooler or how fast you mash the go pedal.

[truckinwagen]

most often there will be more pressure in the exhaust manifold than in the intake.

usually 1:1 is what people strive for, and what compound turbo setups(when high boost numbers are gone for) do very well at.

-Owen

[jasonsansfleece]

Exhaust manifold pressure.
So having a higher boost than EMP is the desired effect, and that would indicate that energy was being disipated in the cylinder rather than just flowing through the engine?
So for any given hard ware ie. manifolds, turbo, header, port work, the boost/emp ratio is not easily adjusted. Fueling and timing being the variables? Would that be right? The ratio is more a reflection on the efficiency of the hardware? (Thinking out loud)

I am still interested in your thoughts on header back pressure. Some/none and why.

Audivwguy, Its a aaz header, or so I was told. It starts at 2 1/4" but reduces to under 2". Smaller k14, Giles pump and a fmic. I get high egts, I could go above 1400* Smoke is grey.

[theman53]

I think the ratio is dependant on go pedal and the turbo / turbine wheel in the exhaust side. I don't know but I would guess in higher boost range the K14 would have more than the K24.

Header back pressure or Down Pipe back pressure? IMHO you want the header/exhaust manifold tubing the same size roughly as the ex ports and the runners as short as possible to get to the turbo. This will give you the quickest boost and most responsive. The downpipe on the other hand is after the turbo, so as big as you can get will help spool and flow. Bigger downpipe less restriction fighting against the turbo.

[blackbird82]

the best thinking for exhaust systems and turbos is like hydraulics.
think of the turbo as a orifice, fixed.
there will always be a limit based on turbine size.
but yes, after the turbo NO! pressure is desired, just a pipe long enough to keep heat after shut down so cold air does not get to your turbo toO soon after shut down!
in high horsepower applications on John Deere's the variable geometry turbo will bypass the turbine, to reduce EMP and let the engine work easier. all while maintaining boost.
Its a sick trick system. can't make it work on VW though, I would do it.

[jasonsansfleece]

So the turbo exhaust outlet is 2 1/4"
If I could install a 3"(largest pratical size) header this would be optimal?
Gasses leaving a 2 1/4" apperture and expanding into a 3" tube.
Least pratical back pressure.

[R.O.R-2.0]

if you want more less back pressure, get a bigger turbo..

oh yea, if someone figures out a way to make drive pressure to boost ratio to a perfect 1:1, let me know..

the most efficient turbo setups out there dont even get a perfect 1:1..

[jasonsansfleece]

Read some more about back pressure on that forum where their turbos are controled by an ECU.
The only reason given for any back pressure was the sugestion that it might help a small turbo from spiking.

[mystery3]

the whole back pressure thing is a myth promulgated by old muscle car guys as far as I can tell, you want ideal exhaust velocity, if the exhaust is too big for a given application you concentrate the heat and too much heat is bad for your motor.