exhaust back pressure in header

[gnpoweredrail]

Has anyone ever measured their back pressure before the turbo?
I'm wondering what I am at with my setup.
.030 over, large roller cam, ported irons and intake with a Turbonetics 70 running 25-28 psi of boost pressure. and a 3"x16" long exhaust pipe

 

[turbobitt]

This is not uncommon. I data log mine but would be little use for you since my combo is completely different. I see about 15 psi over 30 psi boost.
AG

 

[bison]

Yes and there's a lot of variables that can effect it

 

[Duke]

so what is the optimum back pressure for your car Bison? Turbobitt are you there with 45#s ?

 

[bison]

The lowest back pressure you can achieve while still being able to keep the turbine spooled enough to support the mass flow needs of the engine is what's optomal. Everything's a compromise. On a closed system mine is over 2x manifold pressure with a 68mm turbine trapping around 140mph.

 

[gnpoweredrail]

Im trying to be under 50psi in the header crossover pipe as I need to install a bellows and that's what they are rated for

 

[bison]

I wouldn't worry about that. It's not going to be engineered to fail at 50psi. Depending on cam and rpm you will be over 50psi with a closed system with your stuff.

 

[gnpoweredrail]

you say don't worry about it but then say I will be over 50psi????

 

[bison]

you say don't worry about it but then say I will be over 50psi????

It's a non critical component. If it fails it's not dangerous and it won't damage the engine. If you want to avoid ever having it fail you should fabricate something else.

 

[turbobitt]

so what is the optimum back pressure for your car Bison? Turbobitt are you there with 45#s ?

At 30 psi boost I'm about 45 psi. Keep in mind you should compare compressor discharge pressure pre-inter cooler to get the most accurate results.
AG

 

[gnpoweredrail]

^^^ HUH
Im talking exhaust back pressure not boost pressure.

 

[gnpoweredrail]

1

 

[gnpoweredrail]

It's a non critical component. If it fails it's not dangerous and it won't damage the engine. If you want to avoid ever having it fail you should fabricate something else.

Yeah but the fab piece is $200...something I don't really want to ruin

 

[earlbrown]

^^^ HUH
Im talking exhaust back pressure not boost pressure.

They are a function of each other. Exhaust pressure powers the turbo. The more boost you want to make, the more 'power' you have to put into it.

The more efficient the turbo is, the less exhaust pressure it takes to get the desired result.

 

[turbobitt]

^^^ HUH
Im talking exhaust back pressure not boost pressure.

I know you were talking about back pressure but usually need a reference such as hot boost. Saying 50 psi back pressure doesn't tell you much unless you know compressor discharge pressure to get a sense for the ratio.
AG

 

[gnpoweredrail]

oh ok.

 

[gnpoweredrail]

so there is really no equation to figure it out. have to actually install gauges and that's what worries me since if its too much then the bellows wont work.

 

[bison]

so there is really no equation to figure it out. have to actually install gauges and that's what worries me since if its too much then the bellows wont work.

Theres no equation. But at 80lbs/min im over 60psi with a 3 bolt ex housing on a closed system. There are a lot of things that could make it different though

 

[gnpoweredrail]

closed system?? as in full exhaust??
I have a 3 bolt .82 housing

 

[bison]

They are a function of each other. Exhaust pressure powers the turbo. The more boost you want to make, the more 'power' you have to put into it.

The more efficient the turbo is, the less exhaust pressure it takes to get the desired result.

The power required to drive it doesnt have to be from increased ex pressure though. If it was relying on nothing but ex pressure to increase mass flow then the engines would all crap out around the same time due to back pressure. With the correct combo of wheels and some other smarts the backpressure can be dropped while still driving the turbine enough to move enough air. The compressor wheel and diffuser can cause back pressure even though most would not realize it.