EFI University

[Guilt-Toy]

Just wondering what everyones opinion is on exhaust back pressure.

How much pressure is too much, say if you were running 22psi boost pressure in the intake, and you had 7psi pressure in exhaust....

any general rules here that you guys follow?

[Roberto Arano]

I suppose you mean 7 psi in the exhaust after the turbine.

You already know the answer I think, the less the better. If i had 7 psi I would definitley take measures to reduce it. 7 psi is a lot but i bet the car is quiet?

As well as power gains, I think you will notice an improvement in spool up.

You should also measure the EMAP pressure (in the exhaust manifold pre-turbine).

I actually sell an inexpensive fitting kit that allows temporary or permanent logging of exhaust pressure. PM me if interested.
_________________
www.circuitse7en.net
http://stores.ebay.com/Circuit-Se7en

[Guilt-Toy]

I am wondering if anyone checks the exhaust back pressure, anyone with any experience on what is considered excessive and what is OK ?

I think that anywhere from 0 - 5psi is OK, but getting up to 8 would be a bit on the high side.

Just wondering what other tuners thoughts are on this subject ? surely im not the only one to log the exhaust pressures on the dyno?

this is my own car not a customer car, and I am sure the back pressure pre turbine is going to be pretty high, the turbo is small and rear housing is small for good response (drift car)

[nfn15037]

I believe we are talking about EMAP, or pre turbine exhaust pressure. On smaller street turbos I've seen as much as 1.5:1 ratio between intake and exhaust manifold pressure (1.5x inlet manifold pressure in the exhaust mani). Measure the pressure in a Subaru uppipe on a WRX if you want to see a lot of EMAP. Obviously the EMAP/MAP ratio is hugely important to power production and generally speaking, the lower the EMAP the better.

Many quality ECUs have a strategy to take EMAP into account.

[stevieturbo]

[giskard]

On a 1.8L with VVT and small street turbo I got 9 psi of backpressure at 15 psi of boost and 265 whp on a dynapack with a (quiet) 2.5" exhaust.

Upgraded to an (also quiet) 3" exhaust, got 4 psi, and re-tuned (VVT, fuel, and spark), and surprisingly it only gained 2-3% above torque peak RPM. The VVT tho appeared to want less intake retard, (more ovverlap) which is in line with reduced turbine inlet pressure.

Turbine inlet pressure reduced by a little bit more than the reduction in exhaust backpressure.

[stevieturbo]

9psi after the turbo with such low power ?

Must have been a horrific exhaust ?
_________________
LS1, V7 YSi

9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

[giskard]

Custom exhaust, cat and 2 straight-through mufflers. At 220 whp it showed 4.5 psi.

Another car with similar power and a 2.5" mandrel bent exhaust showed 6.5 psi.

[Warpspeed]

Noise is probably the biggest problem with very low overall back pressure.
You cannot get both street legal and ultra low back pressure, just not possible.

So what is this?
The wife's everyday shopping car, or an off road unlimited racer ?

[stevieturbo]

[giskard]

Daily driver occasional backroads/track/autox.
It's a miata so 270 whp is pretty damn quick.

And I like quiet.

BTW adding a spring loaded flapper just before the tip, cut out of a Dynomax VT muffler, worked wonders for reducing thrum in low RPM about-town driving. This thrum only appeared with the 3" exhaust.

[Roberto Arano]

Yes, active flappers are the only way to get low backpressure AND low noise at idle and cruise etc. Unless you have a huge amount of room under the car for a lot of muffler volume. A long time ago I used to use some "heat riser" valve from an American car and use vacuum to move it. Now a days these can be bought off the shelf, no need to cut apart a muffler.
_________________
www.circuitse7en.net
http://stores.ebay.com/Circuit-Se7en

[giskard]

Interesting, could you provide a link?

[Warpspeed]

Flappers "flap" and can actually make the nature of the sound even more objectionable.
The huge pulsing in the exhaust can also cause exhaust restrictor valves to rattle, especially at cold idle.

But I totally agree, that a variable exhaust restriction is definitely the best solution, but it is not as simple to do as you might expect.
Quite a few high end sports cars have these variable exhaust restriction valves, so it is definitely possible to do.
This particular one is off an F355 Ferrari.


Something could probably be made at much lower cost from a secondhand aircraft wastegate, which is essentially the same thing, but these usually use an engine oil pressure driven hydraulic actuator, which is not very useful for us.

[stevieturbo]

They are little different to QTP's electric or pneumatic cutouts. Although they use them to run open pipes. You could just as easily adapt to restrict the exhaust.

But for most cars, acceptable silencing can be achieved with conventional straight through silencers without too much difficulty
_________________
LS1, V7 YSi

9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

[Warpspeed]

Straight through silencers mostly attenuate only the higher frequencies.
They sure kill the bark and crackle that you get from open exhaust ports.
They change the nature of the sound to make it far less objectionable.
But with large diameter pipe, they do nothing to reduce the heavy low frequency pulsing, and that will fail you on a formal EPA sound level test. And the constant droning can really be an annoyance over long trips.

The solution is to use big pipe, straight through mufflers and then have a variable area exhaust flow restrictor right at the end of the exhaust.
It only takes about half to one psi back pressure at small throttle openings to dramatically reduce noise. But that takes a surprisingly small hole to develop even that low back pressure at very small throttle.

The whole exhaust then acts like a big pressure storage tank, with a small constant flowing "leak" at the back to let out the exhaust. Very quiet at idle and small throttle.
Full throttle you just open the restrictor right up, and there will probably be enough natural pressure drop along the big pipe to still keep noise withing reason.

It is very effective, it is how Ferrari can make a several hundred horsepower n/a engine very quiet at normal legal road speeds, and very easily meet all the international EPA sound level limits.
But the power is still there when you open it right up.

[stevieturbo]

I'd have to say that as much as the Ferarri idea is nice. Ive heard of a few getting thrown off track days due to excessive noise.

So even totally standard they are far from quiet. Ive also heard a few Maserati's locally, and they too are pretty damn loud for a factory car.

But adding a simple restriction to the main exhaust should be pretty easy.

This sort of thing. Even handier that it is boost operated, although there are the likes of QTP's electric versions too.
Just add this inline and set the closed position as partially open

http://www.youtube.com/watch?v=C556EHYUijA
_________________
LS1, V7 YSi

9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

[Warpspeed]

[stevieturbo]

Nope, standard cars with standard exhausts.

Not all noise tests are static. Often meters are placed trackside so the car needs to be quiet at WOT as well as when slow/stationary.

Most of the flap style systems are to reduce noise at low loads and allow it at higher loads, so some cars do still fall foul of noise regs.
_________________
LS1, V7 YSi

9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

[underpsi68]

To the original op, anything less than 1:1 bp is considered good. On my setup I am about 2:1. At 15psi I have 31-32lbs bp. The turbo housing is causing my issues. I put down 787rwhp at 15psi through a non locking AOD. I would love to lower the bp but I would have to switch turbos which I really don't feel like doing. I'm going to try to get 900rwhp on my setup with more boost and a larger dp.