MoTeC Global Forum

Hi mates,

What back pressure figures are considered optimal for a turbo / NA engines?

Obviously less is better, but there will always will be some present, so
I would like to know what is considered acceptable. Logically it
will be an IN/EX ratio. Working on the car that fails to make estimated
power and suspecting that turbo exhaust housing is limiting us.

Thanks in advance for your input.

Alex

Hi Alex,
I'm definitely no expert on the matter but I'll tell you some things I have seen. Firstly, many people always refer to having exhaust pressure (before the turbo) close to equal with manifold pressure and say that this is optimal, I supose it sounds logical. BUT, talk to some of the turbo people and it all becomes a big "what do you want to do" question.

A small "restirctive" exhaust housing will mean that the boost can be build really quickly and you simply run out of ability to flow at high engine RPM, this may actually be quite fine for a daily driver. I got sick of the shotgun like response of my turbo so have gone to a much smaller exhaust housing. My car is daily driven so lag was a pain in the butt when you got caught off guard. With the smaller turbo I understand that it may be a problem up high but I have a close ratio 6 speed now instead of the standard ration 5 speed so it should work out ok.

another example is a drag bike I saw where the exhaust pressure was actally lower than the boost pressue. This engine had a really big exhaust housing that was not much of a restriction. Obviously getting the thing on boost is a fairly major task but that is what launch control is like.

So, what do you want to do?

Mark is exactly right, there is no right answer.

That being said, less backpressure will almost always make more power. How much more can be affected by the cams quite a bit. An aggressive camshaft with a lot of overlap will not do well with excessive exhaust backpressure.

We do a lot of turbocharged drag race stuff and on the big power applications, we try to keep EMAP below MAP at full boost. On my 1.9L 4 cylinder race car (Honda), i'm at around 380kpa EMAP and 400kpa MAP. It takes a good turbo setup and a big turbine to do it, but when setup properly, it will spool much better than you would expect it to.

If you don't have EMAP logged, a good sign that you have 'too much' is when you run the boost controller in duty cycle mode with a fixed duty during a pull and the boost tapers off as rpm increases. As long as you know it's not the compressor running out of breath, it's a result of backpressure creeping up with RPM. Since the pressure to the top and bottom of the gate is fixed, the exhaust backpressure starts to push the valve open with the increase in backpressure.

Probably a good bit of advice is if you suspect backpressure is a problem investigate it....I would use a sensor and log it.

And another small idea, try to test it guilotine exhaust valve to see what is the best backpressure. We did try to make our self a throtle body on the exhaust and controlled from the cabin, manually. Faster way to find out problems! Cheers

Hi,

I plan to log my exhaust pressure before the turbo.
Now how do we go about doing this?

Do I use a normal MAP sensor (what ever rating is required) and run some long copper tube in a coil to remove the heat from the gas then connect it to the MAP sensor via a rubber hose?

Or is there a specific EMAP sensor available?

For help with picking an EMap sensor look here http://www.motec.com.au/forum/viewtopic.php?f=23&t=790

For me MAP/EMAP is a measure of the over all turbo system efficiency. The ultimate game is to achieve your target MAP levels at all of the desired specific points but at the lowest possible cost in terms of EMAP, I find it helps to think of EMap in the same way as you would a supercharger drive belt.

Not all turbo's are equal in this department at all! and studying the MAP/EMAP relationship is a really good way of comparing different turbo units for your application. It can also highlight other efficiency problems in your systems, for instance I quite recently saw a significant improvement in MAP/EMAP delta on a high output turbo car by simply enlarging the radius of one of the bends in the intercooler pipework.

Alex B wrote:Hi mates,

What back pressure figures are considered optimal for a turbo / NA engines? I try to Visit this site to learn more about back pain.

Obviously less is better, but there will always will be some present, so
I would like to know what is considered acceptable. Logically it
will be an IN/EX ratio. Working on the car that fails to make estimated
power and suspecting that turbo exhaust housing is limiting us. Y

Thanks in advance for your input.

Alex

Hi Alex,

The optimal back pressure for a turbo/NA engine will vary based on the specific engine, as well as its design and tuning goals. Generally, it is best to keep the back pressure as low as possible, as it can lead to increased exhaust temperature, higher fuel consumption, and decreased power output. In terms of an IN/EX ratio, a good starting point would be to aim for an exhaust back pressure that is about 20-30% of the intake pressure.

When it comes to turbo exhaust housing, it is important to make sure that it is properly sized for the engine in order to ensure optimal exhaust flow. If the exhaust housing is too small, it can lead to increased back pressure and decreased performance. If the housing is too large, it can lead to decreased exhaust velocity, which can also reduce performance.

Finally, it is important to note that back pressure is only one factor in engine performance, so it is important to consider all aspects of engine tuning when attempting to maximize output.

Hope this helps!