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Turbocharger Exhaust Back-Pressure Values (What is a good # to see?)

Boost Control

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Hey everyone,

So I am going to jump right into it and lay the groundwork here so my question above makes sense. I'll do my best to try and keep it short and get to the point of the question being asked.

I have a late model 1.5L turbo four-cylinder, factory turbo equipped, and GDI. I have outfitted the car with 8x AEM auxiliary sensors.

1x each (pressure and temp sensors)

- pre compressor cover (inlet pipe)

- post compressor cover (hot pipe pre intercooler)

- pre turbine (technically in the housing before the scroll, turbo mounts directly to cylinder head so no ex-mani)

- post turbine (in the downpipe exit)

These sensors exist so that I can measure the pressure at all these areas with an emphasis on exhaust back-pressure.

With me so far? cool

The car has been fitted with both OEM turbo and 2x aftermarket turbos at various points in time. The issue is one of the aftermarket turbos had a turbine wheel fracture and some pretty bad thrust bearing wear indicative of too much back pressure or to much axial play that looks to have squeezed the oil out between the bearings and caused a failure. Turbo experts have told me that it was too much back pressure that caused the issue.

So now to my question above, how much back pressure is too much and what value should it be? I have the sensors to read said values but now I need to put some quantitive data to the numbers. I know zero back pressure is goal but not achievable in this sense of a streetcar. I have logs and data already showing some #'s but the OEM turbo values look similar to that of the aftermarket turbo. So again when they tell me that it was "too much backpressure" that cause the issue well then what (#) is too much? 0psi, 1, 5, 10?

I hope this makes sense and look forward to some input.

With most road car-sized turbos, EMAP will be 1.5-2.5 X MAP. With a small modern GDI engine designed for economy I would expect yours to probably be at the upper end of that range.

NOTE, for discussion!

As Adam said, there may be roughly a 2:1* for the exhaust manifold/turbine inlet to impellor outlet pressures, but I assume you're enquiring about outright exhaust manifold pressure?

You don't give a pressure or ex temp'figure, but a correctly selected and installed diesel race turbine could see 200+ PSI which is rather more than you might be expected to see. I would suggest contacting your turbo-charger manufacturer and see what they suggest - different designs of thrust bearings can handle different amounts of thrust resulting from the turbine forces.

Something else that may have an affect is the oil flow through the bearings, as this is important for removing heat from the bearing - especially if it isn't a water cooled design. If you have an OEM restriction and a non-OEM turbo-charger it may not have the required flow volume required and/or may be partially blocked.

The pressure ratio will depend on several factors, and will be different under different loads and rpm. You may be willing to sacrifice the top end flow loss cause by a restrictive turbine housing for the improved low rpm boost and response, or trade of a loss of low end and response for a better top end. As a general guide, you may be best around 1.5:1 in the normal driving range, with 1:1 being about the best? Depending on what you have, a less restrictive (larger) exhaust turbine housing may be expected to increase power and drop exhaust manifold pressure and temperature.

You mentioned your thought that "zero" [gauge?] would be best, but the turbine needs a pressure drop across it to function, so exhaust manifold will always be at least atmospheric and much higher when working.

@Adam@Link_ECU so to make sure I am understanding this correctly. If my target boost was let's say 25psi, then I would expect to see 37.5 to 62.5 psi of pressure in my turbine housing as measured by the EBP sensor in the scroll there. In the small GDI engine as stated likely closer to 60psi or so?

I hope you can see the graph,

So then in this example at 5651 rpm at full bost of ~21.5psi as measured pre intercooler, I see 41psi of EBP in the turbine scroll, which would be roughly 2xMAP, so that is "normal"/good value? Not too much?

Attached Files

MAP = Manifold absolute pressure, and EMAP = Exhaust Manifold Absolute Pressure.

Your log appears to be showing gauge pressure, so assuming you are near sea level, if you add atmosphere onto those values you have a ratio of more like 1.5 which is pretty good. But what you normally see on road cars is the EMAP will "skyrocket" at higher RPM, im not sure your boost and EMAP drop so much after 5600RPM.

Back Pressure blowing the gate open?

I asked a similar question. Perhaps they will answer me so as not to create a similar topic. I am interested in a target exMAP for high performance gasoline engines with a mid-sized aftermarket turbine.

Adam, could you answer?

I am right at 2000ft above sea level.

That big drop is mostly from very small turbo, it basically just really hot air after 5500 rpm.

So it looks like my numbers are "normal" which now has me trying to figure out why I was told too much backpressure.

Back from the dead, So would this present normal?

Attached Files

I'm FAR from an "expert", but that's about what I'd expect from something aimed for mid to upper mid boost, where a little restriction at the top end is traded off for coming onto boost a bit lower in the rpm range.


A sample graph to look at, showing RPM, boost in KPA, and pre and post-turbine pressure in psig


Your EMAP to MAP ratio is less than many modern OEM setups and the raw pressure isn't high either. I can't imagine those numbers being of any concern since I've seen 4:1 ratios not cause issues, 120 psi raw EMAP, for example.

I'm not clear what the failure was from the info provided, but more info might help us suggest other areas to evaluate.

As a general note, common causes of turbo bearing failure are: lack of maintenance (oil changes), oil type, high oil temperature, insufficient OR excess turbo oil pressure or flow, excessive crankcase pressure preventing drainback, or the turbo was simply over-sped.

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