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Exhaust back pressure

Exhaust back pressure

Exhaust back pressure

I always thought I had a pretty good understanding of exhaust back pressure limits on diesel engine and the reasons why, until a recent dilemma came up.. For most manufacturers of high performance modern diesels, seems most spec the limit the maximum exhaust back pressure to about 3" hg.. Yes, some are a little less, and I have also seen some as high a 5" hg for certain applications ( fire truck applications) .........

I could only surmise that if these pressures are exceeded,  performance and engine longevity would or could be compromised....That seems totally plausible to me.....

But here is the dilemma... I found a  technical bulletin  specific to an engine I deal with quite a bit that listed a MINIMUM exhaust pressure limit, and now,  have come upon another manufacturer that also has a "minimum" exhaust pressure limit.................

Yes, we deal with very high performance diesels (most all at 60+ HP per liter, w/ some at close to 80 hp/liter).

Many exhaust systems we design, build, install,  and test in the field actually go "negative" at speeds of about 60-80 load because of a suction that must be being created at high speeds and the way the exhaust exit the system.. We use a differential pressure gauge to measure the pressures and are quite sure of our reading.......

I would like some input from some of the real engineering that read this forum... Please do make this assumption.. My reading are correct, the engines are loaded and applied per the manufacturers recommendations and history of these exhausts and engines have show no issues for 1000's of operating hours.........

My question is: Why would there be a limit or requirement as to minimum exhaust back pressure for a high performance turbo aftercooler 60+++ Hp/liter diesel?? My only assumption is that "they" are afraid of "over boost"........

Please help me to understand what I cannot seem to reckon with...........

Thanks, Tony


Tony Athens

RE: Exhaust back pressure

In my former capacity as chief engineer for DeAngelo Marine Exhaust, I asked my friends that very same question when I first ran into a minimum backpressure requirement.  The US applications engineers claimed not to know; the requirement was just passed down to them like The Eleventh Commandment from the mothership in Sweden.

I have another friend who is an engineer for a major turbo manufacturer.  He asked the question of their pointy- headed gurus.  The answer was some incomprehensible (to us pretty good engineers) techno- speak about keeping the turbo spooled up, or hot, or something.

[ I suspect that the requirement really came from someone who was intensely studying a problem, and got so involved in the details that they missed a major point.  One unrelated example of that is some sparkys insistence that putting a resistor in a stepping motor's supply increases the rate of current rise, which is not actually true, but appears to be so if you plot the two curves in normalized form, i.e. as a percentage of steady state current vs. time.  ]

Unfortunately, you have to meet the spec to pass a sea trial.  I'm sure you know a lot of ways to increase backpressure; in this case you get to do it intentionally.  Easiest way is to just restrict the water bypass until the bp goes up to spec.

Mike Halloran
Pembroke Pines, FL, USA

RE: Exhaust back pressure

Could it be a "sanity" check? To see if your gauge is actually working (assuming factory system). Is there anything related to that spec, like "replace muffler if..."? Do any seals in the turbo require exhaust pressure to keep long-life seal?

RE: Exhaust back pressure

With regard to backpressure "going negative" under certain operating conditions:
If that's what you're seeing,  I would take a VERY close look at your instrumentation, the location in the pipe where you are taking your measurements,  and the pressure tap itself.

Something's wrong.

BTW, heavy truck Diesel engine manufacturers are now "allowing" exhaust backpressures as high as 60 in. H2O,  and these limits are going as high as 100 in. H2O for the particulate-filter-equipped systems mandated for '07.
These are turbocharged engines that see on the order of 48 psi boost and produce up to 625 HP and  2000 lb-ft of torque.

RE: Exhaust back pressure


We test approx 50 engines a year, have been doing so for over 15 yrs, and we are quite confident in our methods and test results. We typically use a Magnehelic differential gauge backed up with a Fluke digital and measure in both water column and Hg.. Our test procedures allow use "to see" exactly what is going on ---

Typically our results are quite linear in that as the power in increased, the pressures goes up.. A typically system may show 2-5" H2O at idle, 18" at 1800 RPM/50% load and 27" H2O at WOT/100% load.. Our exhaust pressure limit is 41" H2O/3" hg... We plot and watch every RPM in 100RPM intervals along with taking boost readings, pyro temps, etc..................Our numbers are good.. We know exactly why we see negative pressures in some our designs as they are designed in a way that we do create a vacuum at higher vessel speeds......That is not I am questioning although you have indicated that this does makes sense--If you were on these tests and saw exactly how these systems are designed, I am certain your thoughts would be different....

 My question is why do some manufacturers have a minimum back pressure and state that a "negative back pressure" is not allowed........... My only thought was to prevent a turbo over-speed or excessive boost.. BTW, many of the engines we test pull 80+++" of Hg boost when pushed all the way to manufacturers specs.. Most average about 60" hg boost at WOT............

Tony Athens

RE: Exhaust back pressure

I'd like to try a couple of your exhausts on my bicycle! J/K!smile.

Specifying a minimum B/P is probably to protect the turbo from mechanical damage and stabilize temperatures. For better throttle response and improved emissions, most automotive turbos have been significantly downsized over the past 10 years. Because of this, engine makers must avoid over speeding a turbo. This can be accomplished on some engines within the intake system and some by the exhaust.

A company I work for has about 150 vehicles with turbocharged (meow) x116 enginessad. The tiniest leak in the engine intake system past the turbo can cause immediate and catastrophic failure of the turbo due to overspeeding. The plumbing must always be sound and the intercooler must be perodically vacuum tested for leakage.

RE: Exhaust back pressure

I still question your negative backpressures;  I've been testing backpressure far longer than you,  and on a great variety of gasoline and diesel engines,  and I've only seen "negative" backpressures in either (a) improperly-instrumented engines,  or (b) glossy advertisements for someone's high-dollar "Super Gee-Whiz Exhaust System."

Do you use a  pitot tube to measure total pressure,  or are you still using a static pressure tap in the sidewall of the tube?  I know that, by selecting a location near a bend,  I can get a negative static pressure reading in almost any exhaust system,  but I assume that's not what you're doing.

At any rate, I would assume that an engine manufacturer's recommendation to avoid negative backpressure is for the purpose of avoiding turbo surge.  We occasionally still experience that at very low engine speeds.

RE: Exhaust back pressure


Totally agree with you, negative backpressure sounds very dubious to me also!

Also agree that min backpressure is usually attributable to turbo life; either leaks past the oil seals, surge in a turbo without a ported shroud or wheel overspeed.


RE: Exhaust back pressure

On a boat, negative backpressures are possible, because of the reverse scoops that are often fitted to underwater exhausts.

Mike Halloran
Pembroke Pines, FL, USA

RE: Exhaust back pressure


Negative pressures are very possible and if I could figure out how to post a picture here of how the underwater part works, I'd do it--Maybe then, are non-believers,  could become "born again"..

A typical vessel that may see a negative pressure on will only go negative above certain speeds / and or under conditions-- We may see +10"  water at 5K's, +15" water at 9K's, 20" water at 12K's, back to + 10" water at 16K's, +4" water at 20K and MINUS 2" at 25K's..........

But again, my only real concern was why the factory specs were "no negative" allowed.....


Tony Athens

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