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Tool Talk Discussion Forum

Automobile Exhaust Pressure

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Glenn F.

03-26-2008 07:02:47




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I was doing soem exhaust work on my Chevy Astro van yesterday. I got to wondering about something....Who many pounds of exhaust pressure does the typical autobobile generate? I'm sure it increases with RPM's and other factors.

Anyone know?


Glenn F.




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buickanddeere

03-27-2008 11:41:15




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:02:47  
Now as long as somebody doesn't try to tell us and engine "needs" back pressure.



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guido

03-26-2008 15:14:13




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:02:47  
Glenn F.
I can tell you how much is to much for a diesel engine.Tipical 900 cu.in. engine has the exhaust back pressure as follows:
1200 RPm'S 1.8 In of mercury column full load.
1200 RPM'S 1.2 in. of mercury column no load.
1800 RPM'S 4.0 in of mercury column full load.
1800 RPM'S 2.8 in. of mercury column no load.
2300 RPM's 5.5 in. of mercury column full load.
2300 RPM'S 4.4 in. of mercury column no load.
All of these figures is the max under the rpm's indicated.
1 in. of mercury column equals 2 PSI. At sea level a mercury column will be 29.92 in.
At sea level the pressure is 14.7 psi.
None of this figures are exact for every engine, but it should give you some idea how much is to much Rounding off both values at 15 and 30, you get a 2 to 1 ratio.
Guido.

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coloken

03-26-2008 17:26:33




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 Re: Automobile Exhaust Pressure in reply to guido, 03-26-2008 15:14:13  
Guido, Will you run that through again..
Isn't it 2 inches equal 1lb.? Pickey, picky.. "1 in. of mercury column equals 2 PSI.
At sea level a mercury column will be 29.92 in.
At sea level the pressure is 14.7 psi."



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guido

03-27-2008 06:04:02




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 Re: Automobile Exhaust Pressure in reply to coloken, 03-26-2008 17:26:33  
Hello Coloken.
My fingers got tongue twisted or something.
1 PSI EQUALS 2 IN HG. THAT IS CORRECT.
THANKS FOR THE CORRECTION! THAT IS WHAT THE BOARD IS ALL ABOUT.
GUIDO.



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Revredneck

03-26-2008 07:08:15




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:02:47  
Glenn, When I was working in a GM shop, we used a back pressure gage to check for plugged catalytic converters. The rule of thumb was that anything less than 1 psi was acceptable, over 1 psi indicated trouble. Naturally, the lower the pressure, the better. Remember, an exhaust system is an open system, and you have to have a restriction to build pressure. HTH. Good Luck and God Bless. Tommy

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Glenn F.

03-26-2008 07:49:12




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 Re: Automobile Exhaust Pressure in reply to Revredneck, 03-26-2008 07:08:15  
Maybe it's a dumb question, but how much pressure would an engine build up if you blocked the tailpipe with your hand?

Glenn



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Rob in Indiana

03-26-2008 14:38:48




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:49:12  
enough the IH used a valve on the farmall A tractor exh manifold to route exh gases into a large cylinder and used it to life implements.... plow, culivator, ect.



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Bob

03-26-2008 08:22:25




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:49:12  
I don't have an exact figure, but likely not as much as you would think.


If the cylinders have to exhaust against too much backpressure, they will not empty, and air/fuel will no longer be drawn in when the intake valves open, and the engine will stall. That is why and engine with a clogged "CAT" will stall when revved up, but will NOT blow the converter or piping apart.

An AIR COMPRESSOR, on the other hand, is designed with very little space between the piston and the head (high compression ratio), and is much more efficient at pumping air.

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Glenn F.

03-26-2008 07:04:21




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:02:47  
Who many pounds...?

Make that, How many pounds?



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RusselAZ

03-26-2008 20:54:02




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 Re: Automobile Exhaust Pressure in reply to Glenn F., 03-26-2008 07:04:21  
If you could hold your hand over it till the engine died the reading in pounds would be just a bit more than a cranking compression test reading.

Hint, you will not get the power stroke pressure.



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