Manifold Air Pressure Gauge

J

Jake8078

New Member
A friend of mine asked me why the MAP measures air from the intake manifold, and not the exhaust manifold. It makes sense to me, intuitively, in that you use it to directly measure a control input, and that, obviously, what's in the intake manifold is what's going directly into the engine.

Yet, I can't seem to answer the question as to why measuring it in the exhaust manifold would be a bad idea? (Fuel injected, naturally aspirated reciprocating engine). Any input?
 
Why the need to re engineer the MP gauge? It seems to work just fine.

I'd suspect the biggest problem is each gauge would have to be tuned/calibrated for each exhaust system.
 
I would say the pressure would be totally different. No vacuum in the exhaust manifold, there's more pressure there than the ambient right? Therefore the manifold vacuum gauge wouldn't work right.

Air into the engine is a good measure of power, but exhaust pressure... :dunno:
 
Jake8078 said:
A friend of mine asked me why the MAP measures air from the intake manifold, and not the exhaust manifold. It makes sense to me, intuitively, in that you use it to directly measure a control input, and that, obviously, what's in the intake manifold is what's going directly into the engine.

Yet, I can't seem to answer the question as to why measuring it in the exhaust manifold would be a bad idea? (Fuel injected, naturally aspirated reciprocating engine). Any input?
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Whats the use. Usually it's only doe to diagnose a problem in the exhaust, like it being plugged up. It would not be as accurate as an intake reading, seeing as how the engine is an air pump, you want to know how well it is sucking. It is also eaiser to diagnose an engine problem with manifold pressure. Things like a leaking intake valve will show as a spike, and flat cams show as a low vacume. Pressure in the exhaust is also directly related to the mixture of things going into the engine. And you may plug the oriface that measures with carbon/lead deposits.

Bottom line, if it ain't broke, don't fix it. and the MAP gauge works fine.
 
thanks guys, what a great wealth of knowledge you are! I'm working on my COMM and CFI simultaneously, and it's good to know I've got you all as a resource.
 
Pressure in the exhaust system is not a good thing. It will rob the engine of power. If you had a exhaust pressure gauge and put your hand over the exhaust pipe, the gauge would show an increase in pressure. The engine would actually slow down due to back pressure in the system, so this would not be an indication of power output.
 
Green Banana said:
Pressure in the exhaust system is not a good thing. It will rob the engine of power. If you had a exhaust pressure gauge and put your hand over the exhaust pipe, the gauge would show an increase in pressure. The engine would actually slow down due to back pressure in the system, so this would not be an indication of power output.
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And with that, we say welcome to JC!
 
tgrayson said:
Doesn't really matter. The gauge would just have to be calibrated differently. The airflow into the engine must equal the outflow, otherwise there would be matter disappearing.
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You can change the exhaust pressure by adjusting the mixture, so I think that would create problems making a simple probe and gauge setup.
 
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