PSI and CFM from Exhaust of Engine

In a properly designed exhaust system the back pressure will vary depending on where you take the reading. About .5 to 3 PSI is a normal average. At 2,500 RPM it's not much.

The volumetric output of the engine is displacement x rpm. Give or take a little because it is momentarily expanded by heat. A 350 cu in Chevy engine will produce roughly 350 cu in of exhaust flow per revolution. Consider it a positive displacement air pump.

I've visited test facilities (in Tony Stewart's home town) and seen the headers glow cherry red wherever there is a bend in the pipe. That's back pressure.

Formula displacement x rpm is right for a 2-stroke. For a 4-stroke it's half that.

Cheers..........Codey

Good catch Codey.

You are right, of course. Good catch. Gettin' sloppy in my old age.

Cheers.

Isn't it a quarter of the rpm for a 4-stroke? - ie exhaust at every fourth stroke.

In a 4-stroke engine, there are two strokes per revolution. Intake and compression in one revolution. Then ignition and exhaust in the next revolution. That gives you the exhaust stroke (and other strokes) every other revolution. Now I can't get that song out of my head.

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No, each stroke takes half a revolution, not a quarter. So a 2-stroke takes 1 rev to do a full cycle, a 4-stroke takes 2 revs. A 4-stroke intakes/exhausts the engine swept volume (= engine capacity) every 2 revs.

Cheers........Codey

Well said LYN, except that you dont mention inch to feet conversion, as in cFm( maybe the OP dont know the difference) and cherry red headers from back pressure? Surely its the combustion heat reddening the headers? I remember in MY younger days in the army, we kept our coffee bottle on the headers of a Bedford truck, and at night, at speed, on a long drive, these headers glowed cherry red until the engine slowed down again.

The engines I observed on the test stands had exhaust pipes with bends in several places along their path. Only the bends were glowing. Straight sections of pipe were a normal color.

These were Ford SHO engines running at wide open throttle.

This facility had a two mile test track. When they took vehicles out for testing, their warm-up procedure was two laps around the track in low gear at WOT.

Lyn, I think what you were seeing was where hot exhaust, being forced to turn a corner became turbulent and because of the turbulence dumped more of it's heat into the walls of the manifold in the turn. This is a lesson I learned when working with heat sinks for electronics. Laminar airflow tends to form a "skin" of relatively stagnant air along the surface of the heat sink (in this case the wall of the exhaust manifold.) which acts as an insulation blanket. But when the laminar airflow is disrupted (by a turn in the pipe or a serrated edge on a heat sink fin) that blanket breaks down and allows the hotter air to impinge on the pipe wall, spot heating that part of the pipe.

Of course. No argument there. But, does not turbulence equal back pressure in this case?

Eh, yeah, but it isn't backpressure itself doing it, the turbulence CAUSES backpressure but it is more of a symptom in this case.

Don't forget throttle position......

Re posts 11 & 12

Obvious - just having a senior moment.

Thanks Codey & Coachdurbin.