As title, I take it there is no measurable pressure in an exhaust manifold on a closed throttle? As there next to no exhaust flow, of course.

I guess there is more pressure (but not much, maybe not measurable) in the post-compressor inlet side than in the pre-turbo exhaust on a closed throttle?

Just trying to work out how EGR Anti-Lag systems work/worked on WRC cars and some of the old F1 Turbo engines. Basically they used a EGR valve to supply air direct from the compressor outlet to turbine inlet, but for that to work the inlet pressure would have to be more than the exhaust, or the thing would flow backwards, messing things up nicely!

 

Some comments:

1) Dry air has a density of 13.076307480840681 cubic feet per pound, at sea level

2) An average sized automobile engine will idle at 750 rpm and will consume between 16 cfm to 20 cfm of air when idling. This is WITH A "CLOSED THROTTLE" which is really open enough to allow the engine to run and consume the 16-20 cfm mentioned above.

If we reference Comment "1" it said One Lb of Dry Air is equal to about 13 cfm at sea level. So our engine, when idling is consuming about 1.5 lbs/minute of air at idle.

In a previous post in this forum we showed that One Lb of Air will make 10 hp.

1.5 lbs of air therefore should make 15 hp.

That means that when the engine is idling the engine is consuming 15 hp worth of mass flow. With 15 hp of mass flow, if you have an exhaust system there will be some exhaust pressure in the system.

When I was much younger used to love being under a Race Car on the ramps of a trailer and putting my hand over the header collector. Man, that big camshaft at idle would make the exhaust pound on my hand and push it away from the collector flange.

There is a lot of "pulse energy" at the open collector of a race car.

Now if you take that same engine and put a full exhaust on it, the back pressure in the exhaust pretty much kills the "pulse energy' and the exhaust seems pretty weak when you put your hand over the pipe. Still, you cannot close off the pipe with your hand, it will blow your hand off the pipe.

So there is actually a lot of pressure in an exhaust manifold and enough flow for a smaller turbo to spin along happily at about 5K-10K shaft rpm.

I have measured a LOT of turbos with an accurate speed sensor that measures the rpm of the blades, therefore I would disagree with the comment:

"no measurable pressure in an exhaust manifold on a closed throttle? As there next to no exhaust flow, of course."

Tom Vaught

Many engines today can cruise down the road at 60+ mph at 1500- 1700 rpm. That is only double the idle rpm in the example above, about 30 hp. Most times the throttle is not depressed very far either.

 

Good info

So though there is plenty of flow/pressure in the exhaust manifold, is there more in the inlet side? I guess there must be?

 

The inlet side has "Atmospheric Pressure" acting on it. "A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 65.5 newtons (14.7 lbf)".
lbf = Pounds force or the pressure at sea level is 14.7 pounds per square inch.

http://en.wikipedia.org/wiki/Atmospheric_pressure

If you were at an altitude other than sea level the pressure would be typically less. (it is possible to be below sea level, though in a deep crater).

The exhaust pressure will always be higher than the atmospheric pressure unless the engine is not running!!! Hope this helps.

Tom Vaught