So help me out here. Exhaust backpressure is exactly that. So why would a freer flowing exhaust cause a supercharger produce higher pressures at the same RPM. I would think it would be the other way around. The higher the backpressure in the exhaust would cause higher cylinder pressures because the exhaust could not escape as freely. Just trying to figure this one out.

 

Cylinder pressures containing spent combustion byproducts devoid of oxygen do not produce power.

 

Correct but if your just talking about flow like a air pump which any piston engine is. It would make sense for the boost to be higher with a more restrictive exhaust.

 

I am not totally sure that what you are saying about a free flowing exhaust is correct.

But if it is I would guess that with less back pressure, intake temperatures are lower. As a result, the compressor runs more efficiently. Somewhat like putting an intercooler into a system. The compressor has to do less work to produce the same amount of work. However, I really do not see this being the case.

 

Superchargers run the same power at a lower boost pressure with a free flowing exhaust. With stock exhaust 350whp at 10 psi. With a good true dual you can make 350whp @ 9psi for example. Hope that helps.

 

I am running a 3.12 pully on my G. I am only seeing about 7.5 psi at 6500 rpm. I was told it was because I am still running the stock cats. I had heard that before but was just trying to get the resoning behind it.

 

when comparing a supercharged car with a stock (catted equipped exhaust) to a big, free flowing dual exhaust with headers..........

the stock setup with show higher manifold pressure! essentialy, the air flow is the same in both motors due to the constant relationship of compressor speed to enginne speed.

this is a great example of why quoting boost pressure is not a very relative marker for engine airflow or power. intake pressure is not related to total airflow! this is how you can make more power with less boost on a supercharged car by improving the pumping efficiency of the total system. by making the exhaust system more effective and efficient, the engine can move more air through its cylinders - resulting in a lower reading of manifold pressure ("boost".)

on turbo cars, it's very hard to know what is really going on since the compressor wheel speed isn't in a proportionate relationship to engine speed. I think a couple companies have come out with rpm pickups for turbo speed now....this is very valuable in understanding what is happening with the turbo.

there's plenty of info on this out there if you are really interested........but to answer the original question...............a more restrictive exhaust setup (ie...stock cats, mufflers, piping) with result in HIGHER "boost" readings.

 

Hi QuadCam:

Makes sense: Same turbine speed + more air flow = lower pressure. I should have thought about that (but I didn't). Thank you for the clarification.

I'll delete my post since it is inaccurate. Glad you're checking!

 

OK that is what I was thinking. So I think I have a boost leak. What boost have people been seeing with the vortech and the 3.12 pulley.

 

When I added cats to my setup, boost pressure (no bc, internal WG springs) was 8 psi. It was 9.3 psi before that (no cats). Back to no cats. Expensive experiment.

 

I hit 10 psi with the 3.12, no cats and a stock exhaust. I was missing one of the bolts in the intercooler during tuning that I later found missing but boost didn't seem to change much but now I need a retune I'm sure.

 

determining actual airflow on a turbo car is harder than with a belt driven blower. you have no idea what your impeller speed is for a given setup to produce a certain boost reading in your manifold.