Help with dyno results

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06-23-2014, 02:18 PM

#46

GlenL

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Pressure drop across an orifice for a compressible fluid varies as the square of the mass flow rate.
http://en.wikipedia.org/wiki/Orifice_plate

It's "geometric" growth and not "exponential" as it's m**2 not 2**m.

Also, it's not so much the higher cylinder pressures after combustion that make a supercharged system different, it's the higher manifold pressure forcing the charge in.

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06-23-2014, 10:35 PM

#47

ptuomov

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Quote:

Originally Posted by Ketchmi

What I have seen and it has been experienced by a couple of folks out there with supercharged cars is that adding my crossover decreased boost between 1 and 1.5psi while producing more horsepower. Boost pressure is a function of mass vs. restriction. If you reduce some of the restriction, you reduce the pressure level while also increasing the amount of flow.

That makes sense. Boost is a measure of a number of things, mostly the restriction that the engine puts on air flow.

Quote:

Originally Posted by Ketchmi

Most exhaust systems do not create horsepower, they allow it. If your exhaust is properly sized for your horsepower application, going bigger is not going to help. When it gets overly large you will lose torque as I found out with my 16v single exhaust I designed and tested. A single 3" properly bent with free flowing mufflers was just too much for early 219hp cars and actually reduced average power. The 240hp cars saw some high end gains but only held their own at lower rpm. The Euro 300 and 310 16v engines loved my 3" single and had gains across the board. It still wasn't feasible to produce one with the Borla setup available as it was a real quality piece ready to go.

I am going to make a claim that after a large enough resonator box, bigger is always better. Except when it starts making the car heavier and causing drag -- not drag in terms of exhaust gas flow but drag in terms of contact with the pavement! ;-)

Before the terminator box, pulse tuning matters. Small diameter pipe generates stronger pulses but causes more drag. In pulse tuning, optimizing that tradeoff is what gives you the diameters for primaries and the collectors.

Quote:

Originally Posted by Ketchmi

As Tuomo stated, forced induction cars do not respond the same as N/A cars to exhaust modifications. Losing restriction in the exhaust will always help but not to the same degree as on an N/A car. Whereas my crossover is based on one pulse creating a vacuum on the opposite side, a boosted car is actually pushing the air through so it's only function at that point is a loss of restriction. (which it still does very well!) A simple explanation would be holding a carrot in front of a donkey or kicking it in the *** to get it to move...

The idea is to generate high and consistent across rpms pressure differential between the intake port and the exhaust port during the overlap. High is hard and consistent across rpms is harder in a normally aspirated engine. There are diminishing returns to this pressure differential during the overlap, because once the chamber has been flushed and unburned charge is flowing to the exhaust, how is this going to help cylinder filling? The only way is if the exhaust pulse is perfectly tuned to push some of the unburned charge back into the cylinder, which few engines can do. So the pressure differential at overlap has diminishing returns.

With a belt-driven supercharger, one has a head start because of the manifold pressure. The cloud in this silver lining is that once the pressure differential at overlap is already there, one doesn't benefit much from additional pressure differential generated from the exhaust pipes.

Quote:

Originally Posted by Carl Fausett

On the other hand - the laws of fluid dynamics say this: start with a tube that is flowing say 1000 cfm, (just to pick a number) and has a certain resistance to flow. Measure the resistance to flow as pressure drop before and after the restriction. Increasing the CFM in that same tube makes that same restriction much worse. The pressure drop before and after the restriction will be much greater as the CFM (flow) increases. I cannot recall if the effect is linear or exponential, but I believe I recall the increase to be near exponential. So restrictions get worse the more fluid you try to move across them.

Ignoring compressibility, flow velocity is proportional to the square root of the pressure differential. For our purposes, it's about the same for compressible fluid. (For more: http://en.wikipedia.org/wiki/Compressible_flow)

Quote:

Originally Posted by GlenL

Pressure drop across an orifice for a compressible fluid varies as the square of the mass flow rate. http://en.wikipedia.org/wiki/Orifice_plate It's "geometric" growth and not "exponential" as it's m**2 not 2**m.

Also, it's not so much the higher cylinder pressures after combustion that make a supercharged system different, it's the higher manifold pressure forcing the charge in.

I basically agree. A supercharger is like a really good N/A intake that is always "in tune" and the engine doesn't have to breathe only off the pipe at any rpm.