Exhaust Backpressure and Scavenging

I am debating on getting an exhaust for my Z.  Some of the forum members say an exhaust needs small exhaust pipes with back pressure to produce maximum power and torque.  Is this true?


Back pressure on the VQ’s exhaust is NEVER a good thing for naturally aspirated (NA) or turbo applications.   It is a widely spread myth that some back pressure is good, but this is 100% FALSE.  There clearly are situations where smaller diameter pipes can outperform larger diameter pipes but this is not because of back pressure.  It is because of scavenging.  When properly tuned scavenging actually reduces back pressure as seen by the engine. 

But be careful in the assumption that smaller pipes automatically equals better performance.  It is highly dependent on where the smaller diameter pipes are being used.  Smaller diameter pipes near the engine can be good for scavenging and power but as the pipes move farther and farther away from the engine the effect of scavenging rapidly diminishes.  If scavenging cannot be taken advantage of, then small/restrictive pipes must be completely avoided. 

Back pressure can only reduce power on a NA VQ engine.  Do not confuse back pressure with scavenging.  Scavenging actually reduces back pressure on a tuned cyclical basis.  With each pulse released during the exhaust stroke of the engine, the pulse travels like a shotgun blast down the exhaust pipes.  This high intensity blast creates a shockwave with a relatively large positive pressure at the wave front.  This slug of gas is traveling so fast that even after the piston reaches top dead center (TDC) the slug of gas keeps moving at high velocity and rarefies any remaining exhaust gas that is behind it.   This is what creates a vacuum inside the exhaust pipes on a cyclical basis.

It is this vacuum that draws out any remaining exhaust gas from the cylinders.  This vacuum also draws more fuel/air mixture through the intake valves during the intake/exhaust valve overlap period.  This is the scavenging effect and this is how the additional power is made.     Some have asked about adding back pressure further downstream in the exhaust to increase performance.   This does not work either.  Adding back pressure downstream can only reduce the vacuum upstream and reduce engine power and torque.

Here is how the back pressure myth started.   It was a misinterpretation of test results.   A long time ago somebody probably did the same series of dyno tests I did on varying exhaust pipe diameters.  Like I did, they probably found that smaller diameter pipes can yield higher HP and TQ. The smaller pipes (near the engine) were interpreted as increasing back pressure, not scavenging, and published their results.   When properly used smaller diameter pipes can provide higher performance via scavenging but small diameter pipes are only desirable when they are very close to the engine.

For example:

I did a series of dyno tests on various diameter VQ test pipes ranging from 2.00″,  2.25″ and  2.50″.

Before conducting the tests, my initial guess was that the larger diameter pipes would produce the highest HP with lowest TQ.   And the smaller diameter pipes would produce the lowest HP and the highest TQ.   Well… I was 1/2 right…

As expected, the dyno testing showed the 2.5″ diameter test pipes made the lowest TQ.  And as expected, the dyno testing showed the 2.0″ diameter test pipes made the highest TQ.  But here’s the surprise.  The 2.0″ test pipes made ~1 more HP than the 2.25″ and 2 more HP than the 2.5” test pipes.  …It left me surprised.   Smaller diameter test pipes make more TQ and more HP.   2.25” will yield good NA results while leaving significant headroom for medium boost turbo applications.

So somebody a long time ago probably misinterpreted the smaller diameter as adding performance by being more restrictive.  But this is not exactly the case.  It is because of increased scavenging.  Smaller diameter pipes near the engine increase the velocity of the shockwave and thereby increasing the effect of scavenging.  It was a misinterpretation of the results.

So I continued down this line of testing at the Y-pipe primaries.  Using the 2.0″ test pipes, I then tested various Y-pipe primary tube diameters.  2.0″, 2.25″ and 2.5″.   The expectation was to see similar results… but not quite this time.  At least not at the Y-pipe.   The 2.0″ Y-pipe primaries did indeed provide the highest TQ, but it brought a good portion of the HP down.  2.25″ primaries were better but could still be improved upon. The 2.5″ Y-pipe primaries provided the best peak power and the best average power.

So dyno testing proved the best test pipe diameter is 2.0″ diameter and the best Y-pipe primary diameter is 2.5″.   I then continued further down this line of testing on the mid-pipe and made some more interesting observations.  Testing mid-pipe diameters at 2.5″, 3.0″ and then a fully open Y-pipe.   Testing showed that there was no scavenging effect possible after the Y-pipe.  There was nothing to gain from the smaller diameter what so ever.  In fact, the only thing that had any effect was simple back pressure.

Using a open Y-pipe as the baseline I found that connecting a 3″ single exhaust had no effect on TQ and had only a small 1.5 HP decrease.  The 2.5″ midpipe slightly reduced TQ and was ~2.5HP down from the the 3″ midpipe.

This series of tests established:

1) There was no scavenging possible after the Y-pipe. 

2) A 3” or smaller diameter midpipe can only decrease HP&TQ 

3) There will be rapidly diminishing returns increasing tube diameter beyond a 3″ midpipe.

4) With power to weight ratios taken into consideration a 3″ midpipe can be considered optimum for NA applications. Single 3.5” or dual 3” yields the same NA performance but allows more headroom for medium and high turbo boost applications.

After these experiments another series of tests were performed at the end of the Y-pipe.  These tests were designed to measure the effect of varying back pressure.

1) Attaching a 3″ diameter butterfly valve with variable position restriction plate.

2) Attaching a 6″ diameter parabolic diffuser which reduces pressure drop below that of a 3″ open pipe.

The purpose of the butterfly valve restriction plate was to directly test the effect of raw back pressure on performance.  And the results were very clear. 


The butterfly valve was attached at the end of the Y pipe and dyno tested at various levels of flow restriction.  From wide open to almost fully closed, as back pressure increased, dyno performance rapidly decreased.

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This data set clearly demonstrates back pressure is not beneficial to power or torque in any way on our VQ engine.

After that series of tests we performed another set of tests were performed.   Tests that decreased exhaust back pressure below that of a simple open ended 3″ pipe.   A 6″ diameter parabolic diffuser (megaphone) was clamped onto the end of the Y-pipe.  This was used to decrease flow resistance below that of an open pipe. Dyno tests show the diffuser produced an instant 4-6HP increase over that of a open Y-pipe.

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So while there are rapidly diminishing returns with going to larger and larger diameter tubing after the Y-pipe, significant gains can still be made by use of a diffuser.  The back to back dyno testing shown below was a simple open Y-pipe as the baseline and then with the diffuser attached.

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To get an idea of how a diffuser works you need to consider all the kinetic energy being pumped out of the exhaust tail pipe.  For a normal exhaust system that uses 3″ tubing connected to a muffler with a normal 3″ outlet with a rapid expansion to a 4″ or 5″ tip, all the exhaust flow energy is wasted by shooting out all that exhaust gas at high velocity into the atmosphere.

If you have ever stood ~20 feet behind a car on the dyno you will see and feel the effects of all that energy being wasted. Standing behind a car on the dyno is like getting blasted by wind on a hot smoggy day.  All that wind is being blasted out of a 3″ tube!  So what is coming out of the tail pipe is quite intense.  It takes energy to move all that hot wind with such force and it’s all just wasted energy.  But if you were to connect a 5″ diffuser to a 3″ exhaust tube the exhaust gas velocity will be slowed down by a factor of almost 3X.

Pressure drop is determined by ρV2/2g. In this case ρ and 2g remain constant and V2 is the important variable.  Not only a diffuser brings the velocity down by a factor of nearly 3X, the total pressure recovery is determined by the square of the velocity 3X.  So the diffuser is recovering kinetic energy by converting Dynamic pressure into Static pressure.  This means your engine doesn’t have to waste energy by pumping out the exhaust gas so forcefully, it also makes it easier for a new fuel/air charge to get into the engine by means of scavenging.

Diffusers are commonly used in jet engines and rocket engines. They convert fluid motion into pressure or Dynamic pressure into Static pressure.  In the case of a VQ engine we only want to convert the dynamic pressure into static pressure before it is dumped into the atmosphere. One way to do this is with a diffuser. For a diffuser to be efficient, it must be designed with the proper expansion ratio and diffusion angle. Like the cone shown above.

If you were to take a common 5″ exhaust tip and bolt it onto a 3″ pipe, it won’t work. A regular exhaust tip has what is called a sudden expansion. A sudden expansion is unable to efficiently convert kinetic energy into static pressure. It just dumps it into the atmosphere and wastes the energy. But a diffuser organizes the flow and gradually slows it down (efficiently) and all that kinetic energy is conserved.  The engine is still pumping out a lot of exhaust gas but a diffuser makes it a lot easier to do.

In conclusion:

A wide range of testing was performed on a range of different diameters and lengths of exhaust pipes in varying locations of the exhaust system.   All of these tests demonstrated different ways of changing exhaust pulse scavenging and backpressure.  In all cases the results show scavenging is beneficial and back pressure is detrimental to engine performance.  Engine performance can be maximized by utilizing a continually expanding exhaust system design with the smallest (optimally sized) pipes closest to the engine and progressively expanding to larger and larger diameter pipes as the flow moves away from the engine.