Eight point seven psi. That was the exhaust
back-pressure of Frank the Falcon’s standard exhaust. (Frank is an EF six
cylinder.) So, is that back-pressure figure good or bad? A bit of background is
necessary to answer that.
The first step before modifying an exhaust should
be to measure the backpressure of the existing system. The higher the
backpressure, the greater the restriction the exhaust is imposing on the engine.
If the back-pressure is low, the gains that you can make in that area by fitting
a new exhaust are also low. On the other hand, a high measured exhaust
back-pressure shows that there’re lots of potential for gains in the exhaust
Measuring exhaust back-pressure is easy. The most
convenient way to measure backpressure is to temporarily remove the oxygen
sensor near the beginning of the exhaust system. The sensor can be unscrewed
from the exhaust and left to dangle in the engine bay. Next you’ll need to find
a bolt with the same thread as the oxygen sensor. Drill a small diameter hole
through this bolt (from the head through the bottom) and weld a short length of
metal pipe to the head. If you don’t have any welding equipment, any exhaust
shop should be able to do this for you in a couple of minutes. An alternative is
to find a plumbing fitting that will screw into the hole. The fitting needs to
be in there for only a few minutes and doesn’t need to be more than
finger-tight, so it doesn’t matter if the thread isn’t identical so long as it
will easily screws in and stays in without leaking!
Run a hose from the fitting to a pressure gauge
located temporarily in the cabin. Any sort of pressure gauge that reads up to
say 100 kPa or 15 psi is fine – it doesn’t even have to be an automotive gauge.
To read maximum back-pressure, it’s then just a
case of selecting an appropriate gear and accelerating at full throttle to the
redline. And, as we said, on the Falcon, that measured maximum was 8.7 psi. So
how does that compare with other cars?
The highest backpressure measurement we’ve seen
came from a bog-stock Holden VL Turbo. Peak backpressure (measured immediately
behind the turbine) was an astonishing 13.2 psi.
A 1994 Subaru Impreza WRX (fitted with a more
powerful Japanese-spec engine) saw up to 8.8 psi exhaust backpressure. Fitment
of a high-flow exhaust then achieved a power increase of ten percent.
The standard exhaust on a 2.4 litre Nissan Pintara
caused a relatively low 5.9 psi backpressure – though this was measured after
the cat converter, so it’s not representative of total exhaust backpressure. The
total exhaust backpressure would likely have been about 7 – 7.5 psi.
An SR20DET-powered Nissan 180SX with a standard
exhaust had a measured peak back-pressure of 9.3 psi.
So you can see that, rather like the Falcon’s
standard intake system (see
Negative Boost Revisited Part 4), the
Falcon’s standard exhaust is certainly not bad.
Despite people claiming otherwise, we have never
seen any evidence that backpressure is good for any aspect of engine
performance. Therefore, the lower the back-pressure you can get (ie, the free-er
flowing the exhaust can be), the better.
Extractors use individual pipes for the exhaust
ports of each cylinder. They replace the cast iron manifolds that are fitted to
most road car engines.
Extractors are available in two different designs
- ‘interference’ where the pipes are of unequal lengths, and ‘tuned length’
where the (usually longer) pipes are of a similar length.
The interference design provides better
results than a typical cast manifold because the less tightly bent pipes flow
better and there is also less pressurising of the exhaust ports of adjacent
cylinders. In addition to these factors, the tuned length designs take
advantage of the reflected negative pressure pulses to aid flow through the
exhaust valve. However, the pipes can only be tuned for one engine speed (just
one rpm), and so will be ‘out of tune’ for other revs. It’s normal to tune the
pipes for the revs at which peak torque is developed. Secondly, there often
isn’t enough room to fit four or six or eight very long primaries in most engine
bays! All this means that in road cars, the benefits of tuned length extractors
aren’t always realised.
In a tuned system the idea of back-pressure is no
longer so important. That’s because the speed of the exhaust gases is now vital
– the rushing past of the gas in one pipe can help draw gas through the
connected pipe. Larger pipes will cause lower backpressure, but they’ll also
drop flow speed. Therefore, it doesn’t make sense to put a pressure measuring
fitting on one pipe in a set of extractors and measure back-pressure. (If you
were going to measure anything, it would be the size and timing of the pressure
pulses running up and down the pipe – and for that you need to use an electronic
pressure transducer and high speed data logging.)
Tuned Length versus Backpressure
In a turbo car it’s easy – you want the lowest
backpressure possible straight after the turbine. But in a naturally aspirated
car, when does the tuned part of the system (the extractors) finish and the rest
of the exhaust system begin? That depends on the last point of pulse reflection
in the tuned system, that is, where the first muffler or cat converter is
located. In nearly all cars where extractors are being fitted, the cat is placed
immediately after the extractors – and so that’s the end of the tuned length
part of the system.
It’s for this reason that you’ll sometimes find
that the outlet of factory extractors is sometimes smaller than the pipe that
immediately follows the cat converter – one is optimised for tuned length
performance and the other just wants as low a back-pressure as possible.
Once, a typical performance muffler used multiple
baffles inside it. The exhaust gas was forced to meet blank walls inside the
muffler, making its way out through holes punched in the tube. Often it would
then have to squeeze through even more holes before it could continue on its
way. Each section of the muffler allowed expansion and pulse reflection,
decreasing noise. These mufflers were reasonably quiet, but very restrictive to
flow. This type of baffled muffler is still currently fitted to some new cars.
Next on the scene was the reverse flow muffler.
This type of muffler doesn’t use baffles to block off flow and so it has less
flow restriction. Instead, it takes the exhaust gas on an S-shaped path through
the muffler. The gas enters the muffler, travelling straight down to the other
end of the muffler, where it is forced to turn through 180 degrees. It then
heads back in a different tube the way it has come, before it is forced to turn
around again. Finally, it flows out of the muffler. The benefit of this type of
muffler is that it is effectively three times longer inside than outside! The
disadvantage is that each of those 180 degree turns causes a flow restriction.
Finally, there is the straight-through design.
This design uses a single perforated tube that takes the exhaust gases directly
from the inlet to the outlet. The exhaust gas can travel through the muffler
with almost no restriction at all. The sound waves expand through the holes in
the pipe and are absorbed in the muffler packing.
These are the three basic types of mufflers but
there are also variations on the designs. Some sophisticated straight-through
mufflers use two chambers, with the exhaust gas expanding into the second
chamber after it has squeezed through the perforations in the main tube. Others
use a 'dog leg' design, with a central open chamber and offset inlet and outlet
pipes that pass through their own respective chambers.
A genuine straight-through muffler outflows any
other type. When compared with the same length of empty pipe, a good
straight-through design flows 92 or 93 per cent of the maximum possible. That is
exceptionally good, and can be compared with the poor flow of a reverse flow
design that is typically down to 59 per cent. Those mufflers using a 'dog-leg'
internal design with offset chambers have a flow of about 65 per cent, while
traditional baffled mufflers can be as low in flow as 38 per cent. These figures
are the result of extensive muffler testing carried out on a flowbench.
Mandrel vs Press Bent
Exhaust pipes can be bent using two different
techniques - mandrel and press bending. (Most factory exhausts are press bent,
like the Falcon’s system shown here.) Press bending machines are commonly found
in exhaust workshops, while mandrel bending machines are much rarer. Press bends
are made with tooling that remains external to the tube, causing some flattening
of the tube as it is bent. Mandrel benders use a mandrel that is of similar
diameter to the inner diameter of the tube. The mandrel is pulled through the
tube as it is bent, forcing the tube to keep very nearly the same inner diameter
as a straight piece of the same tube.
Mandrel bends are generally regarded as being much
superior to press bends, but depending on the tightness of the bend, the flow
difference can actually be quite small. For example, in 45 degree bends the
difference in flow between mandrel and press bends is minor. However, a
180-degree bend flows much better if it is mandrel rather than press-bent.
But there is a very important point to consider if
specifying mandrel bends. Because the vast majority of exhaust shops do not have
a mandrel bender, their "mandrel bent" exhausts consist of many pre-formed
mandrel bends that are cut and welded together to form the exhaust. Unless the
welding is done with great care, so that a bead does not penetrate the pipe and
there is no offset at the joins, the flow may well be worse than would have been
achieved with a continuous length of press-bends! Note also that many exhaust
shops that make an exhaust in this way grind back the welds and then paint the
pipe, so that the method of exhaust construction may not be at all obvious.
If you have a common car (like the Falcon!),
proper aftermarket mandrel-bent exhausts may be available off the shelf.
The most restrictive single element in nearly all
exhausts is the cat converter. Selection of a cat converter can be made on the
basis of diameter and description (eg “3 inch hi-flow”) or, when selecting
factory cats, on the basis of the engine power they were working with on the
standard car. An example of the latter is to take a cat from a 150kW six
cylinder car and put it on a 100kW four cylinder – it’s likely in that situation
the cat will flow well.
The Falcon System
So what did we select for the Falcon? We wanted
long runner, tuned-length extractors to take advantage of reflected pulses to
promote better cylinder evacuation, and following the tuned length part of the
system, the lowest back-pressure consistent with low noise.
We’ll cover the system in more detail next week
(when we fit and test it) but in short, we used Jim Mock Motorsport (JMM) 3 >
2 >1 mandrel bent ‘Race’ headers, an ex-Commodore V8 2.4 inch cat converter,
and an off-the-shelf Mercury 2½ inch mandrel bent straight-through muffler
Next week: the Falcon system plus road and dyno
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