Shopping: Real Estate |  Costumes  |  Guitars
This Issue Archived Articles Blog About Us Contact Us

Frank's Exhaust, Part 1

Why we selected these bits for our EF Falcon's exhaust

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Measuring backpressure
  • Tuned systems vs lowest back-pressure
  • Selecting mufflers
  • Pipe bends
  • Cats
  • Part 1 of a 2-part series
Email a friend     Print article

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 area.

Click for larger image

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.


Click for larger image

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.

Click for larger image

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.

Click for larger image

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

Click for larger image

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.

Cat Converters

Click for larger image

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 system.

Next week: the Falcon system plus road and dyno testing

Did you enjoy this article?

Please consider supporting AutoSpeed with a small contribution. More Info...

Share this Article: 

More of our most popular articles.
The 1100hp Porsche 917

Special Features - 18 April, 2003

The Early Days of Turbo Part 3

A new engine designed to extend the range of plug-in hybrid cars

Special Features - 8 December, 2009

The Lotus Range Extender Engine

A salutary lesson in failure

Special Features - 9 July, 2013

Giving up

Advancing the ignition timing can result in better fuel economy

DIY Tech Features - 28 April, 2008

The 5 Cent Modification

Some of the best wind tunnel pics you'll ever see

Technical Features - 4 July, 2007

Aero Testing, Part 4

Do-it-yourself aero testing on the road!

Technical Features - 13 June, 2007

Aero Testing, Part 1

If you love modifying cars, be sad

Special Features - 12 June, 2012

The beginning of the end of a 60-year era?

Steps in mixing and matching front brake components

DIY Tech Features - 29 May, 2012

Selecting components for upsized front brakes

What are the risks and benefits?

Special Features - 6 August, 2013

Children and home workshops

30 cylinders, 21 litres and 470hp!

Technical Features - 25 July, 2008

The Chrysler A57 Multi-Bank Engine

Copyright © 1996-2020 Web Publications Pty Limited. All Rights ReservedRSS|Privacy policy|Advertise
Consulting Services: Magento Experts|Technologies : Magento Extensions|ReadytoShip