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MG MGF Technical - Exhaust Techie question

Can one of the techies explain the necessity of 'Back pressure' in an exhaust system. I know it's needed and it has an effect on the power output of an engine, but WHY.

There must be a fine balance between straightening the flow of exhaust gases in a sport exhaust and creating enough back pressure.

tim woolcott


First of all we're talking about 4 stroke engines, right?? I thought so... Because 2 stokes' are a complete different subject when talking about exhausts. There you must have sufficient lenght for the expansion of shock waves. And the exhaust has a much more important job.

Your question in particularly it's not a straight forward one. It all depends about how you want those power and torque curves to develop. The idea is to get the exhaust flow quickly enough through the manifold, but also to create enough pressure inside the rear and last 'panel'. Normally it's a trial and error thing, but there are some great softwares to study that.

Cheers, Valter.

Perhaps the term 'back pressure' over simplifies things somewhat.

Backpressure - that pressure that is created by resistance to gas flow (rather akin to putting your thumb over the end of a running tap - the water comes out at higher pressure, but if you measure the amount of water that escapes, you'll find less water leaves the tap as a result of your obstruction) is bad. A system that results in a reduction of back pressure will result in better gas extraction and therefore better power gains. The engine is more efficient because it does not have to expend work on forcing the exhaust gases out past an obstruction.

That's the easy bit.

A four stroke engine releases exhaust gas from each cylinder once every two revolutions of the crankshaft. In a four cylinder engine this is phased so that one cylinder is releasing exhaust gas every 1/2 revolution.

The point here is that gas is not being released continuously, but is being pumped out of the engine in pulses.

Therefore, if you could measure the pressure in the exhaust system, you'd find that the pressure fluctuates with positive and negative spikes about a mean pressure at a given engine speed.

If you were able to image the movement of these pulses through the length of the exhaust system, it would be akin to watching water ripples propagating over a surface of a pond. If a wave hits an obstruction, some of that wave his reflected back.

Now, if you could tune your exhaust system so that the negative waves are bounced back towards the engine so that there is a relative vacuum in the exhaust manifold, just as a cylinder is about to eject exhaust gases, then you can see that you have a situation whereby exhaust gases are *actively* extracted from the cylinder - again to the benefit of engine efficiency and power.

This is the principle of extractor manifolds - and, I presume, the beneficial effects of some degree of "back pressure" in an exhaust.

The MGF's standard exhaust manifold is so poor that it almost doesn't matter how good the exhaust system is - there simply isn't much scope for extra release of power (which is what we saw in the MG World exhaust tests - max output gain was a mere 5 bhp :o(

I hope you followed the above, and is of some help Tim?

Rob Bell

I found a fairly good (i.e. I could understand it!) explanation of back pressure at:

It talks about the effects and the reasoning for valve overlap (inlet and exhaust open at the same time around the top of the exhaust stroke), and then explains:

"You can't control your overlap without changing the cams, but the effects caused by changing the back pressure are the same. When you reduce back pressure, it is equivalent to increasing valve overlap, and when you increase back pressure, it is the same as decreasing the amount of valve overlap. That's why some people will say, "you need a muffler for torque", or, "you'll have more high-end, but less torque, if you run straight exhaust". They are right, but a muffler's purpose is to reduce sound output, not horsepower! By reducing back pressure in an exhaust system, you increase high-end horsepower at the cost of low-end torque."




Hi all,
and to explanations above can be added that most modern exthaust mainfolds do include the catalyst in the total sum for best flow / backpressure - this can be a reminder for those who think that there will always be "free power" when changing to a bypass tube.... It is not allways so, VVC owners will note this for sure!

regards , Carl.


Great explaination, I followed it all the way and combining with Neils web site & comments I think I have a bit more of a clue....

Curiosity satisfied

Thanks guys

tim woolcott

Every wished you never wanted to ask the question Tim? ;o) LOL But you are very welcome :o)

All the above explanations are saying the same thing (which is nice)

Points of back pressure as the exhaust manifold, the cat (as Carl points out) and the back box.

The reason why sports exhausts fail to liberate much power is because of the back pressure intrinsic to the exhaust manifold and to a lesser extent, the cat.

The standard manifold is a mass produced design with circumferential welds found on the inside of the bore. This represents a terrible restriction to gas flow. In fact, a great deal of benefit can be derived from grinding these welds down and re-welding around the outside. Mike Satur can perform this for you. Andy G reports excellent gains as a result of this mod when he fitted one of these modified manifolds to his car.

The other option is to fit a high flow extractor manifold - a 4-2-1 with long primary and secondary pipe lengths. EBD sell these, and typical rolling results show that it is worth any where up to 15 bhp and an equivalent improvement in torque (depending on state of engine tune) throughout the rev range.

It's an interesting area - and I intend to investigate more thoroughly in due course :o)

More power to be had out the standard engine methinks! :o)
Rob Bell

>>The other option is to fit a high flow extractor manifold - a 4-2-1 with long primary and secondary pipe lengths. EBD sell these, and typical rolling results show that it is worth any where up to 15 bhp and an equivalent improvement in torque (depending on state of engine tune) throughout the rev range.

Who are EBD? any idea on price? could this be DIY fitted?

It was only an idle thought/comment/question, problem is you mentioned power gains from Mikes modification or 4-2-1 manifold, this in-turn equals the potential of yet more cash flowing from the bank account.

I'm sure there is a directly proportional relationship between number of BBS threads read and the amount of money spent on 'Toys'

tim woolcott

While we're at it, any idea how much Mike's mods cost - or it just an option on his engine mods?


From my reading of the text I pointed to:

Less back pressure = more power (particularly at high revs), but reduction in torque.

More back pressure = less power, but more torque at particularly at lower revs.

It all depends how you want to use the engine :-)

One of those subjects that sounds simple, but isn't! I suspect there's quite a lot of inconsistancy (and differences between design and fabrication) between exhausts, and it might well be appropriate to "smooth off" rough welding to get the pressure that the designers intended, rather than what the makers managed!


No idea how much Mike's modifed manifold would cost - over to Mike on that one.

EBD (Exhausts by Design) produce a lot of systems for the Elise boys - who really rate it. Web site:

It costs about the same as a sports exhaust (400 quid) - so the order of modifications would probably go air filter/ exhaust/ manifold.

There is a problem with the manifold: a lack of a flexipipe to absorb the movement of the engine on its mountings. EBD sell a flexible cat replacement tube that'll erradicate this problem - but not much use to those of us who need to keep the standard cat. There is a way round this I think - but will need a specially fabricated flexipipe and a post MY2000 compatible back box - plus some attention to the location of the exhaust hangers...

No budget to develop this at the moment - may be later in the year! :o)
Rob Bell

Oops, sorry, forgot to mention that yes, the manifold replacement is within the relms of DIY replacement, but will be a bit of a pig due to restricted access.
Rob Bell

Hmmm. Excellent thread, first time my brain's been stretched on here for a while.

However, whilst I am both tired and a little drunk, it does seem that all the above explanations bar one don't give a good reason for the need for back-pressure; most seem to suggest that the ideal would be 'vacuum exhaust', getting rid of almost all exhaust gasses instantly. My reasoning is that the negative effect of this is hinted at in the valve overlap comment - some back pressure allows the mixture at the moment of ignition in the cylinders to be under greater pressure, thus providing some extra (and more evenly distributed across the combustion period) expansion of that stroke. If ALL exhaust gasses were removed prior to inlet, there would be more fuel/air required to reach this pressure, thus making explosion rather than combustion more likely (because inlet duration would not be changed), and also lacking the added pressure of a resistance to free flow directly through the cylinder during the valve overlap. It's a cleaner cycle with no back pressure, but it's a more powerful stroke with some back pressure.

a. Did that make any sense?

b. Is it right?

E (struggling)
Ed Clarke


I follow this pretty well, so what you're saying is that the remainder of the exhaust in the cylinder 'modify' the explosive nature of oxygen.

So, following these explainations we get to an even more complex situation. The more air we get into the engine through (say) multiple throttle bodies and big air filter, the more exhaust gasses will be expelled during valve overlap (correct?). So again we reach a very fine balance point in the induction/exhaust and valve timing set up.

I'm I following it correctly

Now what about forced air induction....?

This has turned out to be a very interesting thread, Who'd have thought it....

tim woolcott

Well I've learnt something from it - thanks Neil for that web page link, as cam phasing parallel was not something I had previously considered :o)

All the above is complimentary Ed - and has as much to do with how you want to tune the engine (power vs torque) as anything else - which is to say there are competing requirements to achieve a particular goal. But your explanation of the need for some back pressure seems pretty much how I'd understand it too - nice and succinctly! :o)

Forced induction? Argh! I hadn't even started thinking about how super or turbo charging would alter exhaust design requirements... There must be differences between turbos - which must need high velocity exhaust gas flow - and superchargers where one can be presumably more lax with the degree of exhaust and inlet phasing...

Where's me brain pills??? ;o)
Rob Bell

Brain pills? Just Ibuprofen required here...

Three things help get exhaust gasses out of the chamber (unless I've overlooked something): pressure gradient across exhaust valve, compression stroke of piston, and new mixture entering at inlet valve. So you'll never manage to completely remove ALL exhaust gasses before the next ignition. I'm not sure that these remaining exhaust gasses actually modify the nature of the combustion (can't see how that chemistry would work), but there's obviously an impact on level of pressure (which in turn modifies combustion) and balance of mixture (i.e. what is in th chamber is slightly less rich than what enters through the inlet).
This is where timing comes in. In my loose understanding of things, the richer the mixture, the later you need the spark in order that the mixture burns progressively (combustion) rather than burns all at once (explosion). The presence of some burnt fuel gasses within the chamber should help (a little) to encourage the former rather than the latter.

There was a completely wonderful explanstion of this on a site somewhere that I read ages ago... I'll see if I can find it.

Ed Clarke

Found it. It's actually a bike site, and this page is actually about plugs, but it's really well written, and there's a lot of info and good sense.

Ed Clarke


You managed to touch on to another question I wanted to ask. 'ignition advancing' Interesting stuff.

tim woolcott

All I can say is thank goodness to modern technology and ignition mapping!!! Getting distrubuter timing (advance/retard curves) correct is a nightmare! At least with MEMS, we can bolt on a new exhaust, a new filter, new larger throttle body - even a new ported head with cams - and these will all run without compliant on the engine management systems compensating for all these variables.

Fantastic. :o)
Rob Bell

This thread was discussed between 29/05/2002 and 30/05/2002

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