Exhaust flow

There is a lot of technical detail in exhausts, and most of it is experimentation. However, 3" pipes are too large for sensible car engine sizes.

The key thing is a restrictive exhaust can be improved on, but once you get to the ideal size, making the exhaust pipe larger deflowers any benefits you might get from gas speeds or restriction reflections.

Essentially, very large exhaust pipes say, to anyone with much idea, this car has been chavved up.

What sized engine? Even 2.5" may be too big. I've seen far too many sub

1.6L cars fitted with large bore exhausts actually go SLOWER than they did with the standard on.

Depends on many things but mostly engine size and how it breathes.

2.5" should me more than enough for most N/A cars as back pressure is important, you've probably seem many Kev'ed up saxos and escorts with a film of oil all over the arse end of the car 'cos the knob behind the wheel is ignorant of this

Within reason, turbo charged cars are happiest with a large diameter downpipe (the pipework after this is of slightly less importance unless the owner has an Inches complex), this can really aid spool up times

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Depends on many things but mostly engine size and how it breathes.

2.5" should me more than enough for most N/A cars as back pressure is important, you've probably seem many Kev'ed up saxos and escorts with a film of oil all over the arse end of the car 'cos the knob behind the wheel is ignorant of this

Within reason, turbo charged cars are happiest with a large diameter downpipe (the pipework after this is of slightly less importance unless the owner has an Inches complex), this can really aid spool up times

Bull. Turbos work best with big, open or no exhaust. The thing that spins them is the pressure difference across the turbo. The bigger the better.

Yeah, but generally, even a turbo car won't need much more than 3" full system (forget tail pipes) for upto 350BHP. Any bigger is a waste until you are making more power and have the rest of the engine sorted.

Thankyou Carl,

I should put more effort into my phrasing. Problem is that I'm spitting these posts out as fast as possible so noone sees I'm not working!

Thing is, my Celica had a nice film of oil/soot up the back bumper which got worse when I fitted a 2.5" stainless mongoose.

Same diameter, but one box less. It drank/burned oil, yet when the exhaust was off the car, the back of the cat was totally clean without a trace of oil anywhere.

But if you tune the exhaust diam/length to get an impedance match between the turbo/exhaust, then you get a negative pressure (i.e. below atmospheric) on the exhaust side of the turbo, and so a bigger pressure difference across the turbo. Although only for a certain span of the rev-range, of course :-)

Probably, but my drag bike had a ful 3" system, and I replaced it with a 3 inch "banana pipe" that exited near my knee instead and ran consistently a tenth quicker. That had 280bhp. So anything helps it boost earlier! But that was a T4 on a 1.1 motor.

Yer think!

Yup. If you optimise the system to give impedance matching at ~75% of peak-power rpm, then you get a benefit across almost the whole rev-range, which is cool.

My understanding is that most manufacturers optimise for matching at low rpm to give boost at lower rpm than with stright-trough exhaust, but at the expense of giving increased backpressure at and reduced boost at high-rpm. In this case, swapping the manufacturers exhaust for low impedance large diam. one improves high-rpm boost and thus peak power, but at the expense of low-rpm performance.

Glad people have basicly backed up and expanded my limited knowledge :-)

How does it give less back pressure than no exhaust??? I don't understand the term impedance when used for exhausts! The flow from a turbo is all but pulse free and still expanding at full power as the exhaust valve opens before the end of the powerstroke. How can any lump of tubing give less pressure than an open / no / huge exhaust?

Please explain!

AIUI, most fixed-geometry turbos can be modelled as single-pole restrictive orificies, meaning basically that they transmit high-frequency pulses and give you essentially a static pressure drop. As such, the output is not pulse-free, but the pulse amplitude is reduced cf the exhaust output, by an amount dependent on the pulse freq (i.e. rpm...) (which explains why turbod engines make less noise, all else being equal, than a non turbo, even though they exhaust the same amount of gas)

The exhaust is then designed basically as a resonant pipe, giving a standing-wave pressure distribution, with an anti-node at the turbo and normally a node at either the open end or at the silencer. At an anti-node, the pressure varies from positive-negative with the pulse-period, and the system naturally synchs so that the negative pressure-peak occurs at the same time as the pulse occurs.

So whilst it's true that the *average* pressure isn't lower than with an open pipe, it is at the points in the cycle where gas-flow is highest.

This is all from talking to a very clever chap who worked for Ford on finite-element exhaust modelling software - it's not my field at all. Last I heard he'd packed it in and got a job designing prosthetic knees! still, i'm pretty convinced he knew what he was on about... :-)

Pulses I am not so sure about. A turbo alone makes a very efficient silencer! My open exhaust bike was loads quieter than the other "silenced" bikes that I was beating!

Yep thats what I said earlier?

But there is bugger all pulse left? Cant do much...

Well since the turbo and its nozzle is so restrictive and therefore quietens most of the noise I doubt you would ever see any advantage over an open pipe or no pipe.

He may do but any gains would be so minimal that its not worth the effort. Much better to just have a huge or no exhaust to maximise pressure differential at all revs! At least I think so...

Hum. As I say, not my field, but surely there must be a reasonable amount left, since you can still hear a turbo'd engine, albeit singificantly quieter than a NA one? Resonant pipes don't need much to excite them - hence why organ pipes driven by a tiny 20w compressor can make your teeth rattle.

Fair enough.

I think to see if there was a measurable difference either way you would need a very accurate dyno. Like I don't have any longer!

With an exhaust what you essentially want to do for power is to remove the necessary *volume* of gasses as *fast* as possible. The *speed* of the gasses flowing through the pipes creates a negative pressure difference at the engine which helps in cylinder filling (of the combustion charge).

Now, if you're pootling along at low revs the exhaust *volume* is relatively small so a narrow bore exhaust is sufficient to provide good exhaust speed. If you then rev the engine the exhaust can't clear enough volume and the back pressure mounts up and the engine won't produce good power - much of it's energy is being used pushing the gas through the pipes.

People say that back pressure is a good thing but you can see that in itself it's not. However, if you simply fit a big bore exhaust to clear the volume of gasses at high rpm you don't get good gas speed at lower rpm and again power is reduced. This is why some people say that a car will be slower with a big bore exhaust on it. It might give more max power but may well not pull so well as lower rpms.

So, exhaust technology is often a compromise. What you want is the best *flowing* exhaust with the smallest diameter. That means removing restrictions such as bends and joins, hence performance "straight through" boxes rather than baffle boxes.

As for your question, you probably need to talk to other owners of the model of car you have and see what the general opinions are.