Can someone explain exhaust back pressure to me?
Discussion
The Black Flash said:
You could well be right now, I haven't looked at many exhausts from the last 10 years to be honest.
Actually it looks like you're right, from pictures on google, most of them aren't. Mind you, it looks like non-turbocharged engines in mainstream cars will be a thing of the past soon, at which point it becomes something of a moot point.
doogz said:
If you change the exhaust manifold on an Impreza for one with equal length primaries, it’ll still sound like a flat 4.
The exhaust noise will be identical to any other four-pot with equal length headers. The rest of the engine might sound different (different vibrational characteristics, etc.). If anything, you'd expect a boxer to sound smoother because it's properly balanced. Listening to a GT86, I couldn't have told that it wasn't an inline-4.
ezi said:
Hi guys,
Looking for a dumbed down explanation of exhaust back pressure and when it is a good or bad thing. Was reading up on different types of exhaust for my car recently and can't understand whether a little or lot of back pressure is good for a N/A or turbo engined car?
Cheers
Is it for this?Looking for a dumbed down explanation of exhaust back pressure and when it is a good or bad thing. Was reading up on different types of exhaust for my car recently and can't understand whether a little or lot of back pressure is good for a N/A or turbo engined car?
Cheers
Back pressure is never good, however if you reduce back pressure and don't retune and optimise the A/F for the change, then performance can suffer. Tune the motor and it'll see more gains.
High back pressure can induce more low rpm torque, but will stifle power.
That said, an exhaust is an entire system and unless you are doing some proper (and major) mods on your Festa I seriously wouldn't worry too much, if at all. Although it's good to know the theory for when you've get a vehicle with a more substantially powerplant
doogz said:
kambites said:
The exhaust noise will be identical to any other four-pot with equal length headers. The rest of the engine might sound different (different vibrational characteristics, etc.). If anything, you'd expect a boxer to sound smoother because it's properly balanced.
Listening to a GT86, I couldn't have told that it wasn't an inline-4.
By that logic, all 4 cylinder engine's exhausts will sound the same. Which obviously isn't true. Things like combustion chamber size and shape, valve size, and several other things will have an effect on what the exhaust sounds like.Listening to a GT86, I couldn't have told that it wasn't an inline-4.
Edited by kambites on Thursday 4th October 13:37
V8mate said:
McSam said:
V8mate said:
There's a reasonable body of opinion/evidence which shows that an E36 loses bhp when de-catted. This is surely simply a removal of a 'blockage'.
The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Someone designed them properly..The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Which is very irritating, actually
h to fit though, and I just don't know whether it would be a complete waste of time doing so.h to fit, in a lift-the-engine-to-get-more-room sort of way.Still reading the tech side of this with interest
McSam said:
V8mate said:
McSam said:
V8mate said:
There's a reasonable body of opinion/evidence which shows that an E36 loses bhp when de-catted. This is surely simply a removal of a 'blockage'.
The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Someone designed them properly..The outcome has always 'baffled' me though, so I'd be keen to understand what's going on in them there pipes.
Which is very irritating, actually
h to fit though, and I just don't know whether it would be a complete waste of time doing so.h to fit, in a lift-the-engine-to-get-more-room sort of way.Still reading the tech side of this with interest
300bhp/ton said:
Is it for this?
[url][img]http://thumbsnap.com/sc/neyACusC.png[/mg]|http://thumbsnap.com/neyACusC[url]
Back pressure is never good, however if you reduce back pressure and don't retune and optimise the A/F for the change, then performance can suffer. Tune the motor and it'll see more gains.
High back pressure can induce more low rpm torque, but will stifle power.
That said, an exhaust is an entire system and unless you are doing some proper (and major) mods on your Festa I seriously wouldn't worry too much, if at all. Although it's good to know the theory for when you've get a vehicle with a more substantially powerplant
Indeed it is. It's already running a Milltek 4-1 manifold and catback exhaust, was playing with the idea of a decat or sports cat but some people said I'd notice a power loss due to a reduction in back pressure, this is what prompted me to make this thread [url][img]http://thumbsnap.com/sc/neyACusC.png[/mg]|http://thumbsnap.com/neyACusC[url]
Back pressure is never good, however if you reduce back pressure and don't retune and optimise the A/F for the change, then performance can suffer. Tune the motor and it'll see more gains.
High back pressure can induce more low rpm torque, but will stifle power.
That said, an exhaust is an entire system and unless you are doing some proper (and major) mods on your Festa I seriously wouldn't worry too much, if at all. Although it's good to know the theory for when you've get a vehicle with a more substantially powerplant
Bigger things are planned next year (Cams, inlet and map) but wanted to get the car 'breathing' right (So to speak) first...
Edited by ezi on Thursday 4th October 20:25
kambites said:
Yes, but cylinder orientation does not have an effect (at least I can't see how it could).
Only in the sense that a flat engine places some constraints on the design of the manifold which may not be there on an inline configuration. e.g. you couldn't have a 4:1 manifold with very short primaries on a flat 4 - just the kind of manifold that many modern cars with IL4 engines come with.Edited by kambites on Thursday 4th October 13:37
Everything else being equal, a flat four would sound just like an inline four.
PumpkinSteve said:
Is stalling more likely with less back pressure? I only ask because the chav on my street with a 6" exhaust stalls his car about 14 times every morning.
No not exactly, stalling is likely due to a bad running car or some other problem. However a poor A/F ratio could indeed be the result of exhaust induction mods without the proper fueling.PumpkinSteve said:
Is stalling more likely with less back pressure? I only ask because the chav on my street with a 6" exhaust stalls his car about 14 times every morning.
Just because he has a 6" tip does not mean the exhaust diameter is 6" all the way down. More than likely, if it's an aftermarket exhaust on a turbo car, the diameter will be 3" inside the pipes, or if it's an N/A car, 2.5" or 2.25" is more likely. V8mate said:
Oh I'm well aware of the ballache... I had to remove it from the car it was on previously! Steering column dismantled, engine lifted clear AND hitting and swearing!
Sounds about right .. And people wonder why I haven't got round to fixing the small crack in my exhaust manifold.It is quite incredible to see the difference between the M52 and S52 manifolds, though!
ezi said:
Indeed it is. It's already running a Milltek 4-1 manifold and catback exhaust, was playing with the idea of a decat or sports cat but some people said I'd notice a power loss due to a reduction in back pressure, this is what prompted me to make this thread
If anyone ever tells you that you need back pressure or you'll lose power without it, walk away.That said, exhaust tuning is complex and I'm nothing more than an enthusiast.
The manifold you've got is a good thing, although with n/a cars exhaust tuning is more about scavenging than flow. I don't know how well designed the tubular manifold you've got is for this.
As for the cat, well personally I can't see the point in de-catting any car in the UK (unless it's uber easy to refit). As you'll have to faff about come MoT time to get it through emissions testing. Also (assuming it still happens) road side emission tests would certainly cause you an issue.
A good high flow cat will have almost no affect on performance in terms of flow, however in the UK you have to be careful as some sports cats are not actually great flowing and hidden by marketing bull.
Metal matrix cats flow the best, but they are expensive and take longer to heat up. Ceramic core cats (the most common and most UK "sports" cats are these) can be fine. But you'll ideally want one with the biggest inlet/outlet possible. David Vizard suggests that we 2.2cfm flow per hp for a resistance free exhaust system. In reality this means a good high flow cat should not affect flow on engines producing 250+hp. And even if they do produced resistance it'll be minor and progressive.
However there is another side to this, there's a thing called the exhaust pressure wave. Affecting this will affect a cars performance, and adding or removing a cat will affect it. However it is way beyond my skill level to tell you how to tune the pressure wave.
But the real key is make sure when you've done your mods to get the rolling roaded with a custom map - it will make a difference.
ezi said:
Bigger things are planned next year (Cams, inlet and map) but wanted to get the car 'breathing' right (So to speak) first...
This all sounds cool, the only thing I'd say and not wanting to put you off. Doing all this is going to cost a sEdited by ezi on Thursday 4th October 20:25
t load and at the end of the day you'll still have a fast depreciating Fiesta (probably worth less modded than not). And that for the same money you could go a lot faster if you put the money into a better performance platform.I'm certainly not against modding (far from it
). And as versatile as a Fiesta is, they are just a blooby looking fwd shopping car built and designed for packaging not performance. Might be something to bear in mind in the bigger picture as it were.Thanks for the help people, think I understand it a bit better now!
300bhp/ton:
I know where you're coming from with regards to the mods aswell, I'd be inclined to agree more though if I was trying to get more power out of a 60bhp Fiesta LX but as it stands, my ST model is a great platform to get more power out of (In my opinion, of course ). The 2.0 Duratec engine is in lots of other cars such as Caterham builds carrying silly power, so in this car it has a lot of potential, it comes out of the factory with 147bhp standard and with the mods I mentioned earlier there are others out there with 200bhp+ in these cars. Couple that with it's excellent handling, that makes it a bit more than a shopping trolley tbh
Depreciation wise I don't really care, these cars have lost so much value recently there really isn't much more to lose and any modifications can make there money back by splitting it when I part with it in a couple of years
300bhp/ton:
I know where you're coming from with regards to the mods aswell, I'd be inclined to agree more though if I was trying to get more power out of a 60bhp Fiesta LX but as it stands, my ST model is a great platform to get more power out of (In my opinion, of course
). The 2.0 Duratec engine is in lots of other cars such as Caterham builds carrying silly power, so in this car it has a lot of potential, it comes out of the factory with 147bhp standard and with the mods I mentioned earlier there are others out there with 200bhp+ in these cars. Couple that with it's excellent handling, that makes it a bit more than a shopping trolley tbh Depreciation wise I don't really care, these cars have lost so much value recently there really isn't much more to lose and any modifications can make there money back by splitting it when I part with it in a couple of years
Some exhausty stuff here: http://performancecarbscience.com/exhaust-system-s...
I do hear you about the 2.0, but Caterhams are a heck of a lot lighter. And 200hp is only what a standard CTR puts out. I'm pleased you are pleased. But for the same money you could be rolling about in a 400hp RWD performance car.
I do hear you about the 2.0, but Caterhams are a heck of a lot lighter. And 200hp is only what a standard CTR puts out. I'm pleased you are pleased. But for the same money you could be rolling about in a 400hp RWD performance car.
kambites said:
SuperchargedVR6 said:
Why does a chevy V8 sound totally different to a Ferrari V8?
Good question, to which I don't know the answer. I'm not very good at V8s. Is it because they run two largely independent exhaust systems and the firing pattern of each side of the engine isn't consistent? So if you view half of a cross-plane V8 in isolation it will fire twice in quick succession then wait for a bit, then do it again. If that is the case, then you could make a flat-plane V8 sound like a cross-plane with a suitably complex exhaust with manifold branches crossing from one side to the other.
Greg_D said:
kambites said:
SuperchargedVR6 said:
Why does a chevy V8 sound totally different to a Ferrari V8?
Good question, to which I don't know the answer. I'm not very good at V8s. Is it because they run two largely independent exhaust systems and the firing pattern of each side of the engine isn't consistent? So if you view half of a cross-plane V8 in isolation it will fire twice in quick succession then wait for a bit, then do it again. If that is the case, then you could make a flat-plane V8 sound like a cross-plane with a suitably complex exhaust with manifold branches crossing from one side to the other.
A traditional V8 is more akin to two V4 engines sharing a common crank, so at some point you two cylinders in the same bank firing one after the other. This makes them sound very different and makes proper exhaust tuning hugely difficult or impossible.
However there are other reasons, typically a Ferrari is setup for high revs, while many traditional V8's aren't. Block material and general exhaust design also has a significant affect. And while I don't know the reason behind it, I've always thought OHV engines sound very very different to OHC ones.
Go on YouTube and list to any 1996 or onwards Mustang. Nearly all are 2 or 3v per cylinder SOHC motors and they sound hugely different to any GM OHV engine of the same time period.
Same is true if you listen to an uncorked Lexus V8, BMW, Audi or Jag. They all sound good and akin to the Ford OHC's, but they all sound different to the current GM OHV's and past OHV V8's such as Poncho's, BBC, BBF and Mopars. Even the old Rover (aka Buick) V8 makes a brilliant noise and having heard them side by side, the Rover sounds better than the current Jag AJV8.
Greg_D said:
someone will come along in a minute with a better explanation, but it is all to do with the positioning of the lobes on the crank. This dictates the firing order of the cylinders, in chevy V8s there is no mathematical way to avoid 2 of the cylinders firing at the same point, which leads to a slightly louder part of the sound track, hence the characteristic 'bop bop bop bop' noise, with flat plane cranks like the ferrari (and TVRs) pixies come along, alter the crank which changes the noise
I think it's the other way around actually, a' chevy' v8 has shared crank pins (it's cheap and simple to make), so no 2 cylinders can fire simultaneously. In fact, the duration between firing pulses varies, giving rise to the characteristic ' burble'.Flat plane, or 180 degree, v8's have a very complex crank with a firing order/duration that's 180 degrees apart (or 90, possibly), 2 cylinders always fire simultaneously, so there is no burble, and the engine sounds more like a manic 4 cylinder.
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