Can someone explain exhaust back pressure to me?
Discussion
V8mate said:
Indeed. I have a very smart (and probably rather valuable) bespoke stainless de-cat system, including M3 Evo manifold, for a 323/328 in my back garden. It's a b
h to fit though, and I just don't know whether it would be a complete waste of time doing so.
In terms of power gains, it would probably be counter-productive. It would make it much more likely to spit flames, though. h to fit though, and I just don't know whether it would be a complete waste of time doing so.
SuperchargedVR6 said:
I thought the impreza warble was more to do with firing order? I.e. a typical inline 4 fires 1-3-4-2 but the Impreza fires 1-3-2-4.
But I agree, the exhaust tuning will also influence the noise heard at the tailpipe.
It's a bit meaningless to compare firing orders of different layout engines, but why would which number cylinder fires in what order affect the noise coming from the exhaust? They still fire exactly half a revolution apart. But I agree, the exhaust tuning will also influence the noise heard at the tailpipe.
I suspect the reason that flat-fours tend to sound like that is simply that with that layout of engine, the shortest path for the exhaust headers happens to be such that the exhaust pulses reach the end of the manifold in pairs. You could design an I4 manifold to achieve the same thing, although it would be rather convoluted.
kambites said:
It's a bit meaningless to compare firing orders of different layout engines, but why would which number cylinder fires in what order affect the noise coming from the exhaust? They still fire exactly half a revolution apart.
I suspect the reason that flat-fours tend to sound like that is simply that with that layout of engine, the shortest path for the exhaust headers happens to be such that the exhaust pulses reach the end of the manifold in pairs. You could design an I4 manifold to achieve the same thing, although it would be rather convoluted.
I don't understand the science of it but it's just what I read / heard a long time ago.I suspect the reason that flat-fours tend to sound like that is simply that with that layout of engine, the shortest path for the exhaust headers happens to be such that the exhaust pulses reach the end of the manifold in pairs. You could design an I4 manifold to achieve the same thing, although it would be rather convoluted.
Why does a chevy V8 sound totally different to a Ferrari V8? I know one is crossplane and the other flatplane, but they have completely different firing orders. It must have a bearing on the engine tone?
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.
Edited by kambites on Thursday 4th October 12:10
SuperchargedVR6 said:
I don't understand the science of it but it's just what I read / heard a long time ago.
Why does a chevy V8 sound totally different to a Ferrari V8? I know one is crossplane and the other flatplane, but they have completely different firing orders. It must have a bearing on the engine tone?
Big difference in cubic capacity per cylinder and power delivery too? Why does a chevy V8 sound totally different to a Ferrari V8? I know one is crossplane and the other flatplane, but they have completely different firing orders. It must have a bearing on the engine tone?
SuperchargedVR6 said:
I don't understand the science of it but it's just what I read / heard a long time ago.
Why does a chevy V8 sound totally different to a Ferrari V8? I know one is crossplane and the other flatplane, but they have completely different firing orders. It must have a bearing on the engine tone?
I found the best explanation of this in a motorbike mag when they were describing the 2010 Yamaha R1, which has a crossplane crank like Rossi's GP bike, compared to the other litre bikes on the market which had flatplane cranks, and the effect it had on the sound. I'll see if I can dig it out.Why does a chevy V8 sound totally different to a Ferrari V8? I know one is crossplane and the other flatplane, but they have completely different firing orders. It must have a bearing on the engine tone?
SuperchargedVR6 said:
I thought the impreza warble was more to do with firing order? I.e. a typical inline 4 fires 1-3-4-2 but the Impreza fires 1-3-2-4.
Listen to an Alfa Sud, it's a lovely exhaust note that bears little resemblance to the "only firing on 3 cylinders" exhaust note of the Impreza. Fit an equal length manifold to the Subaru and the obnoxious exhaust note disappears.Mr2Mike said:
Listen to an Alfa Sud, it's a lovely exhaust note that bears little resemblance to the "only firing on 3 cylinders" exhaust note of the Impreza. Fit an equal length manifold to the Subaru and the obnoxious exhaust note disappears.
Glad I'm not the only one who thinks it sounds terrible. To understand pressure waves - just look at any jet on afterburner, or ripples in a pond for that matter. Sound and light all travel in 'waves' without exception...Then the descriptions of exhaust scavenging already provided above quickly become clear.
I find 2-stroke back pressure more interesting - The back pressure created by the previous explosion in the expansion chamber travels back towards the exhaust port, effectively sealing it (just like a valve really) thus preventing the fuel/air mix from escaping as the piston travels back up the stroke. It's a fascinating science.
I find 2-stroke back pressure more interesting - The back pressure created by the previous explosion in the expansion chamber travels back towards the exhaust port, effectively sealing it (just like a valve really) thus preventing the fuel/air mix from escaping as the piston travels back up the stroke. It's a fascinating science.
skinny said:
i got a bit confused about turbo back pressure a few months back. my exhaust fell off, after the downpipe (which held the o2 sensor). there was a definite loss in power (not just from me instinctively lifting off due to the noise) and it struggled to idle (lost a few hundred rpm).
I had an downpipe / cat joint blow last december on the Civic Type R as one of the two bolts holding the pipe flanges together sheared off and the gap opened up. The effect on performance was absolutely staggering, no pull at low RPMs and the VTEC felt like it wasn't working, when I got a new bolt in and tightened it up, I drove it away from the garage and the it went from feeling like a 1.0L Clio to a 2.0L VTEC again.skinny said:
i got a bit confused about turbo back pressure a few months back. my exhaust fell off, after the downpipe (which held the o2 sensor). there was a definite loss in power (not just from me instinctively lifting off due to the noise) and it struggled to idle (lost a few hundred rpm).
That will mainly have been caused by the oxygen sensor seeing lots of air being pulled back into the exhaust by negative pressure waves, and the ECU messing up the fueling trying to compensate.
kambites said:
The Black Flash said:
Most production manifolds will be unequal length (because it's cheaper/easier)
I thought they were pretty much all equal length these days as a result of the relentless pursuit of better MPG figures? Those that are normally aspirated anyway. Gassing Station | General Gassing | Top of Page | What's New | My Stuff