Can someone explain exhaust back pressure to me?
Discussion
kambites said:
I didn't think any V8 ever had two cylinders firing simultaneously?
Well, a 180 degree crank, aka ' flat plane' has the crank pins spaced such that there's combustion every 180 degrees of crank rotation. That means that there can only ever be 4 combustion events in 720 degrees or 2 rotations of the crank. Logic suggests therefore that combustion must occur in 2 cylinders simultaneously.It's a complicated and expensive crank design, and every piston uses its own crank pin.
I'm happy to be proved wrong, of course.
Super Slo Mo said:
Well, a 180 degree crank, aka ' flat plane' has the crank pins spaced such that there's combustion every 180 degrees of crank rotation.
In the case of an inline 4 engine maybe, but you seem to be forgetting there are two banks of cylinders separated by 90 degrees in a V8.Super Slo Mo said:
That means that there can only ever be 4 combustion events in 720 degrees or 2 rotations of the crank. Logic suggests therefore that combustion must occur in 2 cylinders simultaneously.
It's a complicated and expensive crank design, and every piston uses its own crank pin.
I'm happy to be proved wrong, of course.
A 90 degree V engine shares a crank pin between adjacent cylinders, and this includes V8s with a flat plane crank. It makes a cheaper, easier to manufacture and stronger crank.It's a complicated and expensive crank design, and every piston uses its own crank pin.
I'm happy to be proved wrong, of course.
The firing events are separated by at 90 degrees in a flat plane V8, and 180 degrees in a flat plane inline 4.
kind of missing the point...
flat-planed V8's are done so that you can use exhaust scavenging like you do with an I4 engine.
with cross-plane V8's you can't do this as the cylinders you would need to pair are on opposite banks of the engine.
traditional cross-plane crank V8's sould like most people imagine a V8 should sound, flat plane will always sound like a very high reving I4, as effectively they are 2 I4 engines running on a common crank.
edit - just to be clear, both flat and cross plane V8's fire at the same frequency, ie every 90 degree's
flat-planed V8's are done so that you can use exhaust scavenging like you do with an I4 engine.
with cross-plane V8's you can't do this as the cylinders you would need to pair are on opposite banks of the engine.
traditional cross-plane crank V8's sould like most people imagine a V8 should sound, flat plane will always sound like a very high reving I4, as effectively they are 2 I4 engines running on a common crank.
edit - just to be clear, both flat and cross plane V8's fire at the same frequency, ie every 90 degree's
I'm not forgetting anything, it's perfectly possible to have combustion every 180 degrees in a v configuration engine. However it's only possible if each cylinder hss it's own crank pin.
You're also making an assumption that all v8 engines have cylinder banks spaced at 90 degrees apart. I hadn't assumed that, but in fact don't know if that's always the case, or if there are narrower configurations.
Tell me then, what differentiates a flat plane v8 engine from a cross plane engine, since I've clearly been misinformed, and was under the impression that it was so called because of my description above.
Of course, not all 4 cylinder engines have a 180 degree crank either. 90 degrees are not unheard of.
Edit: post above mine explains it better, thanks. I'll consider myself enlightened.
I'm on the world's slowest phone connection today and responding is frustratingly slow, so I'm going to sign off I think.
You're also making an assumption that all v8 engines have cylinder banks spaced at 90 degrees apart. I hadn't assumed that, but in fact don't know if that's always the case, or if there are narrower configurations.
Tell me then, what differentiates a flat plane v8 engine from a cross plane engine, since I've clearly been misinformed, and was under the impression that it was so called because of my description above.
Of course, not all 4 cylinder engines have a 180 degree crank either. 90 degrees are not unheard of.
Edit: post above mine explains it better, thanks. I'll consider myself enlightened.
I'm on the world's slowest phone connection today and responding is frustratingly slow, so I'm going to sign off I think.
Edited by Super Slo Mo on Friday 5th October 17:05
Super Slo Mo said:
Tell me then, what differentiates a flat plane v8 engine from a cross plane engine, since I've clearly been misinformed, and was under the impression that it was so called because of my description above.
flat plane crank by definition is one where all the 'pins' are in-line at 0 and 180 degree's, eg.
this one is typical in that the crank pins are shared, but that's not always the case.
Cross-plane (or conventional V8) cranks are 0-90-180-270 degree's, eg.
So do you think someone will come up with a way to mimic the beautiful sounds of the V6S and V8S f-type over the next few years? It would be nice to try and make my V6 sound that dramatic :P although it doesn't have a supercharger... wondering whether a butterfly valve on the stock exhaust will do anything of importance but I think to actually get a higher pitch and then the pops n crackles it will take a lot more work and probably a full new exhaust system haha!
Am i right in thinking that thinner pipes would make for a higher pitch as there is more pressue and the airflow will have to move more quickly to escape?! And wider piping would make for a lower pitch maybe more of a rumble?!
Am i right in thinking that thinner pipes would make for a higher pitch as there is more pressue and the airflow will have to move more quickly to escape?! And wider piping would make for a lower pitch maybe more of a rumble?!
I'm not sure why you need back pressure to get a scavenging effect?
All you need is sufficient flow though a branch of the manifold at the right time to create a partial vacuum in the other branches.
It's a very simple principle, one which we used to use on a laboratory to create a vaccum in a gas cylinder. Water flowing quickly through a length of hose into a drain with a T piece in the middle and another hose conected to the cylinder. The flow of water past the branch of the T creates a vaccum in the cylinder.
I'd imagine that loss of power by fitting aftermarket exhausts and decats has more to do with taking the system outside of the original design parameters and the cars electronics cannot adjust sufficiently.
Edit:
Just read something that makes sense.
The scavenging effect requires sufficient gas velocity. If you fit a manifold that has bigger primary pipes(those that come off each cylinder) then you reduce the velocity of the gas at a given RPM and so reduce the partial vaccum created in the other primary pipes.
However bigger pipes allows more flow so if the size of the primary becomes a restriction at a certain rpm (and above) then fitting a bigger pipe will give gains above that rpm, even if it loses due to a reduction in scavenging below.
As someone else has said this is clearly a complicated subject.
All you need is sufficient flow though a branch of the manifold at the right time to create a partial vacuum in the other branches.
It's a very simple principle, one which we used to use on a laboratory to create a vaccum in a gas cylinder. Water flowing quickly through a length of hose into a drain with a T piece in the middle and another hose conected to the cylinder. The flow of water past the branch of the T creates a vaccum in the cylinder.
I'd imagine that loss of power by fitting aftermarket exhausts and decats has more to do with taking the system outside of the original design parameters and the cars electronics cannot adjust sufficiently.
Edit:
Just read something that makes sense.
The scavenging effect requires sufficient gas velocity. If you fit a manifold that has bigger primary pipes(those that come off each cylinder) then you reduce the velocity of the gas at a given RPM and so reduce the partial vaccum created in the other primary pipes.
However bigger pipes allows more flow so if the size of the primary becomes a restriction at a certain rpm (and above) then fitting a bigger pipe will give gains above that rpm, even if it loses due to a reduction in scavenging below.
As someone else has said this is clearly a complicated subject.
Edited by Devil2575 on Wednesday 7th August 12:20
davepoth said:
Any part of the exhaust system that restricts flow causes a pressure rise.
I remember Autocross minis in the 70s had a megaphone exhaust which got progressively larger from the point where it joined the manifold/downpipes to the exit at the rear. This eliminated any restriction in flow and allowed the designer to control any required back-pressure purely with the manifold and collector!It sounded pretty good too!
Great thread, especially kambites' description. Never really understood how decreasing back pressure could possibly make the performance worse, but makes more sense now 
and sounds like a complex subject!
It would also explain why my old Daytona 675 had a (squeaking) flap in the exhaust, so would happily pull from 2.5k all the way to 14k...
Regarding unequal length headers/manifolds; as lots of cars, including hot hatches, are going for i4 turbos usually to the detriment of exhaust note could the use unequal length headers/manifolds to give the engine a bit more 'character'? Are there any examples of i4's which are like this?
Personally I like off-beat noise of the Imprezas
and sounds like a complex subject!It would also explain why my old Daytona 675 had a (squeaking) flap in the exhaust, so would happily pull from 2.5k all the way to 14k...
Regarding unequal length headers/manifolds; as lots of cars, including hot hatches, are going for i4 turbos usually to the detriment of exhaust note could the use unequal length headers/manifolds to give the engine a bit more 'character'? Are there any examples of i4's which are like this?
Personally I like off-beat noise of the Imprezas
Lets be clear though, back pressure itself isn't good. Pulse tuning is good (because it decreases back pressure on the exhaust stroke!), but it just so happens that pulse tuning also tends to increase back pressure. Thankfully the benefits of pulse tuning outweigh the downsides of the back pressure it introduces.:-)
Scuffers said:
kind of missing the point...
flat-planed V8's are done so that you can use exhaust scavenging like you do with an I4 engine.
with cross-plane V8's you can't do this as the cylinders you would need to pair are on opposite banks of the engine.
...unless you put the exhaust in the V and build some funky manifolds like BMW did for the F10 ///M5.flat-planed V8's are done so that you can use exhaust scavenging like you do with an I4 engine.
with cross-plane V8's you can't do this as the cylinders you would need to pair are on opposite banks of the engine.
(but skip the turbots!)
In very simple terms...
Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?
If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?
Chav consideration aside of course
Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?
If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?
Chav consideration aside of course
texasjohn said:
In very simple terms...
Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?
If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?
Chav consideration aside of course
If its just wadding you're talking about, I can't really imagine it having any effect on the scavengine.Is it true to say that the further along the exhaust system you go (ie closer to the exit) the less impact there will be on the scavenging effect?
If you have a bargain barge and just want to release a little more volume would removing a bit of baffle material from the tailpipe silencer be a good option to avoid mucking up the exhaust tuning too much?
Chav consideration aside of course
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