Straight Through Exhausts = More Power?
Discussion
(Using mum's account
) I'm in my second year of my Motorsport engineering course at Brooklands. One of my lecturers said that mainly straight through exhaust systems lose Horsepower because there is little back pressure which an engine requires. I thought this was 'BS' in V8 engine terms as they are pumping in a lot of air/fuel mixture and the exhaust gasses need dispersing quickly so i thought the engine would 'breathe' better and have a gain in HP. Can anyone help me please? what kind of engines would a straight through system benefit in? thanks
) I'm in my second year of my Motorsport engineering course at Brooklands. One of my lecturers said that mainly straight through exhaust systems lose Horsepower because there is little back pressure which an engine requires. I thought this was 'BS' in V8 engine terms as they are pumping in a lot of air/fuel mixture and the exhaust gasses need dispersing quickly so i thought the engine would 'breathe' better and have a gain in HP. Can anyone help me please? what kind of engines would a straight through system benefit in? thanksI heard that straight throughs lost hp. I personally dont believe it. Cant see why it woul benefit the engine having loads of exhaust about.
edited to add...
Surely you want to make it as easy to lose the exhaust out of the cylinder, otherwise it constitutes resistance Against the engine.
>> Edited by All Terrain on Wednesday 22 October 21:37
edited to add...
Surely you want to make it as easy to lose the exhaust out of the cylinder, otherwise it constitutes resistance Against the engine.
>> Edited by All Terrain on Wednesday 22 October 21:37
You need some backpressure to prevent 'scavenging' of the inlet gases - basically there is a brief point where both the inlet and exhaust valves are open at the same time, and with too little backpressure from the exhaust gases, inlet air/fuel is sucked out of the exhaust valves hence loosing power - it's a well known effect on 1 litre minis.
As for what type beneffits from straight through - turbocharged engines want as little backpressure as possible after the turbo - it provides all the backpressure the engine needs
As for what type beneffits from straight through - turbocharged engines want as little backpressure as possible after the turbo - it provides all the backpressure the engine needs
Don't understand the thing about minis .. there is lift on overlap, you use to to scavenge the combustion chamber of exhaust gas using the momentum of the expelled exhaust slug of gas which causes a pressure reduction behind it, that's a good thing! what's even better is if you can time a positive wave reflection to stop too much fresh charge leaving the cylinder whilst the momentum of the incoming slug of fuel/air keeps ramming it in - free supercharging. You can get over 100% VE on some race engines (maybe not minis though) using this technique.
Fact : engines don't need back pressure to make maximum power, but... If you put a straight through exhaust onto an otherwise unchanged car you will highly likely lose power, that's just because you didn't follow through and correct the fuelling and ignition for the changed circumstances though.
Fact : engines sometimes need backpressure to tune torque characteristics. Some formulae require relatively standard cars ie you can't change gearbox ratios, in these cases you can tune the torque curve using back pressure and the position of the first negative wave-reflection expansion chamber to modify the torque curve to make best use of the available gear ratios. the engine will not make the most maximum power it's capable of since you've limited it's exhaust breathing ability but the area under the torque curve would be such that the car makes best acceleration so is quicker.
So: everyone is right to some degree
Fact : engines don't need back pressure to make maximum power, but... If you put a straight through exhaust onto an otherwise unchanged car you will highly likely lose power, that's just because you didn't follow through and correct the fuelling and ignition for the changed circumstances though.
Fact : engines sometimes need backpressure to tune torque characteristics. Some formulae require relatively standard cars ie you can't change gearbox ratios, in these cases you can tune the torque curve using back pressure and the position of the first negative wave-reflection expansion chamber to modify the torque curve to make best use of the available gear ratios. the engine will not make the most maximum power it's capable of since you've limited it's exhaust breathing ability but the area under the torque curve would be such that the car makes best acceleration so is quicker.
So: everyone is right to some degree
Oh god, not the old "backpressure" argument again?
Its really very easy to explain.
Tell your instructor/teacher to put his hand over his mouth and then attempt to breathe out normally.
He'll soon get the message.
Backpressure is a pumping LOSS!
It isnt needed by any engine, ever! Never was, never will be!
Open pipes show the highest power gains as they have the least pumping losses.
The "reported" power drop offs that are occasionally bandied about, are a result of poor testing.
For instance, different exhaust diameters and lengths change the resonant frequencies which in turn can affect power outputs.
Unless the same lengths and diameters are being tested, then how can a comparison be made?
Also, optimising the fuelling and ignition for each setup is very often not carried out. No wonder this myth about backpressure keeps coming up!
Ok, is that answered now?
Its really very easy to explain.
Tell your instructor/teacher to put his hand over his mouth and then attempt to breathe out normally.
He'll soon get the message.
Backpressure is a pumping LOSS!
It isnt needed by any engine, ever! Never was, never will be!
Open pipes show the highest power gains as they have the least pumping losses.
The "reported" power drop offs that are occasionally bandied about, are a result of poor testing.
For instance, different exhaust diameters and lengths change the resonant frequencies which in turn can affect power outputs.
Unless the same lengths and diameters are being tested, then how can a comparison be made?
Also, optimising the fuelling and ignition for each setup is very often not carried out. No wonder this myth about backpressure keeps coming up!
Ok, is that answered now?
Not sure I realy understand whats going on here with the loss of power thingy 
.
Yes I agree that big bore straight through may apparently lose power and as Jools says the car needs to be tuned to utilise it. The problem as I see it, is that a big bore exhaust is good for an NA engine that has a peak torque at say 8.5K rpm, great for race cars shite for road cars.
Reducing the bore brings this peak down the rev range a bit, so can be ustilised in a road car. Maybe this is where my confusion comes from, in that an engine 'needs back pressure'.
Restricitng the size has the effect of speeding up the flow through it, the net effect of this is to draw more through the cylinder during the valve overlap making for a cleaner burn on the next stroke, thus increasing power.
All based on heresay and IMHO which has no factual basis, but I have heard tell of systems that will give up to 18hp/30lb to RV8's
.
Harry
. Yes I agree that big bore straight through may apparently lose power and as Jools says the car needs to be tuned to utilise it. The problem as I see it, is that a big bore exhaust is good for an NA engine that has a peak torque at say 8.5K rpm, great for race cars shite for road cars
. Reducing the bore brings this peak down the rev range a bit, so can be ustilised in a road car. Maybe this is where my confusion comes from, in that an engine 'needs back pressure'.
Restricitng the size has the effect of speeding up the flow through it, the net effect of this is to draw more through the cylinder during the valve overlap making for a cleaner burn on the next stroke, thus increasing power.
All based on heresay and IMHO which has no factual basis, but I have heard tell of systems that will give up to 18hp/30lb to RV8's
. Harry
All Terrain said:
decreasing diameter makes it harder for the cylinder to remove exhaust gasses. like blowing through a straw. Air flow is faster but not easier!
Think of a venturi, (Pressure is directly proportional the cross section times the velocity; P=a*v) if the pressure source is constant, i.e. the piston/cylinder head then if reduce the area it passes through you increase its flow (velocity) you will not have affected the pressure at source.
This obviuosly only holds true up to a point though, too big a restriction and you create turblence(sp?), poor flow and hence back pressure
. IMHO................
Harry
OKay, half of you have got to half the truth of this but I'm not satisfied with any of the answers completely - the truth is this - a straight through exhaust *of the correct diameter* is the most efficient at removing exhaust gases, and thus makes the most power. Mufflers inhibit flow and lose power. A CAT inhibits flow and loses power. An exhaust of too small a diameter inhibits flow and loses power. An exhaust that is of too great a diameter reduces power because the gases pass through it as too slow a velocity and you lose any scavenging effect.
There is NO benefit to be had from mufflers apart from a quieter exhaust note and no benefit from CATs apart from for emissions. A straight through exhaust of the correct diameter gives more power than any other setup.
There is NO benefit to be had from mufflers apart from a quieter exhaust note and no benefit from CATs apart from for emissions. A straight through exhaust of the correct diameter gives more power than any other setup.
Dunno how relevant it is but I hear BMW Z3M roadsters with the same engine as the M3 produce less power because they have a shorter exhaust system... might have something to do with an NA engine needing some sort of backpressure. Or is it more marketing hype? :/
I don't think it's marketing hype ... the original 321bhp M3 and Z3M had exactly the same quoted power outputs. It's only with the latest 3.2 engine (as found in the E46 M3) where the power is quoted differently between M3 and Z3M (343PS vs 325PS).
Ian.
I don't think it's marketing hype ... the original 321bhp M3 and Z3M had exactly the same quoted power outputs. It's only with the latest 3.2 engine (as found in the E46 M3) where the power is quoted differently between M3 and Z3M (343PS vs 325PS).
Ian.
Fatboy said:
You need some backpressure to prevent 'scavenging' of the inlet gases - basically there is a brief point where both the inlet and exhaust valves are open at the same time, and with too little backpressure from the exhaust gases, inlet air/fuel is sucked out of the exhaust valves hence loosing power - it's a well known effect on 1 litre minis.
As for what type beneffits from straight through - turbocharged engines want as little backpressure as possible after the turbo - it provides all the backpressure the engine needs
This is correct as you can actually lose power on a mini by having a massive 2" bore exhaust on it. So some back pressure is needed, but as I understand, there is a fine line between this. I was told that the a 2" bore exhaust is only really needed on the highest tuned cars and that most cars that have these are actually losing power. This was by a friend who builds Mini 7 Race Engines and did the work on mine.
As for a straight through over a std exhaust, I think your tutor is talking out of his tail pipe, which obviously is straight through.
Model Aero engines get a lot of power from correct exhaust design. Flyers talk about being "on the pipe" which is rev specific and very analagous to drivers being "on the cam". Some racing pipes only work over a very small range of revs.
I've got admit to never having understood any of the articles I've read on this subject
I've got admit to never having understood any of the articles I've read on this subject
A well timed thread for me!
I'm just about to swap my baffled sports exhaust for a straight through. I had a RR power run yesterday and will have another once the new exhaust is on. Car also has a good 4-2-1 and CRP so exhaust system should be as free flowing as it'll ever get with the straight through.
If I find I lose some power then I'll experiment with putting the CAT back or a Sports CAT to test different flow rates.
Probably prove absolutely nothing of course!!
Cheers,
Rob.
I'm just about to swap my baffled sports exhaust for a straight through. I had a RR power run yesterday and will have another once the new exhaust is on. Car also has a good 4-2-1 and CRP so exhaust system should be as free flowing as it'll ever get with the straight through.
If I find I lose some power then I'll experiment with putting the CAT back or a Sports CAT to test different flow rates.
Probably prove absolutely nothing of course!!
Cheers,
Rob.
Not being an expert on the matter I can't say for sure what the truth is behind the argument of backpressure being good or not. I can state my own observations though. Most exhaust systems are built to reduce noise from the engine, not to control backpressure (unless you have an old RX7). A well designed exhaust can be quiet without reducing engine performance (too much) - Carlo Abarth wrote the book on exhaust design back in the 60s and was the first to tackle the process on a scientific basis (the rules basically being constant diameter and smooth radius to generate laminar flow). Most performance exhausts I have tried have improved power but have also been poorly designed in terms of primary and secondary lengths. The only really good exhaust I have had was a genuine Abarth item.
Running an engine with no exhaust is not only loud but gives relatively poor performance - you need a certain amount of pipework before things start to really work well. Whether this is down to primary length tuning or to backpressure is something I haven't had an opportunity to explore.
As already mentioned, on turbo engines the whole set of rules basically get thrown out since "backpressure" is good in the short run from head to turbine. Overlap is generally undesirable since you have a pump pushing the mixture into the combustion chamber instead of the piston sucking the mixture in.
Running an engine with no exhaust is not only loud but gives relatively poor performance - you need a certain amount of pipework before things start to really work well. Whether this is down to primary length tuning or to backpressure is something I haven't had an opportunity to explore.
As already mentioned, on turbo engines the whole set of rules basically get thrown out since "backpressure" is good in the short run from head to turbine. Overlap is generally undesirable since you have a pump pushing the mixture into the combustion chamber instead of the piston sucking the mixture in.
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