useless induction/exhaust ramblings
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  1. #1
    1000+ Posts U Turn's Avatar
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    Default useless induction/exhaust ramblings

    Ok without any proper experimentation, I've just been pondering about the theory related to induction and exhaust mods. Wasn't sure whether this thread should belong in the Toad Pond instead due to it only being half serious.

    With regards to induction, IF the standard system was designed poorly and the oem filter didn't flow well, then a change to a better flowing filter/system should yield decent gains. Because obviously more air will flow into the engine and therefore with a corresponding increase in fuel delivery..more power. How much more, hard to say..but if you could double the amount of air into the engine, well you'd come close to doubling the power produced (though completely implausible with just a filter change).

    With regards to exhaust, say the standard system was designed really well i.e similar to rsc, gti180, gti6 etc with extractors, properly sized main pipe, smooth bends etc, with say the greatest restriction being probably the rear muffler (reflection type, for noise reduction with corresponding med-high back pressure), then my thoughts would be any increase in power you would get due to just changing the rear muffler to say, a straight through low-back pressure type, would only be the amount of decrease in pumping losses from the engine not having to 'push' the exhausts gases out so much. Now I'm trying to, without any proper experimentation or validation, quantify this pumping loss. Surely even at high rpm, it can't be more than a couple of kW's tops? I come up with this figure by imagining the suction ability of a 1800 Watt (1.8 kW) vacuum cleaner, and let me tell you, it sucks in a lot of air, very quickly. So I figure that the power cost of the restriction difference between a standard muffler on our hot hatches and a straight through muffler can't be much more than a powerful vacuum cleaner. I mean, theoretically if you could connect that 1800 watt vacuum cleaner to the exhaust of the car with a standard muffler, wouldn't that vacuum's suction be enough to overcome the standard muffler's restriction and therefore the engine wouldn't have to 'push' the gases out. Hence my piss-in-the-wind hypothesis that the standard muffler would only rob the engine of a couple of kW's?

    Therefore, I postulate that it is much more important to increase flow INTO the engine rather than decrease the restriction OUT i.e doubling the air into the engine will produce much more power than halving the restriction of the muffler. Obviously, this is only valid to a degree..i.e if the said restriction was a bananna lodged in the tailpipe, it'd be pretty important to decrease said restriction I've also realised that I've just invented the principle of the turbocharger 90 years too late.

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    Anyway, all of this is completely useless to anyone, thanks for reading. Does anybody actually have dyno results before/after a muffler change?
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  2. #2
    1000+ Posts PeterT's Avatar
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    I think you're absolutely spot on. Have you read any of Graham Bells' books? A big exhaust for a 2L engine is 2.25".

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    1000+ Posts tekkie's Avatar
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    ... more importantly, does anyone have dyno results before and after the vacum cleaner was connected to the exhaust tailpipe !

    (turbo exhaust?, will we see hoover plastered across the windscreens of wannabe performance hoons?,)


    now to semi serious.
    Tuned exhaust, is as important as tuned intake. Considering the rev range our cars operate in there is no ultimate "best" setup. Each one caters for specific task (low end torque/low end power, mid range torque/high end power, high end torque/highe end power, or a combination of). Add to this various cam grinds, different fuels used, size (or more importantly mass) of cars the possibilities of setting up a better than factory system to cater for all types of drivers become pretty small of anyone short of automotive engineer with fair access to some high end computing power.
    BUT...
    If you know what you want, and where you want the torque and power delivered (at the expense of all else) the range of options narrows and there is a good case to argue that Joe Blow may design better intake/exhaust combo than factory to suit his needs.

    ... now back to that vacuum cleaner idea, what size is the primary tubing, will there be a performance gained if we dont use the corrugated hose?, what if we enlarge the exhaust port and fit KN filter instead of the dust bag?
    .
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  4. #4
    1000+ Posts U Turn's Avatar
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    Quote Originally Posted by PeterT
    I think you're absolutely spot on. Have you read any of Graham Bells' books? A big exhaust for a 2L engine is 2.25".
    Nope haven't read his books. Yeah I remember seeing something on the net a while back which gave recommended sizes of pipe against engine size for road cars. 2.25 sounds about right and I think that's what the 6's diameter is standard. It's hilarious when people put 2.5" on daihatsu charades and it sounds like an air horn.

    Tekkie, ahh but the theory of conservation of energy says that it'll take 2kW of engine power to drive the vacuum in the first place Yeah I agree about the tuned exhaust, but I THINK it only matters up to the end of the extractors. Of course having a restriction back from the extractors/exhaust manifold would have an effect, but I don't think that having too big an exhaust pipe would change the powerband of the engine and the only negative effect would be horrendous noise. Only the extractors would have an effect on the power band of the engine because of the resulting scavenging effect being greatest at a certain powerband dependent on extractor design. I've heard people say, but I don't think it's true, that for example removing the exhaust pipe after the extractors would reduce torque at low rpm.

    Geez I would love to get my hands on my own personal dyno lab, a bunch of cars and equipment and someone to pay my bills while I could muck around with the excuse of testing.
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  5. #5
    1000+ Posts tekkie's Avatar
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    Quote Originally Posted by U Turn
    but I THINK it only matters up to the end of the extractors. Of course having a restriction back from the extractors/exhaust manifold would have an effect, but I don't think that having too big an exhaust pipe would change the powerband of the engine and the only negative effect would be horrendous noise. Only the extractors would have an effect on the power band of the engine because of the resulting scavenging effect being greatest at a certain powerband dependent on extractor design. I've heard people say, but I don't think it's true, that for example removing the exhaust pipe after the extractors would reduce torque at low rpm.
    I was under the impression that the piping past the extractors has as much effect on power/torque as the extractors themselves. I think of it as an extension of the primary and secondary pipes (tertiary?). So if you maintain that gas velocity between the exhaust cycles of each cylinder you will in the end achieve smoother flow of gas through the whole system. Very similar to intake idea. So in the end smaller pipe will give higher gas velocity but limit high RPM, and drainpipe will prevent continous gas flow at low rpm but give unrestricted venting at redline.
    .
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    1000+ Posts alan moore's Avatar
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    I would use the vacuum cleaner to blow in the intake for more gains as the exhaust gas is physically pushed out with the piston, but the intake is only at atmospheric pressure, so that 2lb of pound of boost would add 13% more power, addmittadly still losing two Kw to drive the vacuum.
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    1000+ Posts U Turn's Avatar
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    Quote Originally Posted by tekkie
    I was under the impression that the piping past the extractors has as much effect on power/torque as the extractors themselves. I think of it as an extension of the primary and secondary pipes (tertiary?). So if you maintain that gas velocity between the exhaust cycles of each cylinder you will in the end achieve smoother flow of gas through the whole system. Very similar to intake idea. So in the end smaller pipe will give higher gas velocity but limit high RPM, and drainpipe will prevent continous gas flow at low rpm but give unrestricted venting at redline.
    Tekkie, yeah I see what you're saying. I'm just not sure either way. I definitely agree that the length as well as diameter of intake system makes a difference on the torque characteristics due to the flow momentum of air increasing with a longer intake length, but whether this can be applied to the exhaust side after the extractors, I'm not too sure. My initial thoughts are that it's not applicable, due to the intake being a literal flow of air into the engine, however the exhaust flow is formed by numerous series of pressure pulses and with no scientific basis, my assumption is that though flow momentum of individual pulses are important, hence the extractors, the flow momentum of the overall volume would not really be there since the pulses break up that flow. Would be great to see dyno results from before/after unhooking the main pipe off the extractors.

    I've got a fair experience with fluid flow and continuous flow around free bodies, but I don't know if I'm murdering the theory by trying to apply it here, as I know zilch in the way of, what I believe is non-continuous gas flow in pipes. How good would it be to have input from the very people that design intake and exhaust systems for performance cars.

    Alan, you'd find that the suction/blow of a vacuum cleaner would not add significantly to boosting intake pressure. I don't think it's entirely accurate to think of cylinder filling to occur only by atmospheric pressure as the suction created by the piston racing downwards would be significant. This is why superchargers/turbo's have to compress the air first before any decent power gain.
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    You guys are funny!! It' not enough to have one thread on this, you've seperated input and output to the engine! lol

    Obviously i agree with what tekkie has said. The exhaust may serve a different purpose and be subject to different resonance than the intake, but it's still the same principle applied in reverse. The catch with out road car (and most race car) exhaust systems is that the pipes do converge at some point, generally pass through some baffling/cat/point of expansion and then try to re-establish the system to continue the extraction. If you cut the pipe at the headers/extractors, then yeah, you don't need to worry about the pipe. But just as tekkie said, my knowledge of this says that if you plumb it all together, then big pipe = low pressure(less extraction) but higher overall flow potential, small pipe = high pressure but lower overall flow potential. I guess this is how some people came to use the term "back pressure" when refering to oversized exhaust systems. If you dont establish the right resonance and all of the piping to achieve the same resonant points and pressure, then there will be some boundaries to pass that are ulimtately dertimental to air flow. The n of course there are the twists and bends in the pipe...

    It certainly complicates things well beyond the tuned individual pipe systems you see applied to drag cars, and i suspect it's still a point of much simulation and evaluation between practicing mechanical engineers. I suspect most of the road systems are trial and error rather than pen and paper.

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    1000+ Posts tekkie's Avatar
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    Quote Originally Posted by Cleanup_754
    You guys are funny!! It' not enough to have one thread on this, you've seperated input and output to the engine! lol
    was thinking the same thing !!!
    .
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  10. #10
    1000+ Posts U Turn's Avatar
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    I was confused before I started this thread, and I think I've twisted myself up more. Nothing new for me. Thanks for the input guys. I'm sure somewhere in the deep dark portals of the www there must exist some conclusive results from someone who, in addition to my curiosity, also has knowledge and most importantly, resources to test it out properly.
    Take the long way home....

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    1000+ Posts PeterT's Avatar
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    Quote Originally Posted by U Turn
    I was confused before I started this thread, and I think I've twisted myself up more.
    have a look at this website calculator!

    http://www.btinternet.com/~mezportin...st_length.html

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    Quote Originally Posted by U Turn
    Alan, you'd find that the suction/blow of a vacuum cleaner would not add significantly to boosting intake pressure. I don't think it's entirely accurate to think of cylinder filling to occur only by atmospheric pressure as the suction created by the piston racing downwards would be significant. This is why superchargers/turbo's have to compress the air first before any decent power gain.
    I think Alan's right, though. The only reason the cylinder fills is because the 'suction' causes the cylinder to have less pressure than atmospheric pressure, and the air moves from high to low areas of pressure. So the maximum intake pressure possible in an n/a car (depending on altitude of course) is the ~14psi of atmospheric - which of course it never reaches.


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  13. #13
    1000+ Posts U Turn's Avatar
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    Oh ok I see what you're saying, that regardless of the suction from the piston moving downwards, that pressure differential causing cylinder filling is limited by the value of the atmospheric pressure. I guess a little knowledge is a dangerous thing, and right now I'm freakin' lethal!

    So do you reckon connecting some sort of reverse vacuum cleaner that just blows air at a great rate (not necessarily compressing it first like a turbo etc) into the intake would add result in an increase in power output greater than the power required to drive the vacuum cleaner?

    Peter, thanks for that website. Haven't checked it out fully yet, but looks like it'll be great to learn and kill a few hours to see what results when changing different parameters.
    Take the long way home....

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    1000+ Posts alan moore's Avatar
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    Sometime around 1930, Phil Irving, a reasonably famous Australian engineer, working for Velocette at the time, hooked up a vacuum cleaner to a high performance 350cc single, to test the strength of a new big end set up. This raised the power from 22 to 30 Hp on the engine dyno. This historic engine was No. KTT240.
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    Quote Originally Posted by alan moore
    Sometime around 1930, Phil Irving, a reasonably famous Australian engineer, working for Velocette at the time, hooked up a vacuum cleaner to a high performance 350cc single, to test the strength of a new big end set up. This raised the power from 22 to 30 Hp on the engine dyno. This historic engine was No. KTT240.
    Alan! My vacuum cleaner musings was just an analogy, but it's amazing to find out some guy has actually tried it out!

    Anyway, I've emailed a whole heap of questions to the guys at Puma racing (excellent tech info website) and hopefully they'll have the patience to provide some info back which I'll definitely post here if/when I get it.
    Take the long way home....

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    having havd the oportunity to drive a car with and with out its exhauat in place i can say that it makes [email protected] loads of noise without it and is therefore very hard to tell what the performance is doing as it sounds like you are flying at 1500 rpm. it was an alfa sud by the way. padock bomb gone very bush (was as rusty as... well very rusty) exhast used to fall off regularly. the last time i drove it i drove it home 5km on a public, but dirt and very rural road with the exhaust rapped around the rear uspension. in this configuration the performance is drastically changed. the thing would drive (1.5 litre boxer btw) in fifth gear at 25-30kmph no worries where as with the exhaust attached it didnt like fifth till at leas 50. so talk distribution must have changed. i find this to be rather staggering and makes me wonder what is going on.
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    Greater intake "efficiency" (over atmospheric pressure) can be gained, on a moving vehicle, by connecting the intake\airbox to a forward facing scoop or a duct at a high-pressure area like the base of the windscreen.

    Look at any hi-po motorbike or race car to see it in action.

    So ducting from behind grille/bumper to front mounted air box, or base of windscreen to rear mounted airbox is likely to add more kW at the top end than
    larger filters\airboxes not receiving ram-air effect.

    It will add no kW on the dyno and minimal increase at road legal speeds outside that of the cooler air compared to the enginebay.

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    Quote Originally Posted by PeterT
    I think you're absolutely spot on. Have you read any of Graham Bells' books? A big exhaust for a 2L engine is 2.25".
    What peter said + my :
    Tuned length intake AND exhaust is important as tekkie mentioned. Get the pressure pulses right before you worry too much about what *should* be minor restrictions to the over all average flow.
    Most engine builders will look at the intake side of things when doing a head ie over size the inlet valve pay much more attention to porting well before they look at increasing exhaust valve size to lend a little more weight to your arguement that it is more important to let air in rather than out.
    A little more weight lent to your arguement, inlet restrictors! You cannot suck air at a velocity greater than the speed of sound but your sure can push it at greater than mach1. Thats why those evil little restrictors are put in the intake (and in turbo cars before the compressor and not after) and not somewhere in the exhaust.
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    Quote Originally Posted by 750sport
    Greater intake "efficiency" (over atmospheric pressure) can be gained, on a moving vehicle, by connecting the intake\airbox to a forward facing scoop or a duct at a high-pressure area like the base of the windscreen.

    Look at any hi-po motorbike or race car to see it in action.
    That's just doesn't sound right. On any bike or 'performance' car with an intake/scoop designed to use the vehicles velocity to increase inlet pressure/air flow, you will find the entire intake system is tuned for this. If the system is extremely poorly designed, and i suspect it will be on a lot of older cars (especially with regards to cold air intake...) then you might get an improvement. But on modern cars, the OEM system will not benefit from the addition of an intake tune disrupting forced induction device...

    As for the idea of it being more important to let air in than out... i think this is getting off the plot. The two things inter relate. The difference with most engine designs is more likely the margin for gas extraction is higher than for intake. Seriously, think about what the exhaust gases are made up of - unburnt fuel (good, we can re-use that), carbon monoxide (ahh... no thankyou), sulphur dioxide (no thanks), carbon dioxide(no thanks) etc. What i'm getting at is the bulk of the exhaust gas contains things that do not favour combustion, but infact retard it. If you leave this gas in the combustion chambers, then you're reducing the VE and decreasing the amount of fuel and air you can mix in the chamber. That means less explosion, so less power. You can't have one without the other, but it does seem the case that there is more tolerance in the exhaust systems of modern cars than the intakes - so you start at the intake and work backward.
    Last edited by Cleanup_754; 9th March 2005 at 04:33 PM.

  20. #20
    1000+ Posts Pugnut403's Avatar
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    Many newer cars have a restrictive intake system for underbonnet neatness, intake noise reduction, etc etc etc. By redus\cing this restriction you will gain a bit of power undoubtedly. New cars are not usually set up for maximum power unless they are sports cars. NVH, economy, etc are all usually more eminent in the minds of the designers than out and out grunt so you can improve on modern design if what you want is not what the designers wanted.

    Removing restrictions in the intake system or pushing air in like the "ram air" systems do will help increase volumetric efficiency. the theoretical maximum achievable is 100% VE, i nother words 1 litre of air at atmospheric pressure ofr every litre of cylinder you are trying to fit it into. As the difference between atmospheric pressure and the vacuum in the cylinders is all that is making the air come in, and the piston is only sucking for a short time, 100% is never achieved in conventional systems. On older engines about 85% was the best you could get but some modern intakes might be doing a little better now.
    The problem is that as you get near full, the pressure difference drops and so does the motive force that is moving the air, so the stream slows. That is why long intake runnes improve power low down, as the mass of moving air still wants to push and so packs a little more in. At higher revs it takes too long to get it moving in the first place so shorter runners are the go.
    By pushing the ari in even a little with a scoop or ram you are slightly increasing the pressure and so gaining a little VE but I doubt you would see 100% VE until you were travelling very fast indeed with a lot of air being pushed into the engine.
    A turbo of course just bypasses this totally and achieves VE of well over 100%

    The exhaust is very important for power also, but will rob heaps if it is wrong. 2 strokes have been done this way for ages now, and it is not so critical in 4 strokes but makes a big difference none the less.
    As you get a high pressure pulse of gas it is followed by a section of low pressure. The as is moving continuously but in a slightly jerky fashon. AS these pressure waves reach the muffler or the end of the pipe if there is any restriction some will be reflected back at a predictable velocity. If a high pressure wave reaches the port as it opens then there will be excessive backpressure and less gas will escape but if a low pressure wave is there at the right time than it will hepl suck gases out and improve power.

    This pulse tuning words on inlets too, by the pressure waves created by the inlet valve opening and closing. If a high pressure pulse reaches the port just as the gases are slowing before the valve shuts then it can help to ram a little more air in and increase VE just a little.

    A combination of inlet and exhaust pulse tuning can make a VERY big difference in power output at your chosen rev range, even on new engines. It will if taken to extremes produce a very narrow powerband though. Roundabouts and swings.
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    Having said all of that, how can you just outright claim that adding a ram intake will increase power? This is just like the claimed 10kW improvements for adding POD filters or new panel filters... The end of your very posts seems to highlight the fact these systems are tuned for flow, pressure and - an intergral part of acheiving this - resonance(pressure waves, as you called them). If you add an intake forcing NEW and different pressure waves into a tuned system, then you are disrupting it's established patern. As with the theory of "bigger exhaust/intake is better", you're more likely to lose efficiency and power than you are to gain it. Believe it or not, a turbo'ed engine is a tuned system - a system tuned for a turbo.

    The addition of the ram filter might appear, in your mind, to result in an increase of pressure at ONE part of the intake system. But what about the rest? I suggest that adding a ram intake to vehicles with tuned small intakes, or dual stage intakes, will give you the net effect of creating a back pressure and reducing flow to the manifold where you desire it. Let's not forget that race cars, especially at the top level where things like ram intakes make a difference, have their engines and intakes and exhaust tuned for such modifications.

    I don't think the addition of a ram styled intake system to any modern car will give you net gains in power, and especially not on performance variants. I'm hoping the forum users here can see that if you make one instrument in the orchestra louder, you run the risk of imbalancing the other instruments and ruining the effect...

    Sorry, but i think some of the recent suggestions on this forum are just outright stupid, and i think i'll leave you guys to adding your ram intakes, pod filters and replacement panel filters that gain you upwards of 1second in your 0-100 times, on cars with sub 8 second 0-100 time. Enjoy...

  22. #22
    1000+ Posts PeterT's Avatar
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    Quote Originally Posted by Pugnut403
    ..........increase volumetric efficiency. the theoretical maximum achievable is 100% VE, i nother words 1 litre of air at atmospheric pressure ofr every litre of cylinder you are trying to fit it into. As the difference between atmospheric pressure and the vacuum in the cylinders is all that is making the air come in, and the piston is only sucking for a short time, 100% is never achieved in conventional systems. .
    The latest M3 BMW achieves 105% at some points in the rev range. Very impressive for N/A car.

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    Budding Architect ???? pugrambo's Avatar
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    but is backpressure important ?

    i know in some cars i have driven if there is a hole in the exhaust or the exhaust exits short of the rear of the car they always seemed to backfire and carry on

    they were all carby cars so maybe it was just over fueling

    but i can clearly remember someone telling me once that back pressure is needed in a system to prevent the backfiring or popping on trailing throttle

    can anyone expand on this as i would love to know more about exhaust systems

    in regards to intakes if you can have forced iar like say cold air induction this has to help in regards to getting each cylinder closer to 100% capacity or am i on the wrong track
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    Quote Originally Posted by Cleanup_754
    If you leave this gas in the combustion chambers, then you're reducing the VE and decreasing the amount of fuel and air you can mix in the chamber. That means less explosion, so less power.
    But how can gas be left in the combustion chamber due to a restriction in the exhaust??? The piston goes up, and since piston displaces that volume, it pushes whatever is in the cylinder at the time, out. Sure a restrictive exhaust will make it harder for the piston to push out the gases, but that's just pumping losses. I can't see how any restriction in the exhaust will cause the gas to remain in the combustion chamber and reduce VE on the next power cycle!?
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    1000+ Posts U Turn's Avatar
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    Quote Originally Posted by pugrambo
    but is backpressure important ?
    I wish I knew that answer for sure too! Till recently I was completely sure that backpressure was only important in 2-strokes to ensure fresh intake isn't sucked through at the end of the previous cycle. I always thought it wasn't needed for 4-strokes because they use valves, camshafts but was recently read some info from an engine builder that said 4-strokes also have an overlap period of the exhaust and intake cycle and backpressure is needed to stop the new fuel charge being sucked out as well. Can anyone else add light to this?
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