backpressure

[5lab]

ok, so i know that exhaust backpressure supposedly helps an engine produce more torque, but i cant understand HOW. surely allowing the exhaust gasses to leave as quickly as they possably can is the most efficient way of emptying an engine of gas??

can anyone explain why backpressure is good?

[Rave]

It only helps make it more torque at the lower end of the rev range. It's because to make good power at high revs the engine needs to start opening the inlet valve before the exhaust valve has closed, to make sure that enough mixture can get into the cylinder. At lower speeds this can lead to unburnt mixture escaping out of the exhaust valve. If there's a bit of back pressure it stops this happening, and so more charge in the cylinder = more torque. At higher speeds it's a disadvantage, because the unburnt charge won't have time to escape anyway, while the extra work that the engine has to do to expel the gasses actually reduces torque and hence power.

So basically you want an exhaust which gives good backpressure at low revs and little backpressure at higher revs. Most modern superbike engines achieve this by having a valve in the exhaust which restricts flow at lower revs and moves out of the way at the top end.

None of this takes into account pulse tuning which is a whole other can of worms....

[Tumble]

probably most important on two stroke engines, as the pressure from the exhaust helps push the piston down (no valves) suishing the compressed charge back round the piston quicker and basically makiong the thing run faster.

[Rave]

probably most important on two stroke engines, as the pressure from the exhaust helps push the piston down (no valves) suishing the compressed charge back round the piston quicker and basically makiong the thing run faster.

Nah, pressure in the exhaust would never push down on the piston- at least not enough to compensate for the pumping loss of having to push it out again. However, the exhaust is even more crucial to a two stroke, because they're much more prone to unburnt mixture getting into the exhaust. Modern two strokes have a two stage approach to managing the exhaust pressure. At lower speeds a 'powervalve' effectively makes the cylinder longer so that the charge pushes the piton down for longer before it escapes up the exhaust pipe. At higher speeds the powervalve moves out of the way, and the pulse tuned exhaust comes into play. Two stroke exhausts have an expansion chamber before the silencer- they open up wide before narrowing down again before the silencer. The idea is that the shock wave from the exhaust pulse travels the length of the pipe, then bounces back on itself just in time to hit the cylinder as the next charge is about to leak into the exhaust port. A two stroke without a powervalve would be quite ridiculously peaky, it'd make good power from maybe 9000-11,000 rpm and that would be it, almost no power lower down the range.

[Tumble]

ah well.. I had it explained to me wrong then

[Butuz]

Thats only for N/A cars though, on turbo cars, back bressure is bad at low revs as well as high revs, as the more back pressure, the harder the turbo finds it to spool up. So as free flowing as possible from 1000rpm to 10,000rpm is the name of the game for me and moby

Butuz

[Zak33]

i shant try to explain HOW cos I cant....but I will explain what happens when you get it wrong ...in the world of Honda anyway

3 - 4 miles per gallon down the bog, no low down torque and a noisy exhaust.

Having spent 2 weeks on Honda tech with this exact stuff, you GOTTA fit the right exhaust parts to most cars.

the Kwik Fit/AllParts jobs are all a standard box with simply a different pipe to fit the middle section.

the specific ones for each car are MADLY DIFFERENT inside and it all goes Pete Tong when you try to mix it up

10,000 miles per year x 3 years average exhaust life for a back box.

30,000 miles

30miles per gallon = 1000 gallons of petrol

30,000 miles and 29 miles per gallon = 1034 gallons of fuel

34 gallons at £4 per gallon (average over next 3 years IF we're lucky) = £136 more fuel.

So unless you get an exhaust component VERY cheap, STICK WITH THE REAL DEAL ON UN MODDED ROAD CARS!!

Imagine IF it rusts in 18 months, and weld lets go, OR if it loses 3-4 miles per gallon

or both.

Your Local Anal Honda Institute Muppet...signing out

[Stoo]

So, is that what the dump valve is for at higher speeds then? Dump the excess before it knackers your torque?

Oh, btw Zak, Honda are about to open a mahooooosive place here, cars, bikes, lawnmowers, the whole chuffin' lot

[Rave]

So, is that what the dump valve is for at higher speeds then? Dump the excess before it knackers your torque?

Only turbocharged engines have dump valves. The main purpose of them is to bleed off excess pressure when you come off the throttle. If you don't have a dump valve, when the throttle closes the charge coming out of the turbo will have nowhere to go so you get a massive pressure spike. This will stall the turbo, and if the pressure gets too high it could damage the intake system.

[Stoo]

aha

[Lowe]

This will stall the turbo, and if the pressure gets too high it could damage the intake system.

You're a marketing departments dream

It's all BS, to make you cough up another 100 quid+

Turbos are NOT as weak as people will have you believe. A well made, well balanced turbo will not need a dump valve unless you're running at pressures well exceeding 30psi or even more. Even then a decent 360 degree bearing is up to the job. So long as the oil is changed frequently and is warmed up and down properly the turbo will last.

Most manufacturers only fit valves (usually recirc) them to stop the flutter noise of compressor surge (what people call wastegate flutter for some daft reason cos it's got sod all to do with the wastegate). Audi used to do this since customers kept bringing their cars back saying they were playing up when coming off the throttle!

[Rave]

You're a marketing departments dream

It's all BS, to make you cough up another 100 quid+

I knew if anyone was going to make me look a fool it'd be you Lowe.

TBH my knowledge of forced induction systems is not as good as it should be, what's the difference between a dump valve and a wastegate for example? Is there one? Do turbos actually stall if the pressure's not bled away, or is that a myth?

[Moby-Dick]

You're a marketing departments dream

It's all BS, to make you cough up another 100 quid+

Turbos are NOT as weak as people will have you believe. A well made, well balanced turbo will not need a dump valve unless you're running at pressures well exceeding 30psi or even more. Even then a decent 360 degree bearing is up to the job. So long as the oil is changed frequently and is warmed up and down properly the turbo will last.

Most manufacturers only fit valves (usually recirc) them to stop the flutter noise of compressor surge (what people call wastegate flutter for some daft reason cos it's got sod all to do with the wastegate). Audi used to do this since customers kept bringing their cars back saying they were playing up when coming off the throttle!

but they do sound nice
whats the official word on throttle responce improment ?

I know on some cars there can be a problem with dumping metered air , causing mixture problems.

btw have you got a pic of the pipe your ssqv was on ? I'm workign on how it'll be bodged into my system

[Butuz]

Sureley there is no denying that having compressed air at in my case 13psi, blown backwards through the turbo whilst the blades are doing 100,000rpm is not beneficial and if it can be avoided then it should?

For a start - its bound to slow the turbo blades down, which will increase lag slightly when you slot that gear in and get back on the throttle. Secondly, i am sure it must put just that little bit more pressure on your turbo blades/bearings - in which case, for the same amount of wear and tear on your turbo, why not fit a dump valve/recirc - and run your turbo at a slightly higher PSI than you would have without the dumpvalve?

Interesting discussion though.

Butuz

[Konan555]

Messing with the back pressure (well, getting it wrong) results in 'pulses' of exhaust gas which impeed flow.

This effect isn't so present on a turbo engine as the turbine 'smooths' it out.

Or something along those lines, I'll fetch the text book when I get a chance.

[Rave]

LOL Konan, the reason I'm always outraged at library budget cutbacks is that when I was 17 my local council got me (via a nationwide search) a book which a) secured my offer to attend the university with the best engineering dept in the country and b) taught me pretty much everything I know about swirl, valve overlap, squish etc.

Having said that though TBH, I learned a hell of a lot from the late great John Robinson's articles in Performance Bikes(RIP)....and the recent articles in Practical Performance Car have clued me in some more to the science of pulse tuning