26 replies to this topic

[bigmotherwhale]

im not exactly sure why you need back pressure, but when my exhaust fell off at the bottom of the exahust, performance was affected, i think it might have to do with pulse tunning and reversion, i didnt think you lost power with a strait thru, (no back box) because the tube is the right length

[sidney]

just tell him to turbo it

the turbo acts as a centrifugal silencer and that sounds great through a open pipe

[998dave]

... u need a bit of backpressure with a 5 port/ a series engine for it to run beautifully .

and how does back pressure increase the efficiency of an engine?

It's to do with wave effects in the exhaust system. Any sort of exhaust pipe improves performance, believe it or not, else why would a dragster run headers at all.

If you can set up waves in your exhaust pipe, then you can use reflected waves to scavenge exhaust fumes, and pull exhaust gases out of, and air fuel mixture into the cylinder. This is normally tuned for one particular engine speed, and can be set up to produce maximum power or torque, or just to iron out flat spots.

A similar effect can be achieved using the inlet manifold, witht he added benefit that it's possible to alter the inlet manifold length, to wave tune for several different engine speeds.

Also, original poster, you'll probably want at least an absorbtive muffler to take out some air rush noise, this doesn't need to interfere too much with the exhaust flow, essentially you want a perforated tube inside a muffler, and surrounded by exhaust wadding. If custom built you can hide this up into the rear subframe and shape it to suit the tailpipe exits that you want.

Dave

[jaydee]

Sorry for my poor English, but a friend of me was making a offer. He build a open exhaust pipe, with 50mm exhaust. Without dampers/mufflers. Direct from the exhaust manifold.

I said yes, let's try that.

Now I'm curious how that sounds, the engine is a 1300 Metro A+. I know it will not be a comfortably sound, but is it a impressive sound of racing? Or what? What is your personal opinion?

Anyone?

I've tried a 1,65'' straith through pipe many years ago, it was the louder thing i've ever drived.
I say no, dont do it, performance will be crap and fuel consumption will be a lot higher.

Edited by jaydee, 24 June 2009 - 11:23 AM.

[SolarB]

If you can set up waves in your exhaust pipe, then you can use reflected waves to scavenge exhaust fumes, and pull exhaust gases out of, and air fuel mixture into the cylinder. This is normally tuned for one particular engine speed, and can be set up to produce maximum power or torque, or just to iron out flat spots.

Dave

That's pulse tuning, not back pressure.

[Asphalt]

If you can set up waves in your exhaust pipe, then you can use reflected waves to scavenge exhaust fumes, and pull exhaust gases out of, and air fuel mixture into the cylinder. This is normally tuned for one particular engine speed, and can be set up to produce maximum power or torque, or just to iron out flat spots.

Dave

That's pulse tuning, not back pressure.

Right! And for that you need at least some sort of pipe work (look at a F1 exhaust and it's shape... Fascinating!).

Four stroke engines need anything but back pressure! Read the exhaust chapters in David Vizards book... Fascinating stuff!

Edited by Asphalt, 24 June 2009 - 11:59 AM.

[Ethel]

Yep, It's not back pressure, it's velocity that helps scavenge the cylinders. The faster the gas is going, the more inertia it has, and the harder it is for it to turn tail and head back to the engine that's what creates the suction to "vacuum" the cylinders clean.

Shock wave tuning also relies on inertia - when the inlet closes the remaining gas in the inlet gets squished against it, like shoppers in the January sales, before bouncing off back up the manifold. The aim of shockwave tuning is to keep that "pulse" compressed and return it down the inlet at the right time to make it past the valve.

It'd be nigh on impossible to pulse tune an A series siamese port.

We still want air in the inlet to go through the engine but we just want rid of exhaust gas so there's nothing to be gained from exhaust pulsing.

[bigmotherwhale]

nice explanation ethel, thats kinda how i understood it, what the mini engnine needs is a valuvar conduit invented by nikola tesla its a one way valve with no moving parts, look it up, its a brilliant design tho i think kinda expensive to make.

[91neon]

slight hijack then but if ive understood this if i keep astandard 998 exhaust and change the back box eg cherry bomb or rc40 itl knacker my cars driveability?

[rocket powered]

Sorry for my poor English, but a friend of me was making a offer. He build a open exhaust pipe, with 50mm exhaust. Without dampers/mufflers. Direct from the exhaust manifold.

I said yes, let's try that.

Now I'm curious how that sounds, the engine is a 1300 Metro A+. I know it will not be a comfortably sound, but is it a impressive sound of racing? Or what? What is your personal opinion?

Anyone?

i cut all most the pipe off the other month and started it up (standered exhaust) all it sounded like was an old morris minor.... now i got my new back box on it sounds like a rally car

[rocket powered]

slight hijack then but if ive understood this if i keep astandard 998 exhaust and change the back box eg cherry bomb or rc40 itl knacker my cars driveability?

my dad used to have a mini marcos with a cherry bomb in the middle, he had to put a cooper silencer on, it was still VERY loud, not as loud as his new car tho

[JohnDoe]

Thanks for all your replies. I manage a maniflow LCB and all the stuff of stage 1 kit.

Now I'm curious...