Is exhaust pressure important for fuel economy and performance

kluivert

The manifold on the car is rusty, rusty as when you rub your figure across it the rust actaully comes off on your figure. and the back box is about a year and a half old. Ok condition.

My question is exhaust pressure important for the performance and ecomony of the car. The lack is lacking some performance and was wondering replace the system for a new one worth considering, not a modified system - waste off money on a 1.4 engine.

Any ideas.

Approaching 85k miles on the clock, only another 15k till I hit the big 100k.

Anan1

A leaky exhaust manifold will definitely have a negative effect on the running of the car. That said, just because your exhaust manifold looks like sh1t doesn't necessarily mean it's leaky. Also, I'd imagine you'd hear the difference. I'm not a mechanic, but my understanding is that engines are set up for a certain back-pressure, provided by the exhaust. The further back the hole, I would assume, the less the effect on back-pressure. None of this is gospel.

commited

You'd definately hear it if your exhaust was leaky - trust me, I had a small leak in my manifold and you could hear it clear as day.

To answer your question, yes back pressure does affect BHP. That's why it's amusing when all the chavs stick massive exhausts on 1.4s to make them quciker, when in effect it has the opposite effect.

crosstownk

Anan1 wrote:

A leaky exhaust manifold will definitely have a negative effect on the running of the car. That said, just because your exhaust manifold looks like sh1t doesn't necessarily mean it's leaky. Also, I'd imagine you'd hear the difference. I'm not a mechanic, but my understanding is that engines are set up for a certain back-pressure, provided by the exhaust. The further back the hole, I would assume, the less the effect on back-pressure. None of this is gospel.

Pretty much accurate, Anan1.

The nearer the exhaust leak is to the engine the louder it will be. If the manifold is leaking you will hear a noticable difference. You may also be able to see where it is leaking as there will be some sooty deposits there.

crosstownk

Anan1 wrote:

A leaky exhaust manifold will definitely have a negative effect on the running of the car. That said, just because your exhaust manifold looks like sh1t doesn't necessarily mean it's leaky. Also, I'd imagine you'd hear the difference. I'm not a mechanic, but my understanding is that engines are set up for a certain back-pressure, provided by the exhaust. The further back the hole, I would assume, the less the effect on back-pressure. None of this is gospel.

Pretty much accurate, Anan1.

The nearer the exhaust leak is to the engine the louder it will be. If the manifold is leaking you will hear a noticable difference. You may also be able to see where it is leaking as there will be some sooty deposits there.

I've no idea why this posted twice!!!

SouperComputer

its to do with a sound wave that travels from the end of the exhaust and back towards the engine. The exhaust length dictates at what engine speed the effect is applied.

As a result, valve timing may also be adjusted to make the most of this effect.

So, yes it can have an effect of fuel consumption.

gibbon75

If you use a not properly sized exhaust system,that can effect your fuel consumption.It usually happens because:

1. Because the lack of enough backpressure the exhaust gases will speed up,so they'll leave the combustion chamber thorugh the partialy open exhaust valve too early --> the fuel mixture is still burning inside --> worts fuel consumption,burned exhaust valves in every 5k-10k kilometers (when you lift your foot from the go pedal comes the typical popping noise)
2. If your car has a secondary lambda probe after the catalyc converter,because of the faster flow of the exhaust gases it will sense a leaner fuel mixture -->the probe will send a signal to the ECU and the ECU will add more fuel to maintain the correct fuel mixture

Although turbocharged vehicles not that sensitive for unproperly sized exhaust sytems,because the turbocharger itself generates a huge backpressure.This is why if you fit the same diameter exhaust system on a turbod car it will be less noisier.

kluivert

If i wanted to get a new system are the boys at quick fit and the likes ok or would it be better getting a geniune Opel system or an aftermarket system, something not too loud, I like a quiet car.

AMurphy

gibbon75 wrote:

If you use a not properly sized exhaust system,that can effect your fuel consumption.It usually happens because:

........

Although turbocharged vehicles not that sensitive for unproperly sized exhaust sytems,because the turbocharger itself generates a huge backpressure.This is why if you fit the same diameter exhaust system on a turbod car it will be less noisier.

In addition to all this, you may also upset the backpressure required for your EGR to work properly, this may (will) cause the mix to run hotter.
This and the fact that the gasses flow out faster as explained by G75, can result in overheating Ex valves, which then burn/errode and fail to seal, so after extended running your engine compression fails.
If you don't believe this, I have an injector partially fail, ran that single cylinder lean for a long time and burned not only the valve but the valve seat and head, which required a new head.

usually increasing the intake/ex bore is done in conjunction with adjusting the valve timing, sizes and ignition timing.
some of this farting about if not done properly can not only result in unburned fuel goin out the exhaust in teh overlap, but hot gasses going backwards out the intake valves... and massive backfires as the fuel/air mix in the intake explodes. Which might explain why thorough bred race car engine idle like sheeet and backfire a lot below their operating RPM range,

IDMUD

You never want back pressure on a four stroke, this is a myth perpetuated by people who don't know how an engine works.

What you do want is an exhaust that takes advantage of the exhaust pulses to increase gas flow.

mmenarry

IDMUD wrote:

You never want back pressure on a four stroke, this is a myth perpetuated by people who don't know how an engine works.

Eh? You might want to clarify that, expecially in relation to engines that have the intake valves opening before the exhaust valves have shut.....

IDMUD wrote:

What you do want is an exhaust that takes advantage of the exhaust pulses to increase gas flow.

Yes, hence 4-2-1 manifolds. Spaces the pulses of exhaust gases to flow better down the pipe.

M.

AMurphy

IDMUD wrote:

You never want back pressure on a four stroke, this is a myth perpetuated by people who don't know how an engine works.

What you do want is an exhaust that takes advantage of the exhaust pulses to increase gas flow.

A little knowledge is a dangerous asset.

Would you like to expand that theory to it's fullest, I don't think 2~3 lines does it full justice, You have our undivided attention.