Differences between a petrol and diesel exhaust??

[John LE]

I asked this question a few years ago and got no real definitive answer to my question.

Has anybody seen inside a Vectra petrol and diesel exhaust?

Reason being that I fitted a petrol Irmscher back box to my last Vec (2.2 DTi) and while I didn't feel an instant drop in power rolling roads seemed to tell a different story

Before fitting the exhaust I had 155bhp (can't remember the torque figure) and then after I had 143 (just before getting rid of the car)

Now there had been a good 2 years between fitting the exhaust and getting the second r/r done so i'm not sure if it was the exhaust that caused the loss in power.

Fast forward to this car (1.9cdti 150) and before getting my remap I fitted the exhaust.

My before (remap) torque figure was 316nm (which is about right for a standard cdti right?)

The after figure was 384.2nm which seems to be about 16nm down on what everyone else gets.

The only thing that I can think is that the, exhaust was fine with a standard map but is slightly restricted by the exhaust when remapped.

Would this make sense?

[paulvectrac]

Tbh mate a free flowing or better flowing exhaust is better for eny car tbh but it does seem weired you would loose power especially after you did the exhaust then went for a map when a map is wrote everything in taken in to consideration including any mods is there a chance your using diffrent Dirv ?

[John LE]

My before and after were both done at MAS, the rolling roads were done about half an hour apart.

I just feel that the engine is getting strangled. I'm assuming that a petrol box will have more sound deadening in it than a diesel one and that with soot etc coming out of a diesel exhaust things are getting a bit choked up

[rich r]

A non-turbocharged petrol engine usually has an exhaust system that provides a certain amount of back pressure at low gas speeds, in order to help with scavenging of the exhaust from the cylinders (by promoting a low pressure region between the exhaust gas pulses). Making an exhaust system too free flowing will lose you torque at lower rpm as a result. You can move the peak torque band up or down the rev range quite a bit with just playing with the exhaust pipe diameter, and how the manifold links together. For what is effectively a family car like the Vectra, GM would have aimed to get a good amount of torque at low rpm to help prevent stalling at junctions and generally make it easier to drive at slow speeds such as parking and round town. Obviously that compromises the torque available at high rpm, but most Vectras are probably driven well under half the max rpm most of the time, even the sporty ones.

Turbocharged engines (diesel or petrol) don't need this, as the turbo itself is a fair old restriction unless it's actually up to speed, and so the exhaust system after it should be as free flowing as possible, as you won't get much in the way of gains that way. The exhaust manifold design is about the only tuning you can do with regards to exhaust gas flow.

So I suspect that the silencer meant for a non-turbo petrol car that you fitted may well be more restrictive than the original one, designed for a turbocharged diesel...

[nutron]

I have had a couple of custom exhausts on the 1.9CDTI and have different results. The first exhaust they did what I asked them not to and put a big offset back box on which you could see a mile away and use small pipework. The result was firstly a noticeably poor performance low down and secondly I sued them and got my money back. The second exhaust I got was made much better but still a bit restrictive at the back, both were power flow but this one has worked well on two cars now.

My exhuast at the moment is only standard diameter with no CATs or silencers.

[vectraross]

love to see a video chap

[mickyh7]

On my old DTi the rear box was a straight though job. I dropped a spanner straight through it !
I tried to fit a petrol back box because it was free and it just made the turbo whistle really loudly.
I think they are only shaped the way they are so they fit the underside of the car,which is standard across the model range.
I even ran my car without the back box and it sounded no different !
Unlike a petrol model ?

[Mark-R]

That was like mine when i was putting my dual on my cdti i fired it up with only the flexy pipe on and it didn't sound much different lol

[vectraross]

you would of thought it b real loud tractor like lol weird that

[mickyh7]

you would of thought it b real loud tractor like lol weird that

Aye, the silencer is on the centre section.
Dealers wanted £111.00 for basically ,a straight through pipe.
How I laughed !
I cut off the rusty pipe end and welded the good petrol pipe bit on,lasted 2 years until I part exchanged the car !