Mot Testing with multiple exhaust pipes

[Mirez]

Just a random question and curious if anyone knows how they do this? Obviously they normally shove the unit up the pipe but how do they measure the soot/gas if you have 2 or even 4 exit pipes?

[insanitybeard]

Purely speculation on my part and I suppose it depends on how bothered the tester is but I wonder if they'd stick the probe up one of the exhausts and then stuff some rag up the others to 'force' the bulk of the exhaust gas to come out of the pipe with the probe in, or whether they'd assume each of the pipes would have a similar quality/volume of gas coming out of it and therefore multiply the readings by the number of pipes? I'd be surprised if this was the case though because the gas analyser printout would be showing untrue/ incorrect readings. Perhaps they just get the readings from one outlet pipe and if the readings are good enough for it to pass and there's no obvious signs of smoke or unburned fuel they figure near enough is good enough and let it go!

[bigdave982]

Surely the ppm reading should be the same  regardless of how many holes its coming out of
Only the sheer volume of gas will be lower, not its composition

[insanitybeard]

True enough, the only thing that I can think of that that might not take into account would be if there was a hole or leak in one of the the exhaust pipes/boxes and not the others which might screw up the lambda readings and change the gas composition slightly, depending on which exhaust box/ pipe was being probed.

[Chrispb]

Mark am I right in thinking what you talking about is when say on a V8 the left pipe is for the left bank of cylinders and the right pipe for the right bank in which case there could be a difference in emissions.

[insanitybeard]

Good point, in which case maybe they'd need to run two separate emissions tests, one per side to check for any obvious differences, depending on how the pipework is laid out.

[Mirez]

Chris, its an open ended question - just one I was rambling about with a bud the other day and put here just for the chat.

I think Dave makes a great point, its not the volume of gas but the composition. Since most V6 and V8 engines merge to a single pipe at some point I suppose that also moots that point but what about on a diesel engine where the test is conducted at full revs? With a pipe on either side surely the volume of particles within the gas exiting would be lower?

[insanitybeard]

Yes, I see what you mean- as the diesel emissions test is performed by measuring the density of smoke/ gas transparency within a tube instead of actual gas analysis then a multiple 'exit' exhaust will yield a lower smoke density reading if only one outlet of several is being probed.

[bigdave982]

Chris, its an open ended question - just one I was rambling about with a bud the other day and put here just for the chat.

I think Dave makes a great point, its not the volume of gas but the composition. Since most V6 and V8 engines merge to a single pipe at some point I suppose that also moots that point but what about on a diesel engine where the test is conducted at full revs? With a pipe on either side surely the volume of particles within the gas exiting would be lower?

dont think so ,the composition is the same. If you were trying to collwct a specific amount it would just takw longer  from the single.pipe of a twin pipe system than single. Thw only time it would.matter is from V engine systems with one exhaust system per bank of cylinders, 2.8i capri for exhample

[Mirez]

Ok, so based on that - how to they test them? Two tests?

[bigdave982]

Pr3sumably yes, one up each pipe and average unless one side is  horribly out☺
Idve thought on those pc based testers you just tell it multi exhaust system and it takes you thru the test

Of course im just a lorry driver so probaly wrong

[Chrispb]

Pr3sumably yes, one up each pipe and average unless one side is  horribly out☺
Idve thought on those pc based testers you just tell it multi exhaust system and it takes you thru the test

Of course im just a lorry driver so probaly wrong

That brings another question Dave V8 Scania with twin eminox stacks how to get the tube up there.lol

[bigdave982]

Carefully