The V8 Owners Forum -

My stag recently failed its MOT with emissions of 6.5 - so I bought a Gunson Gastester and plan to adjust carbs as necessary. For V8s with two exhaust pipes I've read that you have to take two individual readings (one from each pipe) and then calculate the average of the two. But I've also read that you have to use the tester on both pipes at the same time to get one reading - this is done by adding two home made pipes to the gunson pipe using a Y-piece. Can anyone confirm which is the correct method. Cheers. Dave

Sorry but this sounds strange
What age is your triumph?

The MOT emissions test should be based on the year of the car or year of the engine whiichever is older

Ian

Hi,

The MOT manual states the following regarding dual systems-

"The MOT limits prescribed relate to the total exhaust gas being emitted by the vehicle.
If a vehicle has a dual exhaust system, then the emissions from the tailpipes should be averaged. This is done by adding together the readings and dividing by two, eg
1st pipe emits 6% CO, 400 ppm HC
2nd pipe emits 4% CO, 500 ppm HC
Average CO reading is
6+4/2=5%
Average HC reading is
400+500/2=450ppm
The Results of this type of test must be entered manually onto the VTS Device."

All the V8, V10 and V12 i test at work have balance pipes fitted so the readings tend to be identical more or less.

Cheers John

Thanks guys for replies.

My triumph is a Stag 1977

You just need to get below 4.5%

No idea on how accurate those Gunsons thingies are though. A simple carb adjustment should get you there.

john 215 wrote:Hi,

The MOT manual states the following regarding dual systems-

"The MOT limits prescribed relate to the total exhaust gas being emitted by the vehicle.
If a vehicle has a dual exhaust system, then the emissions from the tailpipes should be averaged. This is done by adding together the readings and dividing by two, eg
1st pipe emits 6% CO, 400 ppm HC
2nd pipe emits 4% CO, 500 ppm HC
Average CO reading is
6+4/2=5%
Average HC reading is
400+500/2=450ppm
The Results of this type of test must be entered manually onto the VTS Device."

All the V8, V10 and V12 i test at work have balance pipes fitted so the readings tend to be identical more or less.

Cheers John

John

I believe you are a tester and hence would know.


BUT never has my twin pipe GT40 been tested on both pipes
There is no balance pipe fitted so it is 2 by four into 1 sets - sure with cylinders 2 and 3 from each side swapping to the other side to get the correct pulsing - alternating down each tail pipe.

Does this mean it has never been "tested" correctly?

Thanks
Ian

Hi Ian,

Going 'by the book' no, but very few tester i know would test on both pipes, even if they should

The other one to remenber is that if a vech is fitted with a 'Q' plate then only needs a visible smoke test, ie. dont need any form of CO, Lamba or HC test. Just check how much smoke visually is coming out of the exhaust and if your opinion its excessive, any doubt should go in favour of the owner of the car.

Cheers John

Results of tests are:

Right Pipe - 2.5
Left Pipe - 7.7

So average would be 5.1, which is under the max value (6.5). The MOT failure a few weeks ago was a reading of 6.7. I wonder if they only tested one pipe! Anyway, I've now got the left pipe down to 3.6, so should be OK.

Hi,

IF it was to fail again, point out should be an average of both pipes, so two print outs should be available of the test. IF they cant supply this then ask politley if they had done a avarage test IF they refuse to do then ask for an appeal form.

Cheers John

i would of bought a wideband instead of a gunsons thing. The one i had was horribly tempramental.

Wideband certainly more useful.

But it doesnt test for CO.

Whilst the AFR a wideband displays should be enough for MOT purposes....it isnt always.

Then again, I think those cheapy Gunsons testers measure CO2 and estimate CO based on that.
Pretty sure they dont actually test CO.

I would say the correct method depends on the configuration of your exhaust / inlet system.

If you have a dual-plane manifold (I don't know if the Stag V8 does), and two separate exhaust systems, then each tailpipe has 50% from one carb and 50% from the other. So they ought to measure the same....! If it's a flat-plane crank, then each tailpipe is 100% from one carb, so you could tune the carbs independently.

Need to know more about crankshaft, inlet, exhaust....

Chris.

You have something wrong on the left hand bank possibly a Stromberg that is set to rich or faulty,this car as a exhaust balance pipe fitted but it is not enough to get both banks to blend perfectly into one at the tailpipe as far as CO is concerned,this engine should be quite happy to run a idle CO of 2.5% anything more and your just wasting fuel

stevieturbo wrote:Wideband certainly more useful.

But it doesnt test for CO.

Whilst the AFR a wideband displays should be enough for MOT purposes....it isnt always.

Then again, I think those cheapy Gunsons testers measure CO2 and estimate CO based on that.
Pretty sure they dont actually test CO.

As regards the WB sensor, doesn't CO generally follow the AFR on a non cat. engine?

I used my wideband to set my RV8 to an AFR of 14.3 (ish) which the tables reckoned would be around 1% CO. Sure enough come MOT test time that was what i got - actually 1.05% i think in the end...