Can someone Explain the Emission Readings?

2 days back I had the Dax through it's MOT inspection for the first time as it is now 3 years old.

It is a 1990 rover engine 3.9l running the Bosch "hotwire system" with lambda sensors in. The heads have been extensively worked to improve breathing and the whole engine balanced by John eales of Britihs Racing fame

Now the specifications it must meat on emissions are based on the engine year so must meet

Max % CO of 3.5 acheives0.31%
Max HC 0.12 acheives 927ppm

The engine has been idling for about 15 mins with fans cycling on and off before the test so she was HOT

So what do these readings say about engine tune - lean rich or OK?

Thanks for any advice

Cheers
Ian

Lean

To acheive 0.12 HC you must have cat converters.?????
STD camshaft??????

3.5 CO is a huge range to play in if the eng is std.
If you threshold on the HC is .12 that is getting low.
(If you have no cats.)
If CO has a max of 3.5 and you are .31 you are to lean,I would expect about 1.5co and 100-200 HC.
It is likely because of it being so lean it can have a lean misfire and will cause the HC to increase.

Lots of things will effect these readings Ian.
EG
To much ign timing at idle (high HC)
Air leak (low CO ,high HC)
Lean mixture(Low CO high HC)
Contaminated oil usually after an ign miss has been cured.(high CO and HC)
Ign miss (high HC)


Depending on the head work carried out it can effect it as well.
If ports have been enlarged it will drop the air speed and reduce the amount of swirl in the chamber at idle, this in turn will reduce the efficiency of the burn and cause high HC.

Play with your mixtures first,bring them up and the HC should go down.
If it doesnt start looking for a problem.
If you have a bigger cam in this eng with more overlap it may be causing the issue.
If that is the case you will struggle.
A CO reading of 3.5 is easy with out cat converters but it would be difficult to meet the .12Hc with out them I think.

Good luck (hope I haven't confused you)
Just remember HC is a measure of efficiency ,if you have modified the engine it will effect these readings.

One trick is as you have a large range on the CO you can run slightly on the rich side,this will fire the cats up and burn more HC off.

Jim

Many thanks Jim

I'm still learning so will need to read that a couple of times at least.

No cats on the car

Heads extensively ported (full race TVR spec on the build sheet)
Both valves increased diameter
Cam is a JE 102
said to be 43/79/79/43 302 degrees .500" lift

Discussion on the cam is here
http://www.gt40s.com/forum/gt40-tech-engines-induction-exhaust/20038-rover-engine-cam-specs.html


Another thing I noticed
When he inserted the exhaust probe it only went in about 10 - 12 inches before it hit the baffle in the silencer - could it be that this is too close to outlet and also giving strange reading

I don't get pinking so presumably not too over advanced but do get an odd miss at idle ( which is very slow at about 550 - 600 rpm)

So it all seems to give conflicting readings

Ian

The probe in 10" should be ok.
I would bring the revs up to 8-900, you only have to pass then put things back to were you want them.
You will need to fix the miss at idle.
If your manifold vacuum is low due to the cam (text book is 21in") but you may not get that.
If it is at say 14-16in due to the cam you may have a fight on your hands.

.12hc is low without a cat ,did this car pass before(when you first mot the car) or is this the first time it has had to pass this test.

Jim

Jim

This is the first MOT
It passed it's SVA 3 years ago and they checked the emissions then but would not give any results so I asked for a copy at the MOT but then got baffled! (easily done) The 0.12 or 1200 ppm HC does not appear to be a problem attaining with injection - not sure with a carb as I've never tried.

I also had the emissions run at a dyno day last year and it looked reasonable to those in the know over here See attached which seems to show emissions correct (Need to multiply by 14 or 14.6 (cant remember) to get air fuel ratio)

The twin 11 inch fans easily cope with the heat generated at idle and cycle on for about 45 secs then off for over a minute. When in traffic I override this with a manual switch and temp never goes above 80.

So one set of results seems to say lean and another reasonable hence the puzzled look on my face!

thanks again for the assistance

cheers
Ian

Sorry for the confusion

The car passed

The 0.12 and 1200ppm are the same number - just one govt department writes it one way and the Mot stations the other!

So the max is 1200ppm and the car got 927ppm so passed

What I could not understand is that the one reading can be so low (About 10% of the max allowed) and the other reasonable (75% of max allowed)

Perhaps I am just worrying about nothing! Should just drive it more

Ian