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MG MGB Technical - Relation between hydrocorbons and CO

Hi there,

I have a somewhat tricky quetstion considering the relation between HC and CO.
Firstly I must give some background why I need this information. In Sweden we have an inspection similar to the British MOT-testing. When changing engines and upgrading the power output, we need to have a document specifying the effect. That effect (kW) needs to be within the limits of the figures originally specified by the manufacturer. In my case I need to comply with the figures originally specified for the MGB GT V8, ie 137,5 bhp. Since my car is a tourer and is due for a Rover SD1 Vitesse engine, I think it will be quite obvious that the effect estimated by "pants feel" is far beyond 137,5 bhp. My battleplan is to retard the ignition to nil (TDC). My slim knowledge on engine behavior tells me that I will get a higher rate of hydrocarbons (HC) in the exhaust gases while doing so. I also need to go below 4.5% CO to pass emissions.
Q) Does my retarding of the igniton tell a good analyst of what I have been doing by readings the relation between HC and CO?
Erik

What I have noticed on the annual test of my V8 is that whilst the CO (also 4.5%) can be relatively constant from year to year the HC can vary wildly, from 200 or so at the lowest to over 1000 at the highest. I certainly don't alter the timing for the test, finding it and dwell is very consistent from one service to the next, but I do have to run at a higher CO to get decent pickup and avoid significant flatspots when accelerating from a cruise, weakening it using a gunsons CO meter immediately before the test then restoring it afterwards. Since one of the emissions changes to the MGB was to move the vacuum advance take-off from the carb, which gave zero vacuum advance at idle, to the inlet manifold which gives maximum advance of e.g. 20 degrees at idle, I would imagine retarding the ignition will worsen emissions which will require even greater weakening of the mixture to try and compensate for. This may well get so weak that it starts misfiring, which will greatly increase unburnt HC.

If your SD1 is from a later era than the V8 then it should really have lower emissions, much lower with EFI, without fiddling with anything. Unless the rules are so severe that they have to power to scrap a car considered to be a gross polluter just take it along correctly adjusted for running and see what the figures come out at.
Paul Hunt 1

Erik,

As I understand your problem, you want to limit the power (kW) of the new engine to meet the original specs of the GT V8 - 137 bhp.

Altering the timing will not achieve this - set the timing and mixture to specification so that HCO & CO are within limits.

Then adjust the throttle cable so that the throttle is not opened wide when the pedal is fully down - this will limit the power.
Chris at Octarine Services

Paul and Chris,

Giving the throttle cable some slack. IŽll go for that. Forgot to mention some vitals.
The former SD1 Vitesse engine once had the twin throttle disc EFI unit. As I bought it from a fellow country man of yours ( an Englishman abroad), he knew the proper value of the unit and therefore wanted it for his remaining SD1. I plan to run the Weber/Edelbrock 500 cfm carburettor.
I was actually thinking about blocking the second vacuum operated throttle disc, thus hindering the second port to kick in. HavŽnt figured out how, yet, but it might be done too. What do you think?

I think IŽll be able to benefit by combining both of your tips.

Thank you very much.

Erik
Erik

Erik
It seems odd that you should have to comply with the OE GTV8 specs when you patently have a different carb set up. Indeed the tester can very easily establish yours as a higher compression engine with different heads as well. One could of course, understand a law which sought to prevent people increasing the power and Chris' suggestion about the accelerator cable would be the easiest way of achieving this. There is also a variable fuel pressure regulator which works with carbs (basically it can starve the engine of fuel before full power is reached).

On the emmissions side, I suggest you should begin by assuming that the maker's ignition timing will produce something very close to the most efficient/combustion well within the specs of the GTV8. When it comes to hydrocarbons, the tester is basically looking at engine wear - so if the engine is in good condition, you need only focus on the carb and ignition settings.

You may get more power from the weber but I would still give some thought to fitting the fuel injection gear - it's quite a challenge but you would still get a good response from a single port hotwire system.

HTW

Roger
RMW

Roger,

Historicly the Swedish legislators congratulated themselves being on the forefront, fighting polution and promoting safe cars. That was in the seventies and there was even a stop in importing roadsters, claiming safety reasons. At home politicians claimed radical in theirs strive to achieve this. But with a slightly more global perspective, the "new" legislation was a mere copy of the rules in PRC (Peoples Republic of California),ie Ralph Nader. Today we have slightly more liberal view towards historic vehicles. But on the subject of my engine conversion. If MoT says dance, IŽll dance. I will even tapdance. I am not to reason why, just comply.

It had to be said

Regards
Erik
Erik

This thread was discussed between 10/11/2004 and 11/11/2004

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