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MG MGB Technical - EGR Valve

How does the EGR valve work?
Justin Stevens

Try http://www.aera.org/Members/EngineTech/edge1096/Page3.htm

Shows high manifold vacuum e.g. on the overrun i.e. decellerating (possibly also on high exhaust back-pressure e.g. increasing engine load and revs) opening the valve and allowing exhaust gasses to be injected into the inlet. These inert gases reduce the production of NOx (NOx + sunlight = smog), which occurs under lean burn conditions, by reducing the temperature of combustion. Also helps to control pinking/pinging.
Paul Hunt

Thanks very much, Paul. If my understanding is correct, if I disconnected the EGR valve from the exhaust system I could use it as a primitive anti-knock device?
Justin Stevens

Justin:all a disconnected EGR valve does is plug-up holes in your manifolds.What problem do you hope to cure? RIC
R E L Lloyd

Justin - from what I read it is already acting as an anti-knock device, so disconnecting it from the exhaust could make knock worse. Allowing air into the inlet through the valve instead of exhaust gases will weaken the mixture, allow even higher combustion temperatures, and make knock even more likely, I would have thought. What is your theory, and as RLC says, what are you trying to solve? If you have pinking now then maybe your valve isn't operating as it should, or just that the timing is overadvanced.
Paul Hunt

I'm running a lot of compression on my 1950 and the engine pinks under heavy loads, like when going up a hill under full power. I checked the ignition and it is reasonable, advances and retards all work as expected. Fuel mixture is OK, too. No trouble with overheating or hot running, just pinking. Camshaft is stock. If I rig an EGR valve and connect it to the exhaust insteadof leaving it open to the air, will I lose much power when it cuts in?
Justin Stevens

Justin-
Yes, because the EGR valve will decrease the size of the fuel/air charge, you will lose some power. How much you'll lose all depends upon how much of a reduction takes place. However, I suspect that you're trying to treat the symptom instead of the cause. If you're sure that your ignition is appropriate, then you've got to be experiencing lean running at full throttle. Which needle and jet combination are you running?
Steve S.

Pinking just means it is over-advanced taking into account all variables of engine, fuel, altitude etc. The book figure was only ever a pessimistic guide to the best for a particular engine, and after 30 years it isn't even that. A new BL car I had for some years benefited from advanced timing giving better performance and economy without pinking, whereas the roadster needs to be slightly backed off and even then it pinks slightly when on holiday climbing hills steeper than I have at home. This is part throttle pinking that I can stop by opening the throttle even further i.e. I am getting too much part-throttle vacuum advance. Are you using the best fuel grade you can get? Have you tried an octane improving additive? Have you compared your centrifugal and vacuum curves against the book figures? One of the common faults is weak or missing advance springs that allow too much centrifugal advance at too low a speed. Full throttle pinking is easy to deal with, just back it off until it stops.
Paul Hunt

justin:"conect it to the exhaust" where is your EGR valve conected to now? Is it possible you are talking about something else?check a few of the catalogs to be sure.RIC
R E L Lloyd

Steve S-
The needles are #5 and the jets are AUD 9141 front and AUD 9142 rear.

Paul Hunt-
The ignition works as it should.

REL Lloyd-
It is a 1950cc 18GD engine without an EGR valve. I was wondering if I could put one in to stop the preignition under heavy loads.
Justin Stevens

Could it be that with all the various mods and tweaks on your engine, the spark plugs are getting a bit over-hot? It might be worth trying a set which run colder. If the plugs are not the cause, this check will not cause problems, just a possibility that a plug might oil-up.

Exactly what number, and whether up or down the numerical range, depends on what you are using - but what you need is a rather shorter length of insulator visible inside the nose of the plug - not sticking out less, but recessed less, so that less insulator is exposed to the hot gasses.

Might not be anything to do with it, of course ...

Cheers, Tim
Tim Stevens

Justin-
The jets and needles that are in your carburetors are the Original Equipment specification items for your engine in 1798cc form. Their fuel flow isn't adequate for a 1950cc engine running at full throttle. You need to go richer. Which ones you need is dependent on which camshaft you're using? What headwork has been done? Intake valve size?
Steve S.

You are hoisting yourself with your own petard. Increased compression ratio increases the liklihood of pinking. Injecting exhaust gases into the intake has the effect of reducing compression ratio by replacing some of the explosive mixture with an inert gas, and this does sap power. Retarding the ignition also saps power. But EGR valves are carefully tuned by the engine manufacturers to achieve a specific effect, something you will not be able to do. Using a standard EGR valve on a modified engine will not have the same effect as on a standard engine. But by retarding the ignition you can continually tweak it until the pinking just stops. It just seems so much easier and cheaper, than to go to the trouble and expense of fitting an EGR valve for an unknown effect. But then I'm both tight and simple.
Paul Hunt

Steve-
The cam and the head are stock.

Paul-
If I retard the timing, I will lose power, and that would defeat the purpose of building a 1950, right?
Justin Stevens

Justin - as you suspect an EGR valve also reduces power as it is like running with the throttle at less than fully open. I would still expect you to get some benefit out of the larger capacity, just not as much as you might wish, unless you can get 100 octane. If the pinking happens at a range of engine revs then it simply is too advanced for its design and the fuel being used. If only at a certain band of revs then there is the possibility of tuning the centrifugal advance curve to eliminate that spike, as it were, until you *do* get pinking on full throttle right across the rev range. Then you back it off till it just doesn't.
Paul Hunt

Justin-
With a stock camshaft and those smaller 1.565" intake valves your engine must pull like a diesel! However, with such a high compression ratio I'd recommend that you send your head to Peter Burgess to have the combustion chambers modified. If that doesn't solve the problem you can try rigging up a knock sensor to trigger an EGR valve. In the meantime, use the highest octane fuel that you can get and add some octane booster to it.
Steve S.

I would love to rig up a knock sensor, but to control the timing not an EGR valve. With knock-sensing timing you will always be at the optimum regardless of engine spec, condition, throttle opening, revs, load, fuel grade, altitude, and everything else.
Paul Hunt

Paul

Check out http://www.jandssafeguard.com/ They are supposed to have a knock control sensor. Info from another thread on or about 10 Dec 2003. Not sure if it was an A or B or C thread.

FWIW

Larry
Larry Hallanger

Interesting, and not that expensive, I've emailed them. Thanks.
Paul Hunt

I have to agree with Paul. Rigging a knock sensor to a small computer to control a linear solenoid for progressively retarding the ignition timing would make more sense than trying to figure out which EGR valve would dump just enough exhaust gas into the fuel/air mix to prevent preignition. However, it would take some creative design work to accommodate the geometry and rig a solenoid to rotate the body of the distributor. It'll be interesting to see if anybody actually does this.
Steve S.

Steve,
if we start using electronics to control ignition timing, it would be much easier to initially over-advance timing just beyond specs and have a module intercept coil (-) to add an appropriate delay based on rpm, knock severity, cranking. Disconnect dist. vacuum advance and instead connecto to module and you have complete timing control. It certainly would be interesting to see a machanism rotate the entire dist. though!
Joaquin

I realise that is a bit tongue in cheek (I hope!) but BL used stepper motors to control enrichment and fast-idle on carb-equipped cars in the 80s. I see no reason why the same thing could not be used to move the points plate inside the distributor just like the vacuum capsule does now, and (size permitting) mounted in place of it.
Paul Hunt

Steve

The J&S system does it all electronically. To quote from their site:

"Using a single knock sensor, the system detects the onset of detonation and retards the timing on a per cylinder basis. A mode switch lets you select a maximum of either ten degrees or twenty degrees of knock retard. In the ten degree range, the unit retards one, two, or three degrees per ping. Double that for the twenty degree range. The system is always trying to re-advance to stock timing. In the ten degree mode, it re-advances at the rate of one degree every twenty revolutions."

It does appear to open up the possibility that Joaquin suggests. The basic questions are 1) how much change is there in the advance over the entire engine operating range? 2)What should the initial mechanical timing setting be?

If anyone tries this I would be interested in the results, and I am sure others would also. Please post on this BBS.

FWIW

Larry
Larry Hallanger

Good questions, although the first would appear to be answered in the quote i.e. switchable between 10 and twenty degrees. If it is always trying to re-advance to stock timing surely that has to be the maximum possible advance obtained with centrifugal and vacuum advance which can be as high as 39 and 10 degrees at the crank, half that at the distributor. But the 10 degree range has nowhere near enough retard to get back to the minimum of 6 even assuming distributor degrees, and the 20 range doesn't have enough if it is crank degrees. But because it looks to be electronic retard maybe you still keep the centrifugal and vacuum advance mechanisms, it takes the points signal what ever the amount of advance that isn giving, and electronically retard from there only if it detects knocking. In which case ideally you would want to overadvance it such that *without* the unit it would be knocking throughout the range as that would be the only way you could ensure that *with* the unit you were at the maximum possible advance for any given combination of circumstances.
Paul Hunt

Paul

Sounds correct to me. And from the other thread as I remember this is how it was being discussed.

Larry
Larry Hallanger

This thread was discussed between 05/01/2004 and 15/01/2004

MG MGB Technical index

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