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MG MGB Technical - Pre-ignition knock

I brought this up on the MGA board and got some helpful info. I realised that some MGB'ers here may not monitor that board and I would appreciate any additional input.

In my A I run an 18V with 7k miles since rebuilt last winter. 9 to 1 compression, flat top pistons, head "flowed" locally by an experienced shop, Piper 270 cam kit (with springs) origional MGA SUs with "rich MGB" jets, stock exhaust, fresh Aldon 101BR2 point distributor, Ford 5-speed, 3.9 rear. I run the best gas avalible in So. Cal. which is only 91 octaine.

When the engine gets warm I get pre-egnition knock on slight acceleration. On a warm day, after several miles at 70 to 80 MPH it will clatter like a diesel! Seems to have gotten worse lately (which could be due to a bad tank of gas).

I Had my shop dynamicly check and re-set the timing and slightly retard it off spec. I am afraid of damaging the engine. To those who did not see this question before, any suggestions for a fix? Diagnosis? Thank you very much.
Steve
Steve Meline

And well you should be afraid of damaging your engine. Better retard it a bit more. And find a shop that specializes in curving dizzys if your performance drops unacceptably under part throttle/lower RPM.
Tom

Without modifying the combustion chamber possibly quite significantly there is no cure for pinking other than retarding the ignition ... or increasing the octane of your fuel. Retarding the ignition will always sap performance, worsen mileage and increase running temps. Any distributor 'curve' (actually two straight lines joined by a 'knee') are only an approximation of what is ideal for any engine, and will be slightly different for each engine. You will be lucky if the actual curve touches the ideal at two points, mostly it will only be one. If the lines *cross*, that is when you get pinking. When you back the timing off the remove the cross-over there will be gaps betwen the curves at other points which will be causing unneccessarily lost performance etc. Whilst it's possible that recurving the distributor for your specific engine will reduce some of these negative effects, it is a very long-winded, time-consuming and expensive process, unless you do away with the vacuum advance in which case it merely becomes ordinarily long-winded, time-consuming and expensive.

The *real* solution, at some expense and engineering, is to fit a knock-sensing retard ignition system which works on a closed loop basis and automatically compensates for ll aspects of an engines characteristics, fuel, altitude etc. The electronics are available at a cost of several hundred dollars, but you need sensors in the right locations (tricky), and mods to the distributor (not too dificult).
Paul Hunt 2

Hi Steve.

I hope to find the time to investigate the temperature of the air that the carbs are taking in as I suspect that it may be a factor.
It only needs a digital thermometer with a remote thermocouple sensor.. has anyone done this ?.

My experience with A series engines (which burn a little oil, as the 'B' series does) showed that another factor was the rapid buildup of coke, possibly as a result of using modern high detergent oils.

Don
Don

Steve, what thermostat are you running? 165,180,195? Get as much cool, out side air to the carbs. What total ttmming, with vac. working are you running at 3,000-3,500 rpm ? 33-36 deg.? Are you running elect. ing.? whos? Have you re-torked the head & adj. the valves? What year head & part # on the head are you running? cam 1130? & whos head gasket, Payen is thicker & will drop your comp. a bit.
Glenn Towery

Does the knock occur at specific rpms, or across the board? Does it go away altogether above, say, 3000 rpms? If the experience is the most dramatic from 2000-2500 rpms, then you probably need a different distributor. The 101BR2 didn't work for me either, and my setup is very close to yours. Email me direct if you want more info.

Jeff Schlemmer

Thanks for your time men,

Tom, I havn't noticed any flat spots in power, pulls strong to 5500, then I chicken-out. At this point I don't know where to start finding a dizzy re-curve shop in So. Cal. Anybody know of any?

Glenn, I'm running a 165 thermostat with blanking sleeve, I also installed the carb-side air ducts to bring ambient cooler air to the SUs. I don't recall the advance, which was measured a few months ago. It sounded right at the time. The head was re-torked at 1k miles, about 6k ago. The engine and head are matched susposed to be from a '73. I don't know where to look for the head part #, I just found "PAT No 565394", engine #: 18V6772-l26160. I don't know what gasket was used but can find out.

Jeff the knock is more noticible below 3000 RPM, in the range you refer to, but probably because things are quieter then. I believe it continues thru the range. What did you do about a distributor when you pulled the 101BR2? (I thought they were the best).
Would I do any better w/ an Aldon Ignigtor solid state unit?

Paul, I'm considering a knock-sencing retard unit. Jeff Becker is putting one in his S/C 1800 A. (I look forward to hearing from you Jeff on its success.) I agree w/ Bill Spohn's previous statement that I "shouldn't" need one, but I've got to do something. More ideas? Thanx
Steve

Steve Meline

"Clattering like a diesel" sounds like a massive amount of pinking/advance. Going back to basics, I'd be inclined to check that the timing mark is indicating a true TDC position and hasn't got misaligned after all that engine work.
Steve Postins

If it pulls strong, retard it until it quits pinking. You can advance a little then if you want to, now that you know where it stops, and hope for a better tank of gas next time.

When I use thin oil in the winter, my rod bearings complain when the oil gets hot. It sounds a bit like pre-ignition. How's your oil pressure when hot?
Tom

Steve, I tore down my old distributor and recurved it to suit. I used an exhaust analyzer to find the rich spots on my rpm curve and kept changing the curve until the engine would burn lean. I ended up with a great distributor curve and #7 needles in the carbs.

The end product was a distributor with 11 degrees of mechanical advance and Mr.Gasket silver springs from the #928G advance curve kit. Its a great curve that starts AFTER idle (easier to set the timing) up to about 3050 rpms, but its a little different on every distributor. Sometimes you may need to add/remove weight to get it right.

Some people believe Aldon provides the best distributor, but really they produce the only distributor. Who else sells recurves?

The next problem may be your vacuum advance. How many degrees does it offer. Any more than 10 distributor degrees is way too much. I find that the 41339 advance unit with 7 degrees at a low vacuum level works great in performance applications, hooked up to ported vacuum.
Jeff Schlemmer

Thanks Jeff. I will have to get some help with thoses approaches (sp.).

I added some "104+" octaine booster and a little Marvel Mystery Oil to this tank of gas and the ping decreased. Does that help the diagnosis?

Man! Does it run strong with thoses additives!
Steve Meline

Can't speak for that octane booster but the local Castrol stuff seems to make no difference. I can imagine that anything that stops the 'clattering like a Diesel' pre-ignition will improve performance, as a significant amount of energy from the fuel is being used to burn through the tops of your pistons instead of pushing them down!
Paul Hunt 2

This thread was discussed between 06/01/2006 and 13/01/2006

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