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MG MGB Technical - knock sensor

Dear Anyone, particularly you electronic gurus! Has anyone tried to make a 'knock' sensor? I think it might be particularly useful for the engine builder and tuner.I bought a very old copy of 'Zoom', which is an Australian publication. Mainly aimed at owners of 'Japanese' sports cars after more grunt. What caught my eye was an article on how to make a 'knock' sensor, using parts from GM, quite cheap really.But before I embark on this I thought I might ask you all, am I wasting time energy etc? Mike
J.M. Doust

Mike I use a knock sensor on my supercharged B and while it will pick up detonation with flashing lights and audible buzzer it is not as useful as it is on later style engines. There are too many other noises normal to a B that set it off even on over run. I dont think I would trust it to control ignition timing as some other engines do. I leave it in the glove box as the buzzer is enough. I am looking at a new position for the sensor which at the moment is screwed into the block just behind the alternator. I an looking at a position lower down on the block to try to avoid "noise". The unit I use is an MSD and uses the GM style sensor. I was not expensive.
Denis
Denis4

I am curious as to how a knock sensor is used to control timing. So it senses knock and retards it. But what brings it back to normal? It is either knocking or it isn't, there is no smooth relationship for basis a control function. But they do it somehow!
Art Pearse

Knock Link used to supply sensors and detectors. I pondered these for some time, then found when I decided to do something about it they were no longer available. I was after knock-sensing timing where a timer increases the advance bit by bit until knock is detected, then it rapidly backs off a few bits until knock stops, then gradually increases it again and so on. In theory it would automatically take account of any engine, fuel and operating conditions and be running the most advance it could safely i.e. getting the best performance and economy, without having to fuss about with curves, springs, and vacuum advance capsules.
PaulH Solihull

Knock Link used to supply sensors and detectors, the detector had a set of filters to prevent responding to 'normal' engine noises. I pondered these for some time, then found when I decided to do something about it they were no longer available. I was after knock-sensed timing where electronics increases the advance bit by bit until knock is detected, then it rapidly backs off a few bits until knock stops, then gradually increases it again and so on. In theory it would automatically take account of any engine, fuel and operating conditions and be running the most advance it could safely i.e. getting the best performance and economy, without having to fuss about with curves, springs, and vacuum advance capsules.

Early (BL at any rate) 'intelligent' ignition systems were like this, if you suddenly tromped on the throttle you could hear it pink like billy-oh, which gradually faded out as the electronics backed off the advance. Engines of our era can take that up to a point, but modern engine with cats and emissions to worry about have to run with a set of pre-defined maps, knock-sensing is only a last-ditch measure, if something should go awry with some other part of the management.
PaulH Solihull

Thanks for all your input folks. I was about to build a very simple kit from an article in AutoSpeed, based on a hearing amplifier you can get at Dick Smiths over here. I guess it is going to 'hear' all sorts of other noises, so not specific enough. Also it is manual, thus on hearing pinking, one then needs to physically do something about it! Like backing off on the loud pedal for a start. Thanks all Mike
J.M. Doust

Mike
My understanding of the operation of a knock sensor is a little different to Paul's grab of it.
If ignition curve and timing are corectly set then a knock sensor will never have to operate
In operation it won't advance timing ,it will only knock it back
Modern cars use them to advantage by running the ignition curve up past the optimum position to get good fuel economy at cruise etc. and then use the sensor to eliminate any detonation that happens with sudden thrttle movement etc that tricks the map in the pcm
Denis4 , I'd imagine is using his to adjust for little boost and timing spikes which happen with supercharging and probably moreso with turbo cars

Most pcm's knock the timing back in 3 degree jumps when they sense a knock and as far as 9 degrees total til the knock is eliminated then creep back up one degree at a time till normal, unless it starts knocking again, then it goes back again and tries again all in milliseconds

I suppose to get the result Paul is after , timing could be set 5-6 degrees advanced from optimun and then use the knock sensor to float the timing back a bit all the time, I think that would probably work but I doubt it would hardly give enough gain to be bothered with

If you are going to go ahead with it the sensor off a BA onwards V8 Falcon would be a good one to try. They are a fairly noisy engine (mechanically) so wouldn't be over sensitive and the sensor is simply mounted with a single bolt through the centre of the sensor into the engine block about 75mm down from the headgasket face Have fun Willy
William Revit

I think you are quite right William, but then Paul also has a point. I was rather going to use the knock sensor purely as a diagnostic instrument. Not necessarily 'wired' in all the time and neither using it to control the advance mechanisim. After a tune and a play with the distributor, it was merely going to warm me that,' oh,oh, I've overadvanced its pinking'! My hearing is getting duff, and a few months ago I could swear I heard pinking, but was not sure. I was constantly putting my foot down and listening. Course that is absolutely the wrong thing to do! I did back off the ignition, now I don't hear that noise?? But for best performance, I have always found if you are on the edge of pinking that delivers the best. But how to accurately mesure the 'edge'. thanks Mike
J.M. Doust

Mike -
That is an excellent article, been meaning to make one of those. I also read someplace that you can make one from a dead phone. Seems that it would be a great diagnostic tool for all sorts of noise investigations, just be sure to turn down the gain and work up, or you will need the hearing amplifier for sure!
"Pinking" is pretty distinctive, once you hear it clearly. Many cars used to have remotely adjustable timing, would not be too difficult to arrange so you could fiddle on the run.

FRM
FR Millmore

I dont use the knock sensor for ignition control, only as a warning device and I use other measures to control detonation with the supercharger.
You do hear many people say that the engine needs to be on the verge of ping to get the most out of it and in some cases its true, but many engines when run on a dyno will have a spot at peak timing and horsepower, over about 3 degrees where output and exhaust readings remain constant. The B is one of these and its possible to be a little over advanced if you just rely on "ping". A few years ago a popular car mag took three MGBs of different stages of tune did several tests and finished up with all three on the Dyno. What they found that although different all three made max HP at 32 degrees total advance and all could be advanced up to 3 degrees before ping developed. When on a Dyno its a lot easier to pick up (hear)detonation than on the road. Denis
Denis4

Knock sensing *does* retard the ignition until pinking stops, but then it has to allow it to advance again, i.e. remove the retardation, or it will be retarding the ignition over the whole of the curve, and not just where and when it needed to be retarded.

I can see that there could be very little gain over a couple of degrees short of pinking, but then we only have pinking to know how close to the ideal we are. However 3 degrees is quite a bit, almost as much as between using 95 and 99 octane, which has a very significant impact on performance, economy and temperature.
PaulH Solihull

Denis
Yeah, only guessing as to how you are using yours ( I should learn not to do that )
As far as ignition timing goes, I agree fully with what you are saying
I was taught years ago to set timing on the rollers, up to best power and then pull it back till power starts to drop then just sneak back up enough to return to full power.
As you say 32 total is it for a B - no more
Just out of interest , We had a 1000cc morris with 10.5comp on a while back with some Elf racing fuel in it, It ran best power at 30-31deg but because of the top quality of the fuel timing could be run right up to 40deg without any sign of pinging, but there was no gain at all apart from the coolant temp creeping up.
I'm for less is best.
Cheers Willy
William Revit

This thread was discussed between 19/09/2011 and 20/09/2011

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