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MG MGA - RPM indication

Happy New Year all.
I'm looking for concensus on the accuracy of the tacho. Mine seems to consitently overead. Speedo is accurate having been callibrated by Speedy Cables using the Diff ratio, number of turns of drive cable and distance travelled by six revolutions of rear wheel etc, plus i have verified it with a GPS.
In fourth gear, running a 4.1: 1 diff and standard 165 Firestone radial tyres i'm showing 2300 RPM @ 30MPH, 3000 RPM @40 MPH, 3750 RPM @ 50 MPH and 4600 RPM @ 60 MPH. Coupled with the needle floating up and down (not erraticlly) but drifting up and down when doing a constant speed. The engine just doesn't sound as if am actually doing those revs at all. I am going to try and take readings with a hand held tacho (reading the crankshaft revoultions at 2,3 & 4k revs and see what the car's tacho indicates, (removng any possible doubt caused by diff & gears etc) but i'd be interesed in others experience of the indications on their car. When i refurb'd the tacho, i simply cleaned all the innards with carb cleaner and used a high performance bicycle lubricant which doesn't thicken but has high lubricant properties. i fear i am into making the needle heavier by some means as it appears to be affected too much by the passing magnet etc.

regards
Colin
C Manley

My guess is the bicycle lubricant you used. For lubricating the tach and speedo, I used just a dash of watch oil, which I aready had. It is the consistency of water or alcohol. Probably, instrument oil should be used. My guess is the bicycle lubricant you used has a high drag causing the needle to indicate a consistant percentage high.
C Schaefer

Mine has always overread by about 10%, no idea why, I just live with it. The needle is steady. It has probably got a bit tired and needs re-calibration or the needle taking off and re-attaching further round.
Malcolm Asquith

Colin
My Coupe has the 4.1 RA with 155 profile tyres and I have just had a 5 speed box fitted and Speedy cables also recalibrated my Speedo
at 60 mph in 4th my rev counter reads just under 3400 rpm
Paul
P D Camp

Colin I have 4.3 in mine with Continental 165's. Mine runs 4200 @ 60 and 4600 @ 65 and sounds it. Never been able to check since my hand tach is negative ground. Hooked it up in reverse once and ruined it,so I gave up.
gary starr

Gary, according to all the tire size calculators I find on line, the 165SR15 (165/80r15) tires run about 794 revs per mile, give or take. At 60 miles per hour, a tire runs 1 mile in 1 minute.If I multiply the revs per mile by 4.3 for the differential, I get 3414 engine RPM. This is a far cry from your reported 4200. Are you sure of the differential gearing?

Somewhere I read that the stock MGA runs a bit over 17 MPH per 1000 revs. 17 * 3.414 = 58+ MPH This matches pretty closely with the calculations for modern 165 tires.

I have noticed on my car that in 4th gear, the speedometer needle and the tacho needle are pretty much at the same angle. When the speedo is straight up (60MPH) so is the tacho needle(3500). This curiosity should be the independent of the tire size.


Pauls setup (155 tires and 4.1 rear end) calculates out to 3337RPM vs his observation of "just under 3400 rpm".

Colin your car should run very close to 3255 (794*4.1) RPM at 60 MPH with the 4.1 rear end.

I think the math is solid. The instruments are good for about 2% accuracy each if in good working order. I would think that anything more than a 5% discrepency between calculated and actual would be cause for investgation.
C Schaefer

Chuck I have never actually counted gear teeth,but I have marked the wheel and driveshaft and counted the turns and it is a 4.3 . Speedo checks out with mile markers and pacing with another car.Only thing left is to check the tach,and since I'm still on pos ground my digital won't work. And thats where it still stands.Sometime I'm going to change over to neg ground and go from there.
gary starr

Gary, have you tried hooking up your electronic tacho with the red pos. terminal to ground / Mine works ok this way, the car tacho reads 1000 rpm at a true 800.If it is anything like the speedo in my 2005 Subaru it will be even further out as the revs rise. My tacho dwell meter is as old as the hills, bought in the states in the sixties. An earlier life time. Sean
S Sherry

Chuck, Gary, thanks for the info. I clearly have an overeading Tacho - perhaps the return spring is 'tired' and weak, enabling the overead. I will use a handheld tacho over the weekend to identify how much it is out by and consider how to reconcile (wind the needle against the spring one turn!?) I recall past practice where the tacho and speedo needles used to run almost identical angles in fourth. Will update you all next week.
regards
Colin
C Manley

Colin, before you start adjusting the spring tension, I would recommend cleaning again and lubricate using a very thin oil, maybe 3-in-one if you don't have instrument oil. Certainly not any kind of engine oil. Lubricate the pinion shaft ends by dipping a wooden toothpick in the oil and then placing it on the pivot bearing. All that is required is enough to overcome the surface tension of the oil and wet to the pivot. Once you see that (less than 1/10th of a drop) you are done. It doesn't take much. You can be more generous with oil on the cable end bearing.

Bicycle oil, I think you are talking about chain oil, is comparitively sticky. It is made so, to keep the oil on the chain and not get thrown off. As used in a tacho, it will drag the pinion shaft and indicate a higher reading.
C Schaefer

Chuck,
The oil i used was a modern high performance thin teflon type oil that i chose for exactly the reasons you point out i.e. not 'sticky'. I am confused though as to how a thick oil would drag and cause an overead, surely if there is any drag it will cause an underead?
I have dug out my old handheld tacho and with fresh batteries installed it is working fine so i should know what error rate i'm looking at after tomorrow.
regards
Colin
C Manley

Alright, you got me. I have never taken my tacho apart for repair. But I have taken the speedo apart and it is the same basic mechanism. The lower part of the needle's shaft rides in a bearing that rotates with the magnets. The magnets are driven by the tacho cable. The drag cup is attached to the same shaft as the needle. So normal operation is the magnets try to rotate the drag cup and the needle spring resists the movement. The faster the magnets, turn the higher force is transmitted to the drag cup and the higher the needle moves on the scale. Since the lower bearing is rotating, my thought is that if the oil you used was more viscous that the mfgr used, there would be a higher force trying to turn the needle (drag cup) and you would see a higher reading.

Does this make any sense?

Try looking at this site for some good illustrations and descriptions:

http://home.comcast.net/~rhodes/speedo.pdf
C Schaefer

This thread was discussed between 05/01/2009 and 09/01/2009

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