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MG Midget and Sprite Technical - Rev Limiter

Having been to the rolling road with my 1275cc Frogeye I have discovered the limitations of the mechanical rev counter - very inaccurate - so I will have to fit an electrical version. Any suggestions as to type, make etc to look something like the Frogeye original - must go up to 8K. To avoid over reving on the track I was also considering a rev limiter. Has anyone any experience of fitting one of these and availability. Then I thought go the whole way and fit a shift light - agian any advise welcome.


P B Chappell

That engine bay sickens me ..... How can it possibly stay that clean !

Looks simply stunning - any more pics ?

Smiths tachometers are available in 0-8K
http://www.speedycables.com/page16.html
Dean Smith ('73 RWA)

Modern Smiths tachos are execrable and highly inaccurate.

I always use Elliot tachos on my race cars:

http://www.merlinmotorsport.co.uk/INSTRUMENTS-/-GAUGES-ELLIOT-TACHOS/c1_357/index.html

/For rev limiters you can not go wrong with Micro Dynamics systems:

http://www.merlinmotorsport.co.uk/p3978/DIGITAL-REV-LIMITER/product_info.html

They even do 'shift lights' although, as an old school racer I hate those bloody things!
Deborah Evans

I've a micro dynamics one, with a soft cut.
Brad (Sprite IV 1380)

You want a rev limiter?

Pin the invoice of your last engine rebuild on the inside of the winscreen, top right.

A
Anthony Cutler

Dean Outside view. Peter


P B Chappell

God that's awfull!!!!!!





















Not.
Shawn

Preeety car
d brenchley

As an alternative I have one of these: -

http://www.demon-tweeks.co.uk/products/ProductDetail.asp?cls=MSPORT&pcode=OMERLC

According to the tell tale on my Stack tacho it is very accurate at limiting the revs where I set it...

No rev limiter will save an engine on change down however... :)

James
James Bilsland

You might find this hard to believe, but...

I calibrated my std tacho some years back using the 'battery-charger-3K rpm' method. I found that the reading varied over the first few mins and so finalised adjustment after 5 mins or so.

I find it's very accurate.

How do I know? I set the Emerald change-up light to a particular RPM, and increase engine speed until it just lights, and it's always spot-on (wthin 100 revs, which for me is as near as damit).

A

Anthony Cutler

Elliot Design have shut as the owners have retired. So if you want one you will have to get in quick. Merlin have some 80mm and Demon Tweeks have 100 and 80mm. Peter
P B Chappell

Hello Anthony, could you possible explain the battery charger 3k rpm method for checking tach operaton? I assume you are using the pulsating DC signal from the charger with maybe a load somewhere in the curcuit? It would be great if someone could design a signal generator to test a tach off the car throughout the RPM range.Anyone out there into this kind of thing? Bob C
chamberlain Bob

For later 'pulse rev oucnters, -ve earth.

This is the way I checked and adjusted my rev. counter - clearly there are other permutations. Firstly, I decided not to apply the full voltage of my battery charger to the rev counter, since I have a small/cheap one, with poor regulation, and didn't want to risk 'blowing' the electronics in the rev counter with over-voltage. So I decided to use an old halogen bulb to load the battery charger.

I clipped the bulb 'common' connection to the battery earth, with the charger 'earth' crocodile clip. Then I attached the charger positive crocodile to one of the other bulb terminals, also trapping a short piece of wire (insulation stripped 1/4 of an inch) in as well. Disconnect the ignition lead (white, with a black tracer) to the points and also to the coil. Connect the other end of the wire from the charger into one of the spade connectors you've just taken off.

Turn on the ignition to power the rev counter (car in neutral for safety reasons but don't attempt to start the engine). Plug in the battery charger (bulb lights!). The rev counter should now be registering about 3,000 rpm (or 2,000 for a 6 cylinder, or 1,500 for an eight cylinder; it's better to set the counter at a high reading if possible, since this gives the best accuracy, but unless the mains frequency is drastically increased (2 or 4 times!) this will not be possible!).

Leave the system running for a bit so things settle down (5 mins). My reading gradually went from 2,800 to 3,200. I would think that this was the electronics warming up (for anyone with electronics curiosity, the circuit is a monostable / filter combination; a C-R circuit on the monostable provides the adjustment). At this point, if the reading isn't within 100 rpm, you could adjust.

For -ve earth current-loop rev counters:

For current-type/coil connection 'counters (-ve earth):

Connect rev-counter -ve to -ve on battery. Then take the white-black lead off the coil; make sure the points are closed and so conducting. Then +ve from the battery charger to headlight bulb; other side of bulb to black-white cable you've just removed.

The bulb should be bright - then you have current flowing around the 'counter loop. The counter should show 3K rpm (freq of full-wave rectified mains). Adjust to suit.

HTH

A
Anthony Cutler

Bob,

Just to add to Anthony's description, the 60Hz mains frequency in the US will, when full-wave rectified, produce a reading of 3,600 rpm on your rev counter.

dennis
D Stanfield

There are several good websites for re-calibrating the Smith tach...just google if this one isnt up to snuff

http://www.classictiger.com/techtips/motach.html

Prop
Prop

Peter - which RR did you use. Apparently there is one in Ipswich now but I haven't found anyone who's used them on older cars.
Dean Smith ('73 RWA)

Dean,
i've had quite a few RR sessions over the years, Peter Baldwin in Cambridge was the best of them.
Brad (Sprite IV 1380)

I used Peter Baldwin as well and he is excellent. You will need to get in quickly as he becomes very busy at this time of year. Peter
P B Chappell

Dennis

Thanks for that; completely forgot about the 60Hz in the USA...!

A
Anthony Cutler

Anthony,
Have I got this right - the bulb is effectively connected in parallel with the tach, providing the load to the charger?

Guy
Guy Weller

Yes:

- for pulse/voltage (later) tachs, I put bulb in parallel across terminals to reduce volts (maybe not needed, but I'm being careful).

- for current-driven ones, I put a bulb in series, to reduce current since coil not in the circuit (and charger would there be shorted to earth).

Tell me how you get on.

A
Anthony Cutler

How do you know wich type of tach you're dealing with?
I mean, there are some markings on the face wich could contain this information, but what does it mean.


Alex G Matla

Current types have a white/black loop 'connection' instead of the spade connector for voltage/pulse types.

A
Anthony Cutler

IIRC type RVC = current and type RVI = impulse
David Smith

This thread was discussed between 22/02/2010 and 17/03/2010

MG Midget and Sprite Technical index

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