MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Tacho Upgrade with Mallory Twin Point?

I heard somewhere that if you run the Mallory twin point distributor on the V8 that you don't need to upgrade the standard B tachometer. Is this true?
Thanks in advance,

Stainzy
Stainzy

My understanding of a dual-point dizzie is that gives a longer dwell (coil charge-up time) but without having to close up the points gap to get it. This is achieved by having one set of points opening and closing slightly before the other set, with a big overlap between them, so you should still only get one pulse to the tach (and coil) per firing. However if you are talking about using a 4-cyl tach with a V8 then surely you will still need a V8 tach (or one modified for a V8) to cope with the twice as many firings per rev.

PaulH.
Paul Hunt

Paul you're probably right (as usual) but I was hoping that there might be a way of hooking the tach up to one set of points to give me half the firings and therefore effectively a 4cyl tach. Maybe I'll just have to try it for myself and let you know.
Thanks,
Stainzy
Stainzy

The dual point distributor manages to get longer dwell, (points open) by wiring the two points in paralell. When either point is closed the coil is getting juice. The points are mounted so that one point closes a few degrees after the other opens. If you ran 4 cylinders off one point and 4 off the other there would be different timing for each set of 4 cylinders. I think it would be hard to do that with out two coils and a two coil distributor cap.
Barry
Barry Parkinson

Stainzy,
As Barry mentions, the dual point simply overlaps a couple of points to achieve more voltage. There are still only 4 lobes that open and close the points and thus you still only get 4 signals per dist. revolution. If your goal is to avoid having to buy a V8 tach, you can add extra circuitry to effectivly multiply the pulses. I think Leon Zack? has a couple of working schematics for this on his web page.

On a similar note, does anyone know where I can buy the IC that's in the tach board, or give me the internal schematics of it so I can duplicate it?. Tinkertronics (usually has everything) cannot cross reference the number on it Thanks.
joaquin

There is probably one resistor that has to be changed in your tach to allow it to read correctly with your V8. I used an electronic tach that could be used either on a four or on an eight depending on a switch setting for my V6 by picking a value half-way between the two switched values as a starting point, then fine tuned it with a function generater.
George B.

A V8 distributor, of course, has eight lobes and both sets of points would 'fire' eight times if you could electrically separate them.

The standard B tachs pre-date ICs, the discrete diagram can be found at http://www.corpdemo.com/tiger/techtips/motach.html

PaulH.
Paul Hunt

Thanks guys, once again you've thrown a bit more light on a subject for me.

Stainzy
Stainzy

This thread was discussed between 04/11/2000 and 06/11/2000

MG MGB GT V8 Factory Originals Technical index

This thread is from the archives. Join the live MG MGB GT V8 Factory Originals Technical BBS now