British-Cars.org

Welcome to the DMR Site for British Car Information.

Recommendations

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 TD TF 1500 - Tach checking

Time and again, folks compare mph versus engine rpms. Relying on stock speedometers and tachometers makes comparisons pretty much a guesstimation.
Speedometer error can be corrected rather accurately with highway mile markers. Timing how long it takes to pass 10 miles markers at a steady 60 mph reading yields good information.
I recall road tests usually had a “fifth wheel” clamped to the rear bumper. Haven’t seen any of those lately.
Nowadays, GPS devices can provide a reliable speed readout.
As for tach calibrations, a pretty good approximation could be made by crunching numbers for gear ratios given a close measurement of tire circumference and reasonably accurate distance/time, velocity or dx/dt.
My wife enjoyed winding out her ’51 TD (5.12 rear) past the 6000 mark on the tachometer. It didn’t matter how accurate it was or wasn’t, she hit the gas until it wouldn’t good any faster and then slammed an upshift. When she ran around Michigan International Speedway, the GPS said we were running 75mph and the tach needle was down into the clock. Crunching some numbers tells me it was really winding around 5000 rpm.
Now that she’s back into the’50 TD (3.92 rear), I’m a little more curious on the highway revs. The tach reads about 3500 when the GPS says 70mph.
I’ve used an elcheapo optical tach to evaluate car tach error. Once upon a time, a black strip was painted on the shiny engine pulley, but the noncontact laser tach wouldn’t settle down with a steady reading. Strapping on a timing light gives a strong, accurate (as long as it isn't missing!) signal. At idle, I crammed two diametrically opposed spark plug wires into the timing light pickup for a nice readout, but for higher revs, only one plug wire was used for a more stable reading (then multiplied by 2 of course). A more constant engine rpm was had by me holding the throttle open by hand than wife’s foot on the gas peddle.
FYI, a true 775 rpm idle was only reading 300-400 on the tach. That 3500 tach reading (reading or interpolation- give or take 100?)proved to be pretty close, right around 3690 which correlates closely with calculations run using 25” for tire diameter. Curiously, it reported the engine was spinning 2450 on the starter motor.


Jim Northrup

Jim,

A good number of years ago, I took my TD to a friend who ran his own auto repair service. He had all the electronic equipment of the day, which included an electonic tachometer. I had always wondered how accurate the TD tach was, considering the drive mechanism. He hooked it up, and I ran the revs up to 1500, 2000, 2500 and 3000 rpm. It surprised me how accurate the results were. The readings [as best as could be read] were bang on up to 2500 rpm and indicated 2950 rpm on the electronic, while 3000 on the car's tach. This was a very pleasent result as in the past, I had on many occasions, relied on that instrument. It's a big toy isn't it?

George
George Raham [TD4224]

The tach on our 54 TF is way off. I used an electronic tach at a local shop to check it a few mo.s ago, when it indicates 4500 rpm it's really 3800. We have the 4.3 rear end, so that's about 65 mph. I usually keep it a bit below there, and don't get passed too rapidly.
Al
A W Parker

This thread was discussed on 08/08/2011

MG TD TF 1500 index

This thread is from the archives. Join this live forum now