instruments Accuracy of Mechanical Taco / Rev Counter

[oldtuckunder]

Hi All

 

Finally seems to have stopped raining, and the river Severn is back within its banks. So a little later than planned I'm just starting to hook up the data logging electrics on the Vitesse.

 

I had hoped to get the wideband sensors in first as its fuelling I'm most interested in, but have just discovered that the bungs I had welded in the exhaust in preparation last year are too short and there isn't enough clearance for anything longer where they are located. So the whole manifold assembly is going to have to come off, be unwrapped, and new bungs welded on :-(

 

Anyway as its nice to at least get something working before pulling everything apart, I have the TPS sensor working and the RPM logging, which I have initially done with an inductive clamp around #1 plug lead.

 

What I have found interesting is that the electronic RPM readings are about 15% lower than the mechanical taco readings driven from the distributor. Now I know that the distributor is going around at exactly half of engine revolutions :-) so I had always assumed that the mechanical taco would be fairly accurate, or if anything slightly under read due to mechanical loses at the gauge.

 

Anyone else measured the difference between their mechanical taco and an electronic one?

 

I'm just wondering if Triumph did it deliberately so that people didn't quickly notice that the Speedo was grossly optimistic?

 

Or maybe its just my mechanical guage that is tired.

 

Alan

[peejay4A]

As the tachometer uses the same magnet and drag cup arrangement as the speedo it has the potential to be just as inaccurate.

[ianhoward]

Alan

 

The tacho on my car was reading quite a bit out as at 3000 rpm it was indicating around 3500rpm (if I recall correctly - it was a while ago now!). I had it re calibrated and all hunky dory since... .

 

Cheers

Ian

[TR NIALL]

Think All the Mechanical ones Over Read between 500/800rpm,why not use a Half Decent Timing Light with built in Tachometer.

[Alec Pringle]

Back in the day most speedos and tachos were optimistic, it made the customer feel better and reduced the chance of losing his driving licence. Triumph were no better and no worse, as it were, than average.

 

5% as a minimum, but age and wear tend to make speedos and tachos alike less rather than more accurate, and 40 years on an inaccuracy factor of 15% would seem reasonable enough.

 

Jaguars were notoriously optimistic, E-types especially. Lied like the proverbial cheap Japanese watch. When E-types were new kids on the block, my uncle's E-type clocked 150 on the speedo. On the M6, relatively new and unrestricted, his 'Blower' 4.5 Bentley wasn't far behind, and the 4.3 supercharged Alvis just had the legs of the Jaguar flat out. That 150 was 130 at best in reality. Like most things about E-types, over rated !

 

Cheers

 

Alec

[westaj]

You could drive the car in top and check the engine rpm reading against the speedo reading; there'll be a direct relationship depending on your back axle ratio and wheel size.

You might won't to also check the speedo accuracy; drive at a steady indicated 60 over a measured mile and record the time.

 

Just a thought; I'd be interested in the results.

 

Jerry

[Lebro]

Easier to use a "satnav" these can usually be set to show speed over the ground.

 

Bob.

[BlueTR3A-5EKT]

Chronometric instead perhaps?

http://archives.jampot.dk/Technical/Speedo/Smiths_Speedo_restoration.pdf

Open your wallet though. The last NOS 4 inch 0-8000 I sold went to Japan for more than a surrey top is worth.

 

Cheers

Peter W

[Guest]

15%? Interesting... my TR's tacho has the same habit. When idling, my TR's engine tacho points at 1200 rpm, too much, but when I use an electronic timing light with tacho, it reads 800 - 850 rpm. You can imagine how I struggled to get my engine running properly with the needle on the 800 rpm mark... before I used an electronic timing light to check things!

 

Menno