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Which rev counter would you recommend?


David Davis
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Correct name is Tachometer, just a small optical one will do the job. Although I don't use mine every time I do like know what an engine is doing, and they are useful on some engines where the engine will slow down when it's leaned to far before it cuts which can be difficult to pick up by ear.

 

Also useful in gauging engine performance, we had a 61 fourstroke at the club last week that was running very poorly, the tacho confirmed it as we couldn't tune it to run at over 5,500 rpm. Engine has been stripped down and cleaned (there was a lot of carbon build up in the exhaust port), but we've got to retest it yet.

 

Had mine for years, but it looks like on of these https://www.elitemodelsonline.co.uk/Aircraft/Starting-and-Field-Equipment/Tools/153643-/PROLUX-MICRO-TACHOMETER-2-9-BLADES (except mine only does up to 3 blades)

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If you buy a optical  tacho be aware that they will be affected by artificial light such as fluorescent tubes if testing indoors.will show an error or peculiar reading as they will read frequency of the light source. Don't know how they react with LED lighting. Use in day light and you will get correct reading . I obtained one many years ago and it sat in its box for most of that time only coming out for tuning small engines like Cox's when a tacho is handy.

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I do not recommend using a tach to tune engines as it always reads to rev chasing. 

 

Engines will perform differently day to day, on different fuel, props, plugs etc and as they warm up so if you whizz your cold engine to full power and record say 8500 rpm, and then have a good old twiddle on the needle only to find you can now only get 8300rpm you assume its mistuned and have another tweak. All this time the engine is flat out getting hotter and hotter. Now you can only get 8200rpm as you wring its neck looking for peak revs and wondering where your 300rpm went. In reality the drop in revs is simply diue to the engine warming up and the tuning has probably been right all along. 

 

The other thing is that most tacho's respond too slowly to changes in rpm so are useless for finding peak rpm. 

 

All that said i do recommend you at least have one as they can be useful. If a customer calls me saying their engine is down on power i need to know the prop, fuel, plug and top end rpm to make any sort of diagnosis. IF they tell me their 5% nitro fuelled OS F plugged 16x8 equipped 1995 laser 150 is doing 8200rpm then i can say its in the middle of the expected range and isnt too bad. If its only doing 7500 then something is wrong. Its also handy to compare propeller A to propeller B on the same engine. One does 9000, the other 8700 even at the same dia and pitch. You might find that the slower one actually pulls better in the air as its taking a bigger bite hence the lower rpm. 

 

 

 

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8 minutes ago, Engine Doctor said:

If you buy a optical  tacho be aware that they will be affected by artificial light such as fluorescent tubes if testing indoors.will show an error or peculiar reading as they will read frequency of the light source. Don't know how they react with LED lighting. Use in day light and you will get correct reading . I obtained one many years ago and it sat in its box for most of that time only coming out for tuning small engines like Cox's when a tacho is handy.

That is quite a useful feature because it allows you to test the tachograph's function and accuracy against the mains frequency!

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I can see both sides of the points made here and well I have a JP Digital Tacho for one reason. Its a useful tool to have and although I might not need it very often it is essential when I do.

 

Twins

  • Handy for checking the tick overs are both the same (within reason) 
  • Handy for checking the both max out at WOT (within reason again)
  • Importantly that both are close on RPM at 5k rpm 

You lot are better than me, but at 5k rpm I would not be able to firstly tell its doing that and secondly which engine is going slower! I have one twin that is on different channels (Yes learnt my lesson on that!) and its real easy to mess and un-synchronize them. The answer is to remove the cowls and then set them up mechanically once and for all, but its hard enough getting to the main needle to adjust when the engine is running let alone the slow run and throttle arm clevis if it needs a tweak.

 

For singles, only ever use it when trying out a new prop to see how it does, but only out of interest.

 

Opening the box of controversy, you will of course have to have one when it comes to noise testing and over propping the engine so that its busting its balls (bearings to you), way off its tune point and oddly unreliable. That way once you get it through its noise test and like my 150 is doing WOT 4K rpm you know its going to be hard to fly, probably dead stick and wear its self out in no time, the realization is to move on.

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