RPM measurements

[TheBiscuits]

1 minute ago, blackrose said:

 

I've no idea how to adjust the Izusu revcounter.

There's a small adjuster on the back of the unit.  Twist it.  If it gets worse, twist it the other way!

 

Incidentally, what's redline speed on your particular engine?  On the Isuzu 42 it's rated as 2100 max revs and Isuzu say not to run over 2000 rpm for any length of time.

[Alan de Enfield]

36 minutes ago, TheBiscuits said:

Fairy nuff.  I hadn't worked out the percentage differences.

 

 

I had and it decreases on a sliding scale as the revs increase

about 22% at 700rpm, down to 17% at 1800rpm

[TheBiscuits]

1 minute ago, Alan de Enfield said:

I had and it decreases on a sliding scale as the revs increase

about 22% at 700rpm, down to 17% at 1800rpm

I'd recommend checking the belt tension before doing much else then.

[Chewbacka]

18 minutes ago, Alan de Enfield said:

I had and it decreases on a sliding scale as the revs increase

about 22% at 700rpm, down to 17% at 1800rpm

My calculator works differently to yours ?

Isuzu           Remote       Low

700              853              17.9%

1000           1186            15.7%

1200           1445             17.0%

1400           1676             16.5%

1600           1916             16.5%

1800           2105             14.5% 

 

Giving an average of 16.7% 

 

I assumed the variation of +/- 2% as being measurement error, but I could be wrong

Edited November 25, 2018 by Chewbacka

[Alan de Enfield]

3 minutes ago, Chewbacka said:

My calculator works differently to yours ?

Isuzu           Remote       Low

700              853              17.9%

1000           1186            15.7%

1200           1445             17.0%

1400           1676             16.5%

1600           1916             16.5%

1800           2105             14.5% 

 

Giving an average of 16.7% 

853 / 700 = 1.22, so over-reading by 22%

2105 / 1800 = 1.17 so over-reading by 17%

[alan_fincher]

11 minutes ago, Alan de Enfield said:

853 / 700 = 1.22, so over-reading by 22%

2105 / 1800 = 1.17 so over-reading by 17%

But if the hand-held is correct, and the fitted instrument wrongly calibrated, (which seems to be the consensus) then.....

 

853 - 17.9% = 700   So the fitted instrument is under-reading by 17.9%
2105 - 14.5% = 1800  So the fitted instrument is under-reading by 14.5%

So those seem the right percentage inaccuracies to quote to me.

[Tacet]

The under-reading varying by a couple of % is partly explained by reading off the analogue rev -counter.

 

Blackrose reports slightly different figures (about 1%) depending in whether the gearbox is in  neutral or reverse.  But as the engine spins in the same direction, then some form of minor reading inaccuracy is the obvious explanation.

[Alan de Enfield]

10 minutes ago, alan_fincher said:

But if the hand-held is correct, and the fitted instrument wrongly calibrated, (which seems to be the consensus) then.....

 

853 - 17.9% = 700   So the fitted instrument is under-reading by 17.9%
2105 - 14.5% = 1800  So the fitted instrument is under-reading by 14.5%

So those seem the right percentage inaccuracies to quote to me.

But - where was the hand-held reading taken ?

It could be that both instruments are correct.

 

Hand held taken on the crank pulley- correct.

Rev counter takes the reading from the alternator - correct.

 

Due to 'slippage' the alternator is not at the expected revs ?

[TheBiscuits]

17 minutes ago, Alan de Enfield said:

But - where was the hand-held reading taken ?

It could be that both instruments are correct.

 

Hand held taken on the crank pulley- correct.

Rev counter takes the reading from the alternator - correct.

 

Due to 'slippage' the alternator is not at the expected revs ?

The simple way of checking this would be to use the handheld on the alternator fan or pulley and see if it's a lot closer.

[Chewbacka]

20 minutes ago, TheBiscuits said:

The simple way of checking this would be to use the handheld on the alternator fan or pulley and see if it's a lot closer.

But then you will need to know the pulley ratio to a couple of percent to convert alternator revs to engine revs.

I still think the hand held is accurate, and can be checked by the record player method or if lucky the 50/100Hz flickering light.

[alan_fincher]

48 minutes ago, Alan de Enfield said:

But - where was the hand-held reading taken ?

It could be that both instruments are correct.

 

Hand held taken on the crank pulley- correct.

Rev counter takes the reading from the alternator - correct.

 

Due to 'slippage' the alternator is not at the expected revs ?

I'm struggling to see why the alternator would apparently be slipping by a higher percentage at low revs than at high revs.

It's not going to need a  lot of power at low revs, but once charging properly I would have thought the slippage more likely?

Or perhaps not?

My strong hunch is that it is not properly calibrated, particularly as Mike seems to feel that it has always read lower than expected over many years.

[dor]

Before my Vetus tacho completely gave up displaying hours, I fitted a Durite hour counter. The difference between the two is about 6%, with the Durite being apparently slower.

 

And for the pedants that use the term "rev counter" I believe a tachometer reading integrates the rev. count WRT time...

[BEngo]

There is usually an adjustment screw down a hole on the back of the rev counter.

  It may be covered by a bung.  Be gentle with it as there are often no robust end stops.

N

There is usually an adjustment screw down a hole on the back of the rev counter.

  It may be covered by a bung.  Be gentle with it as there are often no robust end stops.

N

[MtB]

10 minutes ago, BEngo said:

There is usually an adjustment screw down a hole on the back of the rev counter.

  It may be covered by a bung.  Be gentle with it as there are often no robust end stops.

N

There is usually an adjustment screw down a hole on the back of the rev counter.

  It may be covered by a bung.  Be gentle with it as there are often no robust end stops.

N

 

Ah now you pressed "Submit Reply" twice didn't you? Because on the first press nothing seemed to happen, so you pressed it again and got the double post. 

[GUMPY]

2 hours ago, blackrose said:

 

I've no idea how to adjust the Izusu revcounter. The plan was to check the calibration of the hand held tacho somehow and if correct just mark the offset on the Isuzu gauge.

There will be a trim pot on the back of the revcounter set the engine at about 1600rpm then  adjust it until it reads the same as the tach.

[blackrose]

2 hours ago, TheBiscuits said:

There's a small adjuster on the back of the unit.  Twist it.  If it gets worse, twist it the other way!

 

Incidentally, what's redline speed on your particular engine?  On the Isuzu 42 it's rated as 2100 max revs and Isuzu say not to run over 2000 rpm for any length of time.

Thanks. I'll try that once I've confirmed the hand held tachometer is correct.

 

On the Isuzu 55 it's 3000rpm max

Edited November 25, 2018 by blackrose

[blackrose]

2 hours ago, TheBiscuits said:

I'd recommend checking the belt tension before doing much else then.

I'm pretty sure belt tension is correct. I think it's not meant to exceed +/- 10mm movement either way in the centre, but I find it easier just to twist it through 90 degrees by hand and make sure it can't twist further than that.

[blackrose]

2 hours ago, Tacet said:

The under-reading varying by a couple of % is partly explained by reading off the analogue rev -counter.

 

Blackrose reports slightly different figures (about 1%) depending in whether the gearbox is in  neutral or reverse.  But as the engine spins in the same direction, then some form of minor reading inaccuracy is the obvious explanation.

 

Also the difference measured between the Isuzu revcounter and the hand held tacho increases until it reaches around 1600 rpm and then it remains at around 300 rpm (if we can say that based on my last two measurements in neutral). Prior to reaching 1600 rpm the relationship between the two sets of measurements isn't linear.

Edited November 25, 2018 by blackrose

[blackrose]

2 hours ago, Alan de Enfield said:

But - where was the hand-held reading taken ?

It could be that both instruments are correct.

 

Hand held taken on the crank pulley- correct.

Rev counter takes the reading from the alternator - correct.

 

Due to 'slippage' the alternator is not at the expected revs ?

 

As I said in my original post, he hand held tacho readings were taken on the main flywheel of the engine. That I assume is connected to the crankshaft? I put the reflective tape close to the outside edge of the flywheel.

1 hour ago, TheBiscuits said:

The simple way of checking this would be to use the handheld on the alternator fan or pulley and see if it's a lot closer.

Ok, I'll try that tomorrow if it's not raining. Do I measure the start or domestic alternator?

54 minutes ago, Loddon said:

There will be a trim pot on the back of the revcounter set the engine at about 1600rpm then  adjust it until it reads the same as the tach.

Cheers

[WotEver]

4 minutes ago, blackrose said:

Ok, I'll try that tomorrow if it's not raining. Do I measure the start or domestic alternator?

Whichever one the tacho pulse is taken from (W terminal on back of alt). But I wouldn’t bother as that’s just going to add the pulley ratio into the mix. 

 

Edited November 25, 2018 by WotEver
Typi

[TheBiscuits]

3 minutes ago, blackrose said:

Ok, I'll try that tomorrow if it's not raining. Do I measure the start or domestic alternator?

 

Neither - @Chewbacka nailed it :

 

1 hour ago, Chewbacka said:

But then you will need to know the pulley ratio to a couple of percent to convert alternator revs to engine revs.

I still think the hand held is accurate, and can be checked by the record player method or if lucky the 50/100Hz flickering light.

 

[blackrose]

1 hour ago, Loddon said:

There will be a trim pot on the back of the revcounter set the engine at about 1600rpm then  adjust it until it reads the same as the tach.

Cheers

Ok thanks everyone.

[croftie]

53 minutes ago, blackrose said:

Thanks. I'll try that once I've confirmed the hand held tachometer is correct.

 

Can you not check the hand held against a car with tacho?

[MtB]

25 minutes ago, croftie said:

Can you not check the hand held against a car with tacho?

 

Car tacho is just as likely to be wrong! Who would ever know if it was?

[Floating Male]

12 hours ago, Mike the Boilerman said:

 

Car tacho is just as likely to be wrong! Who would ever know if it was?

Precisely!

Actually all measurements are 'wrong' to an extent. In the calibration world we talk about uncertainties. As a number have pointed out reading the pointer of the engine rev counter can lead to significant possible errors. The snag is these errors may (will) not be consistent. The same probably applies to belt slip, being dependant on alternator load!

Unfortunately you don't know the extent of these uncertainties in your case.

Reading the pointer, rev counter calibration and possible belt slip are possible issues. However it is still likely that the tacho is the most dependable indication.

Really what you want is to have a rev counter that actually measures revs and not a system that relies on pulleys and adjustments. :-)