How many revs should my engine be doing?

Quidi Vidi

Quidi Vidi

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Whilst trying to sort my engine out today (posted in another thread) I used a hand held laser rev counter to see how many revs we were doing as we don't have a rev counter fitted and I've always wondered about this. At idle we seemed to be doing just over 500 rpm, in neutral at full chat about 1600 rpm and in gear at full chat whilst still tied up just over 1200 rpm. My owners manual suggests max rpm should be 3600 rpm, am I missing something? My knowledge of these things is pretty basic to say the least!
 
Tied up doesn't give accurate measurement in gear, you won't hit the max possible. We tick over at 600rpm but your max seems rather low as we top out at 2200. Difficult to say until you get your air leak sorted.
 
Bower15 said:
Whilst trying to sort my engine out today (posted in another thread) I used a hand held laser rev counter to see how many revs we were doing as we don't have a rev counter fitted and I've always wondered about this. At idle we seemed to be doing just over 500 rpm, in neutral at full chat about 1600 rpm and in gear at full chat whilst still tied up just over 1200 rpm. My owners manual suggests max rpm should be 3600 rpm, am I missing something? My knowledge of these things is pretty basic to say the least!
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If you cannot get the revs up to what they should be the engine will not be developing the power it should . At 1200 rpm less than half!.

While there might be external factors affecting the revs in gear, in neutral it should rev easily to its maximum. There must be something pretty significantly wrong if you can only get it to 1600 rpm.

Are you sure your readings are correct?

Smoking at all?

Is the throttle linkage moving fully?
is there any restriction to the fuel flow. ... Whats this air leak all about?

Is the exhaust system Ok, are the valve clearances correct.

Maybe the injector pump or governor system is faulty .


Are you really sure your readings are correct.
 
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We rarely exceed 1400rpm. At that speed in a calm sea we're doing 6-7kt which is close to hull-speed. I should add that we've got a wonky throttle/gear lever that needs a sandal wedged between it and the cockpit side to keep it still; and sound-insulating the engine "room" is one of those jobs that we've never got round to (in about 30 years!).

I'm sure the engine (a 4 year old Lombardini 18HP) is capable of 3000 revs or more, but the lack of sound insulation, and of need to go any faster; coupled with the wonky lever ... well you know, one day we'll get round to it.
 
Bower15 said:
Whilst trying to sort my engine out today (posted in another thread) I used a hand held laser rev counter to see how many revs we were doing as we don't have a rev counter fitted and I've always wondered about this. At idle we seemed to be doing just over 500 rpm, in neutral at full chat about 1600 rpm and in gear at full chat whilst still tied up just over 1200 rpm. My owners manual suggests max rpm should be 3600 rpm, am I missing something? My knowledge of these things is pretty basic to say the least!
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At 1st sight, I'd suspect your readings to be inaccurate - mainly because most engines tick-over @ about 800 rpm.

Not knowing the engine type, it's impossible to give any valid answers except to say all the rpm figures quoted are extraordinarily low (except for a long-stroke semi-diesel of early 20th century manufacture)
 
Bower15 said:
Whilst trying to sort my engine out today (posted in another thread) I used a hand held laser rev counter to see how many revs we were doing as we don't have a rev counter fitted and I've always wondered about this. At idle we seemed to be doing just over 500 rpm, in neutral at full chat about 1600 rpm and in gear at full chat whilst still tied up just over 1200 rpm. My owners manual suggests max rpm should be 3600 rpm, am I missing something? My knowledge of these things is pretty basic to say the least!
Click to expand...

Prop size?
 
Bower15 said:
I used a gadget that I'd bought off the internet but unfortunately I have lost the instructions. I put the reflective patch on the main pulley where the starter/cranking handle goes. I did wonder if I should double the readings.
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You may well find that is geared down 2:1 so that you get more oomph when you are handcranking. That would give you 1000 tickover and 3200 at full throttle. Much closer to reality, but it should run to rated maximum revs with no load. It is only shortage of fuel that stops it, so you might find the governor is adjustable.
 
Tranona said:
You may well find that is geared down 2:1 so that you get more oomph when you are handcranking. That would give you 1000 tickover and 3200 at full throttle. Much closer to reality, but it should run to rated maximum revs with no load. It is only shortage of fuel that stops it, so you might find the governor is adjustable.
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I have a spare engine in bits in my father in laws garage, I will if it is geared down as that would make sense of the readings, thanks.
 
Bower15 said:
I used a gadget that I'd bought off the internet but unfortunately I have lost the instructions. I put the reflective patch on the main pulley where the starter/cranking handle goes. I did wonder if I should double the readings.
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Google is your friend http://www.ebay.co.uk/itm/New-Digit...ntact-RPM-Tach-Meter-FC-OT11703-/321123339601

Read Para 5:2 Perhaps if you put two reflective strips on and divide by two the readings will be more accurate (ie. the one mark is too slow for the meter, it seems to need a fast cycle to work properly)
Also it recommends making sure the part that is not reflective is genuinely dull, so as not to confuse the meter.
 
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charles_reed said:
At 1st sight, I'd suspect your readings to be inaccurate - mainly because most engines tick-over @ about 800 rpm.

Not knowing the engine type, it's impossible to give any valid answers except to say all the rpm figures quoted are extraordinarily low (except for a long-stroke semi-diesel of early 20th century manufacture)
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They were my thoughts too, and the reason I asked if he was sure that the readings were correct.

i assume his engine is what he says it is ... a 6 hp Petter diesel.
 
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Bower15 said:
I used a gadget that I'd bought off the internet but unfortunately I have lost the instructions. I put the reflective patch on the main pulley where the starter/cranking handle goes. I did wonder if I should double the readings.
View attachment 33562
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I have used the exact same laser tacho borrowed of a friend who bought it from ebay. It only requires one strip of reflective tape to work and didn't seem unduly fussed about the rest of the pulley being painted white/rusty! The readout was exactly what my rev counter indicated, which is what i was trying to check the calibration of!
 
captainboo said:
I have used the exact same laser tacho borrowed of a friend who bought it from ebay. It only requires one strip of reflective tape to work and didn't seem unduly fussed about the rest of the pulley being painted white/rusty! The readout was exactly what my rev counter indicated, which is what i was trying to check the calibration of!
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I will try again tomorrow when I have more time and swmbo with me, we can test it whilst underway then. We did a couple of runs up and down Fareham Creek the other day and measured 4.5kn SOG using GPS so that makes me think that the engine isn't running at half speed. We didn't have the tachometer with us on that occasion but it doesn't sound slow, in fact I'm not sure I'd want to hear it going ad twice the speed.

Just to confirm, it is a Petter AC1WM otherwise known as the mini six.
 
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Those old Lister / Petter low bhp singles were low revving. The old mans 8hp Lister produced max bhp at about 1800rpm if my memory serves me right, so I don't think the readings will be far out. It certainly won't be doing 3200rpm!

Ian
 
ianj99 said:
Those old Lister / Petter low bhp singles were low revving. The old mans 8hp Lister produced max bhp at about 1800rpm if my memory serves me right, so I don't think the readings will be far out. It certainly won't be doing 3200rpm!

Ian
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Download a manual and you will find that the rpm figures Bower has quoted are low. He quotes a max rpm from his manual of 3600 rpm

The manual I am looking at says it develops 2.8hp at 1500 rpm, 3.3 at 1800, 4.0 at 2100, 5.0 at 2500 and 6hp at 3000 rpm.

In view of what he has said about measuring the speed "where the starter/cranking handle goes" I'd think there is a good chance that the crank shaft speed is double the figures he quotes. That would put the true rpms within the range above.
 
ianj99 said:
Those old Lister / Petter low bhp singles were low revving. The old mans 8hp Lister produced max bhp at about 1800rpm if my memory serves me right, so I don't think the readings will be far out. It certainly won't be doing 3200rpm!

Ian
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The Mini 6 is nothing like the old Lister engines. High speed engine (3600). Main market refrigeration units on trucks, portable gen sets etc where light weight and simplicity were important. Lots of aluminium bits and only partly successful when water cooled in marine applications.
 
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