How accurate are built in tachometers?

[Loneranger]

I know at least with procedures such as timing that you need a more accurate tach measurement than the dashboard tach. How inaccurate are dashboard tach? Do they tend to over or under report RPM's?

When I bought the 270 with Bravo 3 it had matched 24 pitch SS props on it. I damaged the prop on a log so I bought a set of 22 pitch which is what the sea ray website states should be on this boat. I never actually monitored the RPM at WOT for the 24 pitch but with the new 22 pitch at wide open throttle I was getting 4200 rpm and 38 mph which is at the lower end of the recommended WOT RPM range according to Mercruiser. I was just wondering if I this data is good enough or do I need to measure a more accurate RPM.

I now have the 24 pitch set refurbished so I may put them on one day to try it again but my understanding is that increasing pitch by 2 will decrease the RPM by maybe 400 which would be well below the recommended WOT RPM.

Thanks for any input.

John

 

[SR Tech]

Decreasing pitch will increase RPM 300-400

Increasing pitch, visa versa

Most tachs I run into are around 100-300RPM off. I've seen some that 1000RPM off.

If you have a shop tach available, yeah check to see how much yours is off. The specs you listed are fairly close to what you should be running.

 

[boatrboy]

I bought a set of 22 pitch which is what the sea ray website states should be on this boat. I never actually monitored the RPM at WOT for the 24 pitch but with the new 22 pitch at wide open throttle I was getting 4200 rpm and 38 mph which is at the lower end of the recommended WOT RPM range according to Mercruiser. I was just wondering if I this data is good enough or do I need to measure a more accurate RPM.

I now have the 24 pitch set refurbished so I may put them on one day to try it again but my understanding is that increasing pitch by 2 will decrease the RPM by maybe 400 which would be well below the recommended WOT RPM.

If you do try them please post the results. I have considered this change over the years to get a little more top end if possible. I am not sure that if I change I will actually achieve the wot in the spec range. I have 22p and get about the same results as you.

 

[Loneranger]

It seems then that the 24 pitch set will get my WOT below the recommended RPM range. So if any change were to be made it would be to a lower pitch set to increase the WOT RPM. For my engine the 7.4L 310 hp 4200 RPM at WOT is the lower end of the ranger. I do not have the manual but I believe 4600 was the upper end of the range.

I am satisfied with the perfomance of the 22pitch.

If I check my tach with a shop tach, and suppose it is off by 100rpm, is the amount it is off the same across the whole range? Or does the error get better or worse at higher RPM?

If I run the 24pitch for a few weeks in the future if my 22 gets dinged up, I should not harm the engine. I will just not be getting all the performance that I could be getting. Is that correct?

Thanks
John

If I do put the 24 back on I will post the results but I am unlikely to do that until next year.

 

[SR Tech]

the 24P will give you a better hole shot. probably run around 4000RPM

the 22P will give you a better top end. With a shop tach your probably running about 4500RPM.

The boat tach will be off the same amount throughout the RPM range.

The only real damage that can be done would be putting a 18P on there, but even then your rev-limiter should prevent any real damage.

They're normally like 200RPM off, but like I said i've seen some off more than others. Mostly found in Crownlines and 4 Winns.

 

[Dirtguy]

Shop tachs are liable to be just as far off as the vehicle tach. Also, digital tachs are not always the best bet. Invest in a small oscilloscope and you can get the actual real rpm. Or an optical strobe. Fluke has a real good Multimeter scope for cheap. Ebay to see the range.

And if it's off at idle, it doesn't mean that it'll be off the whole way up the scale. Might get worser. It could go either way.

dg

 

[SR Tech]

I have a craftsman analog tach/dwell meter, always spot on. I'll agree that digital tachometers suck. Any digital meter that has to read a measure that is constantly fluctuating sucks.

And I was basing the RPM difference in what i've seen. It has always been the same difference all the way to redline upon me testing it.

But I haven't been in every boat in the world so yours may differ as you increase throttle.

 

[Nehalennia]

the 24P will give you a better hole shot. probably run around 4000RPM

the 22P will give you a better top end. With a shop tach your probably running about 4500RPM.

The boat tach will be off the same amount throughout the RPM range.

The only real damage that can be done would be putting a 18P on there, but even then your rev-limiter should prevent any real damage.

They're normally like 200RPM off, but like I said i've seen some off more than others. Mostly found in Crownlines and 4 Winns.

I think you swapped your Prop #s. The 22" props should give a quicker hole shot with less top end. The 24" ones should give you slower hole shot with a higher top end and sweeter cruising range provided you have the power to be within the 4400-4800 WOT Recommendations.

 

[Presentation]

the 24P will give you a better hole shot. probably run around 4000RPM

the 22P will give you a better top end. With a shop tach your probably running about 4500RPM.

I suggest you eadray your anualmay.

[size=+9]Where is Gary when you need him?[/size]

 

[SR Tech]

Shit, yeah you got me.

I'm gonna stop answering at work, apparently there is too much going on...

Appologizes....

 

[Loneranger]

When I bought the 270 with Bravo 3 it had matched 24 pitch SS props on it. I damaged the prop on a log so I bought a set of 22 pitch which is what the sea ray website states should be on this boat. I never actually monitored the RPM at WOT for the 24 pitch but with the new 22 pitch at wide open throttle I was getting 4200 rpm and 38 mph which is at the lower end of the recommended WOT RPM range according to Mercruiser. I was just wondering if this data is good enough or do I need to measure a more accurate RPM.

I now have the 24 pitch set refurbished so I may put them on one day to try it again but my understanding is that increasing pitch by 2 will decrease the RPM by maybe 400 which would be well below the recommended WOT RPM.

I apologize but I misspoke in my initial post. I too got 24 and 22 mixed up.

Searay's recommendation for a 2000 270DA 7.4L MPI MCM Stern Drive (310 PHP) Bravo 3 2.00:1 is a 24 Pitch Set at 4600 RPM WOT. I originally had 22's when I bought the boat and I did not observe the WOT RPM mark before I boogered them up. I purchased used 24's which are the ones I noted 4200 RPM at WOT the other day.

Mercruiser owner's manual recommends a range of 4200-4600 RPM WOT for the 7.4l 310 hp.

So I went to a higher pitch and now my WOT RPM is at 4200 with the 24's. So if I go back to the 22's it should increase the WOT RPM to say 4600, that is assuming my dashboard tach is +/- 100 RPM. If I come across access to a more accurate method I will correlate with my dashboard tach. I guess either prop will work.

To be honest I have not driven the boat long enough with either set of props to really tell a difference. Maybe in the spring I will do a more thoughtful comparison and take written notes. If I do this does it matter about the bought being trimmed, load (i.e. full gas and water tanks), etc. as long as the conditions of the boat are the same during each prop test. I just bought this boat in June of 2008.

Sorry for the confusion but thanks for all the info.

John

 

[Dirtguy]

I have a craftsman analog tach/dwell meter, always spot on. I'll agree that digital tachometers suck. Any digital meter that has to read a measure that is constantly fluctuating sucks.

And I was basing the RPM difference in what i've seen. It has always been the same difference all the way to redline upon me testing it.

But I haven't been in every boat in the world so yours may differ as you increase throttle.

I have dealt in instrumentation and have the ability to compare against a "lab standard.' So with that info, and a huge inventory of instruments to choose from, I have seen
many tachs and their inaccuracies.

Any tach can be fixed to be accurate. It's just that some have trim pots and can be tweaked.

But you need to compare to a standard, not just another tach.

dg

 

[Loneranger]

You mentioned a small oscilloscope or an optical strobe as a better gold standard. I suppose it would work like a timing light? Which moving part would you measure with the strobe?

John

 

[NorCal Boater]

Dirtguy,

I agree with you on the Craftsman analog engine anylizer. I bought one on ebay for like $20 and it is always right on. It worked so good that I bought a second one for $15 just for the tachs. My engines ae '89's and these units work really well for any non-electronic ignition engine tests. I also adjust my carbs annually wusing these tachs.

 

[Dirtguy]

You mentioned a small oscilloscope or an optical strobe as a better gold standard. I suppose it would work like a timing light? Which moving part would you measure with the strobe?

John

The crank pulley/damper would be the most accessible. Clear markings on it will make it very clear and easy to identify the rpm.
For an oscilloscope, you can attach to a coil (-) terminal, (4 pulses per rev) or an inductive pickup on a plug wire itself, (1/2 pulse per rev).

dg

 

[greenstreak]

I just had mine checked out. the hrs on my tach shows 308 and the actual on engine is 310 from computer. That is real close.

art