WHY DO WE BELIEVE THE TACH?

[OB_Rude Dog]

So if we don't believe the tach (mine appeared to be about 200 RPM higher than the one on the dyno at redline) then why would we believe that the rev-limiter is kicking in at 10,700?

I'm not challenging - I'm just curious about what we can really count on.

 

[OB_Johnnycheese]

rev limiters kick in at different rpms in different ECU's or ignitors.I used to test them and find the ones that woud cut out at real high rpm's and put them on our race bike some were 1,000 rpm over others.

 

[OB_tinhead]

The stock speedos read off of moving metal past a sensor, various electronics, then actuating a motor that moves a needle. They're all over the place (dead on to +10% that I've heard of). The tach reads off of moving metal past a sensor, various electronics, then actuating a motor that moves a needle.
Has anyone ever tested the tachs? Am I missing something?

 

[OB_Animal]

Yep!

They can make both tach and speedo 100% accurate if they wanted, as long as we're talking electronic versions.
We're just lucky that there's no good reason that even they can think of to NOT make the tach accurate, contrary to the speedo.
Maybe you should check on Roman Black's posting on the Busa's speedo, way back in time.

 

[DaveO]

Tin,

My tach is off. It revs to about 11,200, since the limiter comes in @ 10,700, it appears to be off 500 up there.

Try yours, ring it in 3rd till the limiter cuts in and look at your tach. Also the limiter comes in softer than my other bikes, maybe cuts fuel, doesn't slam you on the tank...

Most bikes I have dyno'd the tach was off, only one ever on was 900RR.

Dave

 

[DaveO]

JC,

Would have thought todays ECM's would be a little more consistent than 1K. I know on the older R's we used to do that, but their limits were set by a "resistor tweak". They did vary some. I find it hard too believe that bikes as sophisticated as the Busa can't be consistent. I'm surprised the "stepper motor" tach is off too.

On other bikes I have verified rpm's by various means. Dynojets pickup is pretty accurate. Also the Dyna "shift minder" was very accurate <50 rpm. The most accurate tool I have is a calibrated frequency counter, it's accurate to one hertz. Maybe I'll hook it up next time I dyno the Busa.
I have found on most "built" bikes that the tach did not respond fast enough in 1st gear either, not sure about Busa yet....

Dave

 

[OB_Johnnycheese]

Dave you are correct this was on older R's
So far I have not played with the busa tach no need to in a ram air design you bluid HP
till you bump the revlimiter

 

[OB_SUPERTUNER]

I rarely see tachometers accurate. The best thing to do is go through a "calibration finding" on the dyno. It really makes little difference what the numbers are, all that is really important is that you know where on the tach swing it is making horsepower. There just happens to be a number there...what that number is, is of little significance as long as you know where the needle needs to be for shiftpoints etc. The numbers for that matter could be nothing but "hash" marks. It is nice of course to have a number though. I do a few dynoruns in the shop to calculate this by sampling the dyno on and off at the predetermined areas on the computer tachometer, noting the number that I sampled each time on...and off. One at bottom end "on" lets say, then rev to "whatever" RPM
and sample "off"...noting carefully and trying to be very "reaction time detailed" to
hitting the button precisely at the desired RPM. Then just compare this to the graph. All runs should be sampled at WOT. This way every bit of data on the graph will be full power and the hardest inertial accelerating environment. This obviously will not re-calibrate your tach, but it does tell you where you need to be reading it for maximum power output. This has worked well on all our engines. It is particularly handy when we tune a peaky engine. A 4 stroke will not be as critical, but for instance, we get a 2 stroke race engine on the dyno and your "window of opportunity" on the power band sometimes is only 400 to 800 RPM's wide and the above works on even these. A 4 stroke engine will not have nearly as peaky a powerband. If we are off even 200 RPM's on certain 2 stroke engines, it could devastate
the power easily to a 30 or more H.P. loss!
So it stands to reason that this works on any
4 Stroke. Hope this helps.