How Can The Tacho Be So Wrong

[rtune]

My dash indicates an idle of 1200 rpm (sounds more like 900). And in 4th gear at 60kph

(speed checked by GPS) it indicates 2500rpm (but stock ratios for 6 speed and stock diff

indicate this should be 2000rpm).

The dash is the 320kmh nismo dash. How can it get it so wrong - is there an adjustment?

[turbo_brian]

Get an aftermarket Rev gauge, wire to the Revs wire ont he ECU and check the ECU's reading.

You might find that although it sounds like a certain rev, it's actually different, the gauge might be right, and you might be wrong.

I did this with my R31, because it sounded wrong, so I stuck my spare SAFC controller in which has a rev reading, turns out the tacho was reading different to the engine revs, and was mixing with some weird formulae with with regards to auto box, torque converter, etc.

B.

[rtune]

Get an aftermarket Rev gauge, wire to the Revs wire ont he ECU and check the ECU's reading.

You might find that although it sounds like a certain rev, it's actually different, the gauge might be right, and you might be wrong.

I did this with my R31, because it sounded wrong, so I stuck my spare SAFC controller in which has a rev reading, turns out the tacho was reading different to the engine revs, and was mixing with some weird formulae with with regards to auto box, torque converter, etc.

B.

I'm pretty sure it is this far out ..

the turbos which are gt-ss equivalents only climb over 1bar by 5000 rpm on the dash.. which is why I started

to doubt the tacho in the first place.

Is there another possibility, that the nismo dash is not broken, but this nismo ECU is outputting a different rev

signal that is getting misread?

[Bakes]

Not sure man but my 320 dash definately idles below 1200rpm, I think it is 900rpm as you said. With those turbos you should have 1 bar before 5000rpm.

I suggest getting it checked out on a dyno by a pro

[rtune]

Not sure man but my 320 dash definately idles below 1200rpm, I think it is 900rpm as you said. With those turbos you should have 1 bar before 5000rpm.

I suggest getting it checked out on a dyno by a pro

Is your speedo on the money as well?

At an indicated 60kph, I'm doing less than 55kph, and in 4th, tach indicates 2500 rpm, but the

gear ratio calculator says I should be doing 2000 rpm at that speed (55kph) with 245/40s 18"

So yeah, I think definitely that would explain the "late boost". (because it isn't so late if the tach

is out that much).

And as for checking it on a dyno by a pro, well, you know what? I've had a pull at croydon by Jim (no

comment made), a pull at IS Motorsports (comment was it seemed like a power plot from t04s) and a pull

at UAS to setup the boost controller (no comment on the very late boost shown).

To be fair on all three places I didn't walk in saying "hey, why is it boosting at an indicated 5k? wassup

with that?

(and thats what i will now have to do). but I am a tad disappointed (and have spent $150+$150+$300

on dyno time so far).

[Bakes]

yeah that sux if you have been to 3 dynos already. Pick the one that gave you the best service and approach them specifically about the problem I guess as you said. Other than that, I dont know.

Yeah my tacho would be out on speed a bit I rekon as my mates Lexus reads slower than mine does. We both have after market wheels though which adds to the confusion.

[blind_elk]

Remember, it's only a mecahnical gauge. And it won't be perfect.

The tacho reading on my Wolf handset (digital) reads a bit lower than the dial tacho in the instrument cluster (R32 GTS4). And it actually shows around +/- 50 rpm at idle, while the dial basically stays steady.

Get one of the dynos to give you a comparison of what their dyno instruments say against what your dial tacho says.

About the only remedy would be to pull the dial out and adjust the needle.

[rtune]

Remember, it's only a mecahnical gauge. And it won't be perfect.

The tacho reading on my Wolf handset (digital) reads a bit lower than the dial tacho in the instrument cluster (R32 GTS4). And it actually shows around +/- 50 rpm at idle, while the dial basically stays steady.

Get one of the dynos to give you a comparison of what their dyno instruments say against what your dial tacho says.

About the only remedy would be to pull the dial out and adjust the needle.

The thing that gets me is the dyno showing the RPM x-axis - they calibrated their RPM "against the tacho"!

unless a dyno shop tells me otherwise, I think standard operating procedure is to tell the dyno what RPM the

tach indicates at maybe two speeds in the selected gear. So any error in the dial is going to be transferred

straight to the dyno print. Anything else would require them to run a wire into the engine or obd port (nope

didn't see that) or to have a whole bunch of correct presets for exact gearbox, diff, tire ..

And this isn't +/- 50 rpm, this is an error of over 20% ...

Edited January 7, 2009 by r34nur

[blind_elk]

The thing that gets me is the dyno showing the RPM x-axis - they calibrated their RPM "against the tacho"!

Then what's your problem? At least the dyno output agrees with your tacho. It doesn't matter if it was really 4500 and not 5000 rpm; at least when you see 5000rpm on your dial tacho, you will know from the dyno chart exactly what power is being output.

And you are basing your belief of "incorrectness" on a calculation from an unknown source. Maybe you plugged in an incorrect value. Maybe there is something about your car that is different to other similar cars (like rolling diameter of the drive wheels).

If you are so concerned about the accuracy or otherwise of an analogue measuring instrument, get a digital tacho hooked up to the engine, and check the accuracy that way.

(My +/- 50 is the variation at idle, ie it hunts within a 100 rpm range)

[rtune]

Then what's your problem? At least the dyno output agrees with your tacho. It doesn't matter if it was really 4500 and not 5000 rpm;

my original question was asking why it can be so out, and was there an adjustment

option (it seems not). It also seems that it being this much off is a rare issue ..

the calculation from an unknown source isn't untrustworthy: it is a convenience but exposes

the ratios and diff ratio, so I can check it by hand. I'm sure my tacho is out that much.

The dyno output might agree with the tacho but that just means that if dynos calibrate

via the tech eyeballing the tacho then that at least in my case, that was a mistake.

As I said, I'm not concerned about 50rpm errors at idle, who would care? That would

be an error of 5% at most. I was surprised to see this apparent 20% error. When I swap

ECUs it is going to be the first thing I ask the tuner to verify/fix before they do anything

else, how can they go through a tune looking at a rpm vs power plot that is out so much?