legacycentral bbs

Just as it seems, my tach is off after doing my build (mainly because of transmission and rear diff swap) and was wondering if its possible to re-calibrate the tachometer in my '93 tleg. Its pretty even in first, about 150-200 off in 2nd, 300 off in 3rd and 500-700 off in 4th and 5th.


I searched but didn't find anything specific to my issue/question

That's weird. How could swapping the transmission and rear diff affect the tach? And what would the selected gear have to do with it?

The only way a swap should be able to make the tach not work right is if you swap to an engine with a different number of cylinders. The tachometer gets its signal from the ECU, which provides one pulse per spark.

I don't know of any way to recalibrate the tachometer. What are you checking it against?

I agree with vrg3.

The tach reads ignition pulses which won't be affected when doing a gear ratio swap.

If you guys remember my build its a stroker 2.37 now. But i calculated expected engine speed using the transmission gear ratio, diff ratio, tire diameter and vehicle speed to get those above numbers.

Engine size doesn't affect how many spark events there are per crank revolution, which is what the tach measures. Only number of cylinders would change that.

Your tach is accurate.

One of your other numbers you used in your calculations is off. Or your speedometer is off due to gearing/tire size changes or just being old and worn out.

I didn't remember your build. If you thought the fact that it was stroker was relevant you probably should have mentioned it before. But it actually doesn't matter in this case.

What do you mean when you say you calculated expected engine speed? Expected under what conditions?

I suspect this whole issue is based on a misunderstanding of how things work. I'm almost 100% sure your tachometer is working correctly.

Alphius wrote:Engine size doesn't affect how many spark events there are per crank revolution, which is what the tach measures. Only number of cylinders would change that.

Even if its a different crank than stock?

speedo is good, gps verified showed it to be about 1-2 mph off. Tires are relatively new, besides that wouldn't change the rpms that drastically.

Different crank than stock still doesn't change how a 4-stroke engine works or how many sparks per revolution you have. The ECU will always output the same signal for the same crankshaft revolutions for any 4 cylinder Subaru engine in your car, no matter what internal engine parts there are in it.

ok, then let me ask this question, for those with a tleg and a manual, can you tell me what RPM's you are at doing 60 in 5th?

In 5th gear my car is at 2340RPM at 60MPH. That's with stock 3.90 gears and 205/55R16 tires which are larger than stock.

With stock size tires (195/60R15) I would be at 2400RPM at 60.

Go to Good Will and pick up a clip on tachometer that actually reads the pulse.

Alphius wrote:In 5th gear my car is at 2340RPM at 60MPH. That's with stock 3.90 gears and 205/55R16 tires which are larger than stock.

With stock size tires (195/60R15) I would be at 2400RPM at 60.

I have the same size 16" tires as you, 4.11 gears and at 60mph my tach reads ~2900.

How do you know that isn't correct? Do you have another tachometer you're using to compare it to?

You need to verify your tach reading with a laptop and cable or with another working tach.

What vehicle is your 4.11 transmission from?

EDIT:
Looked at your post history, you have a '96 STi transmission. It most likely has a .825 fifth ratio which would put you at around 2800 at 60. US Turbo transmissions have a .738 fifth and a 3.90 ratio which would easily explain your discrepancy. Do you know your trans code? It would be something like this: TY752VB2AA

I think your tach is accurate.

You just have a variable somewhere in the line of things that is giving you different results than you are expecting. Tire size, gear ratio, individual gear ratio, something. I don't think there's a way your tach could be off like that since nothing in the engine affects the rpm reading.

The .825 fifth gear ratio explains the discrepancy at 60. I think the mystery has been solved. And all the other gears are shorter too, which explains why he thought there was a different error in each gear.

Were you using US 5 speed ratios for your calculations?