Dual Needle Tach?

I just had a co-worker ask me an interesting question (to which I don't know the answer, and google only relayed aviation results). Are there any dual needle tachometers? That is, one need is for engine RPM, one is for transmission RPM? He says that it is common in helicopters (to which google seems to agree). It sounds interesting and at the very least extremely fun to watch as you drive.

Well, your speedometer is telling you trans rpm, it's just converting it to mph via a few calculations.

In reply to Strizzo: Sorry no that wheel rpm....Trans rpm would be before the final drive R&P.

But to the original poster no i don't know of any "stock"

44Dwarf wrote: In reply to Strizzo: Sorry no that wheel rpm....Trans rpm would be before the final drive R&P. But to the original poster no i don't know of any "stock"

No sorry, typically the sensor is in the trans, so it's using trans output shaft speed to figure axle speed, and thus vehicle speed based on an assumed tire diameter

If your trans RPM isn't in direct correlation to wheel RPM, either you have a multi-gear differential or something's broken and you are already coasting to a stop.

Thus the difference between a road car and aircraft - transmission output is a direct correlation to vehicle speed. And why we aren't concerned too much with transmission RPM.

Depending on the trans architechture, there can be 2 shafts turning at speeds diferent from the engine and different from each other. Which shaft would such a needle relate to?

There's a few different answers for that. For instance, the engine may turn at X RPM but due to torque converter slip the transmission input shaft speed won't necessarily match X at all times. So if you had a dual needle tach, one for engine RPM and another for input shaft RPM they'd dance back and forth.

Just about every slushbox out there has two speed sensors (sometimes called turbine sensors), one for input, one for output. The PCM's programming tells it that if the transmission is in this gear and the output shaft is turning at this RPM, then the input should be turning at that RPM, calculated from the ratio of the gear the transmission is in. The PCM calculates any deviation from this ratio this as the slip percentage. If it gets too high, it'll stick the transmission in limp mode.

The transmission input shaft signal is also used to calculate the percentage of torque converter clutch slip by, as in the first example, comparing engine RPM to input shaft RPM. As in: if the PCM requests converter lockup but the speeds vary by more than the allowed percentage the PCM knows the TCC is slipping.

They also use the input/output sensors for the adaptives for the shift strategy, not just for diagnostics. It's only when it can't make the shift happen in under about 1/4 second that it starts whining about discrepancies.

Ha, brain fart. It made sense when I was pondering it without really digging. That's what I get working on a sunday I suppose :)

it's an interesting idea.. and best if used on an Autotrans

I thought of MoPar's Tic-Toc-Tach, but that's a tach and a clock.

I never cared for the "tick tock tach" I always thought the clock blocked too much of the tach

Dual needle...
I was thinking of the '79 Honda Prelude with the gauge-in-a-gauge speedo and tach:

Not dual needle, but dual channel...

Autometer Ultimate II tach

Mercedes tried a clock within a tach and a clock next the tach for some of their diesel models. It was a failed experiment as drivers had difficulty distinguishing between the two.

Chas_H wrote: Mercedes tried a clock within a tach and a clock next the tach for some of their diesel models. It was a failed experiment as drivers had difficulty distinguishing between the two.

So, morons drive diesel Mercedes?

My Trans-Am has a clock in the tach and I've nerver had any trouble understanding the difference.

Trans_Maro wrote:

Chas_H wrote: Mercedes tried a clock within a tach and a clock next the tach for some of their diesel models. It was a failed experiment as drivers had difficulty distinguishing between the two.

So, morons drive diesel Mercedes? My Trans-Am has a clock in the tach and I've nerver had any trouble understanding the difference.

I think he meant that the needles on the tach and the clock move at similar speeds.

Josh wrote:

Trans_Maro wrote:

Chas_H wrote: Mercedes tried a clock within a tach and a clock next the tach for some of their diesel models. It was a failed experiment as drivers had difficulty distinguishing between the two.

So, morons drive diesel Mercedes? My Trans-Am has a clock in the tach and I've nerver had any trouble understanding the difference.

I think he meant that the needles on the tach and the clock move at similar speeds.

The calendar function was also confused with the speedometer.....

I had a volkwagen with a clock where the tach should have been.
I had silly plans to replace the clock guts with tach guts and calibrate it to run to 11:00 for redline.