sweanders

It is not uncommon that the stock tacho is off quite a bit and since the needle is on a spring it bounces when it stops.

Don't worry, you hit the rev limiter and it did it's job.

toddk911

So is it a situaiton like the speedo and being off on the conservative side? The tach reads a little lower then actual?

sweanders

I don't know, my guess is that there is signal loss.

Duke

You don't want to shift at peak HP... Look at a power chart and see at what rpm you land on the next gear. You want the maximum amount of average power. And to get that, with almost every engine, you need to shift at redline.

johne

Duke is correct. Maximum average horsepower throughout the rpm range that you experience is what determines your acceleration. And the lower gears offer a hp multiplier due to gearing. HTH.

-John

Dal Heger

Ok, let's think this through logically.

Where does the tachometer getting the signal from? The DME.
Where is the DME getting the rpm data from? The flywheel.
What does the DME do? It runs the whole engine including fuel and spark.
Does the DME need accurate information from the flywheel? You bet.

So, where is the signal loss? It's digital to the DME, and surprise, it's digital to the tach on the instrument cluster! The needle moves and hangs at the correct rpm due to the frequency of pulses it gets from the DME. So, assuming that the engineers got the spring ratio on the little tach needle correct, you should be getting a pretty accurate indication of rpm. The only better way to get the exact rpm (down to the single rev) is to make a digital counter and display it on the dash somewhere. Which would be cool for diagnostics, useless in the real world.

Dal.

johne

Dal, sounds about right. I tell ya what; I've got datalogging equipment hooked up to my girl. How about I do a comparison and see if there's a difference between the signal the DME sees and the signal displayed by the tachometer. My guess is that you called it on the spring being a bit off. If I know German engineers they probably made it just a bit optimistic in the eyes of safety.

-John

Bill

If memory serve me correctly, the factory RPM limit is set at 6,800 rpm. With the stock chips, this limit can not be exceeded.

Custom chip makers can alter the RPM limit or if foolish even remove the limit. I have heard of chip makers raising the limit to 7,200 rpm. I believe some do this for bragging rights. The higher the RPM the greater the horsepower reading. Peak torque is not affected with a stock cam. Perhaps raising the limit would make sence on a race car, where a tiny edge can make the difference. But on a street car, 6,800 rpm is still pretty high compared to other manufactures.

RPM limits and longevity are directly related.

Duke

Just a note, stock limit is 6480 rpm.

ZPmadA

Except in a Honda

VWaddict

Duke and John are correct, but the last part is incorrect, the lower numbered gears do not multiply horsepower.

Horsepower is the amount of work done (torque multiplied by revs/second) and is unchanged. Gearing down (higher numbered gears) increases revs/second, but reduces torque. Gearing up (lower numbered gears) decrease revs/second, but multiplies torque.

It's precisely analagous to step-up and step-down transformers with electricity: Current (I) and Voltage (V) are stepped up and down in the same inverse relationship, but the power (W) remains constant: W=IV. A 100volt-to-10 volt lossless transformer may put out 10Amps at the 10V secondary, but the 100V primary only passes 1Amp. 100x1=100Watts at the primary, 10x10=100Watts at the secondary. -Transmissions are the same and do not multiply power; just torque.

Keith

special tool

The stock tach is nowhere near accurrate.
It can be off by 400 rpm at redline.
I have seen this on several cars ( not a bull**** several cars, a REAL several cars) and had multiple types of datalogging equipment using signals from spark lead #1.
The tach is a piece of ****. Its as though it is powered by a rubber band and bounces waaaay tthe hell up there wherever it wants.

Bill

My tach is powered by my right foot and bounces waaaay the hell up there whenever my right foot wants.

2bridges

WOW, 400 rpm off at redline, I would have never guessed that much

sweanders

As I said I don't know where the error is, what I have noted when driving is the tacho does not show the same value all times that I have reached the limiter. With my previous chips the limit was at 7080 rpm (IIRC) and the tacho showed everything from 6700 to 7200 when I reached the rev limiter.

I certainly hope that the DME RPM value is more accurate than the one showed on the tacho..