**Admins**
If this thread belongs elsewhere, please move.

About a month ago when my car was on the dyno, during a 4th gear pull, the operator stated that he ran the car all the way to an indicated 7500 rpms before he let off. The dyno itself merely registered 7000rpms. I've gathered that tachs aren't completely accurate, especially at the upper rpms, but I would have never imagined they were off *in advance* of actual engine revs.

Armed with this knowledge, I tried shifting at an indicated 7300 at the track the other night. I did this on the first run, from 1st to 2nd. It didn't work. Almost exactly at an indicated 7100rpms I encountered the fuel cut. After that, I didn't try it again. So, the tach appears to be more accurate in 1st than 4th.

So today while getting on the interstate, I strung out both 2nd and 3rd until I hit the fuel cut. In both 2nd and 3rd, it was just barely before an indicated 7400rpms. When I say just barely, I mean it was over 3/4 of the way between 7200 and 7400. So I'd guess around an indicated 7350rpms.

So, in my car at leat(05 6MT Sedan), in first gear the fuel cut occurs and an indicated 7100. In 2nd and 3rd, it's almost 7400. In 4th, on the dyno at least, it's beyond 7500.

One thing to consider is the fact that when I was at the track, I was WOT, obviously. Today when checking 2nd and 3rd, I was *slowly* accelerating so I could more accurately observe the rpms on the tach. The indicated 7400 might well be different when under WOT.

Why the discrepancy? And why does the discrepancy vary depending on gear?

Has anyone else experienced this? I'm specifically curious about the revup, primarily because of the increased redline over the 280/287 motors.

Does anyone else care to try some fuel-cut runs in his/her revup? I'd like to compare numbers.

 

How was the tach signal captured by the dyno operator?

 

Interesting Trey. Look at my video here...

http://titantrucks.net/russ/dynovideos/test5.wmv

4th gear pull by mistake, but look at the rpms.

 

Thanks for the vid russ. Reaffirms my experience.

 

I have no clue.

 

if our tachs are that far off, I must be really short shifting my car at the track!!!!

I have an '05 6MT Sedan

 

I'd like to stress that this is merely something that I have observed on my own car. And with the exception of Russ's video, I am not aware of anyone else noticing the same thing. Also, keep in mind that the discrepancy seems to vary depending on gear. The next thing to determine is if it also varies depending on throttle position.

In the end though, I think those extra revs in 2nd and 3rd will help out a bit at the track.

 

My tach is off by 500rpm up top to . 03 coupe . Confirmed on a dyno , my data logger , and my OBDII . I can run it to 7300rpm [ on the tach ] and it shows 6800rpm on every thing else

 

Is this gap consistent across all gears, and throttle position? Or have you observed this only on a single gear?

 

My 03 5AT's tach is off with varying rpms. In 1st I'd say the tach only goes barely to an indicated 6600rpms (right at red line). In 2nd and 3rd it will happily do an indicated 6800/6900rpms. I think there is some sort of floating rev limiter on these cars which acts as a torque management device in certain gears. The auto would be easy control since most of the system is electronic, but the 6MT is a bit odd. I guess there could be a switch in the tranny that lets the ECU know the car is in 1st therefore lowering the limiter a bit.

 

I havent paid much attention to it , Nor tried to find out exactly when and where in the rpms range it changes

 

I dont think there is some type of floating rev limiter at all . I believe the tach is just off at different levels at different rpms .The higher you go in the rpm's the more the tach is off . And different gears may effect how much it is off also