According to my logging data my tach is WAY off. Everything seems real accurate from 0 to ~1500, but after that it's slow. 1750 is really 2000 (per logging), 2000 ~ 2500, 2500 ~ 3500, 2750 ~ 4000 (usual shift point). I haven't pushed any further becasue i only have 300mi on the new motor and i'm still trying to get it tuned, plus i'm running injectors that are too small. Please help, this makes shifting maunally in the Automatic (ick) difficult. I can hear the motor and have a feel for it, but it's my first experience with SBC motors and it's nothing like the sound of my old 4.3L. Thanks,

 

The tach has nothing to do with the ecm. If you put a v8 in where there was a v6, then yes the tack is going to be way off. One some of the GM tacks there was a selector on the back for v6/v8, but that was years ago, you might try looking thou. It was a *jumper* that slide from side to side. Some of the later ones, allegedly, can be changed, internally, is all else fails might try www.jagsthatrun.com...

 

It was an LB9 stock, has a L98 in it now. Same ECM, same distributor, same everything except for CAM, Block, and few small items but most are stock replacements.

 

Other then it just maybe being the way GM calibrated it, looks like replacement time. *Normal* seems to be a couple hundred faster than actual at the higher revs, but that seems to be a record.

 

When i take it apart and analyze it, i'll let ya know what i find.

 

When my stock tach was going south it was reading about 1500 at an 800RPM idle. It was also pegging out at cruise speeds.

Just a gauge on it's last leg is my guess.....

 

I am going to pull the dash as soon as i get my white overlays

 

I always used to wonder why my GTA would act like I hit the brakes when got her to 5500RPM (as read on stock tach).

Then I started datalogging I found out 5500RPM on the tach was 6400-6500RPM as read by the ECM. When I found that out I put an aftermarket tach in with a shift light.

Problem solved.

I'm actually suprised that motor lasted as long as it did considering I took it to redline about every other time I drove it and it had 160k miles on it.