Tac calibration issue???

[Paul Workman]

When I mentioned to Pete and again to Mark Haibeck (yesterday) that at 7000 rpm I'm only doing about 102mph, the question of tac accuracy came up (Pete). Or, do I have a 4.10 gear? (Marc). I verified my rear gear is a 3.45, and I hooked up my scanner and ran the tac up to a steady 3000 rpm, and the scanner rpm says 2565. Hmmmm.... Pete tells me he's at 122 mph at 7000 rpm with the stock 3.45 rear gear, and if I do the math, 122mph*2565/3000=102mph.

Q: Obviously my tack reads high at steady rpm. Is that to offset tac lag in rpm runup, or is there some calibration issue I might want to address?

What say da Z gurus??

TIA,

P.

[petefias]

What does the tach show at idle? Without the engine running and key on does the needle sit exactly at 0?


If you go into diagnosis mode you can get the exact RPM of the engine to verify the tach.

[Paul Workman]

What does the tach show at idle? Without the engine running and key on does the needle sit exactly at 0?


If you go into diagnosis mode you can get the exact RPM of the engine to verify the tach.

At idle it sits between 700-800 (hard to tell exactly). However, w/ engine off, it reads below zero; approx - 500 rpm. Hmmmm...sez me.

My scanner reads the rpm off the ALDL connector - the reading I was using to compare with the tac. But, I don't know if the rpm reading there is a DC (analog) voltage which is being interprited, or actual digital "blips" being counted - I'll have to research that some (with the FSM), I recon. In any case, the speedo reading seems to suggest the ALDL scanned rpm are in agreement, FWIW.:???:

P.

[Aurora40]

Sounds like your tach is off. I believe that's somewhat common. I do not know what the fix is, though. Mine is within 50rpm pretty much throughout the rev range (looking at increments where the needle is right on a line, otherwise it's just a guess). You can slowly rev it up when parked and look at your scanner to see. There's no lag compensation I'm aware of. It should match what the scanner says, or else your tach isn't very accurate.

[Pete]

Pual here is the hi tech way i check my tech :) if you have the stock chip cut off is 7100 take her out stay in the throttle till cut off look what the RPM gauge is showing.LOL

Even though this works, i'm J/K.

Paul,time for a shift light.

Pete

[petefias]

Easy fix: just shift at 7500.