hello, i posted this on the electronics board without any useful help. i was told to replace my tach with an autometer, and stock tach's are usually off by 500rpm. well mine's accurate at idle and cruising speed, but not when i jam on the gas. here's the original thread:

i'm in the process of swapping to a cluster with the gauge pack, from the dummy light cluster in my firebird. just took it for the first rip tonight, but the tach didn't seem to be working right.

the idle red correct at 900rpm, and it also seemed to be pretty correct with me at 2100rpm at about 110kph. the issue is that when i would mash the gas and run the car up to what feels like what would be redline, the tach will only go as far as 4000rpm. it also climbs very slowly even though i can hear the engine picking up the revs faster.

the car is an 86 v6, and the other cluster is out of a wrecked 86 v6.

does this sound like a common problem, or do you figure the tach is infact reading accurate? is there any way for me to test this?

any help on this would be greatly appreciated guys, even on how to recalibrate the tach. thanks

 

one way you could test it but maybe not higly recommended...stop, pull gear selector into 1st and nail the gas, wait til it hits to rev limiter and see where the tach is at. on an auto v6 (i'm assuming you do have an auto) there should be a rev limiter on the computer at about 6800 roms or so. obviously don't do this too often or anything. actually, i'm not 100% sure if i'd really even want to try it but wait and see how many people shoot down the idea first. but, its one way to test it.

 

actually the car is a manual, from my understanding they didn't have a rev limiter?

the strangest thing i find about all of this is the tach is pretty dead accurate to what it should be when i'm at a crusising speed, it's only when i run the revs up it doesn't seem too accurate

 

i know when i had mine dynoed, it showed about 600 rpms less than my tach did. your car may not have a rev limiter but it should... doward has a 5 speed and he was tellin me one time that he floored it in neutral just to see where the rev limiter was and i hit one...i don't remember exactly where it was though. obvious, like i said before, not a good thing to do regularly but you SHOULD be ok once. i take no responsibility though if something bad does happen though. you could still hold it in first and see how high the tach will go unless the motor is REALLY screaming and still no limiter yet.

 

Sounds like your tach is goofy.

Your rpms should climb faster than your speed. I'm not sure about where the rev limiter is on a stick, Ive been at 6500+ before.

 

when they go off, its off by a percent.

not a set number.





so the amount its off isnt noticable at idle, is only bearly noticable crusing, but the higher the RPMs go, the farther it goes off.

according to my tach, im shifting at some stupid number like 7k or 6500 or somthing, i forget... i ignore it now when racing, i just "know" when to shift by ear... i plan to get a shiftlight from santa.

anyhoo, you have a manual, so put it in a gear and go a set speed where your RPMs are on the high side.... then go back and do the math to figure out what the RPM really was and compare it to what it said.
you'll need to know your tire size, your rear end ratio, and your trans ratio.... im pretty sure 4th is 1:1 with a T5, but i dont know the other gears.

 

I'd like to know how ya did the swap!
I have an 85 idiot light & I got an 85 V6 converted gauge set up.

 

all i did was change out the oil and temp senders, plug in the cluster and go. i'm upgrading to the digital speedo and trying to gather all the parts, until then no speedo. (car is parked anyway)

 

My suggestion is to pick up a cheap tachometer- well maybe not super-cheap, a $12 tach might not be accurate at all- and patch it into your car's wiring. You'd want to tag the black/white wire, off the gray connector that goes to your coil.

Then run both tachs while you rev the motor in neutral and see what's up with your swapped-in tach.

 

Whenever I just "Plugged in" my guage dash set up, I blew a fuse for horn.
Lucky you!