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Tacho Overeading!!

M

mikeyc4

Guest
Hi,ive noticed the tacho on my 1990 is approx 800-1000 rpm over whats its actually doing,ive read about a tacho filter,does anybody know where this is,and can it be bypassed?or could it be something else?My dash is the part analogue,part digital.

Thanks in advance
 

Notorious

Well-known member
Joined
Oct 9, 2005
Messages
53
Location
Confederate States of America
Corvette
Highly modified '95 A4 coupe
These cars are "notorious" :D for tachometer inaccuracy but yours is a bit much. Around 300 RPM or so in excess of reality seems to be the norm. How did you verify this?
 
M

mikeyc4

Guest
Well i 1st noticed it on tickover,its nunning at 1700 rpm,when i connected my timing light,which tells you the true rpm,it was at 800rpm,also when you floor it the rev counter goes off the clock before the box changes up!
 
Joined
May 10, 2002
Messages
2,254
Location
Boise, Idaho
Corvette
2002 electron blue Z06
Well i 1st noticed it on tickover,its nunning at 1700 rpm,when i connected my timing light,which tells you the true rpm,it was at 800rpm,also when you floor it the rev counter goes off the clock before the box changes up!

Does the timing light fire every other revolution of the crankshaft? The spark only fires every other time around on a four stroke engine.
 
B

billv

Guest
My 90 is the same way. You can hit 6000rpm in a heartbeat! It has a tach filter, but I don't know if it can be removed or not. Does anyone know approximately what RPM you should be running in 5th or 6th at 60mph? I have 3.33 gears.
 

jimmers

Well-known member
Joined
Mar 27, 2005
Messages
302
Location
Canada
Corvette
1990 Red Coupe 6 speed
My 90 is the same way. You can hit 6000rpm in a heartbeat! It has a tach filter, but I don't know if it can be removed or not. Does anyone know approximately what RPM you should be running in 5th or 6th at 60mph? I have 3.33 gears.

Mine is the same way. I scan my 90 with Datamaster and the tach is about half off. My tach will say 6k and Datamaster will only show about 2500. I'm pretty sure my cluster is shot.
 

SPANISHVETTS

Well-known member
Joined
Nov 5, 2002
Messages
1,143
Location
El puerto de Santa Maria Spain
Corvette
before 60,63&82 now 90 383 Stroker & 91 ZR1-Sold
[FONT=times new roman, san serif]You can find your gear ratios here

MPH = TIRE RADIUS ÷ 168 × ENGINE RPM ÷ GEAR RATIO
[SIZE=-1] Note: Tire Radius is distance, in inches, from center of tire to ground.
[/SIZE] [SIZE=-1] Note: Gear ratio is rear axle ratio multiplied by transmission gear ratio. [/SIZE]
[/FONT] [FONT=times new roman, san serif]Example:
What is the MPH at 6,500 RPM with a 4.90 rear axle and a 14 inch radius tire in 4th gear, (1:1)?
[/FONT]
[FONT=times new roman, san serif] 14.0
÷ 168.0
× 6,500.0
÷ 4.90
÷ 1.0
_________________
111 MPH





[/FONT]
 

vetteoz

Well-known member
Joined
Jan 20, 2004
Messages
1,564
Location
Not in Kansa anymore
Corvette
1990
Could this happen if the cluster was replaced with a cluster from a 1992-1996 cluster?
Is a popular fix.
Problem is with the '90 dash unit itself ; not the tacho input signal as indicated by the datalogs showing correct RPM as mentioned above
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Rpm @ 60mph

Here are the numbers for figuring out the RPM at 60 MPH

60X.50(6th Gr Ratio)X3.33 (Diff Ratio) X 336 (*)=1398 RPM
24(Tire Diam)


(*) This is a constant number taken from my Auto Math Handbook

Now you have something to compare your Tach with.

Let us know what your actual RPM at 60 MPH is.

OH! OH! I did not realize the age of the original thread. I now see that it was resurrected with a new question being added which I do not have the answear for.
 

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