Doubting my tach
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Doubting my tach
The tach circuit board went out on my 1982.
I replaced it with the little board from Zip. Easy job.
However, I think the tach is now reading high while cruising.
What should my rpm’s be doing at approx 65-70 mph in overdrive? I plan to try and use my timing light to test this weekend. Just hoping to get some other 82 owners input.
Stock Crossfire engine
700r trans
2.87 axle
OEM size tire/wheels
Stock exhaust
I replaced it with the little board from Zip. Easy job.
However, I think the tach is now reading high while cruising.
What should my rpm’s be doing at approx 65-70 mph in overdrive? I plan to try and use my timing light to test this weekend. Just hoping to get some other 82 owners input.
Stock Crossfire engine
700r trans
2.87 axle
OEM size tire/wheels
Stock exhaust
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
With the numbers you provided:
2.87 rear end ratio
0.7 tranny final drive ratio
27" diameter tires
70 mph
0% converter slippage from lockup converter
rpm should be 1750 rpm.
With 5% converter slippage rpm will be 1838 if you do not have the converter lock-up operational, so Richard's guess above is right in the ballpark.
Lars
2.87 rear end ratio
0.7 tranny final drive ratio
27" diameter tires
70 mph
0% converter slippage from lockup converter
rpm should be 1750 rpm.
With 5% converter slippage rpm will be 1838 if you do not have the converter lock-up operational, so Richard's guess above is right in the ballpark.
Lars
Last edited by lars; Oct 31, 2022 at 10:55 AM.
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Thank you both.
Its got me at 2400-2500 at 70-72.
The engine didn’t sound like it was running that high. This 82 really cruises well. I am counting the shifts on the tach also. I don’t expect it to be exact, but it could do a little better.
I guess I will send this little board back and get another and see what happens.
Thanks again.
Its got me at 2400-2500 at 70-72.
The engine didn’t sound like it was running that high. This 82 really cruises well. I am counting the shifts on the tach also. I don’t expect it to be exact, but it could do a little better.
I guess I will send this little board back and get another and see what happens.
Thanks again.
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
With the numbers you provided:
2.87 rear end ratio
0.7 tranny final drive ratio
27" diameter tires
70 mph
0% converter slippage from lockup converter
rpm should be 1750 rpm.
With 5% converter slippage rpm will be 1838 if you do not have the converter lock-up operational, so Richard's guess above is right in the ballpark.
Lars
2.87 rear end ratio
0.7 tranny final drive ratio
27" diameter tires
70 mph
0% converter slippage from lockup converter
rpm should be 1750 rpm.
With 5% converter slippage rpm will be 1838 if you do not have the converter lock-up operational, so Richard's guess above is right in the ballpark.
Lars
Your calculations are spot-on.
65mph-75mph @ 1750rpm - 1950rpm
My tach with the new tach board is consistently reading 300-400 rpm’s higher than the tachometer.
The 82 CFI with 700r4 really cruises comfortably at 70-75mph.
Thanks for your help. I will try to find a working tach board.
Race Director
Joined: Mar 2017
Posts: 11,944
Likes: 4,506
From: Virginia
Get a digital timing light (like an Innova 5568) to confirm your tach readings.
Or better yet, with an 82, borrow one from a friend.
Or better yet, with an 82, borrow one from a friend.
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
Get a digital timing light (like an Innova 5568) to confirm your tach readings.
Or better yet, with an 82, borrow one from a friend.
Or better yet, with an 82, borrow one from a friend.
Race Director
Joined: Mar 2017
Posts: 11,944
Likes: 4,506
From: Virginia
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Le Mans Master
Joined: Jun 2019
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From: Indiana
Make sure ignition on..needle is zero..
then go to post 140 here
https://www.corvetteforum.com/forums...uestion-7.html
then go to post 140 here
https://www.corvetteforum.com/forums...uestion-7.html
Thread Starter
Drifting
Joined: May 2020
Posts: 1,663
Likes: 702
From: Baton Rouge, Louisiana, metro area
I got another tach board from Cajun Tach Shop for about $45. This one came with an adjustment feature and detailed instructions for calibration.
All the adjustment screw needed was 1 1/2 turns and now the Tach is perfectly synchronized with the Tach on my timing light. Took it up 75 mph and 2500 rpm’s. Matched all the way from 0 - 2500.
I got to looking at one from Corvette Central and see it may be adjustable as well; however it came with no instructions - it was just thrown in the box. Now I know.
Either way, got to give a favorable review of the Cajun Tach Shop if you need any Tach stuff.
cajuntachshop.com
All the adjustment screw needed was 1 1/2 turns and now the Tach is perfectly synchronized with the Tach on my timing light. Took it up 75 mph and 2500 rpm’s. Matched all the way from 0 - 2500.
I got to looking at one from Corvette Central and see it may be adjustable as well; however it came with no instructions - it was just thrown in the box. Now I know.
Either way, got to give a favorable review of the Cajun Tach Shop if you need any Tach stuff.
cajuntachshop.com