Aspkiller

After installing the LS6 into my wife's 82 Auto, I did a little calculating the other day because the speedo vs. tach just didn't seem right. It's the same tranny that has always been in the car (2.75 w/1:1 in Drive) and it had 245.45.16s on the rear (which are really close to the diameter of the 225.50.16. that is listed as stock for that year. Never noticed the discrepancy (real or imagined) before.

The Owner's Manual shows the speed to be appx 62-64 mph at 3K rpm. However, every speed/rpm calculator I have says the speed at 3K is more like 80 mph. An the speedo is showing slightly more than that.

I've also confirmed my tach is reading correctly.

Regardless, the book doesn't match modern math. Or am I having a brain fart?

ArthurPE

3000 rpm/(60 sec/min x 2.75) x 3.14159 (Pi) x (((245 mm x 0.9)/25.4 mm/in) + 16 in)/12 in/ft / 88 ft/sec/60 mph x 60 mph x 0.97 (tire slip) ~ 81 mph

give or take

Aspkiller

Yep! I did the math too. 82 book is wrong, as are the 78, 79, 80 & 81 books.

the flyin' scotsman

we all own 928s...............we're all nuts to some degree

mickster

Mmmmmmmmm...Pie.....

hacker-pschorr

Tachometers run off the distributor.
Installing the 85 EuroS ignition system into my 81, my original tach now reads 1/2 speed because the 85+ tach only read half the engine where as the 78-84 count all 8.
To "fix" this I will either install an 85+ tach or a signal doubler between the EZK and original tachometer.

I'm guessing you need a similar tach adapter since the 78-84 tachs normally run off the same signal as the "green wire" = 8 cycles per revolution and my "new" ignition system is only sending four. Since your GM system doesn't even have a distributor........??

So....not sure how you have the tach connected to the Chevy system, but that probably has something to do with it.

Richard S

After installing a complete 84 Euro S engine in my 1980 928, I simply sent my tach to Palo Alto Speedometer and asked them to make it indicate double the received signal.

Rich

hacker-pschorr

Yea, that's what Mike Schmidt (aka Z) suggested I do. Either way I need to pull the pod first

That is a good point, if you send the stock tach out to a shop like them with a description of the new system, they can probably re-calibrate it.

Since I'm now converting my 79 too, I should send out both of my tachs.

Aspkiller

The tach is right, as we can control the pulse flow thru the engine management system. The point of the thread is that the owner's manual is wrong.