Something doesn't add up....

A

Anonymous

Guest
I've been working on this problem and I'm starting to bug out and need someone else to take a look and tell me what you think.

My problem has to be one of three things. I recently start thinking about what I want to do for a rear end. To my knowledge the rear end is stock and according to my axle code (found on the panel attached to the drivers door) says the car is equiped with a 2.83 ratio (looked up axle code in my manual). However, at 60mph my tack is reading around 3200 rpm's. All the research I have done says the 3rd in my C4 is a 1:1 ratio. Problem is the math doesn't work out

60mph
tire diameter = 25"
axle ratio = 2.83
high gear ratio = 1 to 1

With the figures above the car should be pulling around 2200 rpm's. Best I can figure is either the axle is not stock or the final gear ratio in the 65 C4 is more like 1.46 or my tach is not reading correctly. Tach's are not rocket science in terms of hooking them up so I don't think that is the problem. Below is the web site I'm using to calculate the rpm's.

http://www.quiknet.com/~ke6vut/rpmcalc.html

Does anyone have any ideas? The Tach is a sunpro unit. Shifts occur around 5800 RPM's according to the Tach. I've checked the axle for a tag indicating the axle ratio but no dice. Everything on the car was stock when I bought it and I can't imagine the axle is anythign but the stocker 2.83.

Using the URL above I would have to have a 4.1 axle to turn the engine 3200 rpm's at 60mph with 25" tire diameter and a 1 to 1 3rd gear ratio. That better not be right cause even after the aussie head upgrade I'm not completly satisfied with the accelration of the car (it is way better then before but it was a gutless pig before) and was hoping to change rear end's for something like a 3.5 ratio.

Does anyone have any ideas?
 
According to my home-brewed calculator, I show you at about 2200-2300 RPM for 60mph under the stated conditions. What size tires & wheels are you using? I use that data to determine the tire diameter. I'm wondering if there may be a faulty assumption someplace.

Could the tach be calibrated for another type of motor (4 or 8 cyl)? I don't know if such a thing happens, it's just a guess. I seem to recall the wiring for a tach I installed on my old Falcon 8-cyl, and it seemed like it picked up every spark pulse at the dizzy. So if you have one calibrated for a 4-cyl, it's seeing extra pulses. At 2200 RPM, your tach would read 3300. This is a wild-assed guess, but it makes some sense if my assumption about the wiring is correct.

If worst came to worst, you could jack up the rear of the car and disconnect the driveshaft at the diff. Mark a "zero point" on the pinion shaft. Have a helper rotate a tire through 1 rev while you count the diff rotations. It a crude way to figure out if you really have 2.83 gears in there.

--mikey
 
HI
When you installed the tach did you change the calabration on it for the 6cyl.
you well most likely have to open it up and there is just a switch on the back inside and it is mark 4,6 and 8. Sometimes there is a hole in the back where you can change the switch..
Tim
 
Mikey, they are 15 inch wheels. I used a tape measure and the total diameter of the wheel and tire is 25 inches. I will have to wait until I get home to see the actual tire size. I think the tach being calibrated for the proper number of cylinders is probably the issue. I will check it out afte work and let you guys know what's up. It may just be set for a 4cyl because it reads 3200 when it should be reading 2200.

Also I love your idea for marking the pinion and counting the revolutions if the tach checks out I will try that next.

Thanks guys.
 
Chaz, yes the stock wheels are 14" which is what was on the car when I purchased but I have since upgrade to 15's.

At any rate the cylinder selector on the back of the Tach was the problem. It was set to 4 and I moved it to 6 and that made all the difference. I haven't yet drove 60 mph (I will on the way home) but just idling the tach is a couple hundred RPM's lower and around work @ 35 it's way lower so I'm sure on the way home it will read 2200 like it's suppose to thanks everyone.

That makes me feel better. Now I can look at a different rear end with a ratio more for acceleration.
 
If you want those rpm numbers back to the 3300rpm mark at 60mph, a set of 4.11's will do it. :)

--mikey
 
And for aminute I thought you were going to be the first guy with a six that would turn 8000 RPM without blowing up...
 
So at 60mph the tach now reads 2200 like the math says it should. Thanks everyone.
 
You must log in or register to reply here.
Back
Top