C4 transmission question

cheshireccat · Mar 18, 2004

I have a question for everyone with a 289 and C4 tranny. Since I don't have a tach, I must ask all of you folks at what speed does your transmission change gears? Mine changes
1st-2nd 25Mph
2nd-3rd 35Mph

My engine threw 3 rods a few weeks ago and I hear it is from revving at too high of RPM's for too long, I want to know if this was caused by my tranny. well, any idea, email me folks at cheshireccat@yahoo.com

grabberorange · Mar 18, 2004

I have an FMX, unlike its C6/C4 cousin, has a Raveneaux gear set which provides 2.40, 1.40, 1.0 in the 3 fwd gears. Here is the formula that will tell you rpm's based on speed, gear, etc. and and example following it.

The formula is:
MPH * 336.13 * (FMX GEAR) * Axle Ratio / Tire Diameter

An example:
42 * 336.13 * 2.40 * 3. / 24.5 = 4148 RPM

crushnut · Mar 19, 2004

Mine shifts from first to second at 10 mph, and from second to third at 25 mph.

GaPonyFarm · Mar 19, 2004

Mine shifts 1-2 @ about 15 and 2-3 @ about 31... doesn't really mean much though. It mostly depends on your acceleration with the C4. In any case, the tranny didn't cause the damage to your engine. It was probably just its time to go...

cheshireccat · Mar 22, 2004

reply

It was only 15K miles on the engine, so I'm not sure what it was, I just ordered a new 302 and wanted to make sure my problems weren't caused by the tranny. I will post my engine problem elsewhere. but does anyone else feel wierd going from 31 MPH to 95MPH in one gear? I use mine as a daily driver and traffic here ranges from 70 - 90, I know I shouldn't drive that fast though...

3spd on floor · Mar 22, 2004

who built the motor? that could have a lot more to do with it than the transmission. engines dont usually throw rods unless they're pushed really hard. even if you were running 4.11 rear gears and a 24 inch tire you would still be less than 5500 at 95mph, not figuring on any slippage.

gtjomama · Mar 22, 2004

I also have a 67 coupe w/ a 289 and c4. I am not an expert, but I think that the 3 speed limits top speed considerably. If you have the stock guages, you'll notice that even though there is not a tach, the speedometer redlines at 70 mph... I have a hard time getting mine to go any faster than that anyway, and I'd bet a nickel that the rpms required of the engine would be well past what could be considered an RPM redline at 95 MPH. I looking into options for replacing my tranny... either finding something w/ overdrive or if all else fails converting to manual. I'll never make an impressive showing at the drag strip or be comfortable with freeway driving with a car whos top speed is 70 mph before you are in danger of blowing an engine.

gbarber · Mar 22, 2004

I have a '66 with a 289 and a stock C4. I don't know about the '67s, but the speedometer on my '66 has no redline. I run at about 2700 RPMs at 70 MPH, and I'll hit 4300 RPM or so at 100 MPH. Then again, I still have the 2.83:1 rear gears in it.
I drive 37 miles to work on the freeway at 70-75 MPH about once a week. I haven't had any problems yet.

Hydrocarbon · Mar 22, 2004

Well I had a 302 in my 69 and the c4's shift points at WOT were at a low 4500 rpm (because of the rail-rockers I guess). MPH was:
1-2 @35-40
2-3 @55-60
Keep in mind that I have a 2.73 rear.

I don't think the transmission would cause a rod bearing to fail. Its more likely a stretched rod bolt or something. If you feel that your transmission isn't shifting properly when I would take it to a shop to have it checked. You more likely to damage the transmission then the engine if you shift points are out of adjustment.

67coupestang · Mar 22, 2004

gtjomama said:
I also have a 67 coupe w/ a 289 and c4. I am not an expert, but I think that the 3 speed limits top speed considerably. If you have the stock guages, you'll notice that even though there is not a tach, the speedometer redlines at 70 mph... I have a hard time getting mine to go any faster than that anyway, and I'd bet a nickel that the rpms required of the engine would be well past what could be considered an RPM redline at 95 MPH. I looking into options for replacing my tranny... either finding something w/ overdrive or if all else fails converting to manual. I'll never make an impressive showing at the drag strip or be comfortable with freeway driving with a car whos top speed is 70 mph before you are in danger of blowing an engine.

wow my 67 would do 80 before she sounded like she was screamin... guess the 2.73's are handy... of course now with a 5-speed 80 will probably be about right....

GaPonyFarm · Mar 22, 2004

cheshireccat said:
It was only 15K miles on the engine, so I'm not sure what it was, I just ordered a new 302 and wanted to make sure my problems weren't caused by the tranny. I will post my engine problem elsewhere. but does anyone else feel wierd going from 31 MPH to 95MPH in one gear? I use mine as a daily driver and traffic here ranges from 70 - 90, I know I shouldn't drive that fast though...

Its not the 30 to 90 that bothered me... it was the 70-130 that got scary! Got a Gear Vendors under/overdrive and 130 is still too scary in a 66 coupe! But the drivetrain likes the o/d alot better...

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