I have an 89 econoline chassis with a 460 that I slapped a C6 behind. 4:10 gears and 29" tall tires (will be going to 33s) with a ****** truck body on top. My cruise speed according to GPS at 60 mph I'm spinning 3800-3900 rpm. I got this trans and converter from a junk yard and winged it. The trans shifts pretty good for the most part, it's a little confused taking off at part throttle and than jumping on the skinny pedal. What I've looked at according to a calculator I should be around 2850 rpm at 60. Granted no lockup so it'll be within a couple hundred rpm but 1k rpm off seems excessive to me. What can be going on that I can change it to drop a bit. She sounds great and is very snappy. I do not suspect a high stall converter in it because it was huge and will break the rear tires loose just off idle. The motor is stock aside from long tubes and dual 3" exhaust with an H-pipe.

 

If a clutch is slipping the trans will stop working in the first mile or two. So that is not likely. It think the problem is lack of torque converter lockup and lack of overdrive. And it seems that you have a fairly high stall torque converter, too.

 

That rpm sounds like 2nd gear.

are you sure it is shifting into all 3rd?

my 79 has the same gears and similar tires and does not run that kind of rpm. But it is completely stock as far as I know also.

can you post up a video? Having a second set of eyes On it might help

 

Mark I have disc brakes all the way around. My understanding is if I had a high stall converter while doing a burn out standing on the brakes shouldn't it rev much higher before it spins the tires? My tires are spinning within 50-100 rpm off idle. I have been doing a lot of reading since I made this thread and have only come across one person having a somewhat similar problem as I am. In that 10 year old thread they were pointing toward the converter having possibly a damaged/seized stator or stator blades. I have absolutely no slipping that I can feel. Power is instant and even the slightest touch of the gas pedal causes the truck to "jump" for a lack of a better way to put it, instant power down low.

What I haven't checked is the trans temp because I don't have a trans temp gauge. I assume it could be excess converter slipping. At least according to "the internet experts" in all my reading. I should be making excessive heat due to slip but the way I have the lines and radiator plumbed it should be pulling that heat right out.

I also bought a new tach. I do believe my current gauge is accurate but just to double check I bought an auto gauge/autometer tach. I never liked the current one anyway and this was as good excuse to get one I liked.

It being in second gear was my first thoughts also Mike. From a stop and in 1st gear I manually shifted to 2nd then 3rd with the same result of 3800-3900 rpm at 60 mph. Here 2 videos. I was just spinning tires in one and the other is a drive by right after I finished my new exhaust.

 

https://youtu.be/LMN7e-VxnqE

https://youtu.be/HxWz1xI_NkA

 

Cool. How is that relevant to this discussion?

If you are standing on the brakes and spinning the tires 50-100 RPM off idle then your brakes do not work. I'd fix them before driving the truck again.

The way to determine if you have a high stall converter or not is to hold the brakes with your left foot and press the go pedal to the floor with your right foot. DO NOT HOLD THIS FOR MORE THAN 5 (FIVE) SECONDS OR YOU WILL DESTROY THE TORQUE CONVERTER!!!!!!!!!!!!!!!!! Read the max RPM the engine attains. This is the stall speed. If your brakes won't hold it, something is wrong with the brakes.

Then you don't have a C6. The C6 has slippage by design. It cannot operate without slippage. The torque converter in the C6 does not have a clutch so it always slips.

 

How about video from in the cab watching the tach as you accelerate?

With a 29” tire, second gear calculates out to 4300 rpm for a 235/70r16 (29”) 3rd calculates out to 2900 for the same size tire. Tire size makes a big difference here as a 235/85/r16 puts 2nd at 3900 and 3rd at 2600.

 

Disc brakes all the way around is relevant by saying I have the stopping power to hold the rear tires from moving 50-100 rpm off idle. They work, they work very well too but I'm able to spin tires the way I am? Something is clearly wrong. Since I can over power my rear brakes at such a low rpm and spin tires.

Standing on my brakes and pushing the skinny pedal causes my tires to spin directly off idle (not due to lack of brakes) 800ish rpm. If you haven't watched or listened to the video please do. You can hear the motor and watch the tires spin. Granted I do it quickly but the video wasn't intended as a troubleshooting tool when recorded.

This truck acts more like a race car with instant response to any input of the gas pedal yet I can idle with the brakes on with no issues. With that being said I would agree with you about the brakes IF I was just reading this like you are. Since the trans doesn't skip a beat I'm left to believe something within the converter is causing my issues.

 

I used an actual tape measurer they measure 29". I counted the shifts and also calculated what the rpms should be too. The numbers didn't make sense that I'm seeing. Asking the Google what size tires an E350 has I came up with 245/75R16 which are 30.5" brand new. Which would make being in second gear make sense at 60 mph and 3800/3900. There's no doubt that I'm in 3rd gear though. With it being a C6 there's no lock up to confuse with a shift.

 

If you are right, and it is in 3rd, then pulling it down to 2nd at 60 will over speed the engine.

however, if you were to do it around 50mph, you should be cruising around 3100 rpm and pulling it down to 2nd would raise the rpm to about 4600.

try manually pulling it down to 2nd at 45-50 mph and see if what it does. If it shoots ip to 4500, then you’ve confirmed 3rd gear. If it does nothing then you’ve confirmed that you are stuck in second.


as to doing the stall test, the brakes really should hold better than you described. Even my 79 with drums in the back holds better than the way you describe yours acting. If you are holding the brake to the floor, and over powering them with very light throttle, then something is not right with the brakes. 460s make a lot of torque off idle, but no so much that they over power the brakes like that.

Is there any chance that you somehow got a rear end that was regeared for big tires? To be in 3rd with a 245/75r16 would take about a 5.56 gear. Which would be about right for a 40” tire.

 

I actually did do what you've suggested also. There's no doubt I'm making all 3 gears. Before I installed the axle I pulled the cover and verified the gear set, 4:10s for sure. Since the trans doesn't seem to skip a beat im going to get a new converter and see what happens. Worse case scenario is I will know what it is and I still have the same problem. Best case is something within it is broken and it's a different truck with the new one installed.