Not sure if it belongs in this forum. I have an 81 f150 with a 351c and a c6 auto. Would i be better off keeping the auto or switching to a manual, what manual trannies would bolt up? With the c6 anit at 75mph the motor is screaming at 4500 rpm, even at slower speeds it seems to rev to high. do you think this is an issue with the tranny gearing or the rearend gearing?

 

The tranny is 1:1 in high gear. That is not your problem. The problem is going to be the rearend. You could get a overdrive tranny to help it out a little bit, but it will not be enough to bring it down in the 1800-2000 rpm range, which is where you are going to want to be for fuel mileage and highway driving.

The 351c is setup the same as a 302 or 351w. So any tranny/bellhousing that will bolt up to the 302/351w, will also fit the 351c(not the 351m).

 

Do you have reason to believe you are running a 5.33 : 1 rear axle?

It is more likely that you are running a 3.50 or 3.55--AND ARE IN SECOND GEAR.

Those are the two combinations that result in a 4500 rpm 75 mph with 30 inch tires. Second is 1.46:1.

Count your shifts. Got two? You may not even have one shift if your shifter is so worn you fall into "2" instead of "D".

From a dead stop, floor it, listen and count. You will not be able to mistake a shift from 2.46 to 1.46, and 1.46 to 1 is pretty noticable too.

Check it out.

 

I havent driven the truck in over a month but one night on an empty hiway i came to a complete stop and then floored it, I definetly counted 2 shifts. I belive the rearend is geared low but not 5.33. When i get the truck back in a week or 2 i will lift the rear tires and figure out the gearing. What manual tranny would be a good swap? ive read the 4 speed toploader is a good choice but hard to find. I have also read the zf is an awesome tranny, its a 4 with overdrive right?

 

Are you sure your engine is a 351C?

No Ford F Series truck ever came with a 351C engine.

The 351C was used in 1970/74 passenger cars & Ranchero's only.

As original, a 1980/82 F150 would have come with a 300 I-6 as standard equipment. The 302 2V, 351M 2V and 400 2V were optional.

The only A/T is the C6, the only rear axle is the 9 inch. The 8.8 was first offered in 1983.

Most 1980/82 F150's have rear axle ratios between 2.75-1 and 3.50-1.

The rear AXLE code is located on the Warranty Plate. It's a two or three digit code.

On the rear axle itself is a Ford ID tag bolted onto the carrier at roughly 5 o'clock.

The ID tag is self explanatory, stating the ratio, with an L between the first and second digits if Limited Slip (LS) is present.

Examples: 3L50 = 3.50-1 with LS / 350 = 3.50-1 without LS.

 

the engine is a 351c 4v (see gallery) with a c6 out of a 72 mustang. I have already check the axle code and its reads 3.50 but i beleive the gears may have been changed. What 85e150six4mtod says makes sence that its not shifting into 3rd but last time i drove it there was definetly 2 shifts

 

A regular 4 speed is not going to help much at all. 4th gear will be 1:1 just like the c6. The c6 torque convertor slips a little bit, but not much.

An overdrive tranny will help, but it's not going to bring the rpms down a lot. But if you have a true factory 351c 4v, you are not going to want to drop the rpms really low, since that is a high revving engine, and will be boggy down low.

 

3 speed = 2 shifts: 1st to 2nd, 2nd to 3rd.

Maybe you do have 5.33's.

 

Well 85e150six4mtod says 4500rpm at 75 translates to 5.33 gears which sounds kinda unreasonable. I would also prefer to drive a standard over an auto. Is it possible that the high rpm is a sign that the tranny is slipping?

edit: just saw your post. when i get the truck back im going to try and figure out the rear end gearing

 

With 3.50 Gears, 1:1 top ratio in the tranny, at 75MPH, that would be a transmission input speed of ~2900 RPM, assuming torque converter slippage of around 200-300 RPM, you should still only have a top engine speed of 3200-3400 tops. If your transmission was slipping that much, the truck would be barely driveable.

Only thing I can think of is that you do indeed have some insane rear gears, a massively failing torque converter, or like it was said, your transmission is not going into 3rd. I'm not sure if this is even possible, but perhaps something is messed up in the kickdown mechanism causing it to go down to 2nd during cruising speed?

 

The kick down is not hooked up so I doubt its causing the problem. What would 4.10 gears put the rpm at when at cruising at 75?

 

We have some worn-out Fords at work with the P R N D 2 1 shift quadrant on the column. (standard C6 since 1967). These are so worn that you usually end up in "2" when you shift to "D". I've heard guys talking about how such trucks are gutless and won't run over 45 or 50 mph. (300 motors)

You have to jiggle the shifter towards N to make these go into "D".

Since the OP says he feels two shifts, this can't be his problem.

If he had 4.56s and a slipping tc or trans, he might accomplish the 4500 revs at 75, but how long could this be kept up? I would think the trans fluid would boil/burn/blow after about 1 min. of this.

More research of the subject vehicle required, methinks.

 

It has a b&m mega shifter in it and even with the original shifter it still drove like this. The front seal on the transmission just blew and i sent it out for repair so when i go to pick it up im going to mention the high revs to my mechanic.

 

the thing about the shifter bein worn out a little bit. I have something similar i have a C6 tranny in my 82 F100 with a 302 and my shifter looks like its in between N and drive and when i put it in park the little needle doesnt go all the way to park its bout half to 3/4 the way between P and R. just count the amount of times it catches going down the column and remember ur points its really pretty simple. and if you have a tach shift it like a manual without a clutch. start out in 1 and at whatever rpm point u want shift up to 2 and then D and then see how the rpms are doin at 75 mph

 

If you read my post you would see i dont have a column shift. Its a brand new b&m floor shifter, It drove the same even before I installed the new shifter. manually running throught the gears doesnt make a differance in the final rpm.

Im going to have to check the gears, ive been thinking and the previous owner did mention that the truck was being built to race so it is possible that it does have some crazy low gears in the rear but i still doubt anything more extreme that 4.10s