I thought I would share with the world what I would like to do with my pickup. I would like your questions, comments, and suggestions about it. It is a 77 F150 4x4 351, 4spd tranny, 3.50 gears with limited slip front and back. What I am wanting to do is pull the 351 and bore it about .040 over get different heads for it (4 valve) put a perfomance intake topped by a 4barrel. A different cam is an order too. The exhaust will be dumped by headers into 2 1/4 exhaust that I already have on the pickup. Then the tranny I have now will get pulled to put a higher geared one in. The one I have now is geared really really low. Its great for pulling stuff but the engine raps up so much going down the road. The milage is ok (about 12-13mpg) but a different tranny will help a bunch there. I have a higher geared tranny in my garage waiting to be put in so that takes care of the tranny. The axles even thought they already have limited slip I dearly want to put Posi in. So if anyone has any suggestions about how I could make this setup better or any comments please dont be shy I need all the help I can get on this project.

 

Hey Fordman, I may have missed it somewhere, but I thought that when the tranny was in high gear, manual or automatic, the ratio was 1 to 1. You can get lower gears up to high. If you want to slow the engine you will have to change your rears, or go to overdrive.

If I have made a mistake I'm sure I hear about it.

John

 

Fordman:

Do you have the 351M or 351W?

If you have the 351M then stay with the 2-bbl heads you have. They flow just fine. The 4-bbl Cleveland heads will flow better but only at very high RPMs, and they will cost you low-RPM torque.

Regardless of the engine type you have, seriously consider aluminum heads. Maybe even a matched set from Edelbrock including cam, heads, intake,carb, etc.

When you price-out the aluminum heads they seem expensive. That is until you see what rebuilding heads costs. It may cost you $700 to rebuild your cast iron heads (including new valves and bronze guides) and only $1,000 to get brand new aluminum heads that will flow better than your existing heads.

Good Luck!!!

 

3.50 gears are pretty fast. I have a friend with 3.73 gears in his pickup hooked up to a C6 and he can go 115-120. 3.73 are lower gears than 3.50 so the only other thing that could be holding me back is my tranny. If you find any other info on the final gear ratio please let me know.

 

Bruce,

I have a 351M

 

Ok If your friends truck is running 120 mph at lets say 5000 rpm's with the 373 rears...

put your 350 rear gear in, and the mph would go to lets say 130 mph at 5000 rpm's

put a 325 rear in and the mph could go to 135 mph at 5000 rpm.

With the same engine, tranny, and rpm, you will

1- have more pulling power with a higer rear

2- have more mph with a lower gear

You stated that the engine was turning too fast at crusing speeds to suite you. The rears are the way to change that.

Now the 4X4 is the problem, they were built to pull and have pulling gears, not mph gears.

When the tranny is in high gear the imput shaft is turning at 1 to 1 with the output shaft. An overdrive tranny is a tranny in reverse, while in OD, the output shaft is turning faster than the imput, and is at higher mph, at a lower rpm.

There are formulas for the "exacts" on all of this wonderful info that I have just laid out, but I have given you the redneck unabridged version.


John

 

What different tranny do you have? Unless it is overdrive, then a tranny change WILL NOT change your cruise rpm's. Taller tires will work as well as a rear end gear change (and its a lot cheaper) I have a 78 F-150, 3.5 gears, 400M, 4-speed, 35" tires. My cruise is 2100 at 60 mph, and I get about 13 mpg on the highway.

As for that limited slip you want to put a Posi in, Posi-traction is the General Motors term for a limited slip type rear end. So you already have a Posi. It may be worn, and not functioning correctly, but you've got one.

As to engine mods,my 400M has an Edelbrock 600 CFM carb, headers, and Edelbrock performer intake manifold, and an Edelbrock cam. It pulls strong from 1500 to 5000 rpms. It just has the two-barrel heads, and they breathe fine. I can easily spin my back tires in 3rd gear.

 

My other tranny cam from a 71 F100 with a 360 in it. I was told that it was geared high but now that I think about it that pickup had 2.75 gears in it so i guess that wont work. Im not much into big tires so im not going to do that to bring my RMPs down. Does anyone know how much it would cost for new ring and pinion for a 4x4 pickup. I was planning on putting in 3.27 or 3.08 gears. What do you think?

 

Sounds like a good plan. But why not get higher "rear" gears and maybe a small stall in the converter. If you go with posi-trak get the detroit lockers.

 

Hey Fordman:

Sounds like a good project. The only things that seem to be of major consideration are in your drivetrain, specifically, tranny and axles. Keep the 351M! That's a darn-fine engine, and can be built up nice. I, too think you should look into an aluminum head set: they weigh less, are easier to machine, and dissapate heat much better than cast iron. The new aluminum units are even just as rugged as the old cast-iron ones!

First off, here's some math you might be able to use to figure out what you want to do:

RPM=(MPH x gear ratio x 336)/ (tire diameter)

RPM, of course is your engine speed. MPH, of course is your ground speed. Gear ratio is the numerical underdrive (overdrive = less than 1.00, underdrive = greater than 1.00) of the driven axle(s). Remember "high" gears are numerically lower (2.72, 3.50, etc.) and are good for high speed, while "low" gears are numerically higher (4.10, 4.56, 5.88, etc.) and are good for lower speeds requiring more torque. 336 is a mathematically derived conversion factor. Tire diameter MUST BE MEASURED WITH A TAPE MEASURE!!! Do NOT use the numbers on the side of your tire, they are only required to be ± 2" depending on the wheel rim used from the factory.

Using this formula, you can derive any other factor from the formula by algebraically solving the formula for that sum.

Now, your tranny. If you want an excellent OD tranny, contact Advanced Adapter www.advadapters.com, for a brand new New Venture Gear 4500. This is the same 5-speed tranny that sits behind the new Super Duties! Or, you can get an excellent selection of OD/UD units from Advanced, too. If you want BOTH Overdrive (OD) AND Underdrive (UD), contact Branbnod's to learn more about the totally AWESOME Brabnod Mountain Gear www.gj.net/brabnods/

Now, if you are going to stay with a stock 1:1 ratio (No OD), and want more torque for hauling/passing, go for "low" gears, but high speed and fuel mileage will suffer. The exact opposite is true for "high" gears. OD is nice because you can keep the "low" gears because the OD (.76 or "higher", numerically lower) will compensate, too a degree. I'd say 5.88 is the "lowest" you might PRACTICALLY wish to go. But, hey, it's your truck.

I HIGHLY RECOMMEND that you get your hands on a copy of the February 1999 issue of Peterson's 4-Wheel & Off Road magazine, flip to page 18 to see where I got all this info from.

Finally, yes, PosiTraction is a GM trademark for their own engineered limited-slip system. Since it was only installed in Corporate axles (12- and 14-bolt), it will not fit in a Dana (60- and 70-series) or the Ford 9" carrier used in Ford rear-ends. Just for some trivia: the GM GovLoc was GM's try at creating a truly "locking" differential. It eventually was discontinued because it was very prone to breaking (it used spining counterweights to lock the clutches together, whereas the popular Detroit Locker uses spring loaded friction plates [mutch like an engine clutch]). If you are just going to use the truck mainly on the street, get a limited-slip. If, however, you plan on doing any "real" 4-wheeling, GET A LOCKER!!! A mechanical unit will do you fine, but it still might bind on the street, and some units will cut tire life down considerably. The best of the best: an ARB Air-Locker. You can turn it on for the nasty stuff, and then shut it off when you don't need it; for on the street. I have one on my truck, and it is totally awesome!!

Anyways, I know I have talked more than my fair share. But I hope this helps. Feel free to Email me at fuzzyduk(No Email Addresses In Posts!) to let me know how your truck is coming along. Best of luck, and above all, have fun. Isn't that why we have trucks in the first place?

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The only thing I'd like to add to the the other comments is that you have to change both the front and rear ratios or there will be a mis-match when in four wheel drive.

 

Two small items regarding the equation. The 336 factor is for manual transmissions as it assumes that there is no slippage from engine to rear axle; use 355 for an automatic which provides a 5.6% slip factor. This factor could go to 10%; I understand some of the new lock-up convertor equipped automatics get close to a 0% slippage. To know your specific factor, you need to do get speed vs rpm data from your truck and solve for the factor. The gear ratio is really the Final Drive Ratio. If you had overdrive, you would multiply the axle ratio by the trans final drive. Virtually all vintage automatics are 1:1 final drive so you can just use the axle ratio; Ford did make a 4 speed manual which had overdrive final gearing (no granny first gear) which, multiplied times the axle ratio, gives a lower final drive ratio.

 

Thanks, Ole.

I forgot to mention finding the "true" final drive, I appreciate you pointing that out. That is interesting about the conversion factor. I was wondering how that figure was derived.

Thanks a bunch, fellow Ford fan:

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