I understand how to determine your MLTW (maximum loaded trailer weight) tow capacity. My question is, how does Ford determine it? Let's use 2003 for the example. To keep things equal, lets use a 2WD F150, reg cab, 3.55 w/ auto trans.
4.2 = 5,700 lbs
4.6 = 7,100 lbs
5.4 = 8,500 lbs

So am I to assume that the only difference in WHY the MLTW is different is because of the engine/trans combo? So keeping all things equal, if you had a 4.2 F150 and then swapped in a 5.4, your MLTW would go from 5700 to 8500? Again, all things equal except for engine/trans combo.

I ask because I'm considering a TT but the ones I'd like are on the almost-too-heavy side for a 4.6 and I believe it would be cheaper for me to put a fresh 5.4PI (and trans) in my '98 as apposed to selling it (to which I'd lose all of my current mods and wouldn't get much extra cash in selling them) and buying a 150 with the 5.4 already in it.

Thanks

 

THere are a number of things that determine max trailer weight by the OEM. Tire load/size, axle gear ratio, axle torque rating, transmission gears, transmission torque rating, brakes, springs, bushings, frame, engine power, and several more things. Improving just one of these will not magically increase your towing power. Most people recommend improving your truck's towing ability to better handle traielrs within the OEM ratings, not to increase it. Besides, towing more than 6000 pounds with any F-150 isn't the best idea.

I you have a 2wd truck, most cost effective mod you can do is change the rear axle ratio. Going from 3.55 to 4.11 will help your gearing by 15%.

5.4L has a larger rear gear, 9.75" vs. 8.8", so changing your 4.6L to a 5.4L still keeps your axle torque limit.

 

5.4L has a larger rear gear, 9.75" vs. 8.8", so changing your 4.6L to a 5.4L still keeps your axle torque limit.[/QUOTE]



What if he swaps for a salvage 9.25" rear ends too? are the springs different?

 

Yes, the 5.4L weighs more and OEM equipped vehicles have higher spring rates.

 

Thanks for the reply guys.
I'm aware of the rear end, springs, etc....that's why I said all things equal.
However, frame? Is the frame really different between engine sizes? I'd assume the only changes would be between wheel bases and 2WD/4WD. Anyway, I was just wondering if the huge difference in towing capacities is mainly the engine/trans combo.
If I get the donor F150 with a 5.4, I might just swap out all the goodies. I love my truck and I'd hate to get rid of it just to be able to tow a bit more (sissy 4.6!!! )
I am fully aware of how much work the swap would be. I've worked on just about everything you can on a car...I'm prepared for that.

 

Just some random thoughts to keep in mind. I don't know about IL but in CA the max load shown on the door sticker stays the same no matter what mods are done to the vehicle. The s##t could hit the fan in case of an accident while towing and someone found out you were over the door sticker limit. The other problem in CA of course are the smog specs. Those specs are determined by the VIN number and your original engine.

 

Yes, same frame between 2wd/4wd and any engine. Just different frame for differnt wheelbases. Engine, trans, rear axle, springs/torsion bar would be the main differences I can think of between a 4.2L 2wd Supercab and a 5.4L 2wd Supercab for example.

 

All things concidered I think you would be better w/ a 4.6 and the heavier rear end and springs than with a 5.4 swap.

Not a popular opinion, but more horsepower doesn't always mean a better or more capable tow rig.

 

why not go with 4.10 gears it would be cheaper and i bet it would pull better

 

This is why I asked what the MAJOR differences are in the jump in towing capacities between each engine/trans combo.

Because I like to "tinker", I might just do the swap AND 4.10 gears. It all depends on getting the donor truck (free...just title transfer costs).

 

This is true, think of the old duece or duece and a half ton trucks. They all had inline 6 cyls. (newer ones are diesls)Torque is the name of the game, HP is secondary.)

 

4.6L = 231 HP @ 4750/293 ft-lbs @ 3500
5.4L = 260 HP @ 4500/350 ft-lbs @ 2500

The 5.4 has more torque and with the addition of 4.10 (and appropriate wheels/tires), aren't I getting as much torque as possible?

 

First let me asy this I should have edited the quote in my reply, I was replying to the
My apologies.
Second let me say I have the 5.4 and LOVE it, I cant imagine swapping for a 4.6L.

Lastly
There are many things to consider when looking at numbers.
First is to remember that consistancy beats peak.
I'll take a flat line of 200 ft lbs of torqe across the board over 350 in the middle range but only starting at 75 ft lbs form idle. You must first beable to get to the middle range with your load.

Second thing is look how close the Peak Torque is to the peak HP RPM. The further apart those two numbers are the slower your going to be and seem harder to pull.

In the end bigger isn't always better, how ever on the flip side of that coin, there is no replacement for displacement.
also keep in mind that the 4.6 and 5.4 are the same bore size, they share the same pistons, the only difference is the stroke.
Again I say I love my 5.4 and cant imagine swapping it for a 4.6, but I only tow maybe 2% of my total drive time. About 74% Highway, 24% city and 2% of those two is towing.

 

If you follow the 75% rule, you should always be OK.
If the truck is rated for 8,500, you really should not tow more than
6375.