Hi there -

I bought a 20' "pipe top" trailer today with two 7,000 lb axles and towed it home (probably 2,500 lbs empty) with my dinky 2008 F-150 XL 4.6. It went completely fine - in fact it was a dream compared to my 16' angle-iron utility trailer which likes to bounce around like crazy.

Of course, I know with this truck I can't fully utilize the trailer. I'll have to get at least an F-250 for that.

However, in the meantime I would like to use this truck/trailer combination to the max safe potential. I have read countless other threads and I know this isn't as easy as reading the sticker on the hitch receiver. So, I researched all of the limits to find the "weakest link" to make sure I don't go over...

• Trailer placard: 12,400 lbs.
• Weight distributing hitch: 10,000 lbs.
• Receiver placard: 12,000 lbs. (with WD hitch, 8,000 without)

Owner's manual limits (although VIN decoder says 133" wheelbase, the door placard and measuring tape say 145" so I'm going with that. I have 17" wheels and if my door placard is correct, a LS 3.73 gear ratio):

• For 4.6: 6,700 lbs. (12,200 GCWR) *This is what I have
• For 5.4: 9,400 lbs. (15,000 GCWR)

Now, of course I know the standard answer is to say 6,700 lbs. is the max, which is fair enough, but I really want to understand WHY. Is there a structural difference between the 4.6L and the 5.4L that prohibits pulling 9,400 lbs? Or is it based on the engine alone?

I have a guess, which is what I want to run by you guys....

According to my door placard, my rear axle GAWR is 3,800 lbs., and according to my spring code (TTMM), the springs are 1,500 lbs. each (thus, my springs are 800 lbs. shy of the axle limit).

After doing some research I've found that 1,900 lb. springs are commonly available, and it seems (?) that the 5.4L also uses the 3,800 lb. rear axle, but with the 1,900 lb springs.

So, finally to my question(s):

Do 5.4L F-150s in fact have 1,900 lb. springs standard?

If so, and IF I am willing to accept less acceleration and struggling up hills, is there any reason OTHER than the rear spring capacity that my truck can't safely haul a 9,400 lb. trailer (assuming a weight distributing hitch)?

And if you can't bring yourself to answer that question, what, aside from swapping out the engine itself, would be required to "transform" my 4.6L into a 5.4L (with the accompanying 2,700 lb towing capacity increase)?

Knowledgeable inputs very greatly appreciated!

PS: Great forum, by the way - thanks to all of you for creating loads of great questions and answers!

 

I'm going to reply to my own thread here and say the following:

I have read many other threads and am aware a standard answer here is to say, "There is NO WAY to increase your vehicle's towing capacity beyond what the manufacturer states by simply replacing parts!"

I respect that sentiment, but then I would ask, "What if I walk in to a Ford dealership and start buying 5.4L parts and replacing the corresponding parts on my 4.6L until everything but the owner's manual, VIN plate, decals, and door stickers have been replaced? Is the limit still 6,700 lbs?"

Obviously the answer is "no" - at some point in the above example the vehicle becomes capable of towing more weight. The thing I'm looking for here is to understand exactly what those magic parts are... and ideally, why.

Aside from the engine and rear leaf springs I mentioned in the original post, I might expect:

• Rear axle (although I'd be skeptical of this one - both versions seem to have the same 3,800lb axle)
• Transmission (again, skeptical - both seem to have 3-speed w/OD and seem to be interchangeable)
• Front coil springs
• Front axle/hubs
• Other front end suspension components
• Transmission cooler
• Thicker/heavier/additional bolts somewhere I haven't thought of(?)
• Brakes (particularly, increased rotor diameter/thickness, or maybe calipers)
• Electric brake controller
• Frame (if this is it, I'll just pack up and go home!)
• Tires (okay, I'm grasping now)

So, if anyone knows what the real differences are between a stock 4.6L made in 2008 and a stock 5.4L made in 2008 (all else being equal), and how exactly they affect towing capacity, I would appreciate the opportunity to learn. Thanks!

 

Your truck has the 8.8 rear end, the 5.4 has the 9.75. Also, I am not sure the two trucks use the same transmission. But I may be wrong there.

The non HD trucks use the same springs, so that's not where the extra capacity is coming from.

 

Very interesting - thank you! I went on rockauto-com and they actually sell the 8.8 AND 9.75 for both the 4.6 and the 5.4, but reading elsewhere it seems you're right that the 5.4 comes with the 9.75 diff gear standard.

I just counted mine and sure enough, 10 bolts... So yeah, my diff is probably a weak link, if not the weakest in this chain. I wonder if I can salvage a 5.4 rear end?

Regarding the transmission, it seems both come with the 4R75W or 4R75E, which apparently are essentially the same except for a sensor, no? Is there another type of transmission I'm missing?

Also, interesting on the springs. I wrongly assumed they'd be bigger on the 5.4. I might go ahead and upgrade them anyway... They're getting a little saggy (260k miles).

Thanks again for the reply!

 

Kinda the same argument that goes on between F250 and F350 trucks....

Not a lot of different parts between the 2, but for them, it's the GVWR between the 2 that is the biggest difference and therefore, depending on where you live, the registration fees can vary a lot between the 2.

In your case, the GVWR on the 5.4 is going to be the same as your 4.6. The rear axle ratings might be different, but I don't know on that?

I had a 97 F150 with the 5.4 and it had the 9.75" axle, but it's GAWR was only #3200....

The front GAWR was #3500! I think it had the "snow plow" front end? It was a Canadian truck originally...

Anyway, the "tow ratings" on any vehicle really don't amount to a pile of beans...

Well, maybe if you are still under "warranty", but after that, it's mostly dependent on the trucks GVW ratings.. Then, it's just if you wish to stay within them or not..

On that 97 F150 of mine, I towed the #5000 TT in my sig with it and it was over #500 over GVWR and about #200 over it's rear axle rating, but I got over 250,000 miles out of it before I traded for the 13.. And it was still running fine and strong..

I did add some overload springs and was running LT rated tires, so that helped it handle the weight better... Yes, I know it didn't increase any of it's ratings..

But running a tad over was my choice, I knew it, I knew my truck, I knew it's limits and I was comfortable with it..

I've towed other rigs that were way overloaded, so I know what that feels like, and I did not feel that with the 97 at all. In fact, I thought it towed great and enjoyed towing with it.

I only got the 13 because I wanted a new truck... Now, it tows the same TT even better, but you still know it's back there.

Guess where I'm going with this is.... Tow what you have and if you are comfortable with it, go with it.. Sounds like you know the limits of your truck and know that you can't just load up that trailer to it's max and expect it to perform.. Sounds like you expect it to work a bit harder on the hills and that's all being comfortable towing is..

Good luck with it and get some LT tires, get some overloads, get a decent WD hitch and set it up right and you'll likely get down the road with the towing expectations you expect...

Mitch

 

Just as a FYI - When I had an axle bearing go bad and had to replace the axle and axle bearing I found out that there are two sizes of bearings that were used in the 9.75 rear. Mine has the small bearings/axles.

 

Wow thanks so much for the responses, guys!

Okay, so here's the update...

I spent several days last week looking into F-250s and F-350s. Clearly they are more capable vehicles and with a max tow rating of 12,500 lbs, would "match" my trailer perfectly. While I wasn't able to find any that suited my needs, I'll keep it in my back pocket, probably for 2 years from now when I go shopping.

In the meantime, I've done a little more research, and combined with advice from you guys, I've done the following:

- Swapped out the 18" fancy, aftermarket custom rims that came with my truck for 17" stock rims (I "assumed" this in my first post, but today I actually did it).
- Will be getting "E" load tires installed this week.
- Purchased a 15,000 lb. Curt weight distributing hitch (yes, it's overkill but the Harbor Freight version is only 10,000 lbs. and I don't want to repurchase later if/when I get a bigger truck and want to tow 12,400 lbs - the max trailer weight).
- Re-wired my stock 4-pin connector with a salvaged combo 4/7 pin connector. I haven't installed the brake controller yet, but the rewire down at the bumper was time consuming and a little nerve-wracking (even though my truck already had the towing package wiring). The salvaged connector had a completely different harness (from a 99 expedition?) so I had to cut/splice every wire. And since the "downstream" harness that was already on my truck had less wires on the "upstream" harness, I had to do all of the cutting and splicing on the truck's "upstream" harness! Eeek! Fortunately with some careful staring and comparing and a little luck, everything tests out so far. I only made one mistake on the reverse light (black with purple stripe!) but that was easy to identify and correct.
- Lastly, we visited several salvage yards on the hunt for a 9.75 rear end. They are not easy to find in my year group (2004-2008). We're going to keep our eyes peeled and grab one if we find it and swap it out. (Does anyone know if I can use a different year group??)

Hopefully all of that that will be enough to make me "not unsafe" towing up to 9,200 lbs.

I'll try to post an update once I get everything done to let people know what I ended up doing.

Thanks again everyone! And I will definitely try to avoid the 9.75 with "small" bearings!

 

Humm... didn't even think that you didn't have an OEM "tow package" on the truck already... No factory frame mounted hitch and no factory 7 pin usually means no tow package.

There are some additional engine and trans cooling options as well..

Rad size is different, or there is an engine oil cooling option added, I don't remember off the top of my head for that generation of trucks, there is an aux trans cooler, the alternator has more output amps to it.

The trucks can still tow, even without the tow package, since you have the 4 pin and you put a ball in the bumper, but I think it's only up to #500 tongue weight and #5000 trailer weight.

Good luck!

Mitch

 

The larger bearing shown is stock on the HEAVY DUTY PACKAGE. Along with stiffer shocks,upgraded engine/trans oil coolers, stiffer leaf springs.

Just out of curiosity what class of license do you possess at the time?

I talked to a ford master mechanic in Los Angeles where I bought my truck and asked him the same thing about towing capacities. Basically my HD has a 1000# less towing capacity compared to the platinum because the 6.2 is in that truck and my truck only has the 5.0, everything else is the same. The trans on the newer f150 all mount up the same. If anything it's probably a different bell housing.

 

It's also dependent on the wheels and tires.