CAT BACK? Is this a manual california truck?

 

Based on the above I estimate your GCW as about 13,500 lb. Your towing performance only depends on how much FWHP you apply to the road as RWHP, but your transmission gear ratios, differential ratio, and tire diameter determine the MPH speed at which you can apply a given amount of FWHP to the road as RWHP!

My ballpark estimate for your load is that to maintain a 55 MPH speed you need about 75 RWHP to tow that load on the flat and about 175 RWHP to tow that load up a 4% grade. The graph below gives RWHP vs MPH for an auto tranny with a 3.73 diff and stock 31.6" diameter tires.

Note that these RWHP curves are for a 90 HP chip so if your truck is stock you need to subtract about 90 RWHP at the MPH speeds in the vicinity of the RWHP maximums but a progressively less RWHP adjustment is required at MPH speeds further from the RWHP maximums. Also if you have a 3.73 diff and oversize 35" diameter tires all of these curves move about a 10% MPH to the right or conversely just read the RWHP at a 10% lower MPH than your actual MPH speed.

For example at 55 MPH in 4th gear the red curve shows a 195 RWHP but for oversize 35" diameter tires you use the 175 RWHP at 50 MPH. So if you had a 90 HP chip you'd need all of the available RWHP at 55 MPH to tow that load up a 4% grade, but an auto tranny will automatically downshift to 3rd gear before allowing you to fully load the engine and use all the available HP in 4th gear.

So look at 3rd gear and the blue curve gives about 260 RWHP at 50 MPH which is about 55 MPH for 35" tires. Without the 90 HP chip you only get about 180 RWHP at 55 MPH for 35" tires versus the 175 RWHP that's required to tow that load up a 4% grade at 55 MPH. So if your "steep hill" is about a 4% grade and you have a stock truck with an auto tranny in 3rd gear with a 3.73 diff and oversize 35" diameter tires I'd say that a maximum speed of 55 MPH is exactly correct!

The reason you shouldn't do any serious towing with oversize tires is because they reduce your braking ability, increase your braking distance, and on a long steep downgrade your brakes will fade! Please note that changing to a lower diff ratio like a 4.10, 4.30, or a 4.88 won't compensate one bit for the adverse effect that oversize tires have on your braking ability!

 

I pull 5K pounds with a similar load in truck up grades in the Rockies with no problems and power to spare - sounds like you have an issue. At the risk of being redundant - gauges.

 

'02 7.3 here... when I was running full stock and pulling a 3-horse slant loaded for bear and a camper, I was screwing around and passing people at 75+ in the mountain passes.

Now, with improved tuning and several small and cheap mods, I am hitting the same speeds with the same load at 2MPG better and at 150 degrees cooler and faster response.

You've got a performance problem, straight up. You ought to be rocking that payload.

 

wow thank you for all the responses.

ernesteugene: thank you so much so the response. i have read a few of your other posts and can tell that math is your friend.

there seems to be a little discrepancy on what the truck should. i didnt think the tires would make that big of a difference in how it towed, but perhaps it does. as far as braking goes, i know that the trailer brakes werent working and the truck didnt even seem to care.

 

Bubba,

I pull a 9500# car trailer and a 5500# travel trailer with both of my 7.3 trucks and they'll both walk up hills with a load. I'll bet you've got a 3.55 rear end and those 35" tires are just gearing you too high.

However, as good as the brakes are in the truck, I wouldn't recommend towing a trailer of that weight without any brakes.

 

I am with Joe here. I am towing with 315's and 3.73's. I have a steel 2 horse slant and it has a dressing room and a sheotload of tack in the dressing room. It pulls fine. I agree with everyone else that says you have a boost issue, or maybe a fuel delivery issue.

You know how they say, "it ain't rocket science"? He is actually a retired rocket scientist. Math is definitely Ernest's "thing".

 

Believe me the lawyers will care if you hit someone! Since you might not see the other thread I'll also post this here! Also check this thread to see how to test your RWHP... How to get your RWHP from a simple road test... This thread is located at:
https://www.ford-trucks.com/forums/s...5&goto=newpost

Towing up a long steep mountain grade with oversize tires isn't the issue it's being able to safely descend the grade without brake fade and to make an emergency stop on the way down if a car pulls in front of you and suddenly slows or if someone pulls onto the highway from a side road and doesn't get up to speed quickly!

For a given brake pedal pressure which translates to a given clamping force on the disks and a given braking TQ lb-ft on the wheels a tire of radius R ft applies a braking force F lb to the road that's given by... F={TQ}/{R} lb. For a given braking TQ lb-ft on the wheels a 37" diameter tire reduces the total braking force F lb that the tires apply to the road by 17% and a 35" diameter tire reduces the total braking force by 11%, both compared to a stock 31.6" tire.

This means that to maintain a steady speed during your decent you need to work your brakes 11% to 17% harder and this produces more heat and eventually results in hot brakes that fade. In an emergency stop you have at least 11% to 17% less maximum braking force even if your brakes are cool and this increases your braking distance considerably and if the brakes are hot you might not have much emergency braking force at all!

As a retired fulltime RVer I'm not in a rush and I'm one of the most conservative drivers you'll find on the road but in 10 years of towing I've had to make several emergency stops that literally only left a few feet of margin! In other words if I'd had 35" tires on my F350 instead of the stock 31.6" tires I would've through no fault of my own crashed several times over! Not big life ending crashes but significant enough crashes to drastically increase my insurance premiums!

The following analysis is simplified to illustrate a few key points so it neglects aerodynamic drag and rolling resistance. The kinetic ENERGY KE ft-lb of a truck and trailer weighing a GCW lb and moving at a given MPH is given by KE={(GCW)(MPH^2)}/{30} ft-lb. To stop this GCW this amount of kinetic ENERGY must be dissipated by the brakes in the form of an equal ft-lb amount of heat ENERGY.

The maximum braking force F lb that can be applied to the road without losing traction and skidding is F={(u)(GCW)} where u is the dimensionless coefficient of friction. If this maximum braking force F lb is applied over a distance D ft that's parallel to the F the ft-lb amount of WORK W that's done is W={(F)(D)} ft-lb, and the amount of KE that's dissipated by the brakes as heat energy is KE=W={(F)(D)} ft-lb.

If you set KE={(GCW)(MPH^2)}/{30}=W={(F)(D)} and solve for D you get D={(GCW)(MPH^2)}/{(F)(30)} ft, and now substitute F={(u)(GCW)} in to this equation and get D={(GCW)(MPH^2)}/{(u)(GCW)(30)}={(MPH^2)}/{(u)(30)} ft, and since the GCW term cancels this dives D={(MPH^2)}/{(u)(30)} ft. Note that this equation says that if your brakes are powerful enough to apply the maximum "skid free" braking force to the road your minimum stopping distance D ft doesn't depend on your GCW lb!!!

If you assume MPH=60 and u=0.8 then D={(MPH^2)}/{(u)(30)}={(60^2)}/{(0.8)(30)}=150 ft so the laws of Physics say that if your brakes are powerful enough any GCW should stop within 150 ft from an initial 60 MPH speed! I have as much trouble convincing people of this fact as I do convincing them that it's not their engine's TQ that gets them up a hill quickly, and that's why it sometimes takes equations to see these things clearly!

As I'll show no one's brakes are powerful enough to stop their GCW in 150 ft from an initial 60 MPH speed and this is why oversize tires are a big safety NO NO for towing because they further reduce the already inadequate braking capability that you do have!

I was towing a GCW=22,000 lb and that requires brakes that can produce a F={(u)(GCW)}={(0.8)(22,000)}=17,600 lb to provide the minimum stopping distance. At 60 MPH these "optimum" stopping distance brakes would need to produce an initial braking HP of... HP={(F)(MPH)}/{375)={(17,600)(60)}/{375)=2,816 HP. Since some trucks can't even lock their service brakes and stall their TQ converter I don't think their brakes will be providing this "optimum" braking HP of 2,816 HP!

At 60 MPH my GCW=22,000 lb gives a KE={(GCW)(MPH^2)}/{30}={(22,000)(60^2)}/{30}=2,640,000 ft-lb and if I want to stop that GCW in a D=500 ft distance the brakes need to provide a constant braking F of... F={KE}/{D}={2,640,000}/{500}=5,280 lb over the 500 ft distance while dissipating 2,640,000 ft-lb of heat ENERGY. That's the kinetic ENERGY you'd feel if you dropped 2,640,000 16 oz cans of beer on your foot from a height of 1 ft! That's 110,000 cases of 16 oz cans or 146,667 cases of 12 oz cans!

In general the braking distance D={(GCW)(MPH^2)}/{(F)(30)} ft, and substituting the tire force F={TQ}/{R} gives D={(GCW)(MPH^2)(R)}/{(TQ)(30)} ft. The braking torque TQ is limited by the brake design so that increasing R by 11% to 17% by using oversize tires also increases D by 11% to 17%. Now a 500 ft stopping distance becomes a 555 ft to 585 ft stopping distance!

My bottom line is that if you tow with oversize tires you've modified the Ford design and made it less safe and if you crash and cause death and destruction you'll probably get sued big time for having changed the Ford design. I know for a fact that one of the items that's always checked in any accident investigation is the tire size and load range against what the OEM specified on the door jam safety sticker!

Note that the oversize tires also compromise other safety issues like steering and handling. It's harder for me to write equations to address these issues but I'm sure a lawyer wouldn't have any problems addressing them in court!

 

Super Duties did not come with 3.55 gears that I know of. 99.5 to 03, 3.73,for the single rear wheel trucks(250s and 350s, 2 or 4WD unless special ordered with 4.10s) 4.10 for most duallies (although one could order a shortbed crew cab with 3.73s ) or 4.88 for the 450s and up

 

SOunds like you have a cali truck since you have a cat heck it could be clogged I would gut that thing out and leave the body on there for inspection sake. Get some 4.10s and some gauges, check out the IC boots, up pipes and anything else for boost/exhaust leaks because those are always killers.

Also check into changing your fuel filter as well.

 

well i was told when i bought the truck that the cat had been removed and exhaust put in from there back. so maybe not quite a cat back exhaust