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Old 01-10-2018, 03:40 PM   #1
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5.4 with 37's?

Hi everyone,
I’m hoping to get some opinions on whether to run 35’s or 37’s with a 5.4. The main question I have is does the 5.4 have enough power to run 37’s with 4.88 gears and still be able to tell up to 6000 lbs.? Just for reference I calculated the difference in rpm for the stock, 4.56 and 4.88 values. All of these are at 65 mph without O/D.

3.73 – 2771 rpm – 29.4” tires
4.56 – 2837 rpm – 35.1” tires – 1.5 mph lower speed reading
4.88 – 2881 rpm – 37.0” tires – 2.5 mph lower speed reading

The way I interpret the numbers is that it can run 37’s and still tow well because of the slightly higher rpm. My reasoning may be flawed, any advice would be appreciated. I do understand that 35’s and the 5.4 are well matched and many have chosen that option. Still are 37’s a possibility?
Thanks
Darryl
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Old 01-10-2018, 04:38 PM   #2
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Just throwing an additional line of thought into this discussion (before it even gets going, ha....!)

Regardless of the measurable additional "work" that an engine has to do in order to get a larger/heavier set of tires rolling / maintain their speed (and overcome the additional rolling resistance.....)

....If you've geared the final drive ratio properly, then the additional work the engine has to do shouldn't add up to THAT much additional strain on the engine/transmission. (To some other parts of the van, the story is different.....)

Sure, the van might not ACCELERATE quite as quickly. (Since it has to "spin up" and accelerate the additional rotational mass of those big tires.) But once the van is "up to speed," (again if you've geared the van properly to compensate for the taller tires), then the engine should be able to maintain speed (and tow) pretty much the same as it would with the smaller tires.

Again, the biggest noticeable difference will be in how fast the van can accelerate itself. Maintaining similar amounts of cruising speed shouldn't be a huge difference however.

BUT:
Two areas that should DEFINITELY come under scrutiny as you continue to "size up" and head towards taller tires:

1) Brakes

and

2) Strength of rear axles / axle shafts.

Even with a re-gearing (actually, ESPECIALLY with a re-gearing....), the axles will be under increasing amounts of acceleration-driven torsional (twisting) loads with the taller tires. And brakes will lose some of their effectiveness/stopping power (as the taller tire is imparting even more leveraged force to the brake rotor as you attempt to slow the van down.)

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Old 01-10-2018, 04:57 PM   #3
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^I agree with above. Engine can manage it with the deeper gear, although I'd be inclined to even consider 5.13 gear for 37's.

However..... That's a lot of tire for a semi-float rear axle, or a Dana 44.
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Old 01-10-2018, 05:32 PM   #4
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I just put on 34" tires on my van with a 5.4 and 3.73 gears.
I'm definitely going to re gear to 4.10 to bring it closer to original ratio.

FWIW
Anything past 4.10 gears can run very loud. The gear whine can be annoying if you're not use to it.
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Old 01-10-2018, 06:17 PM   #5
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Thanks for the advice. I never thought about the torque forces on a semi-float axle. That would definitely be a concern. I have no plans for a FF or Sterling so I best keep it tamer.

My current van has 4.10's and 33's. I like it for the 4" lift I have. My only complaint is that it sucks towing in the mountains where I live. It will do it but slowly. The van these are going on is a 2013 E350 with a 6" lift. It also has a 3" body lift as part of drop floor for the wheelchair conversion. That means that 33's will look down rite DORKY!

I can live with 35's and 4.56's or maybe 4.88's if it wouldn't kill my mileage when daily driving. Anyone running 35's and 4.88's that can offer some insight on their experiences? Thoughts?
Cheers
Darryl
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Old 01-26-2018, 10:45 AM   #6
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Quote:
Originally Posted by depark View Post
Hi everyone,
I’m hoping to get some opinions on whether to run 35’s or 37’s with a 5.4. The main question I have is does the 5.4 have enough power to run 37’s with 4.88 gears and still be able to tell up to 6000 lbs.? Just for reference I calculated the difference in rpm for the stock, 4.56 and 4.88 values. All of these are at 65 mph without O/D.

3.73 – 2771 rpm – 29.4” tires
4.56 – 2837 rpm – 35.1” tires – 1.5 mph lower speed reading
4.88 – 2881 rpm – 37.0” tires – 2.5 mph lower speed reading

The way I interpret the numbers is that it can run 37’s and still tow well because of the slightly higher rpm. My reasoning may be flawed, any advice would be appreciated. I do understand that 35’s and the 5.4 are well matched and many have chosen that option. Still are 37’s a possibility?
Thanks
Darryl
The calculation is the correct direction, but instead of guessing I would suggest to take a look into the power diagram of the 5.4 engine. I own the 7.3 IDI so therefore I do know how much power my engine has @ a given RPM [@ 1400 max e.g.].


The 5.4 is probably like this one?

@ 2,770 and 2.888 RPM the engine has pretty much the same torque.

Though I don΄t believe that Ford configures an engine with such a crappy torque curve, this can be fake by the "tuner" company - don΄t know.
The source of my picture above is Ford USA.

Actually 100 RPM does not make a big difference, as expected. Especially NOT for an gas engine. More RPM more power, pretty easy - a diesel engine is much more limited by the very small usable RPM range.

PS: Here is better picture, aka true diagram:

This is an interesting one: IF this is your engine there is a peak right @ 2500 RPM. Lower and higher will result in less power!
BUT: This is actually exactly what tractor engines look like > IF more power is needed the engine RPM slows down and at the same time gains more power. So this could work out as better than stock: towing uphill you will NOT get a slower truck [because engine slows down into an RPM range with a little more torque.]
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