Max safe sustainable RPM

[KansasIDI]

So I bought some axles with 4.10 gears that I will be putting in Duchess, my 86 F250, which is technically gonna be an F350 after the dually axles swap.

Truck originally came with 3.55 gears, with slightly undersized tires I was at 2350 at 80 mph.

I run Interstate highways frequently enough, and usually about 5-6 over on highways of any kind, unless it’s really busy or lots of houses…

I’m wanting to go to slightly oversized tires after this swap, but might not immediately, especially since the fronts are new and the rears are 70% tread, and Hankook AT2s are expensive... Calculated 2700 RPM at 80 MPH, with current tire size, and 2450 with 235/85 R16s at 80.

I run 2700ish in 4th at 65 with a trailer, with current gears and tires.

These old engines seem really tough, but would 3 grand literally all day long take life off my engine? These engines don’t seem very well balanced.. it runs good at high rpm but just want to make sure this ain’t shortening her lifespan.

My dad seems to think that I’d gain a lot of power, and lose a lot of life, with lower gearing, not so sure if I agree with him… but I guess the faster it spins, you’d gain some heat and thus some wear… he seemed to think I’d be best off with 3.73s, which does sound optimal, not sure what a gear swap entails…

 

[IDIBRONCO]

Towcat (rest in peace) used to run a wrecker service. He used a F Superduty, maybe more than one, and he ran it against the governor for long periods of time with no bad results. 2700 RPM isn't really that close to the governor. These aren't a Cummins engine! Would it shorten the life of the engine? Probably, but I don't think that it would be drastic.

 

[Greenie]

On vacations towing a 3 ton travel trailer I ran 80/85 mph all day long with an E4OD and 4.10 axle. It didn't appear to hurt it.

 

[KansasIDI]

On vacations towing a 3 ton travel trailer I ran 80/85 mph all day long with an E4OD and 4.10 axle. It didn't appear to hurt it.

E4OD have quite a bit taller overdrives but good to know thanks

 

[KansasIDI]

Towcat (rest in peace) used to run a wrecker service. He used a F Superduty, maybe more than one, and he ran it against the governor for long periods of time with no bad results. 2700 RPM isn't really that close to the governor. These aren't a Cummins engine! Would it shorten the life of the engine? Probably, but I don't think that it would be drastic.

Cumapart, eh?

 

[IDIBRONCO]

He didn't say whether or not he was in drive or overdrive. I was wondering just that myself.

 

[IDIBRONCO]

Cumapart, eh?

No. They are a much slower turning engine than our IDIs are. I'll never badmouth a Cummins. They are good engines. I don't believe they are worthy of the absolute worship that some people like to give them though.
Now the Dodge that surrounds most of the Cummins engines though...

 

[KansasIDI]

No. They are a much slower turning engine than our IDIs are. I'll never badmouth a Cummins. They are good engines. I don't believe they are worthy of the absolute worship that some people like to give them though.
Now the Dodge that surrounds most of the Cummins engines though...

I agree. Just saying they would probably die at 3800 rpms on stock valve springs, which some people do with these IDIs

 

[Greenie]

He didn't say whether or not he was in drive or overdrive. I was wondering just that myself.

I always drive in OD like the manual recommends - unless the transmission is hunting - which I try to avoid by backing off the accelerator when the hill is steep.

 

[IDIBRONCO]

I think that most rebuilders set the governor speed to around 3500 RPM. It seems like that's close to where the were from the factory too. It doesn't seem like a good design to have valve springs that can't hold up to 3800 RPM. Gas engines can turn quite a bit more than that. I think where the valve spring problems come into play is with a bigger cam or with running a lot of boost. I've heard that a lot of boost can actually push the valves back open a little bit with stock springs. I can't give any numbers on that though.

 

[KansasIDI]

I think that most rebuilders set the governor speed to around 3500 RPM. It seems like that's close to where the were from the factory too. It doesn't seem like a good design to have valve springs that can't hold up to 3800 RPM. Gas engines can turn quite a bit more than that. I think where the valve spring problems come into play is with a bigger cam or with running a lot of boost. I've heard that a lot of boost can actually push the valves back open a little bit with stock springs. I can't give any numbers on that though.

These engines have really high compression, if a rod were to stretch just a tiny bit it’d probably be game over?

 

[IDIBRONCO]

These engines have really high compression, if a rod were to stretch just a tiny bit it’d probably be game over?

That's true. Wes from Classic Diesel Designs says that these rods will hold up to over 300 wheel HP.

 

[KansasIDI]

That's true. Wes from Classic Diesel Designs says that these rods will hold up to over 300 wheel HP.

Oh… only 300? Hmm… what are the options for upgraded rods?

 

[IDIBRONCO]

Factory Turbo rods and modified PSD rods. Justin sells the modified PSD rods, but they're not cheap. I think they were over $800 the last time that I looked. If you want to go big, you have to pay! Just so you know, if you're talking about 300 HP at the wheels, you'd be probably looking at at least 450 at the flywheel depending on your set up. 4X4/4X2, standard/auto trans, etc.

 

[Jesus Freak]

Now the Dodge that surrounds most of the Cummins engines though...

Hey, that's my line! It's copyrighted, you owe me $3.