Diesel RPM limitation

[laserjock]

Okay guys, I was just having a discussion with a friend of mine who is trying to work out a gearing problem on his truck. He is currently stuck with really low (like 6.52 ratio) gears and it's not easy to change them out (its not a pick-up truck). That lead us to the more fundamental conversation of what is the actual physical parameter that limits diesel engine RPM. An extra 500 rpm in his case would give him as much benefit as a regear and depending on whats involved to get there (motor work, pump work, etc.) it may be way more cost effective. I know our trucks by diesel standards rev fairly high when compared to something agricultural or a _ummins. Is it a rotating mass issue? A balance issue? A pump issue? Is it that the power curve falls off so why bother? Fuel consumption gets stupid? Reliablity goes way down? Valve train limitation? I'm sure there are plenty of high performance applications that rev out way farther than a stock application but they are probably only expected to survive a short time.

What is the trade space?

I know the answer may be different depending on the application but I'm thinking in general. There are probably some guys with F-Superduties that would kill for 5 more MPH sometimes.

Just thought I would toss this out there to start a conversation and maybe learn something.

Thanks!

 

[Agnem]

Well the immediate factor is the governor settings in the IP. But that can be overcome fairly easily. Beyond that, you would have to expect things to start to fly apart. This must be a really unusual application if gearing cannot compensate. It's a shame to try and run these engines faster, because the amount of wear and loss of economy is going to drastically change with a higher revving engine.

 

[riotwarrior]

IMHO lift foot up, place block under accelerator pedal and limit speed! If truck is geared slow, it was designed that way, acceptance is the key!

In reality our valves and pistons are so close that an increase in rpm could be quite catastrophic and in my mind NOT worth any gamble to get 500 RPM more for more speed. Learn to drive slow, stay calm, and be happy!

What about a tyre change to a taller tyre? That would in essence give a perceived taller gear ratio and provide the needed difference he is seeking!

Next option a Brownie Box with slight over drive, inline in the driveline. This would probably be the best option!

In all honesty, it's just NOT worth taxing the engine at max RPM.

Take it for what it's worth!

JM2CW

Al

 

[laserjock]

Thanks for your thoughts guys. In his specific case its a bean counting exercise. He's considering trying to trade engine RPM for speed and trying to figure out what that means. He's currently limited to about 53 MPH with 42" tires. Like I said not a pickup. Larger tires are an option but they are getting to stupid. He will probably drive the truck a couple thousand miles a year and the majority of that would not be against the governor. It's more of an "is it available if I need it situation" then I just want to go faster all the time. Anyway, thanks for the thoughts and I think it may help push the discussion along. Just like in business, its all about ROI and what is the risk.

 

[89Laredo]

Ouch... bdmt on Rockwells or something?

 

[laserjock]

His truck is actually a Unimog 406. Same kind of idea, just German.

I thought we might stir a more general discussion on performance limitations. I posted up here because the collective here is one of the most diverse and interesting I've run into. Debates tend to be civil so I thought what the heck. Don't know if he's thinking outside the box or just plain crazy. There is a fine line sometimes. I am comfortable with calling him crazy though.

 

[youngunbd]

Send him over to steel soldiers, they have a fair number of unimog folks there. And his engine sure isn't a IH diesel, those Mercedez diesel's are tricky. I think the only way to get more speed on a mog is the "high speed" axles, they are a pretty penny if you can find a set though. Unimogs are neat and all, a pain to maintain unless you have a little german man that can fit in all the nooks and crannies.

 

[drinkypoo]

His truck is actually a Unimog 406. Same kind of idea, just German.

Might be able to install an alternate trans in there with an adapter, and get different gearing that way. Those guys also upgrade to the OM617.951A/B turbo engine, it's a much better engine than what came with the truck. Same theory, more engine. Available in 300D, 300SD, 300TD.

 

[The Warden]

As I've always understood it, there are two basic reasons why you don't want to run a stock engine at too high of a speed. I'm speaking for engines in general.

First, the rotating mass...the connecting rods and piston wrist pins weren't meant to handle the added stress of changing directions more quickly than designed. Given how overbuilt the 6.9l/7.3l IDI are, I don't know how big of a problem this is with our engines.

Second are the valves and the valve springs...if you run an engine too high, you risk the valves not closing in time...at a minimum, you have valves "floating" and not being closed when they're supposed to. Worst-case, the valves stay open so long that the piston comes into contact with the valve face, which can be catastrophic. Also, even if the valve-piston kissing doesn't have a catastrophic result in and of itself, it can unseat the valve from the valvespring, and if the keepers pop off, you risk swallowing a valve. This actually happened to my truck at one point in the late 1980's, according to the maintenance receipts I have

Specific to the IDI...keep in mind that IH designed the engine to run at 2800 RPM; Ford increased that to 3400 RPM without modifying the engine internals. They've proven to hold up at 3400 RPM for many years (just look at the F-450's with deep rear axle gears running on the highway), but I would highly recommend against running the engine any higher than 3400 RPM on a regular basis without at least replacing the valve springs with stiffer springs.

To the Unimog...I don't know much about the engine that originally powered them, but FWIW the OM617 governs out in the 5500 RPM range without a load (I've never seen one run past 4500 RPM in driving). That's one nice thing about German diesels; they over-design them most German diesels run at a higher RPM than we're used to. With that said, I'm not sure how much higher I'd run one..you still have quite a bit of rotating mass, not to mention the timing chain...

 

[kuskoal]

I want a mog! The portal box idea is genius! You can spin a diesel to about 5000 rpm before you need to re-engineer the injection spray pattern. Sheid built a cummins that can spin to 6500? Rpm. So your question of getting the fuel lit off isn't a problem. I think hypermax is doing a dt466 high rpm as well.

If you can get the injection pump redone for higher rpm fuel and timing, go for it. Its a farm engine designed for abuse, dirt and crappy oil.

 

[greenskeeper]

the faster the engine turns, the less time you have to inject fuel and get it to burn efficiently. My 98 Jetta TDI redlines at 4500rpm but the internals are good well past that. Fueling is the limitation and I'd be it's the same problem with any diesel.

 

[The Warden]

Fueling is the limitation and I'd be it's the same problem with any diesel.

Not necessarily...as a general rule of thumb, the larger of an engine you have, the lower the governor's set at. VW and MB diesels are typically governed in the 4000 to 5000 RPM range, but most pickup truck diesels govern out near 3500 RPM. Most diesels in semi trucks don't go over 2000 RPM, and big industrial and marine diesels turn far slower than that...

 

[drinkypoo]

Not necessarily...as a general rule of thumb, the larger of an engine you have, the lower the governor's set at. VW and MB diesels are typically governed in the 4000 to 5000 RPM range, but most pickup truck diesels govern out near 3500 RPM. Most diesels in semi trucks don't go over 2000 RPM, and big industrial and marine diesels turn far slower than that...

My understanding is that it's more related to the size of the pistons than the displacement. Big pistons reciprocating quickly equals a whole lot of stress on the rods and crank.

The MBZ OM617 is three liters by five cylinders and is typically (stock) regulated to 4700 RPM, but many people regularly rev theirs into the low fives. It doesn't really have that much more to give with stock tuning, but by increasing fueling there's probably more up there. But then you want an intercooler, and a pyro, I have neither

 

[Knuckledragger]

Not sure why anyone would want to spin a diesel way faster than necessary. Diesel engines are far more productive in low RPM because they produce massive amounts of torque, which evaporates at high RPM (much like electric motors, which develop 100% of their torque at 0 RPM). Finding the optimum power curve and changing tires/gears/driving habits to make use of that is the best and most economical way to go.

 

[drinkypoo]

Not sure why anyone would want to spin a diesel way faster than necessary. [...] Finding the optimum power curve and changing tires/gears/driving habits to make use of that is the best and most economical way to go.

speaking only for myself, if I were shifting the OM617 manually I would want to rev it higher so as to keep the power up during changes. But since it's hooked up to a slushbox I don't care, the torque converter does the dirty work.