Hey everyone,

I have spent the better part of my life building and racing gas engined vehicles, but the diesel performance and theory is totally new for me (and the reason I am so interested in learning it)

Before I came here, my knolwedge of diesels was limited to my 5.9 Cummins powered truck, transport trucks, and various industrial diesel powered deep sea fishing boats I have worked on and fished on. Redlines at 3,000, and cruising at 1,500-2000 at most. My Cummins Dodge with six speed and overdrive splitter is happily running down the road at 1300rpm at 65mph and can pull mild grades like this when not towing. it has around 350 hp, and 675 pounds of torque.

Now I find these Mercedes engines diesels are revving to nearly six grand and happy cruising all day at 3,000.

I also noticed from mantahead's' challenge thread also, that the torque seems much more in line with a gasoline engine built for torque produces..... a bit more than the horspower rating..... where a Cummins typically produces twice the peak torque numbers as horsepower, and at half the rpm.

So can someone tell me what makes the engines work so differently? Is it all timing and camshaft? Combustion chamber design? Stroke? Can you build a low revving mercedes OM617a or OM603/606 turbo engine to make torque like a cummins for off road truck use?

Thanks, Allen

Thanks!

Torque and hp converge at 5250rpm. So it's just the ability to rev high that's skewing the ratio that way. Also the use of laggier turbos for maximum hp (resulting in no low end torque).

So if you want more low end torque, you could select a turbo for that, but you'd sacrifice high-end hp.

Only IDI / Indirect injection diesel car rev this high easily. The prechamber with the glowing red hot ball can start the burn as the fuel is being injecting before TDC when the cylinder is completely compressed like a gas engine!

DI/ direct injection has to wait till after TDC when the air is hotter to start injecting. So it has a shorter power cycle that's not very effective after 3000rpm

at least that's how I under stand it

My dads old ford 6.9 IDI international reved to 4,500rpm and stock the Mercedes only rev to 5k they need to have the injection pumps modified to rev higher than that! But I have seen Cummins modified to hit 5k rpm for tractor pulling

Also the mercedes are low displacment compared to a 5.9 cummins and torque is relative to displacement I beleive.

willbhere4u, thanks for the great explanation, that makes a lot of sense.. I have never looked at the heads of engines with the different types of injection. I didn't realize it was a difference in the head, I thought it was just a different kind of pump or delivery pre engine..... now it makes sense when I look at the OM352 engine, as I wondered why it didn't rev. I see it is direct injection. I love to learn new things!

So did Mercedes make any small (2.5-3 liter) non electronic, non common rail, direct injection diesels?

It may also have to do with the bore to stroke ratio somewhat.

Ed

Impretty sure thats right as well longer stroke gives more you more torque in general....i think.

(11-24-2011 06:57 PM)willbhere4u Wrote:  Only IDI / Indirect injection diesel car rev this high easily. The prechamber with the glowing red hot ball can start the burn as the fuel is being injecting before TDC when the cylinder is completely compressed like a gas engine!

DI/ direct injection has to wait till after TDC when the air is hotter to start injecting. So it has a shorter power cycle that's not very effective after 3000rpm

at least that's how I under stand it

No, direct injection (TDI ,CDI) also starts BTDC under WOT, about 20-30° depending on engine type....

DI and IDI engines both start injection at ~24° before TDC.

The ability to rev higher or have more torque also depends on the bore-stroke ratio and the balancing of the engine internals.

My Landcruiser engine e.g. has more stroke(103mm) than bore(96mm) and has its max power at 3600rpm, end of rev at 4200rpm. Displacement 3.0L like the OM617a
I almost never rev it over 3300rpm because usable torque starts at 1600rpm; max torque is 300Nm@2400rpm ("flat hill" torque curve)

OM617a has stock 250Nm@2400rpm in the EURO-Version

The OM617a is almost even with bore(90,9mm) and stroke(92,4mm)

The OM352 also does not rev high because of bore(97mm) and stroke(128mm) ratio.

The higher reving OM606 has bore 87mm and stroke 84mm

Gruß
Volker

Bore has less to do with it than stroke. You can get alot more out of the om engines rpm wise but work has to be done on the valves springs etc.