I'm sure this has been asked, but haven't found it. Why are the OTR diesels all straight 6's? Why not a straight 8? Would the internal mass be just too much then?

If I'm right a power stroke occurs every 120^ of crank roatation on a 6 and every 90^ on a 8 cylinder, so the 8 should be smoother, right? My truck is plenty smooth, just thinking and realized I have no idea why the design standard for OTR's is 6.

 

or 5 like the new Chevy gasser vortec I5?

 

A straight 8 would not fit in the engine compartment... that is a very long block. The hood/engine compartment would have to be almost a foot longer to accommodate that.

 

I'm just pulling this answer out of my butt. My guess would be because of the tooling and machining that would be needed to make an engine that long wouldn't be cost effective. I know there are machines out there with longer engine blocks but your talking about trains, etc type stuff. It is hard enough to get a car straight 6 motor to machine properly due to its length, I can't imagine the issues involved with trying to keep the cylinder bores parallel with these big bore inlines.

If you pull up beside a 4.0L jeep (mostly pre 97 models) you almost always hear a rattling noise (pistion rattle) and it is do to the blocks moving, flexing when machining. I am told that most "honest" machine shops will not bore these blocks unless they have a special plate to bolt to the top of the head to keep the block in check when machining. My guess would be the same (just worse) for a inline 8 engine. You probably can get more usable cubes out of a bigger 6 than you could out of a simular sized 8, and less moving parts.

 

As an inline engine gets longer, torsional vibration in the crankshaft becomes more of a problem. All else (i.e., main and rod journal diameters, materials, etc.) being equal, the torsional resonant frequencies of the inline 6 will be higher (which means more of them will be out of the engine's operating range) due to the stiffer crankshaft than those of an inline 8. As crankshafts get longer, their diameters may have to be increased (if possible) to increase stiffness, and these larger main and rod journals can create their own set of problems with increased bearing surface speeds, etc. So, the inline 6 is a good compromise - stiff crankshaft, easy to machine, perfect primary and secondary balance, simple intake and exhaust arrangements, relatively easy to package in a vehicle and so forth.

Rusty

 

Stationary power plants to go straight 8 and 10 but when you've got the thing bolted to the floor (or deck) there isn't any need to worry about hood length and turning radius. in this case engine design/usage is driven by function and style not performance. A radial engine like fighter planes of old actually has the best power to cube ratio and smoothest power transmission. But it you put it in a wheeled vehicle you would have to sit 6 feet in the air to have ground clearance and be able to see over it

 

All designs are a series of trade offs. An inline 6 balances well, and the block isn't too long so it can be built stiff. The typical diesel seems to have a long stroke, which would make for heavy block with a V configuration of similar stiffness.

 

An inline 6cyl engine is a naturally balanced engine (so is a V12). Going to more cylinders does require extra hood length or smaller cab. Maybe that is why Dodge does not have a true crew cab SWB. Just my 2 cents.

 

heavy duty diesels have been primarily 6 cylinders for like 50 years, why fix it if it 'aint broke?

 

The reason that OTR's are straight 6's is because you can always get more torque from them. All of the big stuff run 6's. I have heard of some big tractors running v-8's and they are known to have engine problems. If something breaks on them they just get a new engine instead of fixing it. This is just I have been told though, I could be wrong.

 

Not for OTR use, but the 7FDL engines G.E. makes for locomotives are pretty
8,12 and 16 cylinder diesels (45* V).
7FDL Brochure

~Rob

 

Cooper Bessemer made a series of diesels (also available as dual fuel or spark gas engines) primarily used for municipal and industrial power generation that were available from an inline 6 (LSB-6) up to a vee 20 (LSVB-20) - pictured below from the topside catwalks. The LSVB-20-GDT is rated 8680 BHP @ 400 RPM continuous duty service with a 15.5" bore and 22" stroke.



As an aside, GE purchased the rights to its locomotive engine line from Cooper Bessemer who had previously built and sold these engines in inline (FW-6 and FW-8) and vee (FVBL-8, FVBL-12 and FVBL-16) configurations to GE from its Grove City, PA factory. That's why GE's locomotive engine business is located in Grove City.

Rusty

 

CUZ YOU JUST DON'T NEED ANY MORE!

I had a friend that had a 16V71TT ........... all it did was eat.... and eat WELL!
The more jugs the more $$, this can be taken many ways.

 

Down at work, our old D8N has an inline 8. I've been told it makes 350 hp and 1,000 lb-ft of torque. It needs all of that too move its 30 ton weight. But it definitely moves it well. The engine weighs somewhere around 8 tons so there's your big block and crank for stiffness. Now if I could just find some tires to hold that weight.....I really would have a tank .
Sam

 

they're right down the street. i didn't know anyone even knew grove city existed. the bessemer, as it was called, was a big employer for many years. my old neighbor worked there his entire career. small world.