Should a diesel be longstroke/undersquare?
 

I wonder, because I'm thinking about taking a shot at the diesel/gas engine thing...

I have a few 300 inline 6 Ford engines laying around in various stages of completeness, and they're not really worth trying to sell or rebuilding without a use for them, so they just hang out because I don't want to throw them away, either.

The specs for it are 4" bore x 3.92" stroke, or basically square.

My question is: Should I be working with sleeving the pistons down to 3" or so, or using a longer stroke, or will the 4x4 measurement be OK to use?

From what I've seen/been told, most diesels have undersquare or longstroke configuration, but I see no reason that it can't work out being nearly or completely square.

I'm hoping to be able to use the OEM crank, but replacing the pistons and rods, and having the cam welded/ground to be more diesel-cycle friendly.

I'm kind of wary about posting this here, since I already know that the general consensus is one that it can't be done, don't bother... but I don't care, really. I just want to play with it awhile. Who knows, I might learn something in the process?

So anyway, feedback/help is appreciated here, as always.

My only point of reference here is from Matsu Matsuzawa, where he stroked his 125 to 185cc:
1985 Matsu talks
should help on the low end anyway.

If you could remove 5 cylinders that would help too :)

There might be something about piston velocity too.
http://ecomodder.com/forum/showthrea...peed-1477.html (Or it is just a symptom)

If you want to design an optimized bowl shape in the piston, and get the compression ratio where it needs to be for a diesel, chances are it'll end up being undersquare by default.

...most diesels are low-rpm (compared to SI-engines), where LONG strokes are used to capture & maintain more "flywheel" affect...maximization of torque.

...the faster you want the engine to turn, the SHORTER the strong typically needs to be...of course, at the expense of some amount of torque.

...the REAL question, however, is "...how strong..." is the block because of the MUCH greater cylinderwall pressure of a diesel (20-22:1 CR) over gasoline (8-10:1 CR).

Very curious as to how you will make this work.

It's almost square right now, but by the time you get longer connecting rods and all the other stuff to increase the compression enough to compression ignite the diesel, you will probably have more stroke than bore.

Me too, honestly. It's more of an experiment than anything, I don't have high hopes, but maybe?

Since when do longer rods increase stroke?

To increase compression, I can do quite a few things, ranging from taller pistons, to longer rods, increased stroke, and even decking the head/block.

Some diesel engines run as low as 16:1 compression, while others pull out all the stops and slam out 21-30:1 compression.

There are alot more factors involved than static compression ratios, though.

You're right, making the connecting rods wouldn't increase the stroke length, but increasing the length between the crankshaft and the connecting rod will. That's what I meant I just said it wrong. The stroke should be twice that distance. I'm not sure what that's called though. 12V P-pumped Cummins have a CR of 17.5:1, a 4.02" bore and a 4.72" stroke. A VP-44 SO Cummins has a CR of 16.3:1. Since the bore is the same in both 5.9L engines, the stroke is obviously shorter. A 6.6L Duramax has a bore of 4.06" and a CR from 17.5:1 to 16.8:1 depending on the year models. The earlier Duramaxes have the 17.5:1 and a stroke of 3.9". The older powerstrokes (7.3L's) have a bore of 4.11" and stroke of 4.18". It's CR is 17.5:1. All of these engines are turbocharged.

The older, naturally aspirated engines (generally indirect injection) have the high compression ratios you speak of. The highest I've heard of is about 23:1... not much higher than that though. If you plan on having direct injection as well as some sort of forced induction, good luck keeping headgaskets on that thing without a CR lower than 18:1. Anything lower than 15:1 and good luck starting it on a cold morning.

Thanks for explaining what you meant there.

While I won't argue the point, I will explain my stance on static compression ratios:

If you build an engine with a static CR of 21:1, and allow the cam to hold the intake open beyond BDC, (atkinson cam design) your dynamic compression ratio will be lower. By controlling cam timing events (which I'll have to do with a custom cam anyway), you can effectively control detonation, even running high boost levels against high compression.

None of that really matters so early in the game, though, and it won't make a difference in the end-game, either, since the rods and pistons will be custom pieces.

I do know that the stock crank in a 300 is good for 500+ lb/ft of torque/around 300+ HP (was a factory setup, I'm sure it could do more), and they're already low-speed engines which are durable enough to handle extended periods at 4,000 RPM. Ford used them in 5 ton trucks for their amazing torque capabilities.

It looks like those are all pretty square engines, or very close to it, that you've mentioned there... Maybe I'll be fairly close to where I need to be, and just decking the block and custom pistons/rods will do what I need?

I'm fairly certain I'll need to do some head work, as well, to allow the engine to breathe easier, since it'll be pulling in all the air it can all the time.

Also - anyone know where I can source an injector pump/injectors for a 6 cylinder?

(Cheap, used, free, preferably.)

I'm going to call this one a career project, by the way. That means it's going to take awhile, and there are other projects WAY ahead of it, but as I have time, I'll tinker with it. Especially now, since my Father just pulled ANOTHER 300 out of a truck.

I remember reading somewhere that smaller bore longer stroke engines are inherently more efficient. It was somehow related to vintage race car motors (old Ferrari and Jag engines were WAY oversqaure). Smaller engines are also generally more efficient so I'm not sure a 300 is a good starting point for any vehicle smaller than a dump truck. I used to drive a Ford dump truck that had a small (for the job) car-type (didn't have replaceable bore sleeves) engine in it and IIRC it was around 300 CID and while it wasn't ideal it did move the 70,000 lb truck down the road at 70 mph everyday.

Best parts source would probably be dodge/ cummins stuff since it's also inline 6 cylinder configuration and there is a pretty good after market developed around them. Might could even use internal parts, I'm not sure but it's definetly worth a look.

On gas engines static CRs range from around 6 to 1 up to around 15 to 1 but if you look at dynamic CRs there is a MUCH smaller range, ie it's the dynamic CR that sets the limit and this is largely dependent on the cam timing I would assume diesel is the same way but to what extent I'm not sure.

I think it would be more fun to find a way to bolt 2 of them together and make a flat 12 boxster out of it:D