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

Normally, I'd agree with the size thing... problem is, I don't have anything else readily available (read: free) with an Iron block, heavy duty crank, and decent specs... not to mention multiple copies of something fitting that description.

I guess the 300, for me, is just kind of a "proof of concept" tinker-build.

Also, the 240 is the same engine with a shorter stroke and longer rod combination, IIRC. That would put the next size down to being the 170 and 140 L6, also from Ford.

Remember - Cheaper is part of efficiency. If I have to pay a bunch of money over the course of a tinker build, I'd have to recoup that in the final product. If I spend less money, I can have lower (read: realistic) expectations without being disappointed in the end.

...the efficiency was from reduced sliding friction due to less sliding surface area of the smaller pistons: stroke is constant but bore is squared and times PI/4...ie: area vs. volume.

The older 1st generation cummins (89-93) have what is called a VE rotary pump. It is just a rotary injection pump which (relative to the new Common Rails) have a low injection pressure. These would be probably the easiest to time onto a gasoline engine. There's no electronics to fool with. These should inject PLENTY of fuel to way over-do your 300.

That's what I want, a standalone setup with no computer controls, like my 240D engine. They're easier to work with.

It is worthwhile to mention there are a whole basket of factors determining the bore/stroke ratio of any production engine ranging from legal influences (in England there was for many years a "bore tax" where engines were taxed on the bore diameter and in many countries in Europe the tax on engines skyrockets over about 2.7 litres and 15CV) to manufacturing "conveniences" which is a nice way to say they pick from existing components and do a mix and match to get it all to work ; technical theory and details are often shove abruptly to one side.

Pete.

I'm NO EXPERT but I'd "guess" that the ideal rod/ stroke ratio will be different for gas than diesel.

:thumbup::thumbup:

Also look at Benz I6 Diesels

Similar pump in my VeeDub; just electronic controls on a VE pump
There is an "M" pump for VW that is full mechanical; not that it matters (4 cylinder) but . . . .

I seem to remember an individual I met claiming he converted his metro motor to diesel, he said the compression ratio was high enough to be left alone so long as it was WARM outside.

My estimate would be follow KISS.

Diesels have come many different ways dimensions strokes and compression ratios.

Leave your motor alone, replace the heads to a bit higher compression than you need which tends to hover in the 14to17-1 area depending on how warm it typically is), deal with the less than ideal bowl by placing the fuel intake/"injector" in a "mini" bowl (aka suido precombustion chamber) you bore out of the head and live with the less than perfectness being happy that it works with minimal work. Of coarse you will have to find some very thick heads to fit your unit.

The main issue is getting fuel into the motor at the right time, the rest is moot so long as the block doesn't overheat in the wrong place and break on you.

Good Luck

R/S ratio has more to do with piston speed and engine wear than anything else. The efficiency/power benefits are a byproduct of frictional losses, from what I understand.

...remember, GM "made" their diesel burner engines from basically stock V8 engine block assemblies...and that was why they didn't last (among other reasons).

That's just TOO easy.

It's known among rednecks that any gas engine will run on diesel once it's warmed up, and there have been (failed) engine designs that utilize spark ignition with oil fuels.

I've personally run diesel fuel (and kerosene) through an engine, when I ran out of gas, and it was all I had. There was enough gas in the lines to get it started again, it was already warm, and I just threw a couple gallon of diesel in the tank, and TOOK OFF as soon as the engine started. It ran a bit hotter than normal, but no damage, other than soot in the exhaust and I'm sure some carbon in the engine.

That's a Carb'd 302, though. I'd probably not be as willing to do it with a FI car.

The OEM head on that engine is iron, the block is iron, the crank is iron, so there's the three components that are usable. The pistons and rods, not so much. The pistons won't take the heat for very long, they're thin. The rods are pencils, from what I can remember. They might take the torque, but I might as well just get new ones matched to the pistons. (Maybe I'll try out the OEM ones first, though.)

SO maybe I just need pistons with a higher deck height on the stock rods. I'll have to check the actual dimensions to find out what will really work, though.

The spark plug holes in the head are like 1" long. I was going to hog out about 3/8" into them, then screw the injectors into those holes. They're angled into the cylinders, so they'll still get a non-uniform spray pattern, but it should work out. I won't be able to use glow plugs, though. I'll probably have to start it with some WD-40 or refined oil.

They used the already butter-like 350 blocks. Those things aren't even close to the 300 inlines, but I see what you're saying.

I seem to recall diesels have stronger rods (not a prob as forged rods are readily available for the 300) and stronger head bolts. They should me available, contact Crower Rods, they'll have them.
Think about a turbo to help keep down the smoke, just remember to keep the static CR low.
Yeah those old GM diesels gave diesels the bad name they have here in the states. :mad:
Keep us posted.

Wow, there's something I didn't think about - head bolts.

I wonder if grade 10 studs would do it? Maybe I'll just see about using head bolts from some other diesel installation..

Head Gasket -
Should I just toss in an OEM gasket, find one in MLS, or make one from tri-folded 24g copper sheet?

If you could find a marine head gasket for a 300, that might be a little better. I'm not sure if they're just thicker or stronger or both, but most high performance Cummins running insanely high boost put in marine gaskets and head studs instead of bolts. I'd venture to say that you could get a marine gasket and then bore the holes out for a larger head stud, then put studs in it. You might be okay then as far as the head goes.

Another thing, the first gen Cummins with the VE rotary pump had angled piston faces as well as angled injector ports. The injector does not sit directly on top of the combustion chamber so these maaaay... work in your 300 spark plug holes. That's a BIG maybe though. Any way about it, it's going to take a lot of work.

I would go with a longer stroke, just to take advantage of having the diesel to begin with. I would also go with a direct injection system, NOT an IDI system as was hinted at earlier. Those IDI diesels aren't worth a crap.

If I were you, I would just buy a 4bt or 6bt for cheaper than you're going to fix these 300's, but that's probably out of the picture. If nothing else, I would go buy an old 6bt just as a model so that you would have at least an idea of what your 300 should look like and run like when you're done. Just my two cents.

BTW, if you have a CR of around 15-18:1 and direct injection, you won't need glow plugs or ether or any other assisted starting until you get around 50 degrees. Under that, you could use a grid heater off of an older Cummins... you should be able to get one cheap that you should be able to fabricate onto it. All it is is a set of elements that the intake air goes through to get pre-heated before entering the combustion chamber. Worked on the older Cummins down to about 0 degrees if you have strong batteries. Anywhere below that, you'll want to have a block heater.

Cummins don't have glow plugs, only grid heaters. I started my '99 on weak batteries (dual batteries) once in 7 degrees and it had been sitting at that temp for over 12 hours. It took about 4 revolutions where as it normally only takes less than one, but it still fired up. It smoked for about 2 minutes, but it eventually cleared up.

Think you can find me a Cummins 6BT for less than $500? The biggest expense is the injection pump, and I'm already considering other options, such as air-assisted injection, which will not require a high-pressure injection pump.

Keep in mind, the idea is not to get the ultimate power or performance, it's just to make a gas engine run diesel with some reliability.

I already know that the 300's stock configuration can handle 500+ lb/ft of torque at higher RPM than I'd want a diesel to run constantly, so the parts themselves should be able to hold up to the abuse - the temperature is what I'm really worried about, which is the reason I'm still considering using the OEM rods as well. I think in this case, the pistons are all I'm going to have to really change before I start working out the logistics of putting diesel fuel where gas used to go, and replacing spark plugs with injectors.

Also - What's wrong w/ the IDI Diesel? International seems to be doing just fine with them...

EDIT: Regarding Ether - I never use Ether in diesels, if I can get away with not using it. I've always kept a can of WD-40 or a similar refined oil on hand, starts 'em up just as quick, but doesn't shock the engine on ignition.

Since I don't use WD-40 for lubricant, because it isn't designed for that, that would be about the only use I have for it, other than water displacement and short term corrosion protection.

International? They haven't been IDI since 1993. The '94 PSD was DI and the PSD has been ever since. Either way, DI is the ONLY way to go IMO. Starting an IDI in cold weather is not fun at all.

I just think that by the time you spend all that money on the head, including the studs and gasket, then get custom pistons, and pay for an injection pump of some sort, plus the cost of fitting it somehow to the 300, you'll have been able to buy 2 6bt's out of a junkyard somewhere.

EDIT:
Oh and don't forget about the turbocharger and intercooler plus all the cold side piping if you decide to throw one of them on... plus a stout enough manifold to hold it up. It can start adding up really quickly, where as an older 6bt with 350,000 miles on it will still have more life left in it and probably still run cleaner and more efficiently than a home-built gas to diesel conversion.

That, I can agree with. I'm not sure how much you think I'm going to be spending on the head, though. A new head gasket is cheap, studs aren't expensive either. The injection pump is the most cost-prohibitive part, along w/ the injectors, but I can get IDI injectors from a 7.3 International (From a F-series) for a few bucks a piece, and the injection pump may not cost me that much, either, if I can get one from the junkyard. (Around $30, IIRC.. they have diesels come in once in awhile.)

I'm still looking at the old-school air injected diesel thing, though. I dunno if I can make it work or not, or if I can simplify the design just enough to make it work.

Beyond that, the pistons are the most expensive thing... and with any luck, I'll be able to find 4" pistons in a diesel somewhere that are close to the right specs, and work around it from there. Pistons are also fairly cheap, as long as they're not custom made. If I have to get custom pistons, I'll just wait until I get my mini-forge setup (which could be years, further delaying the project) and cast some myself. (Yeah, I can cast/mill metal, when I have the tools to do it.)

You'll have to machine a pre-combustion chamber into the head in order for the IDI injectors to work. They're not built to take the hammering that a DI injector will. Plus, you'll have to have a glow plug. I'm betting that with a loose engine and all the WD-40 in the world, it's still not going to start under 30 degrees. I've never actually seen how air injection works... never heard of it before.

Either way, I'm not trying to say it can't be done at all. You can probably do it with enough time and enough money. I guess it's just the German Farmer in me that wants to over-engineer everything. If I build something it's because it's going to be better than I could buy it or so much cheaper that I can deal with all of it's crap that I'll have to put up with.

Anyways, it sounds like you have a pretty good idea of what you want out of this engine, so I'll just leave it at that. Have fun!

I'll have to drill out the spark plug holes anyway, and I thought about drilling them out, tapping them, and screwing in a sort of precombustion chamber. The OEM spark plug holes are way too small to screw in any diesel injector I've ever seen. I'm going to have to check clearances on that anyway, to make sure it won't interfere with the two-valves-per-cylinder.

Basically, the thing I had in mind for the pre-combustion chamber was just a threaded bulb fitting with holes in it that stuck down into the combustion chamber only slightly, and the injector screws into the top of it. If you need, I can make a crappy MS Paint rendition of it and upload it, so you can get a better idea of what I'm talking about.

Air injection seems to be out of the question - its' used on slow-speed diesel engines, I can't find an example used on a high-speed engine. (slow speed = 60-200RPM operational speed).

I don't know how well the engine will start in temps below 30 degrees, but my GM 6.2 Diesel started with a single spray of WD40 in sub-0 temps as long as I warmed up the block for about an hour. It would only take about 10 rotations to start it up, with a quick spray of WD-40 in the intake.

I know what you mean about over-engineering things, it's normally a fault of mine too. I overdo it until it doesn't work, usually.

You have to remember, though. This is a TINKER project. I'm not taking it too seriously, I just want to see what will and won't work, first hand, and WHY it won't work. If, in the end, it works, I might build a few of them and sell them.

I've got small (briggs engines, Kohler engines, etc.. ) that I've also been considering similar projects with... I have one specifically that I don't have a use for, so I was thinking about doing a carb'd spark ignited diesel with it. Again, just to see what happens.

Horrible image of home-brew precombustion chambers.
 

Okay, I see what you're getting at with the precombustion chamber. I guess that wouldn't be so bad as long as you kept the block warm enough to start it in colder weather. You'll probably want a block heater on it later. I think that your relatively small head (in comparison to most diesels) might still be too thin to live up to the heat transfered into it by IDI. I could be very wrong though. I guess your next step is to just try it and see if it all works. I wouldn't mill the head just because you need all the material you can get there. I'd change your compression ratio by changing the crankshaft because you're going to need closer to 20:1 for IDI.

dude, forget this this, get that twinstar runnin :)

Find me a key and ignition switch that ISN'T $100... and I'm still waiting on the title.. I think I may have to get one for it via "other means"... and it's not old enough to get a BOS from NY state and do it that way.

I guess I can't really expect much different for the price, though.

I think I can get a title for that Dream 90, though, and it appears to have the same frame as the 185, so maybe I can swap the frame over and use that title... :)

Lol, there were plenty of switches in that old house I bet :)

There were. I have them all, too. They had just been put in, so I pulled them out to sell them. I've sold a bunch, so I don't really have them all... but I did make sure to get every last one, as well as GFCI's and Grounded Plugs.

I'm probably just going to solder wires to the old switch (the wires are soldered in) and tape it off, then run a generic keyswitch until I can find something original. I need the switch cover, too, but that's not a big deal as yet.