As FI mentioned above, when we refer to compression ratio, we're talking about static compression ratio. Simply take the volume of the cylinder with the piston at BDC and compare it to the volume of the cylinder at TDC. That's your ratio.

If you then manage a higher effective ratio via the use of additional boost pressure, that's another condition altogether and it's got nothing to do with the static compression ratio.

Even 25:1 is significant for a diesel when looking at static compression ratio. Clearance for the valves becomes a significant problem. Were these DI engines?

Yes. A piece of thin paper would be the only thing you could fit inside the cylinder for instance. At first, we wanted to be behind an explosion barrier when at first start!

You mentioned that your Cummins has a compression ratio of 30:1. It's presumed that you are not talking static ratio, again. What's the static ratio of this engine?

Does it differ from the typical Cummins 6BT?

I know on the boats with the older 2 stroke diesels they have heaters that you have to use or they won't start. Block heater or air box heaters. They smoke like crazy when they start.

With the new electric 4 strokes I don't know what they use, never had to start one when it was 80 degrees out.


I know the little Yanmars don't have glow plugs or anything, they just have big starters. You crank the heck out of them and they start.

But on a boat in the water the engine compartment usualy stays warmer remmeber, 30's is cold it doesn't get down much colder than that because of the water. A 617 would probably start in the 30's with no glow plugs.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The Static ratio is 22.5:1. In out ward appearances it does not differ from the 12 valve offered from late 1993 to 1998. The difference was in the size of the bore and stroke and these engines were marked by a maltese cross on a certain part of the block stamped into the casting. To be honest, In my opinion, they didn't make the greatest dino-diesel burners and were awfully loud which is why they were discontinued at least for consumer use. But they were rock solid with an inline DI pump that looked like the Benz 300D pump with an extra injection sleeve added and made great veggie/ plant fuel burners and were flex fuel ready in 1994. My truck runs better off of biodiesel or veggie oil than dino and my fuel economy is better too. Not every truck came with one of these. Remember the Cadillac fiasco of the late 1970's when Cadillac owners sued GM because when they opened the hood of their cars, they saw Chevrolet on the valve covers? Simillar situation. Not widely advertised on the service books and definitely not in the sales pitch, but GM had a bunch of overstocked Motorhome 350 V8's they needed to get rid of and they were installed in the Cadillac! Most who got their engines exchanged now find their cars in the junkyard, but the ones (and only a few) who didn't care about having a Chevy engine in the engine bay are still driving those Caddy's around with 300,000 miles or more on them and they run great even some 30 years later! Sometimes a mistake will turn out to be a bank error in your favour!
Now for the counter-rotating diesel.... Picture this....Your standing in front of your 300D or whatever watching your engine run, block and head plus accessories just sitting there motionless while everything inside is whirring and clacking away, right? Now if you could take that same OM 617 and install it in a steel sleeve with an outer hull and cylindral rod bearings in between, your motor would have the crank turning to the left while your block and accessories turned to the right, right in front of your eyes, your motor is running in two different directions with a DI pump for each cylinder and your transmission could harness the output from two rotating forces going in opposing directions!! Wouldn't that look COOL! Since the pressure exerted from the ignition of each cylinder forces a powerstroke and stress in one direction, the rotation in the opposing direction counteracts it and you can increase your output and RPM! Sort of like a reciprocating turbin hybrid! How Cool is that!!!! When we built one of these realising that if we didn't do our homework, parts flying off during a self destruct phase could kill us, I really thought that if I could be a part of this and see it and help build it, it would be worth the risk! I've seen stuff most people dream about!
However, right now the weather is good and I'm building a small storage barn here in my backyard and need to get cracking on it, but i'll be back tonight. I love talking about this stuff and even though there is a lot I don't know, it's a fun topic huh?