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6BT 12 valve Marine Pistons


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There may be existing threads on this subject, but when I had my engine rebuilt, I was unfortunately convinced to go with Marine pistons. Now, more than a year later, I am still not happy with the way the engine runs (I have been through a lot sorting out mistakes that were made, too numerous to go into). In any case, the rougher idle, lower fuel economy and higher egt are all negatives. As I understand it, marine pistons require marine injectors. My question is, short of pulling the engine, yanking the pistons and going back to stock, are there any options to using a more efficient and smoother running injector? Thank you

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I am not sure on those pistons but from what I have just read throughout the net, they have a bigger bowl to handle the fueling from the 370 injectors since the marine engines were rated at 370HP. The bowls are slightly offset which would definitely mean you need marine injectors. The only problem is that I for 1. am not an injector guru and 2. do not even know of any marine injectors under 370hp. Those things are huge and run hot from what I understand, which doesn't sound like the kind of thing you want. I will keep digging.

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I did a little reading, and it looks like some of the marine pistons had a lower compression ratio, and lower compression height. I can see where an extreme race motor might make use of the lower compression by upping boost significantly, but its probably not great for a regular motor. What I don't understand... mabe ISX will find something... Is what is it about the marine engines that made Cummins put these in? Are they running some really high boost at sea level with these old motors or what the induction setup is. If I were to guess, they are probably running a large very laggy turbo and the power band on the engine may be a bit "peaky."

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I don't know about the Cummins Marine set up, though I had done basic maintance on the couple we had at our docks. Marine engines are always under load pushing, more of a torque situation than HP. My reading also indicates LOWER compression on these marine pistons and altered nozzle patterns. Definately above my pay grade but I do think different nozzles would be worth a shot. Considering what you've got into it already, I'd be trying to locate some injector people familiar with this issue. DO let us know what you learn & how you make out. Others might go the same route without understanding the full consequences. (Soounds like something I'D DO.) Best Wishes,Russ

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Thank you all; have sent a message to Contagious Diesel Performance. I did find a feference in another forum (not a member so couldn't pursue) to a different timing setting as the stock setting causes firing when the piston is in the wrong position for optimal results. Any thoughts?

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Hmmm, I have no idea how I could change the advance curve? Advance setting (timing) is straight forward but curve? I don't know enough about the P7100 pump. I would have no idea what the right curve would be even if I knew how to change it. Thanks, though

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Hmmm, I have no idea how I could change the advance curve? Advance setting (timing) is straight forward but curve? I don't know enough about the P7100 pump. I would have no idea what the right curve would be even if I knew how to change it. Thanks, though

Me neither... I only have VP trucks. But from a general standpoint, less compression means slower burn rate and slightly less efficiency.
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I don't know about on the 6BT, but on the HPCR the compression difference on the high HP marine motors is from the head gasket, and not the piston itself. Marine motors are under constant load at a specific rpm, and the prop demand is not linear its an increasing slope. So you can have a motor that doesn't make as much hp at 1300 rpms but meet the prop demand hp and the load is constant so the turbo doesn't have to react quickly like in a pickup. The compression ratio is lower because the build them to run at rated rpm, and even without huge boost they last longer with a sligtly lower compression ratio. Before getting a new piston set I would try a 370 marine nozzle, cheaper then R&R and parts on a new piston set.

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  • 1 month later...

I know it's been a month or so but I have the 370 marine injectors in my truck with the stock pistons. It runs hotter than it should and smokes more too. I'm getting ready to replace them because I'm wasting fuel since the spray pattern is outside the bowl on the piston. And risking cylinder wash. If you haven't tried another set and are interested in these let me know.

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  • 11 months later...

I know it's been a month or so but I have the 370 marine injectors in my truck with the stock pistons. It runs hotter than it should and smokes more too. I'm getting ready to replace them because I'm wasting fuel since the spray pattern is outside the bowl on the piston. And risking cylinder wash. If you haven't tried another set and are interested in these let me know.

Not sure if this helps now.... but stock timing on a Marine engine is 20° BTDC.... So setting it to 27° BTDC is nothing irregular.... With Marine pistons, you will see crazy EGTs with your stock timing...
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Not sure if this helps now.... but stock timing on a Marine engine is 20° BTDC....

It really hard to compare marine engine timing to truck timing. The prop demand curve doesn't match the hp curve, so you aren't using full fuel for a rpm until at or near rated rpm. A pickup may need full power at any rpm, if you work the truck, and that's where the advanced timing takes a toll on lower rpm use. Marine engines also run higher sustained rpms when making power, which is evidenced by the 20° if timing.
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At least in gas marine engines... the rpms were kept low. Used to be set to WOT (Wide Open Throttle) at 4000 rpm. After I got out of the business, it was upped to 4400 or so. The prop (load) was selected so that the motor could not rev above that limit with the vessel in question. In that era, the equivalent auto/truck engine would red line at 6000.

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