DD Field Service Manual

[bostonhatteras]

In light of the post Timeless72 made about installing larger injectors over what the engine data plate lists, I thought it might be interesting to look into this small book I picked up some years ago. Although the specific engine and Hp that the listed injectors go into isn't made known, there are some relatively big injectors on the list. Whereas this list is for only 53 and 71 engines, some military engines must be using far larger injectors than pleasure, and especially industrial, engines. If anyone is interested in inquiring what else is in the book just pm me and I'll look for you. In total its 110 pages so too much to pdf the entire thing.

 

[SKYCHENEY]

More fuel requires more air for complete combustion. I don't see where that air comes from on a Natural.

 

[bostonhatteras]

Gasoline needs close to the 14.7 ratio but doesn't diesel use the air in the combustion chamber as necessary, but not more, to burn the available fuel sprayed in? There is a threshold for not enough air when increasing fuel without introducing a means to cram more air in but my belief was that there was some excess of air in nearly all injector/supercharging variations. If the blower on a natural is always pumping same volume of air for a particular rpm and the governor is able to vary the fuel amount without creating dangerously lean or rich burn then its because the required air, but no more, is used to burn the fuel. Some air pushed into the combustion chamber isn't actually used to support combustion, its along for the ride. Thats the way I always thought it worked.

 

[Timeless72]

Thanks for posting! I'm going to give it a whirl and see what happens. Made dumber mistakes with less. Will post results in the spring.

 

[SKYCHENEY]

Gasoline needs close to the 14.7 ratio but doesn't diesel use the air in the combustion chamber as necessary, but not more, to burn the available fuel sprayed in? There is a threshold for not enough air when increasing fuel without introducing a means to cram more air in but my belief was that there was some excess of air in nearly all injector/supercharging variations. If the blower on a natural is always pumping same volume of air for a particular rpm and the governor is able to vary the fuel amount without creating dangerously lean or rich burn then its because the required air, but no more, is used to burn the fuel. Some air pushed into the combustion chamber isn't actually used to support combustion, its along for the ride. Thats the way I always thought it worked.

Maybe, but then why the need for turbos? At some point a natural is getting so much fuel that it needs more air.

 

[Sonic Boom]

Thanks for posting! I'm going to give it a whirl and see what happens. Made dumber mistakes with less. Will post results in the spring.

Go for it and let us know how it does! Maybe install EGT gauges to protect the engines?

 

[bostonhatteras]

Maybe, but then why the need for turbos? At some point a natural is getting so much fuel that it needs more air.

Turbos allow even more air, for even bigger injectors. The 92 series blower bypass arrangement is an interesting case of this, with the blower resulting in restriction in front of the huge turbos. I wonder if there are tables available to the public stating the air introduced into the combustion chamber for a given supercharging arrangement. The DD Marine Pleasure Craft Library has a section on setting up air intake piping for given models/Hp levels but that's too coarse a comparison. Air temperature/density plays a minor part as well. Anybody think engines with cockpit air perform better than with engine room air?

 

[rsmith]

I’ve seen the blowers sped up with a gear change. Blevins or someone here in Florida a long time ago. But the blower isn’t really compressing anything just blowing straight through the chamber getting the exhaust out and the new air charge in.

 

[cww]

I've upsized the injectors in the naturals in my last boat with very good results, it really does work and didn't cause any smoke or overheating like some people said it would. The military section of the service manual is where I got the timing specs for them. I had a good experience with it.

 

[rwaldrop13]

Any thoughts on where we might find hard copies? The only field manual I've found was the special forces booby traps manual. Yes, I did download it. Not joking. But I had a major flashback when I saw that booklet. I had an almost identical booklet for the .50cal HMG.