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Detroit Diesel 8V-92TA injector sizes and HP ratings

Cloudsurfer355

Registered
I have a pair of 8V92TA engines in a full displacement motor yacht. Their data tags say 735HP. From what I can tell, these use the 9225 injectors.

Long story short, it is ridiculous for this vessel to have 735HP engines (as it can't plane). If we were to re-prop the thing for less pitch, and drop down a few injector sizes (I'm thinking 9G90s), what would the effective HP rating of the engines be?

I don't want to get into getting away from the twin turbo setup and after coolers and bypass blowers and all that, just drop injector size.

As far as I can tell, to achieve normal hull speed in the vessel, with less pitch on the props, with a few sizes down in injector, you'd cruise a few hundred RPM higher to make the same HP, with, in theory at least, more airflow for lower EGT, and, also in theory, cleaner burns and more efficiency.

Thoughts?
 

Birken Vogt

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If you don't get a satisfactory answer here, you might try Heavy Equipment Forums. Mostly trucks and tractors but there are some Detroit guys on there.
 

ronm

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Last Subscription Date
07/10/2019
The Diesel Doctor would be the guy to ask...do a search or look at his posts in the never-ending Detroit oil thread, & shoot him a PM.
 

The Diesel Doctor

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12/28/2019
Aloha , If you went with the smaller 9G90 you would loose your current top end hull speed. If you have the right propellers, reduction gear and a clean bottom then there should be 200 RPM difference between no load and fully loaded RPM. There is quite a bit less fuel available with the smaller injector. Your engines are set up with advanced camshaft timing for the 9225s. Most Detroit turbocharged engines are STD timing. It could be an issue. What is your cruising RPM? ALIMG_4231 (1).JPGIMG_4232 (1).JPGIMG_4218.JPGIMG_4229.JPG
 

Cloudsurfer355

Registered
Aloha , If you went with the smaller 9G90 you would loose your current top end hull speed. If you have the right propellers, reduction gear and a clean bottom then there should be 200 RPM difference between no load and fully loaded RPM. There is quite a bit less fuel available with the smaller injector. Your engines are set up with advanced camshaft timing for the 9225s. Most Detroit turbocharged engines are STD timing. It could be an issue. What is your cruising RPM? ALView attachment 375361View attachment 375362View attachment 375363View attachment 375364
I'm not exactly sure what their thinking was way back when when they engineered/ built this thing, but, in my opinion it's a bit ridiculous haha. She's about 65' at the waterline and 80,000lb, and normal cruise is 8-9 knots. At 8-9 knots the engines are turning 1200 RPM or so.

If you work the required HP math backwards, I'm sure this thing would happily cruise around with 200-300HP engines just fine (with, admittedly, substantially reduced top end speed- but, really, what's the point to go faster than hull speed in a full displacement vessel?).

I've never actually wound these things out to the pins, but, supposedly, she's supposed to do 17-18 knots topped out. I see no reason for this thing to ever go past, say, 12 knots, and, if dropping an injector size or two results in a cleaner and more efficient normal cruise, I'd gladly take that trade.

I'd think that re-pitching the props/ dropping enough injector to get up to 1500-1800 cruise would be much more efficient and cooler running (due to actually spooling the turbos and having more air thus lower EGT).

In theory, what should these engines make with 9G90's? Somewhere around 450-500HP? If we were to go down this road, any other injectors come to mind besides the "common" 9G90?

I greatly appreciate your insight into this.
 

The Diesel Doctor

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Last Subscription Date
12/28/2019
I'm not exactly sure what their thinking was way back when when they engineered/ built this thing, but, in my opinion it's a bit ridiculous haha. She's about 65' at the waterline and 80,000lb, and normal cruise is 8-9 knots. At 8-9 knots the engines are turning 1200 RPM or so.

If you work the required HP math backwards, I'm sure this thing would happily cruise around with 200-300HP engines just fine (with, admittedly, substantially reduced top end speed- but, really, what's the point to go faster than hull speed in a full displacement vessel?).

I've never actually wound these things out to the pins, but, supposedly, she's supposed to do 17-18 knots topped out. I see no reason for this thing to ever go past, say, 12 knots, and, if dropping an injector size or two results in a cleaner and more efficient normal cruise, I'd gladly take that trade.

I'd think that re-pitching the props/ dropping enough injector to get up to 1500-1800 cruise would be much more efficient and cooler running (due to actually spooling the turbos and having more air thus lower EGT).

In theory, what should these engines make with 9G90's? Somewhere around 450-500HP? If we were to go down this road, any other injectors come to mind besides the "common" 9G90?

I greatly appreciate your insight into this.
 

The Diesel Doctor

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Last Subscription Date
12/28/2019
Can i assume your engines fairly fresh and run with a clean exhaust at 1200 RPM to move your yacht 9 knots? If so, they would be about 40% loaded. If you go with smaller injectors, de-pitch the props and run the engines at 1800 RPM you will get more engine noise and use more fuel to get the same 9 knots. I do not have a 8v92 TA HP and fuel consumption chart for your engines. This one for a 16v-71 is as close as i can get to yours. As long as the engines are loaded they are going to run well. I would keep those extra ponies in the closet. AL
 

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ronm

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07/10/2019
Are those Detroits happy at 1200 RPM? I realize a boat is a different animal, but in a truck or tractor they seem like they aren't even getting their breath at that low an RPM?
mikebohman, that's about as UNhelpful a post as I've seen in a while...
 

Vanman

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Last Subscription Date
07/10/2019
I had to laugh at the idea of replacing a superior two stroke cycle Detroit with a Four Joke. ;)

Yes! They work fine at 1200 rpm, if the power required is consistent with the power developed. They used to build 1200 rpm DD generator sets.

I was thinking the same thing as Diesel Doctor. If the engine pulls the load at 1200 rpm, “downshifting” to 1800 rpm will only use more fuel. Just like a car. You don’t drive with your engine near the red line on the freeway. I’m going to gear my bus so my 671 turns about 1200 rpm at 60 mph in high gear. That’s all it needs on level ground.

Keith
 

The Diesel Doctor

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Last Subscription Date
12/28/2019
Are those Detroits happy at 1200 RPM? I realize a boat is a different animal, but in a truck or tractor they seem like they aren't even getting their breath at that low an RPM?
mikebohman, that's about as UNhelpful a post as I've seen in a while...
In flat calm water the load on the engines is constant and determined by the speed desired. Think of them as being on a water dyno. Being two cycle the blower is supplying most of the air to support combustion and give you the needed HP at lower RPMs. The turbos are turning and adding very little air at this point. As the load increases you need the help of the turbo to produce more air. On the chart (post #8) at 1200 RPM you have 300 SHP available to do a 100 HP job. Nothing wrong with that. Low fuel consumption, less noise and longer engine life. AL
 

Cloudsurfer355

Registered
Can i assume your engines fairly fresh and run with a clean exhaust at 1200 RPM to move your yacht 9 knots? If so, they would be about 40% loaded. If you go with smaller injectors, de-pitch the props and run the engines at 1800 RPM you will get more engine noise and use more fuel to get the same 9 knots. I do not have a 8v92 TA HP and fuel consumption chart for your engines. This one for a 16v-71 is as close as i can get to yours. As long as the engines are loaded they are going to run well. I would keep those extra ponies in the closet. AL
The engines have about 2500 hours on them, however, they start immediately and throw almost no smoke on a cold start. I've been told by the local DDC guy that "if they don't smoke when they're cold don't worry about cylinder wear." I've had the airbox covers off to take a peek at the liners, and things look good.

Just for warm fuzzy feelings, I wouldn't mind rolling in new main and rod bearings though.....

Being a full displacement motoryacht, I'm sure these engines have spent their entire lives "loafing along." Seems its the sport fishing and express cruiser guys who are always burning up engines given their "need" to go fast and run 100RPM off the pins.

All that being said, it sounds like maybe I'm looking at this from the wrong angle? Based on what you're saying, dropping injector size is NOT going to get me cleaner and more efficient operation? I have far more knowledge on modern, DI injection diesel stuff, where, yes in those cases you can tune an engine to make 500HP but only use 100HP of it on a regular basis and not have much downside in terms of fuel burn- given that these Detroits are mechanical unit injectors, I had thought that since the injector can only do so good of a job at metering fuel when you get down to the 10-50% scale of its maximum, that being at 60% of max on a smaller injector would be better than being at 30% of a larger?
 

Cloudsurfer355

Registered
I had to laugh at the idea of replacing a superior two stroke cycle Detroit with a Four Joke. ;)

Yes! They work fine at 1200 rpm, if the power required is consistent with the power developed. They used to build 1200 rpm DD generator sets.

I was thinking the same thing as Diesel Doctor. If the engine pulls the load at 1200 rpm, “downshifting” to 1800 rpm will only use more fuel. Just like a car. You don’t drive with your engine near the red line on the freeway. I’m going to gear my bus so my 671 turns about 1200 rpm at 60 mph in high gear. That’s all it needs on level ground.

Keith
While I completely get what you're saying here, the difference is that in your bus, once rolling down the highway, the required power is very low (same for a train). In a boat (or propeller driven aircraft), the required power to maintain cruise speed is much, much higher.

People don't realize this, but when we're in cruise flight in a jet aircraft, we're sitting at about ~90% of rated RPM (which works out to about ~70% of max rated thrust).
 
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