[rgsuspsa]

Originally Posted by MDS

Fuel used is controled by the throttle. Simple. Installing injectors smaller than what is curently being used is not going to save anything. It takes (X) ammount of fuel to run at 1800 rpms at a specific load. If you remove larger injectors all you will be doing is limiting the full load rpms and full load capabilities of said engines. The ONLY way to reduce fuel consumption is to limit speed or load. Example would be running at lower speeds or reducing the weight or removing pitch/diameter of the props.
I have covered this so many times it makes sick seeing someone spend thousands of dollars detuning detroits. These 2 strokes will load up and will need to be run above 1800 rpms to clean them out. Its just the nature of these beasts.

The above statement by MDS is correct. A given diesel fuel flow rate will produce a given horsepower, irrespective of injector size. Also, Detroit Diesel injectors are size rated by the volume of fuel discharged when at maximum rack position, and the units of measurement are cubic cenitmeters (CC) per 1,000 strokes of the injector (which corresponds to 1,000 engine rotations). The usage of the term liters-per-hour (LPH) as cited elsewhere in this thread is incorrect, and is an invalid measurement of fuel injector flow rate capacity.

Ron Sparks

[MDS]

Lets look at it this way. #1 the prop moves the boat. Next in order to move this boat at 10 knots you can use a 550 hp engine to turn a 26X26 prop at 1000 rpms engine speed and 500 rpms shaft/prop speed given a 2:1 reduction in the transmission. The engine burns 10 gallons per hr at this speed.
Now take a 735 Hp engine turning 28X30 prop in the same boat. If you run it at 1000 rpms with a shaft speed of 500 the bigger prop is going to push the boat faster. Correct? This engine is using more fuel at this rpm because it is turning a bigger prop. Bigger prop is more load. More load means more fuel burn. Lets say it is using 15 gallons per hr due to the bigger load.
Now slow down the 735HP engine so the boat make 10 knots. The fuel burn will be close if not right at 10 gallons per hr because the load is now equal. The thing you are not getting or maybe we just dont understand each other is it takes the same amount of fuel to move this vessel at 10 knots with a Detroit diesel 2 cycle engine no matter what it can do at the top end.

[MDS]

Hello Mr Sparks. Good to see you here. I was wondering where you were hiding. If anyone doubts what I am trying to say or convey then just listen to Ron. No questions asked Mr. Sparks knows what he is saying and it will always be spot on accurate!

[Capt J]

I agree with both statements, but the equation you're failing to realise is that it takes VERY little HP to move a 50' Bertram at hull speed. For example, you're probably doing 10 knots on 200 total HP, 8 knots on 175 total HP. So the difference in HP needed to push the boat at displacement speed between both engines is relatively little in rpm's with the different propellors. However the smaller injectors will meter the fuel better, the engine will run cleaner, and you will save fuel.......it's not proportionate. When you get in the really high hp ti's and ta's the amount of fuel you throw in there to make the additional HP is not as efficient for the amount of gain, a lot of it is wasted cooling off the additional heat created by the larger turbo's and not converted to HP. The lower HP engines have a higher efficiency.

I ran a boat they did this on many years ago. A 58' Hatteras YF with 8v71 TI's. The boat with the origional 130 injectors cruised at 16.5 knots (1950rpm's) at 45 gph. With the 90lph injectors and appropriate prop change it cruised at 15.3 knots at 1950 rpms burning 30 gph. At 1000 rpm's with the larger injectors it did 9.5 knots at 9 gph, at 1000 rpm's with the smaller injectors the boat did 9.2 knots at 6 gph...... It also ran crystal clean idling all day down the intracoastal or at cruise.......DD factory recommended the change in injector size based upon the owner's usage when they did majors on both engines......

I can't remember the actual injectors used, but I think they went from 130's to 90's.......Might've been from 110's to 90's....but I don't think so......The rating went from 550hp to 435hp if I remember correctly........

actually you're right on the fuel injector rating......90's should deliver 90Millileter's of fuel per 1000 strokes, 110's should deliver 110 Milliliters of fuel per 1000 strokes and so on and so on......

Here is a spec I found off of the internet for the various injector size and GPH differences for a 12v71. I highly doubt you'd get much of a HP difference between a N90 and 7N95 injector and take note of the fuel burn difference.
WITH N90 INJ /18-2100 RPM,GPH 22-24 PER ENG.
7N95 INJ /18-2100RPM,GPM 34-38 GPH PER ENG.
M15 INJ/18-2100 RPM ,GPH 38-44 PER ENG.

[rcrapps]

Originally Posted by jhall767

There is no need to detune. The most you could do is replace the injectors with much smaller ones and that MIGHT save you a couple of percent in fuel burn. The fuel savings, if any, would take thousands of hours to earn back the cost of 24 injectors. If your transmissions and running gear permit it you might want to consider running on a single engine at a time. This should give you a modest boost in fuel economy.

We run a 58 Bertram MY with 12v71ti's. I have even considered removing the twin installation and going to a single center line installation and keeping the second engine for a spare.
Then we had to come in the St.Johns inlet one day. Sure glad we still had all 1300 hp. I learned allot that day. We still cruise at almost 10kts LSD. But when needed, all those ponies do come in hand.
We have experimented with running one engine at a fast idle. With small rudders, we learned to use the down wind (current) engine. Engine temp does stay up. If we really get hard up for fuel we will do this more, but at that speed, snailbotes leave us (i don't deal with embarrassment well).
Our advice; Ensure your injectors you have are running the best they can,, Then slow down. A flow scan will really show you the real MPG with out guessing. Anything you modify on those engines is gone forever and it will take a LONG time before it's paid back if at all, and your not running the engines per design.
A previous comment was something about getting a snailbote to save more fuel. You may $till not come out ahead with even that idea and snailbotes in a following inlet sux.

Some re-powering articles in the May Yachting mag that may be of interest. But again, where'$ the payback?

KISS

,rc