Detuning 12v92 for fuel savings

To truly detune one for fuel savings isn't a simple job and certainly more effort than justifiable for a single trip. If you wanted to permanently change the engines it maybe be worthwhile but the difference in fuel burn will take a long time to amortise unless you run a lot of hours. After detuning them you will also have to retune and reduce pitch on the wheels as they wil now overload the engines.

To reduce fuel consumption, you would need to change both the amount of fuel and air going into the cylinders and keep the proper ratios. Meaning you need different injectors, rack settings, and most likely turbos and possibly changing the drive ratio of and/or the blowers themselves.

Unlike electronically controlled engines, your mechanical injectors will fire the full fuel load into the cylinder with every stroke, and she will load up after extended low RPM running but blowing her out after 6 hours or so will clean her just fine and won't cause any appreciable harm. Much like sportfishing boats trolling all day and running home, gradually increase rpms to WOT for maybe five minutes or until the black smoke is gone.

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.

This is not entirely true. I ran a 58' Hatteras YF with 8v71TI's that were majored by DD and they detuned them. They had the 110 LPH injectors put in, turbo's were not changed as far as I know. They went from 50gph at 1950rpms to 30gph (for both). BUT, 1000 rpms and under they ran extremely clean. However, cruise speed did go down along with the fuel burn but still a good savings. However, when you consider the cost of re-pitching propellors, new injectors, tuning the rack etc......it would take a lot of engine hours to equal the savings.

On the other hand, 12v92's have such large injectors that they a lot of times don't atomize the fuel properly at lower rpm's and have a tendancy to wash the cylinder walls, especially the JT's with 160 LPH injectors versus the standard 140 LPH injectors. JT installed larger injectors using the same turbo's to eeeeek out a little more HP.

How cheap do you think you're going to run a 70' Motoryacht for? At 1000 rpm's and 30mins cruise every 6 hours, you're going to average 1.5 GPM. I would not recommend running the boat on one engine. Pushing a twin engine boat with one engine is going to lose some economy because the propellor is not center line, and the rudders are going to be over. Also, it's an overloaded situation for the 1 engine because it's propped based upon running both engines. If you want to save even more fuel, run it at 800 rpm's instead of 1000 rpms,

1.2-1 GPM at 800 rpm's. If you have to slow down that much, you might want to consider a sailboat or something. I think there comes a point in time when the ends don't justify the means.

Hi,

Here we go again.

This has been discussed here before.

Briefly, the long and short of it is-

You can run a twin engine install on one engine and not overload the running engine, you can reach a happy medium where it works well and fuel is saved at the expense of speed/time to get where your going.

It is not exactly straight forward to get it right first time but a bit of experience and observation of the engines parameters will soon have you finding a happy rev range.

Agree! 800rpms are just 150 above regular idle... this is not good for engines since it is like trolling or less.... These DETROITS have their best moment at 1200 (by bench tests) but around 1200 to 1800 rpms... their sould be your optimal cruise rpms.... however there are plenty of external issues like transmission ratio to propeller configuration that change all calculation... and when all matches... the hull starts to interfer,,,, is it clean or not?? Good luck!

Regards from Venezuela

O.K..enough is enough...if that was a 27 yr old 36-24-36 gorgeous blonde who really wanted your high tuned "body" would you want to DE tune that ??

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.

This is simply not true. When you detune the engine, you need to re-prop the boat with less pitch so the engines still see their 2300 rpm redline and are not overloaded. You will lose speed at a given rpm.

However, with mechanical injectors and especially the 140 or 160 LPH injectors used in the 12v92's, they have a really hard time metering the fuel accurately under 1000 rpm's, unlike the electronic diesels that monitor load factor, exhaust gas temp, and more. If you're planning on running at 1000 rpm's and below for most of the time, going to smaller injectors will make the engines run considerably cleaner (you'll still need to run them up every 6hrs), and be more fuel efficient because at hull speed you won't lose hardly anything with a few inches of less pitch, and they'll meter the fuel much better.

The J+T 12v92's were notoriously bad at washing liners out with the 160lph injectors on sportfish that were trolling at 600-700 rpm's all day most of the time. I ran a 58' Striker 1991, that by 2004 had major overhauls on both engines 8 times, basically every 500-800 hours because of this and the 160lph injectors JT stuffed in them.

However are the fuel savings worth it after spending the money to detune them, if you don't need to change injectors anyways, and aren't doing the great loop or a very extended journey, probably not for 95% of yacht owners........

Also, when running a twin engine on only 1 engine it is overloaded, because the boat is propped for running on both engines, and the propellor load curve is WAY off. In fact most of the CAT electronic engines will limit RPM's to 1200 rpm's and throw diagnostic codes when you try to do this, because they sense a very high load factor that is not correct for the rpm's.......Usually at 1200 rpm's running only on 1 engine you'll see a 90% load factor.......give or take, if you pay attention to the digital display.

Ok then. With your example lets do this. Take a 50 foot Bertram with Detroit Diesel 8v92 TAB's rated at 735HP@2300 rpms. Using 28X30 props. This engine came with 145 injectors or transitional 7125 injectors and burned 58 gallons per hours at 2300. Lets say this specific vessel made 30 knots at 2300 rpm. Now you go and put lets say M95 injectors in it and the new horse power rating is 550 at 2300 rpms and you cut the props down to 26x26. The vessel will no longer make 30 knots because the HP is down and the props are smaller correct! Lets say the boat now makes 22 knots and burns 40 gallons per hour because thats all the fuel the injectors can put out. If you take the first setup rated at 735 HP and run the boat at 1800 and burn exactly 40 gallons per hour, you are close to if not right on at putting out 550 HP. Same fuel burn, same speed and all you did was slow down.
Detroit Diesel knew that the large injectors had a metering problem early on so they designed a transitional injector that kept the fuel metered properly at low rpms or at the low end of the rack travel. Nothing new here and not an issue.

You are correct, but the point you're missing is that at low speeds such as 1000 rpm's and under, the engine with the smaller injectors will move the boat at or below displacement speed the same as the higher HP version, using less fuel and runing/burning cleaner to do so......The main poster is NOT talking about cruise speed or fuel usage at cruising speed.........So while what your saying is correct, it is off topic and has nothing to do with what the poster is asking which is "will detuning my 12v92's make them more efficient at low rpm's". The answer is yes detuning will save a good amount of fuel at displacement speed and depending on the boats usage it makes a heck of a lot of sense for 5% of owners.

Take the boat you are giving the example on. If the boat does 9.5 knots at 1000 rpm's with 28x30" props and the 750hp engines, with 26x26" props and the 550 hp engines at 1000 rpms, it will probably still do 9.7knots but using a lot less fuel per hour.......Also the detuned engines will last a heck of a lot longer because they're not being pushed to the max to make 750hp.......

Capt J, at 1000 RPM is the 550hp motor producing as much HP as the 730hp motor at the same RPM? You say that the detuned motor will last much longer which i agree but if you are not pushing the engines to 2300 RPM or even above 2000 RPM then the motors aren't really ever seeing the 730hp and should last just as long no?

I am not questioning you, i am merely asking for my own knowledge.

At 1000 rpms the 550hp motor is producing slightly less HP than the 730 hp motor. BUT, not proportionately less because both engines aren't utilizing the turbo's at this point, or injector size for HP. Both engines should be propped to still achieve 2300rpms at WOT. However, the 550hp engine will last longer when run at 1900 rpms or all of the rpm ranges over the 730hp version because it is making less HP across the entire rpm range and not being pushed as hard throughout the entire HP range. Fuel consumed over it's life and at each rpm point is much less, as well as the engines internals are not being pushed as hard, so it should last considerably longer. It's definately not the right thing to do if cruise speed is a major consideration for the owner.

Do a search for the 3208 CAT thread, and the general consensus on here, is to steer clear of the 425hp version as it's being pushed too hard, but the 375hp version is a good motor.......similar situation.

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.

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.

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.