Need power or torque data for Detroit Diesel 8V71 !!

[JimGnitecki]

The fire truck having a higher governor speed seems reasonable to me. Since a fire truck puts on relatively few miles compared to either a bus or a Class 8 semi, the reduced engine life would be inconsequential.

I suspect that for most motorhome owenrs the same logic applies. I personally will be surprised if I average over 5000 miles per year, so a reduced lifetime for the overengineered million mile engine is not an actual "problem".

My engine was rebuilt during the 1990s, and has only 29,000 mile since then. 2500 rpm governor speed would sound just fine to me. How costly is it to make such a change to the governor speed?

Jim Gnitecki

[TexasBorderDude]

Simple adjustment.  Gotta have the tool.  I just paid $281 to Bert at Stewart and Stevenson in El Paso, Texas and watched him make the adjustment to 2514 rpm without load. The hardest part was he had to hand grind 8 shoulders on the socket by hand (without any indexing) to accept an end wrench on the socket barrel.  He couldn't get the socket drive in because of clearance issues with the air ram.  Bert is one of the last of the great 2 stroke wrenches.  Highly recommended.  And.... he really likes to work on them!

dg

[RJ]

Your 5.5 to 6.5 mpg prediction is excellent as it turns out.

It should be - I spent 25 years in the bus industry!

[JimGnitecki]

Jim,
   The  torque peak for the "318" is at 1400 Rpm.  The "318" uses "advanced? injector timing to allow the use of larger injectors  (N65 s) while the typical bus engine uses " standard" injector timing which limits injector choices to the N60 or smaller (going larger makes lots of smoke).  The standard timed engines have a lower torque peak, 1200 RPM and a flatter torque curve which gives better performance with 3 and 4 speed transmissions typically found in buses.   You'd have to pull a valve cover and look for markings on injectors to verify that you do really have a 318.   With a 4 speed manual or automatic you are best off with the standard timed engines.  The 318 setup is best for trucks & a few buses with 10 or more gears.
Regards
Jerry 4107 1120

Jerry: The records I received with the bus are VERY meticulous, as it turns out. They include an invoice from late July 2008 that shows engine  tuning work was done after the previous owner complained of smoke and reduced power. The parts list for repair parts provided at that time includes new fuel filters and new "N65" injectors. The N65 injectors are consistent with the bus dealer's description of the engine as a"318".

There is also other corroborating evidence that the injectors had to have been changed from the original 272 hp ones: without trying anything as foolish as a top speed run in a 34,180 pound bus, I HAVE seen more than 85 mph on the speedo on level highways with no tailwind, when I have failed to watch my speed or have passed slower vehicles. I HAVE checked the speedo for consistency with Interstate mile markers over 5 mile distances, and while there is a 2mph or so error at 73 mph, it's no larger than that.

Caterpillar, in its booklet "Understanding Coach/RV Performance", shows that a 34,000 pound coach (mine weighs 34,180) requires 259 hp NET AT THE WHEELS at just 80 mph, AFTER deducting the power lost to the automatic transmission and axle, and the power to turn the belt-driven fan, the alternator, and any other engine accessories. The same graph shows you need about 305 NET hp at the wheels to hit 85 mph.

That 85 mph repeatedly observed speed is inconsistent with the 272 hp original engine rating, but is fully consistent with a 318 being revved to 2500 rpm or thereabouts (with 3.73 non-overdrive gearing and 478 rev/mile tires), where its power output should hit about 338 hp, despite the falling torque, assuming the injectors don't fall flat on their faces at that rpm.

I'm right now doing some fuel mielage calculaitons that are proving interesting, and I will post those as soon as I have satisfied myself pf their accuracy.

Jim G

[JimGnitecki]

Jim, if you know the injector size we can give you a answer the 318 hp is actually a 302 HP.If you buy a series 71 manual it will have the HP and torque rating for the injector size and timing has graphs and charts     good luck

The Detroit Diesel borchure for the 318 hp 8V71 clarifies the power measurement, and why a "318" sometimes is descirbed as a "302" or "304".

It turns out that the 318 hp rating is obtained using the SAE "STD J607" method which assumes ambient conditions of 60 degrees F, 29.92" barometric pressure, and zero percent humifity.

The 302 or 304 rating is obtained by using a modified (old) version of the SAE "SAE" method, which assumes ambient conditions of 85 degrees F, 29.00" barometric pressure, and zero percent humidity.

Modern cars and trucks are emasured by the factories using a more current version of the "SAE" method ("J1349"), which assumes 77 degrees F, 29.235" barometric pressure, and zero percent humidity.

The significance of the changes in ambient conditions is that:
- Higher temperature reduces engine power (lower air density - fewer oxygen molecules per cubic foot)
- Higher barometric pressure increases engine power (higher air density - more oxygewn molecules per cubic foot)
- Presence of humidity reduces engine power (the water vapor displaces pure oxygen, and has less oxygen than oxygen molecules - water vapor has ONE oxygen atom per molecule, while oxygen has TWO oxygen atoms epr molecule)

Thus, the STD J607 method ends up delievring results that are generaslly higher than the "SAE" methods, with the difference depending upon the actual conditions at the time of the test before "correction" to either of the standards. In general, the STD J607 teasting yields results UP to 4% or rarely even up to 5% higher than the SAE methods, BUT the difference does vary depending upon the actual conditions before correction by the dyno operator.

I have found over years of computer modleing and actual road testing that the STD J607 method is the one used by almost all aftermarket service and dyno shops, because it reports higher numbers which make owners feel better, makes the dyno tuners look better, and provides better bragging rights.

However, since most of the actual experimental tesitng is also done using STD J607 based data, it is also the right one to use for modeling, if you want consistent results.

Also, the Detroit Diesel engines (and many other diesel engines used commercially) are NOT limited in their power output by the amount of air and fuel being ingested by the engine, but rather by the governors. You can get much more than 318 hp out of a 318 if you spin it faster (higher rpm) than the 2100 rpm it is rated at and governed to by the factory. However, you encounter some deterrents:

1. You will reduce the life of the engine
2. You enable an unwise or non-alert vehicle driver to much more easily exceed the heat dissipation capabilites of the engine and its cooling system, and thus risk engine destruction
3. You enable an unwise driver to hit vehicle speeds that are inappropriate for the conditions, against the law, wasteful of fuel, or counter to his or her employer's policies.

All 3 of these are very important to commercial owners, but are relatively far less applicable to motorhome owners. Motrohome owners will put on so few miles compared to buses or Class 8 trucks (5000 to 15,000 miles per year versus 100,000 miles per year) that the reduced life is meaningless. A motorhome owner may willingly assume the repsonsibilities for both heat monitoring and safety, where a comemrcial owenr would not generally trust a driver to be responsible if the driver's income or work hours required depend on his or her road speed.

So, riaisng the governor speed, and consequentialy the power output, of the engine, maight be perfectly acceptable and even desirable or advisable for a motorhome owner (once the vehicle is outside of manufacturer's warranty, which msot of our buses are ). This is in fact probably why fire truck governors are set higher - the annual mileage is low AND the drivers are by definition EXCEPTIONALLY careful, repsonsible, and alert - and the extra power and speed are beneficial to the public.

There are 4 things I can think of to watch out for:

1. the injectors can run out of capacity, depending upon high MUCH you raise the governor speed: A diesel engine has no throttle plate, so it will always get 100% of the air it can ingest, so the power produced is a function of how much fuel you add via the injectors, which are controlled by your right foot. If you have the injectors delivering 100% of their capability at say 2300 rpm, there is no point in trying to spin the engine any faster, as you are unable to add any more of the ingredient needed for more power: more fuel. I have no idea how a mechanic determines this in actual tuning.

2. The engine may not be able to ingest enough air to make good use of the extra fuel: If you are ALREADY getting enough fuel for the air you are able to ingest, or a bit too much (evidenced by black smoke), adding more fuel will just add more black smoke. Again, I don't know how a mechanic determines this otuside of a road test or dyno.

3. Don't go too crazy on rpm: The Detroit Diesels, and I assume also the Cat and Cummins engines, are designed for low rpm operation compared to the engines in a car, and have both long strokes and very heavy reciprocating parts. As you increase the rp, some of the forces beign generated are increasing at the square or cube of the change in rpm! At some point, you go beyond simply reducing engine life to making engine life REALLY short! Again, I have no idea what rpm range that sort of destruciton starts to occur.

4. Watch the engine temperature: As stated earlier, making more power produces more heat, and you need to watch to ensure that the heat is beign efefctively shed by the engine. If it starts to increase in temperature towards its upper approved limit (supposedly about 210 degrees on a Detroit Diesel 8V71), you need to BACK OFF or you will hurt or destroy the engine.

Jim G

[buswarrior]

Jim, you do know that all of this engineering info has been done for you by Detroit, and is available in a number of places on the internet free for the reading?

Just plug in the parts and settings you desire, no more analysis required, as you are attempting to redo what a building full of engineers did over the course of a 60+ year manufacturing run.

But, the fun is in the journey, not the destination?

What did Caterpillar include in their calculations for aerodynamics? Effective frontal area quite different for different models of coaches...

happy coaching!
buswarrior

[JimGnitecki]

Caterpillar assumed 90 sq ft of frontal area and a coefficient of drag = 0.60.


WHERE specifically on  the Internet is this sort of info present? I haven't found much of it yet, and am eager to!

Jim G

[buswarrior]

ok, folks, wade in with your favorite links.

tejas coach is a nice place, where else?

happy coaching!
buswarrior