Detroit 71 vs 92

No, 71's last a long longer than 92's
No, 92's are a little more costly to rebuild (not a whole lot more).

71's are considered a much more durable, lasting engine than the 92 series which was designed for the military to produce as much HP in the lightest weight package at the time. And ease of rebuildability in some aspects (o-rings) to seal the heads. The military had full time mechanics that could rebuild them in the field.

+1 on that

I am not sure where the "throw away engine for the military" comments come from but I doubt if this was the driving design/business case to bore the 71 series to the 92 series. I suspect the business case was driven by the on-highway trucking/transportation market, as this new model became available in 1973/74 right after one of the first major fuel crisis in the states.

It is true that some 92 series models are built for the military today, as well as some 53 and 71 series product, as the US can claim a National Security Deference regarding emission requirements on engines that can not be sold into other markets.

The 71s remain a favorite, as long as they were not the highest hp rating for that model. Walker Airsep technology did wonders for controlling and cleaning up 2-cycle engine blow-by. But it is was Penske (in 1988) who consolidated the many verisons of the DD product (J&T, Covington, S&S just to name a few) and created a single factory model for each engine line and gave the consumer a more of a one stop shopping approach. The 2-cycle product was legislated (righfully so) out of existence by EPA emission levels and Penske/DD did not have any strategy to replace the product line, allowing CAT and Cummins to take over market share in the 400 - 1000 hp range. At one time Hatteras was 100% DD powered............

But the 92 series started the high power density trend we now enjoy in the marine market. You could get an 8V-92 at 735bhp or a 12V92 at 1040 bhp and these made a huge difference on boat performance. These engines led the way past the 30 knot barrier. Remember the 8V-92 powered 55 Ocean or the 12V-92 powered 54 Bertram or 61 Buddy Davis - those were considered "rocket ships' for their times. Cat or Cummins did not have anything to compete at that time, and it took MAN and MTU to raise the bar again, into the 35 to 40 knot barrier. Now, MAN, MTU and CAT powered vessels (typically SF) can break the 42 knot barrier.

Their still is absolutely nothing as soul stirring in the powerboat world as lighting off a 2-cycle DD at o-dark thirty in the marina as you get ready for another day on the fishing grounds. But really, a throw away engine for the military? I don't think so....

Lots of myths abound about these engines, some repeated in this thread. 71's that have been juiced up way beyond the "1 HP per cubic inch" level will blow well before a conservatively set up and run 92. There are no rules of thumb. I have 650 HP 8v92s with almost 3000 hours that spec out beautifully.. compression, oil analysis trend, lack of soot etc etc. I run them nice and easy and take care of them. I have seen some with 4400 hours same way. Just ignore the old wives tales and have the engines under consideration thoroughly surveyed by someone who specializes in that.

All these big engines can be a nightmare. Just takes the right criminal technician to take you for a ride when promising to fix it. You can say goodbye to $50k-$100k with little to know recourse.

I have the 12V71TI engines ... they are fine (crooks aside who took me for a ride on one of them) I have told the long story already so I won't bother telling it again.

There is no way I would call these engines bulletproof ... fuel can get into the crankcase, radiator fluid can get into the crankcase and seawater can get into the crankcase. The radiator fluid into my crankcase on my fresh $65,000 overhaul is what took my "bulletproof" 12V71TI down.

The right mechanic and some personal knowledge/experience is far greater factor with regard to your engine experience than whether you end up with a 71 or 92 series.

All I'm saying is that there is a lot going on. I'm in no position to judge the design of these mammoth 4,500 lb monsters. The sheer size and weight of them is one real problem for many mechanics. Ever try to wrestle a 300 lb cylinder head out of an engine room without damaging anything on the way out? It's not really for skinny guys ... not really for fat guys ... gotta be big and tough but not too big to get in there. When I was looking for somebody to overhaul the engine (the third mechanic) ... had 2 turn me down because they didn't want to deal with the "basket case" engine. That included Johnson & Towers! A third mechanic was getting over an injury and didn't want to deal with the huge parts in and out of the engine room. Ultimately, John Carey of Careys Diesel overhauled the engine ... did it in 28 days ... made it seem easy. Installed the completed engine through the salon window without even taking the boat out of the water. Just took the right guy. John Carey of Carey's Diesel in Leipsic DE is a real gentleman.

Not sure what facts you have on this matter, but I disagree on your statement. Military time between overhauls are significantly less than any other markets (they can run as low as 100 hours depending on the application)and even today, the military has requirements for their engines to run on JP5 (similar to Diesel fuel No. 1), which is no problem for a 92 series engine. The 92 series liner design came straight from the 53 series approach.

There is a big enough population of 92 series that for every story you hear about one letting go early, you'll hear that many more which have a tremendous amount of hours. Not every application was picture perfect from the boatbuilders side, so there are a lot of variables, especially sufficient air intake/air exhaust design.

We had a pair of 8V-71's pulled out of a Pacifica 44 and replaced with 550bhp 8V-92's and consistently (twice) saw 3500 - 4500 hours between overhauls.

They couldn't make a DD 2-cycle EPA Tier 1 compliant no matter how much R&D money you had, it was a dead end street no matter what. It did not come down to Roger not wanting to do it, he couldn't. Aftertreatment, including SCR and PM Filter technology, was not available at that time and would have only added $$$ and taken away valuable ER space.

As with any diesel engine, even today, if you want the longest life between overhauls, do not get the highest power rating for that model. But if you have that need for speed............

Hi,

Where do you see that the engines can get close to Tier 3?

If this were the case surely they would have been available as Tier 2 engines until recently.

All of the JT 12v92 boats I've run have had to have majors every 1000 hours. I ran a 1994 54' Hatt DDEC boat that had majors at 952 hours by the dealer. The port engine grenaded on survey/seatrial at 1938 hours. I ran a 1991 58' striker that by 2004, had had both engines majored 8 times. Yeah there are exceptions, I ran a 1987 63' Ocean SF that the owner owned from 1988 to 2009. The origionals went 5670 hours towing a 35' Marlago most of the time and over propped. The vast majority of 12v92's were marinized by JT. It's very common for a 12v71 TI's to go 5-6000 hours

The 8v92's seem to last a little longer and they seem to go 1800-2000 hours between majors typically in SF. Sure there are exceptions, but this was what 8 out of 10 of them saw, hours wise.....It's very common for 8v71's TI's to go 4-5000 hours

I was told by a FDDA mechanic that the JT 6v92's had to be majored every 200 hours. It's very common for 6v71's to go 10,000 hours......

Johnson Towers tended to put larger injectors and turbo's on the 92's compared to everyone else (Covington, S+S, etc.) and they tended to last less. 92's have a big problem with injectors being too big and washing the liners out if they're run at trolling speeds a lot. 92's also have had many issues with head o-ring failures if you're not religious about changing coolant every 2 years, or if you use the wrong coolant (green) instead of the DD nav-cool or etc.....

In my personal experiences the 92's ran like raped apes the first 200 hours, then they seemed to lose 20% of their power somewhere and even after doing rebuilds they still never saw that HP again and it's very very common to see 92 boats running a few inches less pitch than they came out of the factory with. Not to mention most 92 owners run them at 1850rpms, negating any of the extra HP they did make over running a 72 engine at 1950...... personally I think the 92's last longer if you run them at 2000 rpms, they don't seem to run as rich (fuel-wise) and don't pollute the oil as much, in most boats......

Hi,

I am interested to know where the information supporting the near equivalence to Tier 3 comes from.

If that were the case compliance with Tier 2's lower requirements should have been a shoe in.

Hi,

Thanks for clarifying things.

When I turn up to do a survey or audit these days the techfile and adherence to it is an important matter.

If you come across it I would be curious to see what the numbers actually are.

No need to yell, and look, I like factual information on these boards as much as the next person, but you are wrong. JP5 is a refined kerosene, No.1 is a kerosene based fuel and they have a lot in common. Are they 100% molecularly the same - no, but they have significantly more in common than not.

Fuel Oil

It is well known that you can have a drop in power on JP5 as well as a loss in lubricity, that has not prevented the Navy for making it part of their requirements. To it's credit, the 92's injector system can handle JP5, No.1 or No.2, and various other grades of kerosene, so I really don't understad your point......

That's an oversimplified version of what went goes on in the emissions world. First of all, EPA will set emission level requirements and the engine manufactureres will have to meet them to get a certificate at whatever power rating and rpm they are targeting. The EPA will require 5 points along the power curve to be monitored and tested, not just the highest ouput power. If it was just as simple as putting on a new Turbo, I am sure it would have been a done deal, but it was a lot more complicated than that.

Penske was always the consumate business man, and they tried to make a business case for spending development money to get EPA compliant 2-stroke engines for the marine market. But the potential sales targets would not justify the R&D $$$ to get the 2-cycles compliant. They would have had to have all been converted to DDEC, with new turbo/piston/exhaust/cooling development and it was deemed too expensive to do ascross the entire product line. A side affect would have been an increase in specific fuel consumption to get a cleaner burn, which the constumer would then see as a drop in fuel economy (more fuel burn).

The corporate marine strategy turned into focusing on the new 4-cycle Series 60, and Roger did a short lived joint venture with Perkins to satisfy the lower power ratings. Since it was just a JV, I understand that CAT was able to go through the back door and bought the Perkins side leaving DD with nada, one of the few times Penske has been 'trumped"! At the time, there was really no other viable engine line to "pick-up' and run with.

Nobody has looked back and said "oh shoot, we could have just made the 2-cycles EPA compliant with a new turbo and continue to sell product". The 2-cycles are no where near EPA Tier 3 on their exhaust emissions even as measured at the single highest ouput point on their power curve. You have to understand that the EPA has cut emission output levels in approximately half at every Tier level starting with Tier 1, and there is a big difference between the old Tier 1 and the upcoming Tier 3 requirements. Every maker is adjusting there product line to meet the challenges of emissions.