Operating RPM for 6-71ti

[LJohnson]

Hello,
This is my first post on this site so I hope I'm not starting with a dumb question. I just bought a 1981 37c w/ 6-71ti rated @ 390 hp. Had my mechanic go through a full tune up. Engines are healthy and have 1700 original hours on them. Was told by mechanic that if I ever want to get 3500 hours out of them I must never exceed 1900 rpms for more than a few minutes. I read elsewhere on this site that 2100 is acceptable cruise. There is a significant speed difference between the 2 rpms, I would prefer running at 2100 @ 22 mph vs 1900 @ 17.5 but would also like to put off $25,000 in rebuilds for as long as possible. If I run at the higher speed, will I really be significantly shortening engine life?

[Nick]

I have a similar set up on the 37. I have the 450hp 671 TI . At 1900 the boat is just not planing right, even with the tabs extended. At 2050 rpm, the cruise speed is about 19-20 kts. I have been told by the Detroit mechanics that 2150-2200 RPM is closer to the right RPM depnding on load and conditions. The turbos are pushing more air and the engine runs well, exhaust gas is cooler. This assumes that your cooling system is in good shape and the water temps are about 175F.

Lets see what others here think!

[jkp1]

I run 2050 - 2100 with 671ti's @ 410 hp. My boat & engines seem to like it there. Pascoe on www.yachtsurvey.com recommends running at 80-90% WOT which puts it between 2000-2250 RPM. (Assuming you can reach rated WOT of 2500)I would not recommend cruising at 1900 if you're running bow high. Push it up a bit so you level out, this will reduce drag and allow the engines to breath a bit easier. Engine temp may also drop some.

[Genesis]

And follow them.

You will see as you advance through the RPM range that EGTs rise, and then drop a couple hundred degrees - then start rising again.

That "knee" is a place you DO NOT want to be running at. You want to be either well-clear of it on the lower RPM side, or in the "trough" on the other side of it.

This is particularly important for Detroits, which pump a lot of extra airflow through them due to their 2-stroke design.

Pyrometers for 6-71s (you only need two probes and gauges, since they're inline engines) shouldn't set you back more than $400 or so, and it'll be the cheapest $400 you'll ever spend.

For the V-series engines with twin turbos its much tougher since you really need one probe for each bank, and twin gauges - which means double the price - but they're still worth it.

If you can't reasonably mount them on the helm then at least put them in the engine room so you can get baseline numbers and check them once in a while.

Pyrometers are arguably one of the most important gauges on any turbodiesel engine, and yet a huge number of people don't have them. They'll detect problems with the engine that will destroy it before almost any other gauge once you know what "normal" is for your engines.

[hATTISFACTION]

Genesis,
your last reply will be very helpful to me in figuring out
problems with my current 671TI's . I have the Pyrometers
but did not know about looking for that trough.
Thanks

[Genesis]

The "knee" happens when the boost comes on. Marine Detroits are typically non-waste-gated turbo systems, which means that they cannot build boost down low, because you can't size the turbo that way - if you do, then it would blow up at higher RPMs.

So the turbo is sized to produce the proper boost at WOT, under maximum load - and not "too much" boost! This means that at the lower end of the operating range it produces basically NO boost - you are operating naturals up to 1600-1700 RPM or so with 6V92s, and as high as 1800-1900 with 6-71s (which rev 200 RPM higher). Detroits LIKE boost, as the extra airflow keeps the engine's piston, exhaust valve and cylinder temperatures under control.

This is not to imply that you should run "in the corner" though!

A very instructive test is to stick some pressure gauges on the airhorns and go for a run. Come up 100 RPM at a time, while you record airbox pressures. You'll find that there is close to zero boost until you cross a threshold RPM, at which point it starts to come on. If you have pyrometers, record those temps. When the boost shows up you'll find that the pyro temperature should drop - even though RPM has increased!

Detroits tend to run significantly lower EGTs than 4-stroke turbodiesels, again due to their extra airflow.

The behavior of the boat also matters here. There's a "transition" point in a planing boat's operation where you do not want to operate. Either be running as a displacement hull or as a planing hull - NOT on the cusp!

Unfortunately a lot of these boats have their hull's "transition" point just at the top of the "no boost" operating RPM region for these engines. If your boat is one of these - and most older Hatties are to some extent - operating there will blow up engines. These older boats are power pigs, as while their hullshapes aren't bad from an efficiency standpoint (mid 80s on are better, as Hargrave redesigned the bottoms), they are HEAVY with a lot of wetted area and are designed for seakeeping ahead of horsepower efficiency. You couple this with the fact that Detroits have a lot of iron in them and they are very heavy, along with being highly stressed at their usual power ratings, and you have a recipe for possible trouble. This is one of the reasons that many people don't get the expected life out of 2-stroke Detroits - they don't understand how to run both the engines and the boats that they are in.

This is also why "nailing" the throttles on a Detroit 2-stroke boat is so destructive - there's no way for the engine to respond to that properly since it cannot build boost until it gets turning. As such you are overloading it severely during that period of time, and we all know what that does to engine life.

[jkp1]

Genesis,

Wouldn't 1 pyrometer sensor & Gage be adequate for a pair of 6-71's? I would think both engines would behave similarly. Even if optimum EGT was slightly different you would still cruise with the engines in sync.

[jim rosenthal]

..the best supplier for pyrometers? I can get these and fit them during the winter layup, I am going to be in the engine room to take out the AirSeps and fit CCVs anyway. Where does the pyrometer probe go in the exhaust manifold?

[Genesis]

You ideally want a pyrometer for each CYLINDER! That's impractical though, so you usually do one per turbo - which means on V engines with twin turbos you can have two dual reading pyros.

You want at least one per engine, because an engine can be malfunctioning and it shows up first on the pyro most of the time.

If you exhaust system doesn't have ports for pyros (most do), they're somewhat of a pain to add. You drill a hole and then weld on a threaded bushing to add them - ideal is before the turbo, but most installations are after the turbo for installation reasons (its difficult to drill cast iron without cracking it, and its also difficult to weld it successfully.)

Isspro is one well-known name, and Gaffrig makes some nice ones too. If you want fancy Hoyt instrument makes some GORGEOUS electronic ones - including ones with memory for highest temperature achieved and other features, along with "dual" models (two display heads in one unit for either two engines or dual bank motors such as twin turbo engines)

Figure $150-200 for a single gauge and probe for the "simple" type, up to $400 for fancy electronic ones, and $500 or so for dual-probe electronic models.

Installation is trivial if you have the ports on the exhaust collectors already. Standard sizes for the probe mountings are 1/4" and 1/8" NPT. Non-fancy ones (e.g. meter movement style) are self-powered and need 12V only for illumination. Electronic ones require 12V and ideally want isolated power since small amounts of noise will influence readings.

One warning - NEVER, NEVER solder the lead wires. These units work on a dissimilar metal junction and soldering wires creates another one - which will throw off readings!

[LJohnson]

Wow, more info than I was expecting and honestly more than I understand. I guess my take away is that I should be OK running to -about 2100 but to really target the "optimum" RPM I should install a pyrometer and plot a temp vs rpm curve. Shoot for the dip somewhere between 2000 & 2250. Is that correct?

[Soffer]

That's pretty much it...put a pyrometer on each engine.

Welcome to the board. Don't be shy about posting - the only dumb question is one that doesn't get asked.