Turbochargers and intercoolers.

[tjshuler]

So, these are new to me. Previous boat had 6-71N's. The new boat has 8V71-TI's.

I have read that running the boat in displacement mode 8-10 knots is hard on the subject parts and will cause added maintenance. Urban legend?

Also, before shutting down to let the engines idle and allow the turbos to cool down.

What do I need to know to keep them happy and not drain my wallet on a regular basis?

 

[saltshaker]

Most of these MYs are run at hull speed. Not a problem as long as they are at and hold recommend temp. Run them up to 2000-2100rpms for 15-30 mins every few hours or near the end of a long slow run. Idling before shut down isn't usually necessary. The reason for doing this is to cool down the turbos before shit down. Most of us can accomplish this when we are comming in. Unless you are running hard up to your slip, the approach and docking should be more than adequate for cooling things down.

 

[rsmith]

The cool down is an aircraft thing so the oil dosent burn and coke in the bearings. I'm always amazed at how people in these turbo cars zip into a parking lot and shut the car off almost while they're still moving. You don't here a lot of turbo problems there.

 

[rmatt]

Exhaust driven turbochargers can spin upwards of 30,000 rpm. When the throttle is cut back you reduce the oil pressure to the turbo bearings. Best to anticipate the slowing procedure to keep the oil pressure up and let the turbo slow down.

 

[saltshaker]

The cool down is an aircraft thing so the oil dosent burn and coke in the bearings. I'm always amazed at how people in these turbo cars zip into a parking lot and shut the car off almost while they're still moving. You don't here a lot of turbo problems there.

That's the main reason why most manufacturers of turbo charged automotive engines recommended synthetic oil. Earlier Porsche 911 turbos had this problem and required racing oil. Same with the early Saab 900 and Audi A4 turbos. My Audi dealer said their service department kept busy replacing bad turbos that were coked up.

 

[workingonadream]

agreed with all of the above a little wide open throttle is good every so often. I cruise my 8v71s about 1800 rpms, she seems comfortable there. you will figure out what her comfort level is, so you will know where to be. as for cooling down this depends on what saltshaker said. if you have a short trip between your slip and coming off the throttles, then let it run for a few minutes and if you are at idle for sometime coming in, then I wouldn't worry about it, it all depends on your destination. good luck, those are good engines

 

[Steve Mannshardt]

So, I just replaced all risers and had the turbos and after coolers rebuilt on 8V-71s x 2.
The biggest mess were the water cooled elbows. Surfaces were coated in thick soot/ fuel soaked gum.
Clearly the result of low RPM running. I agree that 1800 is a good load range for these engines.

 

[SKYCHENEY]

There is no reason that you can't run at hull speed 12-1300rpm but you have to run them up and clean them out by getting them up to temperature at least once per day and more often if possible on a long run.

 

[Boatnut]

While we are on the subject of turbos, which is gives the best performance, intercooled, or aftercooled

 

[saltshaker]

While we are on the subject of turbos, which is gives the best performance, intercooled, or aftercooled

Don't know the mechanics or science behind this but the aftercooled versions of the same engine are typically the higher HP versions VS the intercooled ones

 

[rmatt]

Intercooled and aftercooled do basically the same thing. They cool the air going into the intake. Cooler air is more dense therefore the denser the air can handle more fuel thus more HP. Aftercooled engines cool the air after being compressed by the turbo. In the case of turbocharged Detroits the air is compressed by the turbo then cooled by the intercooler then compressed by the blower and into the engine. That is my understanding.

 

[MikeP]

DD Aftercoolers are better at "engine management" than DD intercoolers. Intercoolers cool the intake charge air more than ACs which sounds like a good thing performance-wise but it actually makes the mixture too cold/dense at other than very high throttle settings, causing more smoke/inefficient combustion. ACs heat the air somewhat so at low/mid throttle settings the fuel/air mix burns cleaner.

In theory, the colder air of an IC would make more power at WOT with all things are equal but other settings affecting fueling, boost, etc can make the HP whatever is wanted with either configuration and a cleaner fuel burn for the AC at lower throttle settings.

I have found over the years that even in proper tune, IC DDs tend to smoke a bit at idle even when warm where AC DDs don't.

The downside of ACs is they are much harder to work on since they are inside the engine as opposed to "hung on" externally. I'd rather WORK on ICs but I'd rather HAVE Acs (but I don't)

 

[Boatnut]

Thanks for educating me guys, I really appreciate it.

 

[rmatt]

The downside of ACs is they are much harder to work on since they are inside the engine as opposed to "hung on" externally. I'd rather WORK on ICs but I'd rather HAVE Acs (but I don't) [/QUOTE]

Maybe on Detroits but not so on Cat's. Can be serviced without touching anything else.