Join Date: Oct 2009
Location: Gloucester, MA
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Re: Repower confusion (horsepower)
To add the jrd22's comments on running an engine with a blown turbo, it is a bit different in a sailboat than in an OTR vehicle. In an OTR vehicle, you are simply down on power. In a boat, without multiple gears, the load on the engine is determined by the rpm, the condition of the prop and the drag of the hull. The problem is that you can end up being unable to get the rpm up very high so you are not up in the power band meaning that your power is much more reduced than in an application where you have gears and can decouple the relationship between load and rpm.
There are really 2 reasons why you are down on power when you have a blown turbo. One reason is simply that you cannot get enough air into the engine so you will not be burning the fuel at all efficiently. The other reason is that except for the really old turbocharged engines, all engines have some form of fuel limiting feature which is based on airflow. On older mechanical injection diesels, it will be an air fuel control which generally has a line coming from the intake to the injection pump. The line is pressurized by the intake and it operates a diaphragm in the injection pump controlling your maximum fuel delivery. On newer electronic injection engines, it is more common to have a MAF sensor and control your maximum injection based on that. Performance guys tend to disable these features and I have actually seen a diesel run so rich that it shut down.
There is a slight danger with exhaust gas temperatures if you are running with a blown turbo. Most stock engines won't let you run hot enough to have a problem here but some could cause piston and valve damage.
The real problem with running with a blown turbo is that the most common thing to go is the bearings. When these go, the turbo shaft will get a lot of play and it will often start to leak oil into the intake which can lead to a runaway situation. I make a point of grabbing the compressor wheel and checking for radial play whenever I have an intake off.
The good news is that turbos are remarkably durable. Excluding the new variable geometry turbos which can stick, almost all of the blown turbos I have seen have had a ton of hours on them or been in a bombed (better of modified baby) engine.
Sorry for the slightly off topic post but I hope it is helpful.