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Marine AFR

Understanding AFR

Discussion and questions related to the course Understand AFR


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I build engines for boats both N/A & blown. Some of the blown engines have a intercooler and some don't. We typically do not use an intercooler unless we will be over 5psi boost, & we never exceeded 12psi. On a N/A engine I typically target a 13:1 AFR at WOT &13.5 - 14 in the idle and lesser load areas. On a blown engine I have been targeting a 11.3:1 at WOT and the areas below the transition are similar to a N/A engine. I'm just curious what you would be targeting in these conditions? These are all big block chevy engines and most are in speed boats. The thing is that most boaters do not understand the stress that a boat puts on an engine, & they will run WOT for an hour or more. Also when I target an AFR on out engine dyno and everything looks great, as soon as the engine goes in the boat the AFR will be at least a point richer. So from a 13:1 on the dyno to a 12:1 in the boat on the same tune. I have not been able to figure this out yet. What are your thoughts?

I'd suggest that your targets are probably quite smart given the way an engine can be run under full load for extended periods of time in a marine application. You're probably sacrificing a little power but if that provides reliability then I'd be happy with the sacrifice. The only reason I can think of that you're seeing the AFR move that much richer on the water would be because of some change in conditions compared to your dyno. If you're running water into the exhaust system and then having this exit at or below the water line then this could be the explanation as the exhaust back pressure would be higher than what you're seeing on the dyno.

I had the same thought about the wet exhaust so I built wet tails for the dyno. I still have the same results with wet exhaust as I did with dry exhaust. I have talked to other marine engine builders and they say that they have the same results.

Are these intakes open to atmosphere or under cowls?

How does fuel and coolant/intake manifold temp compare in the boat?

I'm sorry that I am so late back to this. We have fixed the issue that we were having. We bought all new Rottler equipment and started doing our own machine work. Now that we have done that our AFRs are the same on the water as they were on the dyno. We believe that this is due to having the proper cylinder wall finish, as we no longer have any blow by or low oil pressure issues either.