Best RPM for 5.7

[marshbob]

I’m new here and I’m sure someone has asked this before but I haven’t had much luck searching for this question. Any suggestions on searching would be appreciated.

I have a 1994 330 with twin 5.7 engines with v drive. This is a new boat for me. I’m wondering what RPM range I should run at for best fuel economy and where the top limit might be. I have been running about 3200 RPM averaging about 20 knots.

 

[Spot]

That is my sweet spot also

 

[CV-23]

I think the 4bbl opens at 3400rpms on my 1988 5.7L in my Carlson, Mike.

 

[JohnEGraham]

3,000 rpm get me 22 to 24 MPH 98-5.7 EFI

 

[Go Aweigh2452]

Take a run out and have a buddy with you. Take your Spark arrestor off the carbs and get her up on plane. Bring the rpm's up until your buddy sees the secondaries open up. Note that rpm on each engine. Should be very close on both engines. Now take the rpm's down until the secondaries show no gas flow. That will be your best RPM... You should be on plane. Mark that ROM on each gauge and you should be good to go.

You could install a floscan and verify the amount of fuel you are running for the best planing speed but it should be spot on the above approach.

With my old 5.7 boat, My RPM for staying on plane and not having the secondaries on was 3250.

 

[dpmulvey]

To add to Go Aweigh2452, make sure you are reaching the MFG recommended WOT. If not you will labor the engine and waste fuel. If the WOT RPM is above the MFG recommendations by more than say 200 RPMs you are wasting fuel and running the engine faster than necessary thereby shortening the life.

 

[rcknecht]

I used to run at 3200 on my 96 300 with 5.7s... That was my sweet spot...

 

[sprink56]

With the Marine Cam grind that most of our 5.7 engines use, 3200 RPM is right at the peak of the torque curve. A maximum WOT RPM should be near 4800~5000 RPM. Any more than that is just spinning the engine faster making no additional or even less power.

 

[marshbob]

Thanks for the help I’ve targeting 3200 RPM for cruising and that sounds about where I should be. I think I have a flow scan in my future purchase but that’s for another thread.

 

[Phlorida]

Thanks for the help I’ve targeting 3200 RPM for cruising and that sounds about where I should be. I think I have a flow scan in my future purchase but that’s for another thread.

Based on this thread I question whether that is necessary. Shouldn't knowing when the secondaries open be good enough for optimizing the cruising range?

 

[Gofirstclass]

I had a 96 330 with 5.7's that I put a Floscan on. The good news in putting one on is you know how much gas you're using. But that's also the bad news.

I found at idle speed I was getting right at 1.1mpg. Above that, when the boat was between displacement speed and planing speed, the fuel economy dropped dramatically, as one would expect.

Once I got on plane the boat got ~1.0-1.1mpg all the way up to about 4000 rpm's where the secondaries start to open. At that point, up to WOT around 4500, the economy continued to drop off.

Your most efficient cruising speed will depend on your load of fuel, water, passengers, gear, etc. If running with 1/4 fuel, 1/4 water and just two of us on board 3200 was the sweet spot. When I was full of fuel and water and had a couple of extra people on board I needed to cruise up around 3600 to stay comfortably on plane, but economy was still around 1.0mpg.

Play around with your trim tabs once you get on plane and you'll be able to hear the rpm's pick up a bit and the engine sound smoother when it's planed out correctly. Also, listen to the sound of the engines to make sure they're synchronized. Engines that aren't synched will cause you extra fuel consumption. When they're synched, you won't hear any harmonic distortion between the sounds of the engines. Pay more attention to that than the synch gauge or the throttle positions.

GFC

 

[JUST PULLIN MY DINGHY]

I’m new here and I’m sure someone has asked this before but I haven’t had much luck searching for this question. Any suggestions on searching would be appreciated.

I have a 1994 330 with twin 5.7 engines with v drive. This is a new boat for me. I’m wondering what RPM range I should run at for best fuel economy and where the top limit might be. I have been running about 3200 RPM averaging about 20 knots.

With usaully 4 on board plus the lil fur ball 3200 keeps me on plane and has been working great sometimes running with current i can run 3000 my wot i believe is 4400.