My Boat should Get 7650 RPM's But I'm only Seeing 7100 riding solo?

[holdnon72]

My 06 Sportster 155HP 4-tec should get 7650 RPM at WOT but I'm only seeing about 7100 RPM but I'm reaching 49MPH at WOT What causes the engine not to reach top RPM? My oil is not overfull cause I allways check it before each outing and its in the middle of the 2 marks when warmed up. Can it just be the Tacometer is not real accurate? Thanks for any info...

[VX1]

Is that GPS 49MPH?

Has the sound or feel of it changed at all?

Have you changed ANYTHING since last time you got 7650?

If not, I'd vote that something's knocked a prop blade out of kilter.

The consensus seems to be that the tach is very accurate. Hell, I've even finally got my speedo accurate.

[holdnon72]

No GPS just the Sportster speedo and I know it's not the most accurate thing. Only thing different is the Mobil 1 10W-40 Synthetic oil. But I was getting the same numbers with the BRP oil also. Thing that sucks is I never paid any attention to the tac the first season so I'm not even sure if I ever hit 7650 RPM's. How about the throttle cable think it might need to be adjusted? Second season and Ive never adjusted it. Other then that boat sounds the same and runs awesome!!! I was just wondering why I don't max out WOT at 7650 RPM's. I'm not complaining cause at WOT she screems!!!! I just wanted to find out what could cause this? What about the wear ring does that have anything to do with it? I checked it but still looks nice...Thanks for any more info

[VX1]

Not very scientific... now I REALLY don't believe you, back when you said you were getting seven miles to the gallon on that thing!

I wouldn't try for 7650 rpms; your engine is probably producing the most horsepower at around 7450, that should be more your target. You've got to remember that in the SCIC version [of the boat], it is only doing about 7900 stock. It takes some mods, especially intake, to get it in its sweet spot at 8100.

Heat is one thing, you're simply going to be down 100 to 150rpms this time of year.

Other than that, you could probably use a pitch adjustment to your prop. I'm still willing to bet that one of the blades is off a significant bit. I pitched my 13/18 this weekend, and one of the blades started out -2mm from the others. I looked at my stock prop though, and all the fins were dead-on where they should be.

You can't worry about it until you GPS it though. If you're really doing 49mph, that's fast for a stock 155, and she's givin' you all she's got.

[TM1]

I generally only see 7400 rpms with a top gps recorded speeds in the 48's and an occasional 49. Your speedo is probably very close, mine is useless, it's either 10 or 15 mph too slow or buried at six o'clock, depending on which setting I put the switch on the back of it.

The "missing" 250 rpm got me to thinking about the impeller though; instead of looking for more power to get to 7650, why not go with less pitch and get there that way? Should also improve acceleration. I'm not sure how well the rev limiter works; I've seen 8000 when cavitating due to seagrass in the pump and I had to back off, the rev limiter function was noticeably absent.

I'd like to see someone experiment with a lower than stock prop pitch (13-23) on one of these boats and see the results, I nominate you

Btw, saw your exhaust drain tube thread. I have a manual I could look at at to find what you are looking for, but I'm not sure what you mean by exhaust drain tube?

[holdnon72]

thanks for your reply. My manual says to remove and clean the exhaust drain tube every 10 hours when operating in salt water. Problem is my manual does not show where the location of the exhaust drain tube. Maybe I missed it and you could take a look in your manual. Thanks for any info...

[TM1]

Well, I see in the owners manual the section you are referencing, but I see no mention of it in the shop manual (which I recommend that you pick one up, the owners manual is not much use), what is strange is that it's listed under the cooling system, which of course is closed and therefore would'nt be affected by salt water anyway. BRP does make mistakes in their printed material though, so it wouldn't surprise me if it were incorrect.

Back to that rpm question; I did see where it lists max hp @ 7300 rpm, so depending on how accurate your tach is, you might be off a few but I would check speed via gps for confirmation; if you get 48+ it's doing just fine.

[holdnon72]

TM1 What year boat is your manual for? I just relized my 4-tec shop manual was writen it 2005. Where do you see 7300rpm? My 05 shop manual says 7650rpm but my boat is a 2006. Maybe the 2006-2007 boats are 7300 and that would make more sense about my numbers. Thanks for all the info..

[TM1]

My shop manual is for the '05 Sportster na & scic and also the C180. In the owners manual it lists the max hp @ 7300 rpm, the rev limiter is set for 7650 rpm, meaning that hp drops off after 7300 rpm. Without mechanical differences in the engines themselves I don't think there would be any difference in the horsepower curve of any year 4tec.

[holdnon72]

Thanks for the info that meens I'm good the then. My next step is to gps it.
Anybody know of a decent GPS that won't set me back a ton of money?