Why would I suddenly gain 400RPM???

[skibum]

Ok... For the past 3 years, I have been consistently getting 5000 RPM at full throttle. My GPS is usually reading between 44 and 48 MPH after about a minute or so, but I really haven't held it there long enough to see how fast it would really go. I've never gone FT for longer than 2 minutes because quite frankly, its just too fast for my taste.

Today, I was cruising down the Bay in very flat water. I had a 300lb buddy on board, a couple of coolers, a half tank of fuel, 1/4 holding tank, and a full water tank. I had some tab down, but not much. I decided to firewall it. When I did, I watched the tach climb right on past 5000. It easily hit 5200 and was still climbing. I am sure that 5400 was doable, but I chickened out and backed off. The GPS was reading 52MPH, and she was drinking 30.2 GPH. :wow:

So... After 3 seasons with multiple full throttle runs that always yielded 5000RPM, why in the heck would I suddenly gain 400 RPM? My props have NOT hit bottom this year, so I can't believe that they are bent. I'm thinking that the throttle lever itself may not have been fully opening her up in the past. It felt like I may have gotten an extra 1/4" or so of travel on the lever this time. Can anyone think of any other reason that his might have happened? Fuel mixture getting lean, maybe? Can't see how that could be possible at 30.2 GPH. :huh:

Looks like I'll need to take the props in and have them cupped or something. I need to make up 300- 400 RPM, so I'm thinking the jump from 26p props to 28p props is looking reasonable.

It could be worse, I guess. Too bad I like to cruise at 26-28MPH. Its like I'm not using half of my engine most of the time.

 

[arcticspud]

Temperature? If it is cooler than "normal" the engine gets denser air and could develop more power.

 

[Turtlesboat]

Temp would be my first guess. I run faster in the fall than summer.

 

[EricW]

Overrev alarm didn't go off?

 

[magster65]

What is the rev limiter set at on a 496? It can't be much higher than 5400!

 

[skibum]

It only got to about 5200-5300 before I backed off on the throttle, but it sure looked like it was easily going to hit 5400. It was above 5000 for less than 2-3 seconds. Not sure how long it needs to be above spec before the rev limiter decides to kick in. Apparently, I am less tolerant than it is.
I think I am going to try a set of 28p props in the spring. There are a couple of guys at my marina with them and I think I could borrow a set for a few runs. When I took my 26p props in to have them tuned up last year, the guy at the prop shop took my specs, and mentioned that he thought I should have 28s on there. I'm really beginning to think that WOT wasn't really WOT due to something going on with the throttle. I took a few minutes to look at the throttle lever itself the other day. It sure looks to me like it is travelling further than it used to based on how close to the cup holder the handle gets.

I did get a chance to look at my props and there isn't any obvious damage.

Mike may be right. Engines love cooler temperatures. It was in the mid to upper 70s on the day this happened.

 

[magster65]

I had to look it up.
There's so much 'junk' information online... but this seems to be the case;
The HO 496 has a 2-stage rev limiter. The first stage is a 'soft' limiter; it's set at 5150 rpm... this shuts down cylinders to keep the engine from building any additional rpms. The second 'hard' limiter is set to 5250 rpm. This shuts the engine off momentarily.

I'd prop it up for sure!
That's some great speed... you gotta' be happy with that!
:smt038

 

[EricW]

I just noticed that you have the Mag HO -- I thought you just had the Mag. What a cool package -- a 260 with 425 horses. I think I'd have to try a set of 28p props. WOT revs will prob drop to around 4800, but you'll be flying. With a light load and cool weather you might hit 55. Wow. I guess you should keep the 26s, for a little while at least, in case you want them back when its 100 degrees and you're fully loaded. But, I bet you'll still jump on plane with the 28s.