newbie with ?'s about the 23 savage

[BigLew]

It seems that the performance should be about that of the Scepter. Check the photo section and there is a guy in Scituate, MA that has one and pop him an email or PM. If you have any problems let me know. He can probably be as objective as any and I'd be happy to hunt him down.

I have a Scepter I/O with 260 HP pushing it. I get about 23 Kt. Cruise and top out about 32+. I plan to go to 315 plus. Not so much for speed, but just to "loosen" it up a bit. I cruise it about 3200 rpm and want to turn a higher pitch prop so I can get it up a bit shallower in the rear and take more advantage of the hull design. It just runs great as far as ride, but I believe even at 23' it should be a bit loser up on plan and going.

PS - I burn about 5-6 GPH. Am I running it too conservatively?

[eggsuckindog]

Your correct lew they do like HP to work

[ScottM]

PS - I burn about 5-6 GPH. Am I running it too conservatively?

Lew,
My dad's friend has the same exact setup as you and I believe he runs the same cruise but gets a WOT of around 35 knots. He's on this site under the name "Jack" so you may want to contact him for better info.

[Bushwacker]

. . .PS - I burn about 5-6 GPH. Am I running it too conservatively?

It's EASY to find out how hard you're running it! Just install a vacuum gage and see how much vacuum you're running at your typical cruise rpm and load. If you're at or above 7" Hg at cruise, you're ok. Mercruiser used to recommend 5"Hg minimum as a Max Continuous Power setting.
Less than 5" and you can expect reduced exhaust valve life. The higher your cruise vacuum level is, the cooler the exhaust valves will run.

A vacuum gage is also a very sensitive diagnostic tool that can pick up subtle performance changes that you otherwise might not notice until it gets a lot worse. I'm amazed that more folks with inboards don't use them. All the engineers I worked with at Pratt, who developed new engines for a living, and ran inboards, had installed vacuum gages and used them to monitor engine performance/health.

[BigLew]

That is some great information, thank you. I will get one and add it to the Flow-Scan I will be installing.

[lost2a6]

It's EASY to find out how hard you're running it! Just install a vacuum gage and see how much vacuum you're running at your typical cruise rpm and load. If you're at or above 7" Hg at cruise, you're ok. Mercruiser used to recommend 5"Hg minimum as a Max Continuous Power setting.
Less than 5" and you can expect reduced exhaust valve life. The higher your cruise vacuum level is, the cooler the exhaust valves will run.

A vacuum gage is also a very sensitive diagnostic tool that can pick up subtle performance changes that you otherwise might not notice until it gets a lot worse. I'm amazed that more folks with inboards don't use them. All the engineers I worked with at Pratt, who developed new engines for a living, and ran inboards, had installed vacuum gages and used them to monitor engine performance/health.

Not to hi jack this trend, just curious about vacuum readings on an engine that has some valve overlap. My 502 barley has 7" of vacuum in gear idling. I've never checked it on plane at cruise, however I could only imagine that it would continue to drop from its idle vacuum as I give it throttle. We might have to go on a certain percent of an engine's max vacuum rather than a fixed number such as 7". Every engine model is going to have a different vacuum reading based on cam design, engine displacement, ignition timing, etc.

[Bushwacker]

Steve, the cam you have in that engine is optimized for fairly high rpm, so I would expect vacuum to increase significantly as you rev it up. Check out the numbers Connor Davis got on his EFI 350 in this post Although no-load readings don't mean a whole lot, notice that he got the highest readings in the 2000-2500 rpm range. I suspect the cam in that engine is probably set up for max torque at fairly low rpm, and I would expect to see the highest no-load vacuum at the rpm where the engine makes maximum torque. Manifold vacuum is a direct indicator of pressure in the cylinder when the engine is under load, so the absolute level of vacuum is meaningful on any engine. A "manifold pressure" (vacuum) gage is one of the primary instruments on all piston engine aircraft for that reason. The cam characteristics will simply determine the rpm at which you'll see the the highest reading.

[lost2a6]

Ok, I'm curious, I'll have to check it the next time that I run it.