Im looking for some answers hope you guys can help...I am installing a new roller SBC in my boat and am wondering how to dial in this thing. I should be up about 80-100Hp. How do I determine the Max Rpm. The motor is built to handle 6000Rpm's but I figure maximum hp will be around 5200Rpm's. The cam specs says the rpm range is 1500-5200. I am running Stainless Marine exhuast thru-transom and upgraded heads that flow alot better. Since the motor will hold up till 6000 should I just keep running higher Rpm's on test runs till my mph drops? Or am I thinking to simple. With the stock motor and a semi-cleaver (attwood balistic) 17" prop I am turning 4900 rpm's. I have been told for my application the ballistic is not a good choice (Heavy deep Vee and consertive X-dimension)I have a 21" Mirage I figure will get me in the ballpark. Is there any ballpark figure to use on horsepower increase and prop pitch increase? Any ideas would be helpful.

Thanks
"Bad-Habit"
86 F3LS

What is your current speed, weight, and power? Do you know the drive ratio? Along with RPM and pitch this will baseline the boat. What is the expected horsepower peak of the new engine? My guess is that whatever you did to the engine to get another 100 HP also raised the RPM at which the peak HP occurs. That means you don't need to increase pitch as much as you'd think. If you are really spinning a 17" cleaver at 4900 RPM now, I'll bet you won't need more than a 19" Mirage at 5100 RPM. Post your data and I'll finetune these numbers.

Current Specseep V with a very conservitive X-dimension. Wt is 4000lbs (loaded)Hp is about 270 (Stock 260hp 5.7 with complete fresh tuneup also exhaust was changed to straight thru-hull using Stainless Marine manifolds and risers. Also K+N Airfilter). Propeller is a Attwood Ballistic which is a semi cleaver design it is a 17". If I understand this design lowers your Rpm slightly over a traditional design. I am not sure of the outdrive ratio though. (1:3 or 1:5) Im running 4950 Rpms and 50.2 mph (GPS) New power will be real close to 360hp. I have a 21" Mirage (not Mirage Plus) which seams to be the best design for my boat after checking with similar setups on the same hull. Though this prop has never been used on my boat yet. Im not sure of the expected peak Rpm range. Guessing around 52-5400. It is a Jasper Marine performance short block. with stock heads,8:75 compresion and milder camshaft they rate it at 325hp at 5400. Im using L31Vortech heads (20-30hp increase) upping the compression from 8:75 to 9:25 and going from flat tappet hydraulic with 224 degrees intake and exhaust duration with .450 I and .460 E lift on a 114 Lobe center to a hydraulic roller with 218I and 224E duration with lift being .502 I and .509 E all on a 112 degree centerline. It is a Comp. Cams marine grind and the cams operating range is 15-5200Rpms. I figure the heads might increase the range slightly. According to several people who have used this setup I should expect 360-380hp.

What is the outdrive? I'm going to guess and say an Alpha1 Gen1. It probably has 1.5 gears in it. Merc put 1.32 gears in the Alpha when they put it behind the big block, until the Bravo came out. I had an Alpha big block combo in a 22 Formula and never had any trouble.

I would not want to spin an Alpha much above 5000-5200 rpm for very long, unless I carried a spare with me. If you are going to try to run 6000rpm with your Alpha put a drive shower on it and check/change the drive oil a lot!

I think the 325 hp with the Alpha will not be a problem, unless you hammer it out of the hole and/or are getting lots of air time. You might look into an Alpha SS drive, it is 1 or 2 inches shorter and will bolt right up to a Alpha Gen. 1 gimbal. I've talked to a couple of people running 450 hp big blocks through them with good succes.

Good luck

mike

F3LS - Based on your data, I would lean toward the 19" Mirage since the new engine will like to rev. The 21" will lower your RPM to 4800 with the new engine. You will go just as fast, but you may be lugging at lower speeds. Borrow a 19" and try both. In fact, I think your boat would run better with the 19" and the old engine. Here's your graph. Good luck!