SBC guys!
 

The pointless incremental improvements thread got me thinking.

As some of you know I'm planning on doing some upgrades to my motors. For the rest I'll lay it out.

The boat is a 1986 Four Winns Liberator 241. It has stock 260hp SBC's with brand new Alpha's. It spins 21P Mirage's to 4600 RPM @ 59.6 GPS lightly loaded.

The plan:

1. Vortec or World Products cast heads
2. Cam - not sure which one yet
3. Performer RPM Intake
4. Roller rockers
5. Imco Thumper or EMI Thunder exhaust

Do you guys have any idea what kind of horsepower I will be putting out after this stuff. What type of speed gains should I expect.

Thanks, James

[ 01-16-2002: Message edited by: Milord ]

Start with a cam. Comp Cam 280H. I havent done heads yet, but the cam has made a nice difference

Jason

Milord, (see no F this time) ;) I have 409's with the XM262H Comp Cam. According to Desk Top Dyno I get 425 HP. Now thats with ported and polished heads. To get to 409 I have .040 400 bore. I have the same intake and roller rockers. Remember the 262 is not a roller cam, so you dont get that benefit. But it does a nice job for me.

Tom Z has a .030 over 350 bore with a 3.75 crank and that gets him 380 HP dynoed on the same cam, intake and vortech heads. Before you go with the Vortec's check you compression ratio. I had to use the World Torquer II's because my comp ratio was going to be pretty high. With the 76CC chamber I get down to just under 10:1.

There are a lot of variables here, but your list of mods should put you in the 375+ hp category. A lot depends on your cam. With an aggressive cam you could probably push it to 400+, but then you have to start thinking about rebuilding the bottom end so that it can hold up.

Regards
Mike

Nice jr, good to see you are paying attention in class! :D

Thanks for the input guys. I think that's more than enough hp to justify the cost of the mods.

jr are you saying to test my compresion before starting any of the mods. I would like to build motors that I can run on 87 octane or 91 at the most. Fuel is frickin expensive in Canada.

Before I started ripping things apart, not that thats not a good thing, but do a leak down test to see what shape things are in. Because unless you want to and you dont have to mess with the bottom end. So you save the cost of rings, machining and so on.

You said that your Liberator is an 86. How old or many hours are on the engine? You just maybe better off doing a complete rebuild. But that adds up into bucks. I took my one engine apart with the intention of boring it out and replace pistons that were slapping. Then I found all the problems. Then the second engine was worse than the first.

But back to your question. You need to calculate what your compression ratio will be so that you can size the heads, or get the right pistons you get a good compression ratio. If you plan to keep the pistons you have then you need to size the combustion chamber on the head so it doesnt give you too high a C.R.

Jr, the engines are 86's as well I believe. They have around 780 hours on them. They are probably due for a full rebuild though they still run like tops. This will be my 3rd season with the boat so I'm not sure if they were freshened before I bought it. I would assume not.

I hear you on the C.R. thank god for this site. Man it sure saves a lot of headaches and money, especially for someone like me that doesn't know the first thing about engines.

Tell you what. I've never had problems with fixing mechanical things, or working on cars and that sort of thing. But I never had done an engine. But the SBC's are so simple, and have been around so long they are the simplest thing to work one. Mine came out great. And I saved a ton of cash doing them myself.

I hear you. It seems everyone says they are simple and I have a great friend that's a mechanic that has done lots of boats that is more than willing to work for free. I let him use the boat now and again so he really helps me out. I won't have any labour costs, it's just buying the parts. I wish I could find a good distributor out of Canada. The US dollar really hurts to buy! :)

To answer your questions about HP and speed, I used Desktop Dyno to build two engines; a 260 HP and one with your parts, a mild hydraulic cam and 9:1 CR. The second engine came out at 360 HP. More cam and/or a roller cam and the HP predictions of 375-400 are supported. When you look at the two HP curves, the bottom end is the same, but the 360 keeps breathing after 4600, where the 260 falls off. This will affect prop choice.

Then I ran these two engines on my boat speed program. With the 21 Mirage the 360 hits 70 mph @ 5400 RPM; with a 23 it hits 70 @ 4900 RPM, but due to the lack of improvement in the bottom end, this might not be the best prop.

Since I don't have any other data points on your hull other than the one you gave me, I really don't know how efficent it is. So the prediction of getting +10 mph with +100 HP per engine may be a little optimistic. If anybody else with this hull and bigger power can post accurate data about speed, RPM, prop and power, I can adjust the hull efficiency factor for this model and give you a better answer. Then we can find out how fast you will go when you add the blowers! ;)