400 hp small block repower questions

[Wellcraft260]

I am repowering my 26 Wellcraft Excel stock V/P 5.7 SX-S with a 383 stroker SBC that produces slightly over 400 hp @ 5200 rpm. The engine has a very sturdy bottom end and is designed for big smallblock torque. I have changed the head gaskets to the marine style and replaced the freeze plugs with the proper V/P part #S. All external bolts are ARP stainless and the Alt. is a brand new V/P Part #. I was going to use a 670 holley street avenger carb and a high torque mini starter and would like to know if Im looking for trouble by using these automotive pieces instead of the marine specific gear. Also is this too much power and or rpm [5200] for the SX-S drive? The current gear ratio is 1.60 and it was swinging a 19 pitch aluminum three blade at sea level. My hope is that a 21 or 23 High Five will work out with the added HP or am I better off changing the ratio ? Most of my boating will be on our family vacation down in Kingston ONT. ELV. 300 ft. but I do some boating at 2100 ft. here at home in Alberta. This altitude change was good for 300 rpm on my 20' Campion 305 Alpha with a 21 High Five! Also is there a way to figure out your most efficient cruising speed without a fuel flow meter ? I feel guilty bugging you guys with all these questions but hopefully many will benefit from your expert answers! Thanks in advance.

 

[ZmOz]

Re: 400 hp small block repower questions

Depends on your boat. If the engine is just sitting out there in the open, you should be fine. If it's enclosed, it's a bad idea. Marine carbs are designed so they never spit gas out into an enclosed engine compartment, and marine starters are designed to not make sparks in an enclosed engine compartment. Like I said, if it's open, then it's really no more dangerous than putting the same things in your car.

 

[Wellcraft260]

Re: 400 hp small block repower questions

Thanks Z , the engine is enclosed and I guess the checkbook comes out once again ! Unfortunatly out here on the prairies it seems the markup on marine specific stuff is much higher than down east where there is more competition in the industry.

 

[ZmOz]

Re: 400 hp small block repower questions

Look online, I'm sure you'll get a much better deal on those parts than anywhere local.

 

[Don S]

Re: 400 hp small block repower questions

After you get your carb swapped out, you might want to look at pulling out the ole checkbook and changing the lower to a dual prop setup. The SX will hold up, the DP will put more power to the water.

 

[Wellcraft260]

Re: 400 hp small block repower questions

Duo prop is on my list of wants ! What improvements can I expect with this system? I have heard they create more lift and lower fuel consumption while adding speed. Is this true? Is my upper gearcase as strong as an original DP? What will be the weak point in this drive as it is ? Thanks men.

 

[Don S]

Re: 400 hp small block repower questions

The upper is the same. They ran the 7.4 and early 8.1's on the DPS drive for a long time. <br /><br />

I have heard they create more lift and lower fuel consumption while adding speed. Is this true?

Yep, it's true.

 

[Wellcraft260]

Re: 400 hp small block repower questions

OK , SX-S lower to DP lower, anybody know where I can find a reman. at a resonable cost? Mail order Alphas seem to be readily available on the net but the economics of scale tell me I'll be hard pressed to find a DP for under 3500 usd. If anyone has or knows of some comparison data between single and dual prop systems on the same boat I would love to see it and evaluate the fuel cost savings over time and planing times. I have asked local marine dealers about this and they tell me a DP or BravoIII outperforms the singles but no real #'s to quantify the performance enhancement offered by this expensive upgrade. I understand the pitch of the prop is reletive to the theoretical # of inches the boat will move forward in one propeller rotation with 0 drag and also understand that there must be some slippage due to the friction imposed by hull/water contact while cruising at a steady rate. Accelerating the boat to this steady cruising speed must also introduce a whole new set of dynamics to the party but I guess my question is percentage wise, how much slip is produced for instance with an SX single compared to a DP on a small cruiser such as my new {to me] 26 ft. Wellcraft? I will be recording my speed vs engine and propshaft rpm using my GPS while I still have the SX and post the results here for others. It will be interesting to see the efficiency improvement with a duoprop later [when I find or can afford it!] By the way Don S. your input here along with Bondo and the many others is what makes me love this expensive hobby so much. I have met so many friendly boaters over the years in marinas and on the water that are willing to help out or just share their knowledge and experience such as you and the others here do so faithfully, that I cant imagine ever giving up this pastime just because it is costly. Its guys like you that make that proverbial hole in the water a little bit smaller! I have never seen a forum quite like this, where experts will advise others just for the sake of helping out someone that wants to learn about their new hobby and maybe save a little money at the same time! Cheers everyone, my hats off to you all!

 

[Olds Eddie]

Re: 400 hp small block repower questions

I just repowered with a SBC 400 this spring. It makes lots of torque and dosen't seem to use any more gas than the 350 I swaped out. You probably out to stick with the Volvo outdrive as I understand they are much stronger than the Alpha 1. You must run a marine carb as the bowl vents internally and it has a port to hook the fuel pump vent to whick the automotive type does not. A permanent magnet hi-torq starter would be safe but if you have insurance you want to run a marine starter. I am running a marine starter that came on my original 5.0 and it does fine. I am running aprox 8.7:1 compression ratio.

 

[whywhyzed]

Re: 400 hp small block repower questions

with the 400 SBC, doesn't the flywheel have a balancer/dampener... as opposed to a 350 etc where the dampener's just on the front of the crank.<br />I have wondered what happens with marine install of a 400 - do you use the auto flywheel? Does the marine starter match it?

 

[Peter J Fraser]

Re: 400 hp small block repower questions

Yes the 400SBC has an external balance on the flywheel / flexplate.<br />The ring gear is the same size though.

 

[mlslocks]

Re: 400 hp small block repower questions

He is not runninga 400sbc. wellcraft260 has a 383.

 

[Wellcraft260]

Re: 400 hp small block repower questions

Yes I had to change the flywheel to an automotive 400sbc for use with the stroker crank.

 

[Olds Eddie]

Re: 400 hp small block repower questions

My engine builder allowed that there was plenty of meat to drill out of the 350 flywheel to get it balanced. I did have to work the mount for the raw water pump a little to get it to clear the front 'flywheel'.

 

[Wellcraft260]

Re: 400 hp small block repower questions

Hey Olds,what goodies do you have on your 400? How about a before and after comparison. Im still working on my stroker swap and will be dropping it in on wednesday. Im hoping for 50 mph with 400 hp. 1.60-1 ratio and 21 pitch 4 or 5 blade. {5200rpm] The boat weighs 5000 lbs dry so I may be a little optimistic. I would also love to know the performance differance between a single vs dual setup on the same boat?

 

[Olds Eddie]

Re: 400 hp small block repower questions

I am running SRP forged pistons at 8.7:1 with 76cc big valve iron heads. The rods are Manly I-beam 5.7". My builder says that the H-beam rods are too heavy as are 6" rods. The Comp Cams xm270hr roller cam has a nice lope to it. The exhaust flows through stock Mercruiser manifolds with IMCO stainless 6" risers and comes out below the swim platform so I have no problems with reversion or the noise police. My builder gets the big money assembling and running an engine in on the dyno, but he allowed that it will probably make around 400 hp. He didn't recommend any higher C/R as a marine engine is under load at all times and heat could be a problem without aluminum heads. My boat weighs around 4000lb and will do 50mph at 3/4 throttle. I haven't got enough time on it to push it any harder and my Alpha1 outdrive is not rated for this much motor anyway.