400 small block?

[jhawk73]

Hello,
I have a 1988 fourwinns 245 vista it has the cobra 5.7/ OMC (350 chevy) in it and am considering swapping the 350 for a 400 small block I already own. What are the ups and downs? If not what can I do to the 350 to get better performance out of it? Does switching from the log type to center dump manifolds help much? I am an adequate auto mechanic but this is my first boat. Any advice or opinions would be great. Thanks

 

[TilliamWe]

Re: 400 small block?

Hello,
I have a 1988... cobra 5.7/ OMC...

There's your real problem right there.

But if you want to spend the time and money, make sure your current 5.7 is the older 2 piece rear main seal style. Cause I know your 400 is. I actually doubt your 5.7 is a two piece rear main seal. If the valve covers have four bolts in the center (not around the perimeter) then you have a 1987-1995 engine. This could make switching flywheels and engine couplers more difficult. It would also make intake manifold issues, as you would have to re-use your 5.7's as the heads on a 1987-1995 engine have two intake bolts per side at an odd angle.

Furthermore, as a car guy, forget almost everything you know about hotrodding a car engine. for a boat you need Torque, and lots of it, down low. MAX RPM at WOT needs to stay at 5000ish. No long duration, long overlap camshaft, as water reversion through the exhaust is an issue. And water cooled marine exhaust manifolds are MUCh more restrictive than anything in a car, so getting a boat motor to breathe is the hard part. Don't over carb either. 650cfm is about all you'll need. Stay away from 750 double pumpers, they are useless. Stay away from components that are not "marine" as well.

 

[ramster]

Re: 400 small block?

What is reason for the swap? 350 getting weak, or just the need for speed? The 400 will make a good boat engine as long as it's built right.

 

[rodbolt]

Re: 400 small block?

and again, the amount of stupid is awesome.

 

[jhawk73]

Re: 400 small block?

thanks for the feed back. Is the flywheel for a boat different than automotive flywheels?

 

[Alpheus]

Re: 400 small block?

Yes. The problem you are going to have is the 400 SBC is externally balanced and the marine flywheels are not. Not saying it couldn't be done but you would have to have a marine flywheel balanced to your crankshaft...

 

[zbnutcase]

Re: 400 small block?

No. But IF you do this swap, you need to use the 400 flywheel (and damper) as the 400 is externally balanced. I always thought the 400 made a better boat anchor than a boat engine.

 

[jhawk73]

Re: 400 small block?

zbnutcase,

I thought in the automotive field 400 small blocks are notorious for making considerable amounts of bottom end torque, more so than the 350? That same enhanced torque does not carry over into marine applications?

 

[TilliamWe]

Re: 400 small block?

zbnutcase,

I thought in the automotive field 400 small blocks are notorious for making considerable amounts of bottom end torque, more so than the 350? That same enhanced torque does not carry over into marine applications?

True, they would make more torque. But so would a 383.

 

[Alpheus]

Re: 400 small block?

Sure does. That big stroke of the 400 has is allot of torque. You can use it its just the flywheel issue you have to deal with...

 

[proshadetree]

Re: 400 small block?

Most boats use straight shift flywheels, automotive style. You will need a marine stater that lines up. If you have a complete 400 then you can swap it in. Brackets should line up. Qjets will almost self adjust if right. If you need to you can make one flow 750 to 790CFM. Check your choke hook up and your shifter hook up. What about throttle? The old 350 intake will fit if modified but I think it will choke the crud out of a 400. I would check everything first, and remember it will take a load of horse power and torque to feel much on the water. That is one thing I will swear to you. 50 horse in a car will wake it up 50 in a IO will go unnoticed.

 

[skargo]

Re: 400 small block?

and again, the amount of stupid is awesome.

Do tell...

 

[jhawk73]

Re: 400 small block?

Most boats use straight shift flywheels, automotive style. You will need a marine stater that lines up. If you have a complete 400 then you can swap it in. Brackets should line up. Qjets will almost self adjust if right. If you need to you can make one flow 750 to 790CFM. Check your choke hook up and your shifter hook up. What about throttle? The old 350 intake will fit if modified but I think it will choke the crud out of a 400. I would check everything first, and remember it will take a load of horse power and torque to feel much on the water. That is one thing I will swear to you. 50 horse in a car will wake it up 50 in a IO will go unnoticed.

VERY INTERESTING you will not notice it plane faster? Or cruise slightly faster or "easier" Thanks for the input proshadetree. If it takes 20 to 30% increase in power and it go unnoticed it is not worth it...

 

[coheej]

Re: 400 small block?

If the 400 flywheel will not work; you could take a 350 flywheel and your 400 flywheel to a machine shop that does balancing and have the 350 flywheel changed to the 400 specs.

 

[Lyle29464]

Re: 400 small block?

The 3/16 of an inch between the cylinders would worry me. The marine engines have a bit more problem with head gaskets than a car.

ok ok I know its more than 3/16 but it does not look like it.

 

[jhawk73]

Re: 400 small block?

I have searched a lot of forums and boats, none with a 400 must be a reason....lol

 

[thunderbirdbill]

Re: 400 small block?

One of the reasons is 400 sbc are getting scarce.Another is excessive rust in the block on raw water cooled that will plug up the steam holes in the heads.I would keep the cooling system self contained.350sbc intake is the same as 400sbc

 

[metalwizard]

Re: 400 small block?

As many have mentioned the 400 is externally balanced. so the flywheel is going to be a problem. I don't know if a 400 chevy standard flywheel will work with the OMC starter and bellhousing. but they are available. they were used in chevy trucks in the mid to late 70's. if the ring gear is the same as OMC I would say it might work.

For a few hundred dollars you can have your 400 internally balanced. It does tend to make them last longer.

the 400 makes more low end torque than a 350 does, mostly because of the longer stroke, and the additional 1/4 bore. BUT the draw back is the 400 has shorter connecting rods. This means the rods are at a steeper angle at full throw. This means you cannot rev a 400 as much as a 350 without causing lots of stress. so make sure your boat is going to run at the RPM that the 400 makes it's power. and consider you are going to loose a few hundred RPM for your max (unless you make lots of internal changes such as longer rods special pistons etc.) also the main bearings are larger diam. this is a plus it slows down bearing speed, which means less wear in the same amount of turns and slower bearing speed.

If you want to make those changes contact me, I have a 400 small block in my chevelle that is pushing 460 HP and has a redline of 7200 RPM. so it can be done.

One last BIG thing to worry about on a 400 the cylinders are Siamesed. this means they are so close together that there is no cool jacket between them instead there are two small holes drilled between the cyl's. these also need to be in the heads you are using and the head gasket. You can drill 350 (or other) heads for the steam holes but make sure whomever does it know what they are doing one hole is straight the other is tilted about 15 deg.

Anyways. answer yes you can do it, BUT make sure you know what you are doing.

Someone mentioned a 383 a 383 is essentially a 400 with 1/4 less bore. you still have all the same problems.. externally balanced etc.
you can go the other way and use the 400 block with a 350 crank and rods. you get a 377. these are really strong runners. its like taking a 350 and boring it 1/4 over sized.. there is no replacement for more displacement. in other words the easiest way to make more power is more cubic inches..

good luck. if you don't want the 400 block I could always use another.

One question does it have 2 or 3 freeze plugs on each side of the block?

 

[coheej]

Re: 400 small block?

and the additional 1/4 bore. BUT the draw back is the 400 has shorter connecting rods. slows down bearing speed

The 400 bore is 1/8" larger. the bearing speed is faster.

 

[Aloysius]

Re: 400 small block?

Metalwizard addresses the REAL problems with a 400...short rods, siamesed cylinders. The good thing about a 383 is you can build a long rod version pretty inexpensively. A longer rod will produce more torque with less sideload on the pistons.

A long rod 383 will produce more power than a 400, using NEW, readily available, and relatively inexpensive parts.