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400 SBC building advice.

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400 SBC building advice.

Sandmanute Sandmanute
New User | Posts: 10 | Joined: 11/08
Posted: 05/07/09
01:30 AM

I am building a 400sbc for my ski boat to replace a tired 383. I want it to be as budget as possible but still with reasonable performance. So I was going through all accumulated/left over parts to see what I could build.

The boat does most of it's running between 2000 and 4000rpm, although it sometimes gets some flatout blasts to around 6500rpm.

I want to keep it running on premium unleaded, so CR will have to be 10.5-1 or less.

I've got 2 sets of heads with similar portwork, flow around 500HP. One is open chamber and the other is closed chamber.

I was thinking of milling down a set of dished 400 forged pistons to make them back into flattops and using the open chamber heads and a 5.7"rod.
The main disadvantage is that the top ring is getting close to the top of the piston. 110".
Is that too close for reliability?

Or would it be better to run the dished piston with the small chamber heads and a std length 400 rod?

Also with similar flowing heads, is it better to run the small chambers and dished pistons or the flat top and open chamber?

The rest of the combo is Bowtie Victor Jnr with 780 Holley, small solid roller cam, 246/254@.050" and .580"lift, large port set of water cooled exhaust manifolds.

Goal is to make 450+HP, run on pump fuel, be reliable, still be strong enough to spin to 7000rpm if it makes enough HP.  

Pontiac4 Pontiac4
User | Posts: 105 | Joined: 01/08
Posted: 05/07/09
12:56 PM

Heads do not flow HP they flow cc or CFM.
Milling down a dish top piston is a bad Idea.
Yes it would be better to run a dish piston with small chamber head and std length rod.
What is the head casting numbers.
Turning a 400 to 7000RPM is asking alot.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

bdvdw bdvdw
New User | Posts: 34 | Joined: 12/08
Posted: 05/07/09
07:35 PM

i heard closed chamber heads are bad for the flame front.

doesn't a boat use lake water for coolant?

that cam is MASSIVE!!!
narrow power band. hope your not using factory chevy heads, cause if you are, it will be a boat anchor.

be carefull of your valve to piston clearance, make sure  you measure it.
10.5:1 compression with that big of a cam is pretty low, i'd use 11:1 minimume.
check dynamic compression ratio calculator. 8.5:1 dynamic compession is factory safty standard.  i've been seeing alot of 9.3:1 dynamic compression ratio, on pump gas. but with aluminum heads, deburred combustion chambers, and a tight quench, fast burn style.  

Sandmanute Sandmanute
New User | Posts: 10 | Joined: 11/08
Posted: 05/08/09
03:10 AM

Pontiac4, Is it just because of how close the top ring is going to end up that is the reason you reckomend not machining a dished piston, or are there other issues.
I was hoping I could run the longer rod to help at higher revs. Also the CR will only be around 9.3-1. As I said it will only see an occasional blast at high revs and it would only be for a short period, not for an endurance race. I've run my 383 to 7000 rpm heaps when it was in a car. The rev limiter is set at 7000 and I just sit it on the limiter when I'm doing my burnout.

BDVDW. yeh the water for cooling comes from the dam. The cam isn't too radical. In fact it seems a lot tamer than an F244 crane that I was running in a streeter.
Must be the characterisics of a good roller huh, as they are similar sized cams.

A boat needs a bit different power curve than drag racing. As long as it's got sufficient torque to pull a couple of skiers up without dragging them through the water for ages,not like leaving the line in a drag car, but to get the boat out of the water at high revs takes a fair bit of HP. At about 6000rpm it's working pretty hard, but once it's up, out of the water and sitting on the cav plate and prop it will start winding up and 7000rpm is around the 100mph.
I don't use it, or plan to use it for racing. It's just fun to go for a real fast blast sometimes.
I've got a new scat crank for the 400, plus it's a 4 bolt block with arp studs throughout.  

Pontiac4 Pontiac4
User | Posts: 105 | Joined: 01/08
Posted: 05/08/09
03:27 PM

Yes that is one of the reasons.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

bdvdw bdvdw
New User | Posts: 34 | Joined: 12/08
Posted: 05/08/09
05:44 PM

a low static compression ratio, will pull your torque curve down, alot.
static compression is calculated for the whole cylinder, dynamic compression ratio is how much volume the cylinder has when your intake valve closes.
Cylinder Head Volume 70(cc)  Cylinder Head Vol
(cubic in.) 04.270
Piston Head Volume 5(flat tops)(cc)  Piston Head Vol(cubic in.) 00.305
Gasket Thickness .039(in.)  Swept Volume(cubic in.) 50.847
Gasket Bore 4.165(in.)  T.D.C. Volume(cubic in.) 05.214
Cylinder Bore Diameter 4.155(in.)  Gasket Volume(cubic in.) 00.436
Deck Clearance .015(in.)Note: Neg. nubmer above deck, Pos. number below deck  Deck Volume(cubic in.) 00.203
Stroke 3.75(in.)  STATIC COMPRESSION RATIO 10.752

Rod Length 5.7(in.)   Adjusted Stroke (in.) 02.822  
Intake Closing Point 69(degrees)ABDC @ 0.050 lift plus 15 degrees   DYNAMIC EFFECTIVE COMPRESSION RATIO 08.339  

10.5:1 compression = very safe on pump gas. with that late of an intake closing point.

focusing on dynamic compression is part of my next build, i'm shooting for 9.3:1 dynamic, but then again i'm running fuel injection, knocks ensor, and will be looking into water injection. as a safty precaution. i might get stuck with 8.45:1 dynamic, due to flat tops, or dome pistons, obviously the lower is flat tops, but better flame front. hmmm.

just trying to help.  

Sandmanute Sandmanute
New User | Posts: 10 | Joined: 11/08
Posted: 05/08/09
07:17 PM


Pontiac4 Pontiac4
User | Posts: 105 | Joined: 01/08
Posted: 05/09/09
09:43 AM

Any thing else just ask.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

ranchopower ranchopower
New User | Posts: 14 | Joined: 09/08
Posted: 06/03/09
05:51 PM

boat engine will need power like the OP mentioned 2/4000 rpm.
400 chevy will work great with the 5.565 rod and dished piston and closed chamber head. I would have the quench very tight .035 piston to head. shoot for 9.75:1 compression and a cam with 220/230 degrees@.050 and a wide lobe seperation like 112/114.
after all that a good tune and ignition curve will make the boat jump out of the hole and put smiles on everyones faces!

rooster63b rooster63b
New User | Posts: 1 | Joined: 09/09
Posted: 09/02/09
02:40 AM

can someone help me? i am trying to build a sbc 400. would like to get at least 500hp. i am building this for my 1969 firebird.  

gtomustang gtomustang
Enthusiast | Posts: 457 | Joined: 06/09
Posted: 09/02/09
02:07 PM

Marine engines are typically heavy duty b/c the propellar can go from load to no-load, depending upon wave chop lifting the boat out of water.  So, let's assume you're focusing your budget on HD parts first, power second.

Do you want torque to accelerate the skier out of water?  Will he be sitting on a deck?  Do you want HP for high speed, and hope he doesn't fall (the water won't break the same at high speeds)?

Long connecting rods, is turning back into theory.  Some dyno tests show there isn't enough difference to be worth the cost.

I would say, take what worked for the 383...and just plug it into the 400.  Don't mill the pistons, just buy aftermarket slugs designed for the longer rod.  If you want a high compression, small chambers and dished slugs are best, b/c then the combustion isn't occuring inside the head and pushing on the head, but happening inside the piston dish, and pushing on the piston.

Finally...don't forget to drill steam holes in those heads, and if you use the 383 reciprocating parts...I do believe the 400 is externally balanced, while aftermarket 383 reciprocating parts are internally balanced...  
Just because you can't do it...doesn't mean its impossible