Exhaust Manifold/Headers

[Klondike Jim]

Repowering my airboat from a 350 crate motor to a 383/500hp engine. Want to put new headers on it. I see headers with 1 1/2” and 1 3/4” primary tubes. Which would be better? BTW, I use 3” collector on current setup.

[SWAMPHUNTER45]

The 1 1/2 should meet your need for a 383 spinning 5400-5600 rpm.

In a natural aspirated engine a narrow long tube will aid in building torque as the scavenged exhaust gasses develop a flow and help to enhance your usable power. The use of an oversized or wider short tube will aid in making extreme high RPM horse power.

[White Hawk]

I put some long tube norman clay headers on my 383 and the torque increase mulled my seat mounts off. I switched engines and still have those headers, they have about 6 rides on them. Norman Clay has been building them long enough they are done properly. I had the bullet mufflers welded on to the headers.

[Slidin Gator]

The 1 1/2 should meet your need for a 383 spinning 5400-5600 rpm.

In a natural aspirated engine a narrow long tube will aid in building torque as the scavenged exhaust gasses develop a flow and help to enhance your usable power. The use of an oversized or wider short tube will aid in making extreme high RPM horse power.

Swamp,
You should know better than this, no way a 383 makes 500 Hp at 5400-5600 RPMS. Your recommendation is correct (as is the long tubes from Norman Clay). But finish off the shopping list, dual plane intake and a wide torque band cam shaft to match.

Klondike, This heavy 18 footer ain't going to be a race boat. Limit the speed to 5,500 RPM and tune to match if you want this setup to be reliable. 450 Hp would be more in line with expectations, but the boat will respond much better and last vs. pushing it for the last 10%.

[Klondike Jim]

My boat is a fairly normal style for Alaska. When built they came from builder with a 350 Marine @ about 350 hp. I have been running the crate motor currently in the boat for almost 8 years. I can normally run 24mph @ 4000 rpm. Motor still running good, just can’t haul enough people. I am not after speed, I am 62 and run a river with a lot of large rocks that will cut through aluminum like butter, so speed is not good. And to get to that river I have to run fast deep water. I was just trying to find out biggest difference between 1 1/2 & 1 3/4 inch short tubes dumping into 3” collector.

[Slidin Gator]

Klondike,

Understood, that is the basis of my recommendations. If you were to setup for true 500 Hp I'm pretty sure you would need to turn a minimum 6,000 RPM. You can gear and prop for that, but will find your cruise RPM just jumped to 4,500 with a lot of time in the 5,000 range. You would need a bigger fuel tank.

For your purposes you do want to cruise below 4000 with max power around 5,500. What I am suggesting is that you want a good torque band that starts around 3,500 range and runs out to max power at 5,500. That setup is a different cam, intake and header setup vs. the true 500 Hp 383 race motor.

The smaller diameter, long tube headers are a good start, but you need to head in the same direction with the engine build.

[Klondike Jim]

Slidin Gator, The engine is a MAS Performance 385/500 tuned to run pump gas (87 Octane). Engine is in build stage right now so don’t know numbers yet. Running pump gas it will probably max out at 470 hp, I have no plans on running high enough rpm to hit that hp. I hope I can run mid 3000’s for cruising to the cabin.

[bkmail]

It will Jim.
Bk


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