Preuni

My freshly built 383 SBC gives me trouble...lots of money invested but still no breakthrough in performance... heres my story:

Tore down my tired 350 Mag because it was not good for more than 52 GPS on my 22 Pachanga.....honed the block 030 over, new forged pistons, 5.7 rods, cast 383 stroker crank, cr. 9,5:1. Heads stock 76cc, large valves.
EMI Thunder exhaust, CompCam XE 262 H, Merc high rise manifold, Q-Jet with DA powertips, initial 10 degrees, fully dialed in 32 degrees, K&N Airfilter.
Did everything in accordance to Dennis Moores SBC bible.

Was hoping for mid 60s at least, theoretically the engine should put out 350 hp minimum.

BUT-all I get after 15 hours of break in is 58 GPS, spinning my 23 labbed Mirage only 4400 rpms.

Checked compression-perfect, tried different powertips on the carb, readjusted ignition, but to no avail.......

Is it possible that my stock heads are the "bottleneck" in my setup? That they simply wont allow enough airflow to take advantage of all these mods? Could this be a Carb problem?

What speeds are you guys running with your 383s in similar boats? Any help appreciated, I am really out of ideas!

liquid lounge

What's your exhuast like after the manifolds? If I remember right, some of those older Pachangas had a unique, but restrictive exhaust, exiting at the rear corners of the boat. Water lift??...somthing like that.

Preuni

liquid lounge, exhaust is open, threw away these ugly drum-style mufflers.....installed inline baffles at the tips, they are not very restrictive, but their effect in noise restriction isnt too restrictive either...

liquid lounge

How are you getting to the tips? If you are using 2 90Deg elbows
to the tip, and they are tight 90's, this will degrade a high performance motors' output more than most would think. Reason I think this could be a problem is I just went through this very thing on a side exit exhaust, the motor[509] just wouldnt spin past 4700rpm's. Had a custom riser made and straightened my exhaust plumbing, now 5200rpms.

ForrestC

Play with the ignition timing. You should be able to go as high as 36-degrees BTDC without a problem, but don't go any higher than what is necessary to increase performance. If you still need more a little more top end, you may need a little more camshaft. Consider using a Crane H272-2. This cam idles well, especially in a 383, and you don't need long pipes to prevent reversion. Here is a link to the Crane H272-2 camshaft card.


I run 383s in my 27 Magnum and my 18 Donzi.

liquid lounge

Preuni- just re-read your post. I dont think your that far off speed versus HPwise. It would not be unusual for a boat like yours to require 15HP/MPH , that would be 90HP in your case...pretty close to the 350hp you were hoping for[assuming your old mag was the 250hp version] Also, your rpm's are a little low...this is why I thought about the restrictive original exhaust...try dropping down in pitch as I think your mouse would like 5000rpm, if it still wont get there, then you have another problem i.e. exhaust, induction, ignition,etc
Just a side note- I think Dennis Moore makes for some great and valuble reading....the only thing he's trying to sell is his books, but I don't think he is the end all, gospel, King Neptune of marine engine building.

Havasu Hangin

...you might need more flow with the extra cubes.

I went a little bigger than the one he recommends, with a little rumble at idle...you definately know it's there.

With that cam and those heads, depending on carb and timing, you may be making max hp around 4,600-4,800 RPMs. Where do you want to make the power?

I'm not sure what heads came stock on that 350 Mag, but if it were me, I would look into some heads with 170-180cc intakes, and bigger valves, smaller chambers (more compression) along with a little more cam- but that's just me.

My 383 in my old 4,000lb 23' slug spins a 23" 3-blade around 5K RPMs.

TomZ

Trash the heads and look at getting something with a more modern combustion chamber (Vortec is first on the list, World Products if you want to spend more). Also, that cam has a power range of 1300 to 5500 RPM. I'd expect maybe 52-5300 as the max with a 383 or 400 so somthing is not right. Bump that timing up there to 36* and see what happens (run high-test fuel and watch/listen for detonation... very important with those older style heads... Vortecs will allow an easy 10:1 btw). You're holding it back with the timing.

KAAMA

Preuni,

Bruce Wolff (S.W. Michigan) just finished a pair of naturally aspirated 383sbc's for a dude in Chicago that owns a 27' Magnum that I think runs at about 78mph now. He's done several small block chevy's for marine apps. He has several combinations that he builds from mild to wild. You might try giving him a call. Shop # is: 616-422-1090

Cord

What was your prop before you changed the motor? You say that you're spinning 4400 with a 23" lab mirage. My old 24' Concord with a 454 mag would turn a 23" @ 5200, so I'm thinking that you're over proped.

If you do decide to change the heads, look at the new vortec heads. They are a excellent flowing head and the price can't be beat. They're only a couple of hundred from the dealer. Add a performer rpm intake and you'll definetly gain power. Just be sure to recalc the compression ratio.