square port intake on oval port bbc heads

[3584sst]

has anyone done this with any luck or power advantage.a 454r intake on oval port bbc. the motor dynoed at 430hp@5200 505ftlbs@3600 with a oval performer rpm but doesnt want to pull past about 4400rpm. boat is a 226 baja.prop is a 25 mirage plus.tried smaller prop with no luck.can anyone steer me right?

[Floored]

you don't want to do this. if it isn't pulling it isn't the intake. is the carb opening all the way? check the tune up, does it feel like the engine is laying down? what type of carb do you have?

[DansBlown73Nordic]

Yes you can run that set-up. Oval with square will work just fine. Im sure you have other problems.

[UtlGoa]

I have a friend who runs that set up and his CP. Boat goes well over 100mph.

[502procharger]

make sure you check you timing. i had it way down and it ran perfect except it had nothing past 4400rpm's i put it back to the right place and ran hard all the way up to 5200. just a thought

[502procharger]

option #2 is i have a set of 500hp heads rec port on e-bay right now cheep. Shameless spam. wink wink
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2421281978&ssPageName=ADME:B:LC:MT:1

[3584sst]

timing is set at 32deg total i think this should be good.also have tried 2 carbs 825 demon and 750 holley.the demon ran a little better.

[Racing Ray]

3584sst
The prop does sound too big for the engine and boat. If you don't mind maybe you can tell us more about the other prop/props you have tried.

[wsm9808]

My 22ft Scarab with stock 300hp 7.4 turns a 25p prop to 4300rpm and 64 mph. With 430hp you should be able to 5000 or more rpm with your 25p.
With a 23p prop mine turns 4700rpm, Did you get an rpm increase when you ran a smaller prop?
What year is boat? what ignition? Same ignition on dyno as using in boat? what compression? what exhaust, did you use the same exhaust on the dyno that you use in the boat? Open exhaust, or throught the prop? What size cam?
Something has changed from when it was dynoed to now. For example, if you have 10:1 or higher compression running pump gas and a newer mercruiser ignition, the engine would probubly ping a little when loaded. This would make the built in knock sensor retard your timing to stop the knock, and at the same time kill a lot of your power.

[gnarley]

3584sst:
the motor dynoed at 430hp@5200 505ftlbs@3600 with a oval performer rpm OK so what else is different from the dyno to the boat? What exhaust was run on the dyno? Are you using the same spark arrestor on the boat as was used on the dyno & is it clean now? Don't forget that your propshaft HP will be 25-50 or so HP less than the dyno also.

[Snorider]

25p mirage seems like a lot for that motor.
We have a '97 baja 24', with a beefed up 454mag, about 485hp, and it will turn a 24p 4blade around 5000 (merc. bravoI brand prop);
waiting for it to break in more before the 23p 3-blade goes back on, a stock 454mag will turn a 23p mirage+ to 5000-5200rpm.

[1FastHallett]

i have a bbc in my jet boat, with oval heads and a victor junior oval intake. i cant compare to a prop but i can say that with an 850 demon carb, 38deg of timing i turn an A impeller to 6,500 rpm. the biggest problem you will run into with using a rectangle port intake is turbulant air flow, but you have dyno numbers to prove it works.make sure your secondaries are opening up all the way and the power valve is working properly. you may want to try more timing also if your set up can handle it.