Hostile Takeover

Dennis Moore mentions in his BBchevy marine book that the chevy rectangle port heads 320cc flow a ton of air but are unsuitable at the rpm range for marine motors. (I assume this is a vellocity issue) Can I keep these heads or modify them?

I have a mk IV 454 w/rectangular port heads, forged aluminum pistons, 8.8:1 compression, forged 4 bolt crank and unknown cam and intake manifold. It is currently quoted as 350 hp. I would like to get close to 500 hp and retain the ability to use 87 or 89 octane.

My plan includes Imco or stainless marine manifolds and risers (I have captains call) MSD ignition, distributor and rev. limiter, a new intake mannifold, roller rockers, roller cam mild enough to stop reversion and re-jet the holley carb. Will this do the trick? Where do I stand with the heads?

mcollinstn

Sure, you can keep em. While they were originally intended as a factory 6500 rpm drag head, they aren't all that bad on a boat.

Just do some unshrouding and bowl work. Leave the ports alone. Stick with a roller cam with fast ramps, lift of around .570", favor the exhaust duration by about 7 or 8 degrees, and keep the lobe center bigger than 112 degrees. Knock about .010" off the deck surface and bring the compression up a quarter point. Pujp gas will still be fine.

Aftermarket heads are ALWAYS better, else they couldn't sell em. But your iron rectangles are okay. Don't throw a lot of money at them, and if they need more than a simple freshening, you'll be better off to sell them for a few hundres bucks and apply money towards some value aftermarket heads.

Raylar

Good solid advise from McColliston. As for exhaust that can keep it simple but give you a soild performance look at the Dana Marine Manifolds with their actual 4-tube design. They will give you a hot look in polished and a little more bang for the buck. Your heads will benefit from a longer runner manifold that will increase velocity at midrange rpms and will definity develop more torque that way. Make sure you spend good money with someone like Nickerson to get your Holley really dialed in. 500HP + is not a unrealistic target here with the right work and parts.
Good Luck,
Raylar

Hostile Takeover

Raylar, I heard about the Dana manifolds but could never find the website. Just got the google to cooperate. They look awsome!!! sepperate runners would seem to be a big advantage with any V8 due to exhaust impulses.

Have you guys got any reccomendations on a particular make and model intake manifold that will fit the rectangular port heads.

Crazyhorse

Just because Dennis Moore says something doesn't necessarily mean you need to accept it as the gospel.
If the rectangle-port heads were that bad I'd have to think that Mercury Marine would have come up with another plan.
And as the guys above have mentioned, the aftermarket heads are very reasonably priced after considering how much money you'll spend on modifying the factory heads.

blue thunder

If you are into head snapping low rpm torque, the rectangle ports may not suit you. If you like WOT max speed, I like the rect ports with good runnered exhaust and a hotter cam. I assume you have a 365hp mag engine with the .510 lift stick. So, it you are after head snapping torque at low rpm, keep the cam you have and run an airgap intake. If you are like me and after top speed and could care less about low rpm torque, maybe a nice single plane similar to the hp500 piece with a cam as mentioned in a previous post, in the .570 inch range. I run dual plane performer rpms on my engines now w/ rect port edelbrock rpm performer heads and am seriously considering the switch to single plane. Dual planes are falling off at the low to mid 5k rpm range where single planes are just coming in. You will need to spin the mill up in the 53-5400rpm range to get where you want to go, and with your parts, no problem.

BT

Payton

How good is the stock 454 Mag intake? I was told it is the one that came on the LS7?? Is that right? The Merc 454/420 uses the Weiand Team G, would that be a good upgrade to go with the better cam and exhaust?

KAAMA

iTrader: (1)

Hostile,

On a 454 I believe the stock GM rectangle port head is a bit too large---but they will still work. Some bowl porting and short turn radius work would be benneficial---especially on the exhaust port side. Also, if you retain the heads for your 454 then I would at least re-use the stock dual plane intake manifold. Put a Holley 750cfm carb on, and go with a larger hyd 222*/230* duration cam on 112* lobes. A good after market exhaust system like Stainless Marine, Dana, GIL, etc will really help wake up your engine mods by making it breath better. I would prefer the Stainless Marine exhausts over the IMCO.

If you want to go a little further, JimV can give you an option of installing some of his wedges in the intake runners of the GM rect port heads you have.

Have fun!

Tinkerer

Jim V put wedges in my stock heads and believe me they work.
I am now running two sets of JIM V prepped PRO 1's.

Crazyhorse

Payton, the 365 intake is the same basic intake from the mid-to-late sixties, having been a stock GM item on the 396ci-375hp, 427ci-425hp, 427ci-430hp(L88), 454ci-450hp(LS6), 454ci-465(LS7), as well as some other big-block engines like the crate 502. The marine intakes such as your 365 have a brass water passage insert.
By the way, the "W" on all of these intakes does not stand for Weiand, it is the trademark from the Winters Foundry, which supplied them to GM.