getrdunn

iTrader: (7)

The rpm's dictate what cams we use so with that said most on average aren't more than .575-.700. Larger runners are for higher rpm and higher lift cams. But even then if they don't have the felicity they aren't worth road kill in in the road. Back in the 80's several great engine builders failed with marine performance engines until they finally took off their tight jeans but realized bigger wasn't always better.

getrdunn

iTrader: (7)

That was a happy day. Realistic 120 hp plus gain. That's why I take my jet pack with me on the lake. It's not unusual to pick up a couple in just jetting or lose if you don't.

MILD THUNDER

iTrader: (3)

I agree Dan. At least for the average offshore boat. The majority of guys, including myself, got our basic engine knowledge from being into cars before boats. The average cam tech guy is used to camming a car, and so on. Lets take a look at an average Chevelle big block build for a guy looking to go cruising. He maybe has some 3.08 rear gears, an automatic, wants to take the kids for ice cream on weekends, and blow the tires off at a stop light. Having a large, poor low rpm airspeed style of cylinder head, especially when couple to a cam that closes the intake valve very late in a quest for upper rpm power, will be dissapointed in his performance off idle. The engine will be sluggish down low, until it can build up enough piston speed, then it will come on like a monster, not the best for a street car, or even a strip car looking for good ET's.

This small port high velocity head thing is nothing new. Its been going for 50 years plus. The infamous little peanut port truck heads, were a perfect fit for what they were intended to be used on. Your typical 70s/80s' 454 truck engine with peanuts, made peak torque in the 1600RPM range, close to 400ft lbs. 400FT lbs of torque at 1600rpm, was impressive for a low compression smog gas engine. It worked great for pulling a trailer.

My opinion is though, is that intake port runner volume, is just one area of concern when choosing a cylinder head, and not the end all be all. The stock GM head mercury used on most of their engines, had a runner volume of around 325ish CC. When put up against those 360cc Trick flows, 355 Edelbrocks, 369cc Pro Comps, or 335 Darts, the GM head lost EVERYWHERE, including torque output. Just because it had a smaller runner, does't automatically mean its filling the cylinder better at lower rpms. And just because a runner is smaller, doesn't automatically mean it has better airspeed.

Of course when one doesn't have cylinder heads, and needs to purchase some, there's a good reason to choose a cylinder head that is best suited to the size engine/rpm/power level. But, more often than not, guys here ALREADY have cylinder heads. I've seen guys throw away/give away, some really nice cylinder heads, because someone told them their port volume was too big for the cubic inch/rpm band, etc that they will be using.

A good example is the old 454 magnums, 454 420's, 454 440's. They all had a runner volume of 325CC, and sub 9:1 compression, as low as 8.6:1, and never turned more than 5200rpm. The 454 Magnum, has probably pulled more tubers and water skiers, than any other engine out there. The 400hp, 420hp, were great old offshore engines. Were they power houses, heck no, not by todays standards. Were they good running engines, hell yes. Now, if you took that same 454, 420HP Merc, and bolted on a set of Dart 325's, everyone would call you an idiot, and tell you how bad the engine would suck. Ignoring the fact, that while the Dart 325 has the same runner volume of the stock heads, it flows a hell of a lot better, and will show a nice increase in power.

Am I suggesting bigger is better, absolutely not. But I believe if you are cammed correctly, with the right induction setup, being a little too big (within reason), isn't going to make the engine a failure, or unuseable.

endeavour32

iTrader: (4)

Didn't you end up getting Jim's marine head? I wanted to look at a those as well, but I heard they are no longer available.

MILD THUNDER

iTrader: (3)

I agree, the afr 290/300 is a great cylinder head.

Not everyone is buying new heads from scratch though. I mean, certainly if you are buying new heads, by all means, go with the proper runner sized head for the application. We all , well most of us, understand bigger isn't always better. The question I bring up here, is at what point, and at what level, are the results dramatic enough, to make one switch out a set of heads, for smaller ones.

Icdedppl had some 540ci roots blower engines someone else had put together. They had 345 Iron eagles. Textbook would say, that head is a bit too large for a 6000RPM 540. I knew that, he knew that, Mike Tkach knew that, the guy who built the engines knew that. But, they were there, and they got used. They ended up making 900HP on a 7.5:1 540, on pump gas, and pushed a heavy old tank Cigarette very well. Would he have been better off with maybe some 320cc heads, very possibly, but where, how much better, and what would the cost have been. Nobody has those answers, unless, they've done a back to back heads up cylinder head swap, with no other changes made. Most of us, don't have the time or money to do that on the dyno.

MILD THUNDER

iTrader: (3)

my buddys 454's that make 450hp had ice on his holley 750's one day while I was out there helping him tune the boat. Those engines are nothing fancy. Stock heads, cam and intake upgrade and 750 holley. Boat runs 63mph.

getrdunn

iTrader: (7)

Like rookie said the real deal is how the particular head works on the water. Plain in simple. I don't agree with the so called hp and tq numbers achieved at 6,500 so why believe what else the article claims. BTW I personally would only look at flow numbers as reference. The day I throw on an out of the box ready to go set of heads I'm done. There is a lot of power to be made their and would never ever bolt and go. There are some really good heads out there. Some castings are designed for porting in areas others aren't so it all depends on wheat your wanting to achieve at the end of the day.

I bet valakos NA 565 has no more than 310/320 max intake runner and he's well over 900 hp on pump gas. That certainly wasn't achieved with just out of the box anything. He prefers darts cause there's enough casting to port where necessary so he can achieve flow numbers like non others.

getrdunn

iTrader: (7)

You been to jv's lately. Just wondered if you seen him flow those pro filers? I was shocked when he said to purchase them. I've read a little about them but no first hand knowledge of or experience with other than jim advised to get those for my 565's. I thought about just using my pro 310's for the builds and spend the time and money snuffing those up.

getrdunn

iTrader: (7)

Why was it merc ever got away from the old 049 castings anyway. I'd choose those on a 454 any day over the rec ports. Wonder if it was only cause of the lift of the cams to make the extra hp. But even then your not talking much difference. The 049 castings were done at .550 lift max and probably closer to .525. Did they ever use the 049's on a 454/365 mag? I don't remember. I didn't think they did depending on what year the mag was introduced. Wasn't that mid 80's?

MILD THUNDER

iTrader: (3)

The only heads I knew of Merc to use, was the peanut ports on the 330's, and the Rectangles, on the 365+ engines.