Dry. It was all about the temperature apparently.

While paging thru some stuff for reference.
HP425=GIL manifold
HP500 = GIL manifold
Early 525sc and 600sc=Gil Manifold
500EFI=headers
525EFI=headers
575 Carb=headers
575SCI=headers
600SCI= headers
700SCI=headers
800SC=headers
850SCI=headers (Big tube)
900sc=headers (Big tube)
1075SCI=headers (big tube)

Back when Mercury was putting the packages together, I would assume they did lots of R&D on the exhaust systems. At no point, above the 470HP, 5200RPM max, HP500, were manifolds utilized. Once you got to the 850sci, big tube headers were used. Which I believe were 2 1/8, or 2.25" primary ID. The only exception I see was the "early", 600sc carb engine and 525sc, which they later switched to headers on.

Theres always talk about cylinder head airflow, camshafts, intake port velocities, intake manifold design, carb CFM size, and so forth. Rarely talk about exhaust.

So, lets say, you are building a new engine package, for a twin engine boat. You're buying some new heads. CNC ported option is on the table. I'll use Dart and AFR for comparision using summits prices (yes, I know they can be found cheaper elsewhere)

AFR 305 as cast = $2,083 per pair
AFR 315 CNC = $2,912 per pair.

About $829.00 to go CNC ported.

Dart 325 Pro 1 =2,106 per pair
Dart 335 CNC =3,376 per pair

about $1,270 to go CNC ported.

Now, lets look at exhaust systems. I'll use diamonds pricing.

Thunder N Lightning cast header, full length runners, pretty much header performance. $3,400

Stainless marine hi torque with long riser $2,600

Imco powerflow $2,500

CMI E Tops $3,329

Lightning all stainless header 3,400

Lightning freshwater header 2200.

So, what I am seeing here, is a cost of upgrading to a better flowing cylinder head, to be close to upgrading to a better flowing exhaust. (800 bucks per engine lets say). The real question is, which one is more HP per dollar, the exhaust upgrade, or the head porting upgrade?

If I was a betting man, on your typical 600+ hp big block, you'd see a larger gain from a better exhaust, than the head porting. For example, a 540ci with 305 afr's and a true header design, vs a 540 with 315cnc heads, and a basic Gil style manifold, I would be betting on the 305 setup. I could be wrong though. Having both is great, but of course, most of us are budget limited.

The best heads/cam dont do well if you stick a 650 carb on top, or , cork the exhaust system. And the best exhaust isn't going to do much, if you have crappy heads/cam. I just feel the exhaust systems are often overlooked, esp considering most dyno their new builds with car headers, then proceed to bolt their boat exhaust on and go, never really seeing the real results.

I forget the diameter on my big tube primaries but if at all possible I will be finding out what the difference is between those and the automotive headers on the 305 heads pretty soon. Edit- I'll also be checking out the 4150/ 4500 Vic Jr/ 454r. Busy day

I think you mean Saxman he got confused and changed his name and agreed, the Saxmobile is insane!!! Just wait till this summer

If I could only afford either CNC ported cylinder head or a proper exhaust, I'd spend the money on the exhAust and then spend some of my time on the heads.

Those heads you mentioned don't need to have the wheel reinvented, so to speak. Some time with a die grinder and some sanding rolls will go along way to achieving good results. No need to go hog wild and screw up the short turn or hog them way large. Just a general clean up and some bowl blending. Maybe have the chambers CNC'd if money is tight and do the ports by hand.

I ported a pair of aluminum GT 40 heads for my small block ford with some tutoring from the local machine shop and lots of reading and they came out nice. Most amateurs ruin the short turn, so be careful.

Really, a CNC head should be hand blended to get the most out of them.

You trying to wake the bears?! Thats a bold statement around here!

I agree for the most part. Most heads today are pretty decent out of the box design wise. Where i think alot are lacking, is the finishing touches. Of course certain heads can wake up big time with some porting, some more than others. I'm sure plenty of guys go in there grinding down the short side radius, when their running a .480 lift cammed low rpm engine, hurting flow where they actually need it. Probably the most overlooked area is the valve job, seat blends, valve shrouding, etc.

I do believe a good cylinder head guy, can tailor a head for the application, just like a cam guy, can tailor the cam for the application.

Hey Joe, you gotta slow down here my calculator is burning up!!! change exhaust, professional cyl. head guy, headers, then I suspect new ign to light it off, prop change, coolers, your costing me to much $$$$$.. I have to stop reading this NOW.

Interesting. Yes, we know hot is better than cold, but would not expect that much of a gain. By any chance where the collectors/risers/tails larger than 4" ?

Sorry, I don't know all the details. I didn't really pursue the conversation any further at the time. Like a lot of blower guys I guess, he was concerned with keeping the engines cool and realized the headers worked better with more heat in them. Based on the engines, power and money involved, I'd assume at least that size if not more.