Why does everyone say if your building a BOAT motor that will not exceed 5100 rpm you need to use a dual plane intake? However, if you look at Mercuiser they use a single plane intake on all their carbed blue motors that do not exceed 5000 rpm. I don't want to buy an intake that I will need to replace later down the road because of intake limitations, plus you can buy a single plane manifold for half the price of a good high rise dual plane. Anyone have any thoughts???

General consensus is the dual plane creates more torque down in the lower revs thereby making your engine work a little better down in the cruise ranges of 3-3500k rpm. by 5k is a wash, and single plane will go to 7k.
It is odd though what goes into factory engines. Mine started life as 375 hp Innovations. They were single plane intake, large square port heads (which arguably aren't suppose to work below 5k rpm) Seemed ok. Followed a Lingenfelter build program and used large oval port head (049) and dual plane intake and made over 500hp with 107% VE.

http://www.youtube.com/watch?v=1_5TnlX-Ams Here is a very informative video on intake manifolds and the process in which they work.

Life is different now.

500+cid's are the norm.

High flow cyl heads are the norm

High lift camshafts are the norm.

That's why you see way more single plane intakes nowadays on 'lower' rpm motors than you used to. Simply, because that's what the engines want.

If your building a motor like the old days, then the single plane vs dual plane laws (based on rpm) from the old days still apply.

So my next question is if I get the single plane intake like the Victor Jr. will I kill my overall performance (fresh 365 mag motors, square port heads, 280 adv. cam, built 750 marine carbs, stock exhaust manifolds with stainless risers)? I like to cruise my boat at 3800-4100 rpm (the boat just seems to run very smooth there) it will pull up to 5100 rpm max. I do plan on buying full headers and better heads down the road and later buying a full race prepped short block such as high compression internals and more of a higher rpm motor. I do not want to sell my boat, I love the older Fountains and I want to just keep increasing the performance that I already have but not ditch everything every time I upgrade.

The single plane wont do a lot for you now. You're probably better off saving them for when you wanna get a more serious set of engines.

Realistically, there isnt much need for a whole new "Race prepped" shortblock. You have a good foundation to build onto. A set of modern cylinder heads with a smaller combustion chamber will get your compression up using your stock bottom end. The right cam, intake, exhaust, you can make around 550-575hp fairly easily, and get your Fountain into the 80mph range.

In that application, I would go with a dual plane. If you get some heads with better ports (higher velocity), then the single plane will would work better. The dual plane will bandaid the weaker port of the stock head and not give up too much if any to the single plane. my 02

Sorry but after designing heads and intakes blocks you naame it on an engine porting heads and intakes for 45 some years cant go along with a lot of whats in your video couldnt watch it all.
When you got into the part about using a dual plane for a SEFI each of those runners is going to flow a different amouunt of air. I have flow tested quite a few intakes bolted to heads. oNE ENDS UP NEEDING TO BE ABLE TO ADJUST EACH CYLINDERS fuel according to runner flow as each will fill the cylinder a different amount. Been there done that years ago but used a system that I could adjust each cylinders fuel.

I've talked to the techs at Edelbrock and they say once you get much over 500ci in a performance application than the single plane is the way to go.

But it's on the internet??? (my brain broke about 15 minutes in)