exhaust modification

[NEW 2 RIVER]

I currently run a 5.0 engine and have my exhaust run through my prop.I have a 2bl carb and want to change the set up to a 4bl carb.I have been told by some people that if I dont change the exhaust to a dual port through transom set up the modification wont give me any gain.Any advice?

[stressedout]

We have done this on 5.0 motors in the past,against our advice but they wouldn't listen and we changed from 2bl to 4 and went thru hull without changing the exhaust manifolds.The only thing they gained was noise and a little better gas cosumption at cruising speeds. No mph gain. A 5.0 has small valve heads that don't flow that well,without changing the cam and heads you are wasting your money.

[BOFH]

Remember the old saw about a chain and it's weakest link? :-) Stressedout has a good point about the heads, but there are some things you can do. I have 4 bolt on mods I would do in this order...
Fix the exhaust! Not a big gain by itself, but it opens the door, and sounds good. :-)
High ratio rockers. It ain't big valve heads, but it is the best you can do without pulling them. Make sure you use a feeler gage to measure spring clearance, or you can get a chance at a whole new engine. :-)
Cold air kit. This is just a box around the carb that sucks cold air from outside the boat. Makes a big difference.
Carb swap. Most expensive, and the least gain on your engine with out all of the above and more.

[Whatboat]

BOFH
Been looking for a cold air kit for twin 350's. Who? What? Where?

[SUPERCREWJOHN]

I would save your money and trade the 5.0 for a 350mag or a 6.2. You will on reap nominal gains doing what you propose so it not really worth it. My rule of thumb have always been 15hp/mph gain provided that the engine produces sufficient torque to push a bigger prop. With the amount of money that you will spend to get maybe 3-4 mph you may as well trade the engine..
In what type of hull does this motor reside?

[BOFH]

Buy? I have always just made them. :-) One method is to go to a junk yard and get an air cleaner from an older car (80's Caddies are good) with the right size base. Then just get some side vents from Boat/US and run a duct. Make sure you have a box exit, however. It is easy to accedently make it a Ram Air box, and mess up your carb settings.

[Cheyenne19]

BOFH
how would a ram air affect your carb settings? And how do you adjust them to correct it?

[mysharona]

I also have a 95' 5.O( Volvo Penta/ Cobra SX drive, 3900lbs loaded) in a 20ft Regal Bow Rider(I'm new here and posted the same ? on the Outdrive forum)
I"m tryin to keep my hole shot for skiing and improve my 53mph top end to 60 +,-. ( we all got dreams, mine are biiger than my wallet)
I've got $1500.00 to spend
Today WOT is 5300rpm with a 3blade 141/2 x 19p stainless
Until I read this forum I Planned on going with 4150 Holley and finding a bigger prop
?? BOHF, what's the deal with new rocker arms, are you getting better breathing the heads?, what #'s do you go with?
??, I don't know if I can live with the noise of thru hull all the time an I don't think I can afford selective exhaust, can I improve the flow and still go out underWater, any help or direction to resources is apopreciated, if I got this in the wrong thread, be easy on me
thanks

[BOFH]

A lot of stuff for this one. :-)
Cold Air Box vs. Ram Air - The problem with a poorluy designed cold air box is that it becomes ram air. This can pressureize the throttle bore, and (dpending on how it is vented) not pressurize the float bowls. Obviously, this hurts fuel flow. A good ram air box encloses the entire carb, but this makes linkage a pain in the @ss. All of this is not a problem on EFI, however, as the mixture is calulated and direct injected.
High ratio rockers - To start, you have to understand that most stock marine engines are undertuned. This is because most boat owners abuse the hell out of them, and manufactures want them to last the warentee period. :-) 8 to 1 compression and small valve heads is the norm. That said, there is a lot of room for improvment, if you are willing to do it right, and maintain it. The steps for rockers (in my opnion) are;
1: Measure lift at the valve, and spring clearance when fully open. (A sample of 3 should be fins, no need for all 8)
2: Do the math! Factor out the ratio of the stock rocker (usually 1.1 to 1) to find lift at the cam, and factor in the new rocker (usually 1.5 to 1) to find new lift at the valve. Subctract the old lift at the valve, and the total better be less than the clearance! Otherwise, you need new valve springs to accomidate the rockers. A leakdown test kit is great for changing springs on an assmebled engine. :-) Once installed, rotate the engine by hand. ANY BINDING IS BAD! It means you may be hitting a piston top! This is highly unlikely with a stock 5.0 engine, as they usually have flat pistons for low compression, however. Depending on valve size, and obscure things like schrouding, you can get a vast improvment!
3: Exhaust volume - I saw the slickest thing the other day. A go-fast boat had the exhaust outlet just under the water level at rest. When on plane, it was out, dry and loud. When in the marina or channel, it was just under the water, and a quiet bruble. Quite slick! Of course, you have to watch out for reversion, and you better have damn high risers, but it sure was slick.

[mysharona]

thanks for the feedback, lots to think about, but after valve work, exhaust and intake, maybe I should save for the bigger motor!!!