2 Barrel to 4 Barrel??

[goleft24]

I'm not sure if this has been a topic but here goes-I have a '96 Sea Ray 21.5 Express Cruiser <br /> with a R.W. cooled 260 hp 5.7 Mercruiser (gen 2 drive). I love the boat and it's performance, however it <br /> only has a 2 barrel carb. Is it possible to upgrade to a 4 barrel Rochester Quad without <br /> having to change the intake manifold? Is there an adapter plate or kit that will fit? Also, I use<br /> a 14/17 prop to stay within 4600 rpms-Will the increase in horsepower allow me to move<br /> up in pitch to 14/19?(Just thought I'd throw that one in). Any help would indeed help! Thanx

 

[Buttanic]

Re: 2 Barrel to 4 Barrel??

If you want to change to a 4 barrel change the manifold too. A 4 barrel on a 2 barrel will not give you the full benefit of the 4 barrel. If you have to buy a 4 barrel go with a Holly. <br /><br />Buttanic

 

[bluewater19]

Re: 2 Barrel to 4 Barrel??

I did a 2bbl to 4bbl carb swap on my 4.3 merc. I didn't need to switch props but my top speed and rpm went from 38 mph at 4200 to 44 mph between 4400 and 4500. the swap is the same on your boat. Easy!!! you might want to change the cap and rotor as well as the thermostat at the same time your taking the distributor a nt therm housing off anyway. just take your time and it goes very smooth. I found my setup on Ebay really cheap. It is a holley 4160 600cfm vac second marine carb off a volvo 4.3 with the intake also. you can find alot more for your 350.

 

[Capt. T]

Re: 2 Barrel to 4 Barrel??

Bluewater:<br /><br />I've got the same engine and have been thinking about the same swap. Two questions for you. How did it affect mpg? Do you notice any difference in holeshot?<br /><br />Thanks for the encouragement, autos I know, marine is still a new world to me.

 

[bluewater19]

Re: 2 Barrel to 4 Barrel??

Hole shot is a little better than before with fuel consumption being alot better.Remember WOT should be4400-4800 rpm.I was a little over proped but now I'm in the sweet spot. The 4bbl has smaller primaries than the 2 bbl. I cruise at around 30 mph so my secondaries arn't kicking in. Fuel consumption goes up alot at wot. I need to tweek the spring on the vac second to open a little sooner.I am thinking of getting the moore performance manifolds and risers and going to thru hull as my stock risers are starting to get hot and they are 10 years old.That along with a port and polish on the heads, roller rockers and new comp cams marine cam and timing set should bump the performance up to around 50mph.I noticed that alot of people on this website think that hopping up your boat engine is a bad idea But if all you are doing is making the engine run more efficiently and not raising compression putting in a high lift cam I see no reason why your reliability would suffer.