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Merc 260 bolt ons?

Old 08-09-2012, 10:41 AM
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Mr. Scarab, the easiest gain I made on my 85 Chris Craft 260 was to replace the standard iron intake with the intake from the Magnum. This one change alone made a huge difference in torque. Before the boat would run up to 4200 r.p.m.s and I could trim it out to 4600 r.p.m.s. After the intake swap it would run right up to 4600 r.p.m.s without trimming the drives out. I next added the Magnum flame arrestors (much taller) but this was a step backwards in performance until I put the Magnum metering rods in the secondaries. The Carb Shop in Ontario California set me up with the correct rods. This was the first steps I made in increasing the performance of my boat. The stock heads were prone to cracking exhaust seats in the mid 80s so that was the main reason I went to World Products SR Torquer heads. While I was into the engines that far I changed cams and added roller rockers. I know the ratings for the Magnum during the 80s was only 10 h.p. over the 260 motor but those had to have been the biggest, strongest h.p. I ever met! My guess is there was closer to a 30-40 h.p. difference.
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Old 08-09-2012, 04:12 PM
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Perfect inexpensive recipe....

Mid-90's 350 from a 1/2 ton truck, check out the bottom end. If it checks good, polish and put back together. The GM engine manual says that the bores should be left alone unless they are out of spec. If in spec, use new rings and put her back together.

Heads... Vortec. Even if the engine you buy has Vortecs on it, sell them and buy new over the counter pieces. You'll be better off not messing with valve jobs with these heads. Then, purchase a set of Z06 LS1 springs (yellow bee-hive) and the associated Comp retainers. If you use a 1.5 rocker, you will not have clearance issues.

Cam... GM LT4 "Hot" cam. Will work with the stock exhaust.

Intake... Edelbrock Performer RPM AirGap

Carb... Holley 750 Double Pumper or equivalent. I'm sure the QJet will work just fine.

Easy 350 HP set up.

When I blew up the 385 in my Stinger, I slapped this one together in a weekend. Fun, powerful, reliable. Ran in the mid-60's and sipped fuel. Estimated around 350 HP (a friend's made 380 with 1.6 rockers) . I think it cost me $900 to put it together.

Tom
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Old 08-09-2012, 04:35 PM
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Back in 2000, my friend and I took a brand new Rinker 232 captiva with a 260 hp 5.7L Vortec and swapped out the intake, 2 bbl carb, and flame arrestor for an Edelbrock Performer(RPM or RPM airgap ... don't remember anymore) intake, 1" port matched 4 hole phenolic spacer, Holley 600 square bore 4150 marine carb, and K&N marine air cleaner/flame arrestor, if I remember all of this correctly. Back to back runs with ~12 hours in between start to finish, we went from 48 mph (gps) to 53 mph (gps). Was able to go from a 21p AL prop to a Merc 23p SS prop. Couldn't spin the 23p prior to the change, and after the change the 21p would just hit the rev limiter. That really "woke up" that engine. Bought all of the parts from a Chevy High Performance shop in Colorado, including the "recommended" grey GM silicone sealer, GM Vortec intake gasket set ($$ ), carb, spacer and intake for right at $450 (seemed really low, but they delivered!). HP increase was estimated to be from 260hp to ~300 - 330 hp. We found out then, that a Rinker wasn't really meant to run at those speeds, but it was a fun 1 day project.

Last edited by bob_t; 08-09-2012 at 04:48 PM.
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Old 08-09-2012, 04:48 PM
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If the motor checks good on a leakdown, you can do what has been suggested and BOLT ON a 177 blower. Run 3 to 4 lbs of boost, and don't run over 4800 rpm, and you can make good power without pulling the motor and going thru it.

Stock ignition, stock heads, stock cam, stock valvesprings. Stock fuel pump, stock exhaust. Bring static timing back to 4 degrees BTDC (from 8), and get somebody you trust to rejet the stock carb.

The blower/intake, the drive snout, pulleys and belts, gaskets, and a new prop are all you HAVE to buy. Anything more than that is optional.

Blowers are the best bolt-on going.

3 lbs of boost will get you to 300hp.

MC
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Old 08-27-2012, 12:59 PM
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Originally Posted by TomZ
Perfect inexpensive recipe....

Mid-90's 350 from a 1/2 ton truck, check out the bottom end. If it checks good, polish and put back together. The GM engine manual says that the bores should be left alone unless they are out of spec. If in spec, use new rings and put her back together.

Heads... Vortec. Even if the engine you buy has Vortecs on it, sell them and buy new over the counter pieces. You'll be better off not messing with valve jobs with these heads. Then, purchase a set of Z06 LS1 springs (yellow bee-hive) and the associated Comp retainers. If you use a 1.5 rocker, you will not have clearance issues.

Cam... GM LT4 "Hot" cam. Will work with the stock exhaust.

Intake... Edelbrock Performer RPM AirGap

Carb... Holley 750 Double Pumper or equivalent. I'm sure the QJet will work just fine.

Easy 350 HP set up.

When I blew up the 385 in my Stinger, I slapped this one together in a weekend. Fun, powerful, reliable. Ran in the mid-60's and sipped fuel. Estimated around 350 HP (a friend's made 380 with 1.6 rockers) . I think it cost me $900 to put it together.

Tom
I just finished a similar build for my ’85 Chris Craft Scorpion 230;

- 4 bolt main Vortec 350 from a 2000 Chevy Express 2500
- Stock bores, just honed
- O.E. type Mahle ring set
- Sold the Vortec heads and bought a set of 64cc RHS Torkers
- LT4 Hot Cam
- 1.6:1 Crane roller rockers
- Holley 650 (vacuum secondaries)
- GM Performance low-rise aluminum intake

After some break-in, no real tuning or much trim adjustment, I was able to turn 53 mph on the GPS at 4200 rpm with a 19” prop. That was with 3 of us onboard and some fuel and equipment.

Last edited by Lennox60; 08-28-2012 at 02:27 PM.
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Old 08-28-2012, 04:33 PM
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Thanks alot guys I am kinda stuck for the time being untill I get it back to running again but that is another thread : (
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