350 Mercruiser to 383

[Jeff Fro]

Just picked up a complete running 84? Mercruiser 260 (350ci) with unknown hours. This will be going into my 85 wellcraft 260 Aftcabin. I will be tearing this down and checking it out and replacing gaskets. Looking into a little bit more hp/torque for the 5000# boat. Current thoughys are to go 383 with a 268H comp cam, pair of camel hump 291 heads, through the hull exhaust, high volume oil pump, and MSD ignition.

Flame suit on...how far off am I from getting a dependable, long lasting engine?

 

[Scott Danforth]

Skip the camel humps. Vortec heads, LCQ pistons

MSD ignition wont get you anything over a stock T-bolt ignition....but it will make your wallet lighter

I myself would get a post 88 block from a truck (machined for roller cam) go roller and run the XM270HR

Install a heat exchanger, and run an RPM airgap intake

 

[alldodge]

Agree get a newer truck motor with vortec heads, TB-V ignition

Use flat top pistons, taller valve covers for increased valve lift, and if your going to build some major power, then get larger oil pan and high volume oil pump

 

[Bondo]

Ayuh,.... What Scott said,... Get a roller cam block, 'n Vortec heads, with matchin' pistons to get the quench factor to .040", 'n static compression to 10:1 or slightly under,....

 

[Jeff Fro]

So....350 Vortec heads, LCQ pistons...skip the msd (wallet will be sufficiently lighter after this project anyways)

How much difference in hp/torque between the hyd lifters and the roller lifters for the XM270HR cam?

I'll need to Install a heat exchanger? and run an RPM airgap intake? Better intake flow?

 

[Scott Danforth]

RPM airgap makes a butt load of mid range torque, however not a marine intake. The heat exhchanger allows you to run non marine intakes as well as increase operating temp from 140 to 160-170 which allows the motor to get up in temp to where it will last longer

 

[Scott Danforth]

A new pair of vortec heads is only $500 and they greatly out-perform the old camel humps.

The LCQ or inverted heart shapped dish significantly help quench

 

[kenny nunez]

Also a belt driven sea water pump pulling through the bottom via a swing valve and a sea water filter if you have the room. MSD = My Spark Disappeard.

 

[alldodge]

Also going to need an oil cooler

 

[Jeff Fro]

Looks like I have some more research to do.

1...Belt driven sea water pump pulling through the bottom via a swing valve and a sea water filter?

2...RPM airgap intake and heat exchanger?

3...and oil cooler?

Give me a bit...thanks for everyone's help

 

[alldodge]

3...and oil cooler?

The more power you build, the more heat is generated. There is also an increase in heat due to load. Being a 5K boat the SBC will work harder the a BBC with the same load, and will generate more heat. Suggest installing a temp sender either before (oil filter) cooler or after, which ever is easier to install

Example: I have a 11K boat with a single 530HP motor. This amount of HP in combination with the weight has increased the heat load. Had a 12x1 1/2 cooler and installed a 2x18 cooler and temp gauge. Gauge shows while cruising with larger cooler oil gets above 280 degrees, so smaller cooler was roasting the oil. Now installing a 2 3/4x18 cooler and hope this will keep oil under 200.

 

[Scott Danforth]

I have a 3600# boat and was goung to build a stroked sbc....then i built a BBC because bigger is better

 

[Jeff Fro]

Been doing some more reading and looking. I'm starting to understand these items a bit better.

1...Belt driven sea water pump pulling through the bottom via a swing valve and a sea water filter?

This increases the water flow for better cooling by bypassing the water intake on the lower drive. Just not too sure on the swing valve through the bottom of the hull. I haven't found any pics of this. I would imagine that its setup similar to a jet boat intake.

2...RPM air gap intake and heat ex-changer?

Changing the intake to an automotive intake sounds like a good thing, however, I'm not seeing how the heat exchanger is increasing the cooling. I do see that a heat ex-changer is required as part of the closed cooling system.

3...and oil cooler?

Haven't seen anything on this at all. Would this be an aircooled cooler or a water cooled setup?

 

[kenny nunez]

You also can mount a water pick up tube on the transom. Check with Teague, Glenwood or CP performance , they also have oil cooler kits that are water cooled. If you are in a fresh water area you could set up your engine like the 350 was cooled and save some $ . But you will definitely need the volume of a belt driven pump, the stern drive pumps are barely enough for stock 260 horse power.
My Donzi was was set up by someone who did not know what they were doing and were trying to feed the belt drive pump with the drive pump. It now has a through the bottom pick up and a valve to close off the intake to service the pump.

 

[Jeff Fro]

I'm not too wild about an intentional hole in the bottom of my hull. However, I did like the ones I have seen in a few different posts. Up to now, the largest boat I have had was a 1959 18 foot ski boat with fins...lol.

 

[Scott Danforth]

I'm not too wild about an intentional hole in the bottom of my hull. However, I did like the ones I have seen in a few different posts. Up to now, the largest boat I have had was a 1959 18 foot ski boat with fins...lol.

many boats have thru-hull pickups

they are actually safer than anything bolted on the mercruiser stuff because a true thru-hull pickup has a seacock and room for two hose clamps