Help With My Head Dilimea

[rpss4844]

I swapped engines in my boat because of a cracked block. I went with a fresh 4-bolt 350 bored .30 with older style heads. I am unsure of cam size but i know it isnt for a boat and it shows big time!! So my dilema is whether to stick with the older style heads or swap to my previous vortech style heads, or swap to another style head thats even better. im gonna run a comp xtreme marine cam 462'' Intake / .477'' Exhaust 262° Intake / 268° Exhaust advertised duration.

 

other: quadrajet marine carb (unsure of CFMs) Weiand stealth intake or stock intake.

 

please all engine junkies chime in and let me know what ya think. hoping for around 300hp with this setup.

 

from comp cams:

Operating Range:1300-5500 RPMDuration Advertised:262° Intake / 268° ExhaustDuration @ .050'' Lift:218° Intake / 224° ExhaustValve Lift w/1.5 Rockers:.462'' Intake / .477'' ExhaustLobe Separation Angle:112°

Edited June 19, 2009 by rpss4844 (see edit history)

[Jaymz]

boat lenght ? open or closed bow ? you have trip tabs ? whats your prop pitch ?

 

volvo or merc drive ? I'm good with boats, just need some more info

[rpss4844]

sorry its a 20' open bow, no trim tabs but do have trim of coures. heres a curveball for ya it has a Yamaha stern drive and on the previous engine i was running 24 pitch for a heavy load, tubing, knee boarding, etc. 26 pitch for small load and just joy riding or poker runs aka hauling balls across the lake. GPS at 62 mph with 3 persons and 26 pitch prop. since the engine swap i cant even plane out with the 24 with just me onboard. moved down to a 19 and it took forever to plane out with just one person. i am pretty sure that my problem lies with the cam being for a car. it idles fine and revs great on the hose but in the water with a lot of exhaust back pressure it will hardly stay running and is hard to start. i have checked and adjust timing several time and rebuilt carb...neither seem to help.

 

boat orginally came with a 5.0 210hp, i swapped that engine last year to 350 and used the cam that was in the original engine.

 

I was also looking at some comp roller rockers, said they would gain 15-30 hp. think that is a good estimate power gain?

Edited June 19, 2009 by rpss4844 (see edit history)

[Jaymz]

Your problem isn't in your heads, it's that cam

 

for that pitch prop and a 20' boat your going to want a camshaft that produces power off idle up, or your not going to get out of the hole and get over the curve.

 

with a 355, your going to want to stay in the 21-24 pitch prop area IMO. Jump to a 4 blade balanced stainless, you'll be amazed at the difference.

 

You might be having cavitation issues with that yamaha drive as well. Try a extended caviation plate, keeps the surface air away from your prop..

 

FOR REFERENCE: My 19' sidewinder had a 315HP 350. I put a crane cam "truck" camshaft in it 500-5500 and it worked perfect. Jumped out of the hole like a animal with a 4 blade 21 and still ran 58-59 loaded. The word "marine" camshaft is generally geared towards the LSA and idle.. They're usually fairly calm and well mannered. I had hydraulic trim tabs.

 

You can typically use any camshaft you want, just watch the tollerance to throttle response/CFM usage/and operating RPM's

 

I used 76cc 1.9/1.6 heads ( standard GM early 70's heads ) Produced good power, but you can run cheap pump gas all day long.

 

If you get me the casting off your heads, I can tell you exactly what they are

 

Good luck

[malibutwins]

The heads you had on there were better flowing heads. If it had vortec heads originally. Boats usually have a torque cam in them for low and mid range power. I would check out a rv grind. Also check out the rotation of the motor somtimes they run backwards especially if it has twin motors. If it is a reverse running engine you will need to get a marine grind cam.

[rpss4844]

what do you think about that comp cam xtreme marine cam i mentioned? Also about the roller rockers? would it really pick up 15+ hp over regular rockers?

 

Its gonna cost me about $200 to get the vortech heads worked some at the machine shop, would it be worth the coin to swap? Keep in mind im only tryin to get 300hp out of this set up and not trying to build a race motor or anything.

 

I forgot to describe what the boat was doin in my first post. When i hook it up to the hose at home it will start perfect everytime with just a quick turn of the key and idle perfect. it will rev excellent and go rite back to idle. as soon as i take it to the lake it is hard to start, even harder to stay running and most of the time wont idle in gear or even idle very well no in gear. the hole shot is where the problem is, this thing wont rev past 3300 rpm and just plows thru the water and will never get on plane. it just stands the bow up kinda like i have it trimmed up on take off. Once it finally does get on plane it will rev fine and rpms will max when trimmed up. it will not run at max rpm trimmed down even the least bit, like it is over-propped, but i went all the way to a 19 pitch.

 

[Buyfire]

i'd keep the cam mild if you choose to replace it. especially if you have thru hull exhaust that's not too far off the waterline. i've seen several times where coming off plane you can end up with water intrusion & bent internals if there's too much overlap in the valve opening/closing.

 

also, 26P prop seems way extreme on a 20 footer with 300HP. is it an OB prop with a small diameter? my guess is that once you have the motor set up right you'll be looking at about a 21-22P prop.

[rpss4844]

well i dont know a lot about cams and what would be considered mild. I am interested in the cam mentioned in the first post, is that a "mild cam".

 

As far as my prop i am running 14 1/4 x 24 or 26. Just depending on what i want to do, i only run the 26 with just 1 or two people on the boat and doing poker runs and such. its not that great of hole shot but really adds to top end and only drops my rpm by a few hundred. most of the time i run the 24 and it does great as long as im not fully loaded and trying to pull skier etc. I have never tried a 21, but there is a prop shop here that will let you try before buy on all there props.

 

My exhaust is all under water, no thru-hull. i think this is why im having such a hard time idleing in the water, because of all the extra back pressure. it idles fine on the hose where the exhaust is open.

 

Does anyone have any opinions or facts on roller rockers, is 15-30hp gain feasable by just swapping from conventional rocker to rollers? How would they hold up in a marine application?