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Old 08-22-2013, 12:52 AM   #1
Omarsvette
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i want to start of by saying i know nothing about boating!! only cars and i have experience in chevy and big blocks. i wanted to build a 496BBC when i found a guy on craigslist selling a 496 BBC. i paid $3500. which i thought was a deal. i could not have build this motor with its parts. he actually seemed upset selling me the motor especially when i gave the the money to his wife to verify and she was pissed about the amount.

eagle stroker kit
h beam rods
probe piston
brodix rect port 305
edlebrock vick jr
quick fuel 850
all comp interanls no cam specs

he had this motor professional built. he didnt know the cam specs. he said he had it on a boat with twin impellers and that one impeller broke so he pulled the motor. i cant remember the size of the boat but i remember him commenting a long boat. i verifyed the internals when i changed out the huge oil pan and the head gaskets. i put the motor in my 73 vette with a m21 and 3.36 gears. which im currently switching to 3.70.

now my question is this... the motor seems to be slugish at low rpms (with 3.36 gears). but on the freeway she starts coming alive at 70 mph. and 90 feels like nothing to her. if i want a strong take off i have to rev the motor high then punch it. problem with that is i broke the passenger side half shaft the first month doing that, never did it with the stock 350 or the beef up 383. i have played with timing a bit have her at 24 or 26 initial. she dont misfire or ping at all. im hoping the switch to 3.70 will improve low end speed. i wish i new more about the cam. maybe thats my issue???? i have been driving her for about a year now. any comments on cam or timing?? maybe its just gearing. compression seems tight no oil being consumed. he said motor had under 5 hrs. which i thought was wierd, didnt know boat motors were measured in hours. externally and internally the motor is very good which led me to believe this motor was not a high mileage motor.
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Old 08-22-2013, 12:54 AM   #2
Omarsvette
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also im currently rebuilding the entire front n rear suspension. awaiting my new trailing arms and composite rear spring.
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Old 08-22-2013, 05:52 AM   #3
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I have no building experience but I know that boat motors are low (static) compression with a camshaft that delivers power at +2500RPM so what you're experiencing is what you would expect from a boat motor.
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Old 08-22-2013, 06:16 AM   #4
Jig A Low
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yep, the cam in the motor is designed for high rpm and low torque, boats routinely turn 4000+ rpm and the motors that are built for boats have cam that are suited for that rpm range. I would do a cam swap and put something in there that makes more torque down low. changing the gear ratio will help some but to be at it's full potential, I would swap cams.

and yes, boat motors are tracked by hours. it's not hard to find a fishing boat that is 15 years old and only have a hand full of hours on the engine, basically you run 15 or 20 minutes and sit fishing for hours and hours, then run 15 or 20 back.

Last edited by Jig A Low; 08-22-2013 at 06:18 AM.
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Old 08-22-2013, 07:10 AM   #5
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Quote:
Originally Posted by Danish Shark View Post
I have no building experience but I know that boat motors are low (static) compression with a camshaft that delivers power at +2500RPM so what you're experiencing is what you would expect from a boat motor.
This isn't true. Boats use cams designed for higher RPM's and have relatively high compression ratios. Switching to a cam with a lower power band can induce severe detonation.

You need to find out what the static compression ratio is before you do anything with the cam.
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Old 08-22-2013, 07:29 AM   #6
carcraze
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I am no expert on them but I have played in the boat side of things for a while.

Performance engines in boats are tricky and generally use a marine grind cam which is done to try and prevent reversion which is water being sucked back in the exhaust at idle. This is combated through the type of exhaust and generally cams with less than 240 duration numbers. The higher the cam duration the closer to reversion you get generally speaking.

A stock boat motor like the 350's in mine are generally built with a cam spec close to that of a truck or RV. Torque is king in a boat and WOT RPM on the 350 is 4500 to 4800 and the boat is proped to run that.

It takes high performance out drives and engines to run high RPM's on boats most cruise at 3000 or so and top out 4500 to 4800.

Most performance rigs are using big cubes for the torque. Compression is also sometimes of concern as detonation can also be an issue due to the heavy loads they run in.

That is my take on it anyhow.
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Old 08-22-2013, 07:54 AM   #7
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Quote:
Originally Posted by carcraze View Post
I am no expert on them but I have played in the boat side of things for a while.

Performance engines in boats are tricky and generally use a marine grind cam which is done to try and prevent reversion which is water being sucked back in the exhaust at idle. This is combated through the type of exhaust and generally cams with less than 240 duration numbers. The higher the cam duration the closer to reversion you get generally speaking.

A stock boat motor like the 350's in mine are generally built with a cam spec close to that of a truck or RV. Torque is king in a boat and WOT RPM on the 350 is 4500 to 4800 and the boat is proped to run that.

It takes high performance out drives and engines to run high RPM's on boats most cruise at 3000 or so and top out 4500 to 4800.

Most performance rigs are using big cubes for the torque. Compression is also sometimes of concern as detonation can also be an issue due to the heavy loads they run in.

That is my take on it anyhow.
Since the OP stated that the boat broke an impeller, sounds like it was a jet drive. Jet drives are extremely strong and require high RPM engines unlike out drives. I've had several jet boats.
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Old 08-22-2013, 08:01 AM   #8
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I saw the impeller comment but the jet drive did not click figured he broke a water pump impeller on a twin and decided to part out as opposed to replace 1 motor.

I had a couple big block jet units from the 70's jacuzzi era they do run higher R's.

I built one up once for my jet and found out about reversion the hard way.
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Old 08-22-2013, 08:26 AM   #9
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I built one up once for my jet and found out about reversion the hard way.
Were you running headers with water injection or transom exhaust?

I never had that problem even with water injection, though on the real big motor I ran dry zoomies.
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Old 08-22-2013, 10:39 AM   #10
Omarsvette
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This isn't true. Boats use cams designed for higher RPM's and have relatively high compression ratios. Switching to a cam with a lower power band can induce severe detonation.

You need to find out what the static compression ratio is before you do anything with the cam.
he told me compression was at 10.25 with the dome piston (probe, bore 4.28, single valve relief, head volume -18, comp distance .220) a 119cc heads it made sense. the piston look like it pops out of the cylinder pretty good before i put the head back on. i want to do a compression check but MAN IS THIS A TIGHT FIT!!!! ive had other C3 with BBC that werent this difficult to change a spark plug. the heads have a .600 raised exhaust runners that prevent me from using side pipes to i used sanderson block headers wich was the only thing that fit. even after putting some dents in the header. i cant even get a comp tester to screw in. never done a test without headers. is it possible?? things is thought i need to lift the motor to remove driver side headers.
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Old 08-22-2013, 10:45 AM   #11
Omarsvette
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Originally Posted by Jig A Low View Post
yep, the cam in the motor is designed for high rpm and low torque, boats routinely turn 4000+ rpm and the motors that are built for boats have cam that are suited for that rpm range. I would do a cam swap and put something in there that makes more torque down low. changing the gear ratio will help some but to be at it's full potential, I would swap cams.

and yes, boat motors are tracked by hours. it's not hard to find a fishing boat that is 15 years old and only have a hand full of hours on the engine, basically you run 15 or 20 minutes and sit fishing for hours and hours, then run 15 or 20 back.
well that explain the BIGG pushroads it has. they were way fatter than stock but i thinck they were shorter also. not sure on that. i figure the next step would be a cam swap but do you think my timing is too high??
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Old 08-22-2013, 10:48 AM   #12
Omarsvette
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Since the OP stated that the boat broke an impeller, sounds like it was a jet drive. Jet drives are extremely strong and require high RPM engines unlike out drives. I've had several jet boats.
then maybe my cam is ok. my tranny m21 and 3.36 is a bad combo!! m21 need 3.70 and higher. well see once shes on the road.
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Old 08-22-2013, 01:59 PM   #13
Gale Banks 80'
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There is no way to figure out the Compression Ratio useing a Compression tester with out knowing the Cam Specs. If one knew what he was doing the the Cam could be figured out by useing a Dial Indicator on a push rod. But for me it would mean taking off the Timing chain cover and looking to see whats stamped on the Cam. Same for C/R. I would be removeing a Head and CCing the chamber. As for rear end Gears I would be installing a 5-6 speed Trans, as the 3:36 ratio is perfect for one.
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Old 08-22-2013, 01:59 PM
 
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