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Old 08-21-2019, 08:24 PM
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I have a 355 SBC i rebuilt about 5 years ago. Had very little mileage/hours on it. Never "ran right", I believe cam was too/big agressive but it did maybe 375 fwhp/420 fwtq on dyno. Anyway vortec heads cracked in a few spots on the rocker bosses from either the high lift cam and/or poor machine work when the machine shop installed jobber screw in studs. flash foward to now, a friend needs a engine for his boat. He got a set of non cracked vortecs rebuilt that were measured @ 62 cc. Engine has old (about 12 years ago) speedpro flat top pistons that are advertised @ 5 cc, but are probably closer to 6 or 7 cc. Pistons are anywhere from 0.018" to 0.028" in the hole. I have an LT4 hotcam that I was going to put in this engine originally, heads are setup for this cam with LS springs. I also have a set of 1.6 pro magnum full roller rockers. I was going to install the cam "dot to dot" with a gm performance tining set.

I have read and read about quench. With a felpro 1094 0.015",quench would be in the "ideal" range, but compression will be near or at 10.5:1. Now marine gas up where I am is 89 octane pure gas (no ethanol), no other options unless you trailer itn it to a gas station. The other option is to run a felpro marine gasket 0.039" thick gasket that will bring compression down to 10:1. I talked with Cliff Ruggles when I ordered my Qjet kit and we discussed at length with him about quench. He said don't worry about static compression and octane, the tighter quench will make more power and fight off detonation vs a thicker gasket to bring static compression down. Local machine/race shop said to run a 0.039" or thicker headgasket and didnt seem to believe to much in the tight quench theory.

I have no issue exparimenting with the thinner headgasket, I told my buddy if it doesnt work I will tear down and replace witht the thicker headgaskets. My engine when originally assembled had the 0.039 headgasket and AFAIK didnt detonate and I ran it on 91,never tried it on anything lower but it was never tuned "right".

One last thing, we are running a CLOSED LOOP cooling system, no raw water cooling in the engine.

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Old 08-21-2019, 08:55 PM
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Quote:
Originally Posted by tayto View Post
Pistons are anywhere from 0.018" to 0.028" in the hole.
So, theoretically, there's a 0.010" slope from the front to the back of the block? Who allowed the motor to be put together like that?

I would side with Cliff Ruggles rather than some "local machine/race shop guy".
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Old 08-21-2019, 09:57 PM
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Quote:
Originally Posted by techinspector1 View Post
Quote:
Originally Posted by tayto View Post
Pistons are anywhere from 0.018" to 0.028" in the hole.
So, theoretically, there's a 0.010" slope from the front to the back of the block? Who allowed the motor to be put together like that?

I would side with Cliff Ruggles rather than some "local machine/race shop guy".
The block was "decked" by the original machine shop 6 years ago. I found out after the fact that the original machine shop was not very good. now keep in mind im using stock rods and cheapo pistons.... new machine shop is much better the engine i am building for my squarebody was out 0.002-0.003" across the whole block.
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Old 08-21-2019, 10:29 PM
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I will recheck the piaton depth tomorrow I think I just checked the 4 corners and picked the lowest and highest value for compression ratio calculations
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Old 08-21-2019, 10:49 PM
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FIRST!: Marine engines are not cheap engines
Use nothing but premium parts
I would run an absolute maximum hi speed cruise at 3600 rpm. 3900 redline
after market forged rods
steel crank
best bottom end caps
marine air cleaner
roller rockers
roller cam
3:2 psru with adjustable pitch blades

350 cubes maybe too small?
choose engine based on power required and remember you want it by 3600 rpm

lets talk details and let Richard paper a great buils?
opening bid will be 25K
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Old 08-22-2019, 04:14 PM
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Pretty much the same rules apply as a street vehicle, but the prop becomes the gears and hydraulic coupling.


To a big extent again controlled by the prop a boat engine is loaded higher at any RPM, that meaning its ratio of RPM to throttle opening is typically using a lot more throttle for the RPMs, thus the higher fuel consumption for distance traveled compared to a land vehicle.


What this gets the engine into is a higher operating cylinder pressure which increases the detonation tendency. One can generalize that a modern cast iron head is pretty well optimized at .040 squish/quench which adds around 4 to 5 mechanical octane to the fuel. That would make an 89 act like about a 93 octane.



To really get at where safe ground is requires knowing the cam timing events in crank degrees to calculate the dynamic ratio which has a high influence as to where this can go with the static ratio.



High squish/quench is not a universal solution against static compression ratio. These things have intertwined relationships that also vary with other events like operating temperature, mixture ratio, air inlet temp, ignition timing, load on the crankshaft, and the list goes on.


Aluminum heads because the move heat faster let you get away with more compression ratio and sins against these other factors.


Bogie
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Old 08-22-2019, 08:40 PM
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Wet exhaust? Depending on the exhaust manifolds you can suck water into the chamber once you start getting past the mid 220's on duration. Most cams made to work with wet exhaust are very very conservative as far as duration and overlap working off a 9.5ish cr.

This engine in a lightweight speedster maybe, but in a heavy boat that's going to loaded with people and fishing or ski gear it's not an ideal engine combo. Boats are propped to cruise at the 3.5k range and where they need peak torque to be more efficient. They need as much bottom end torque as they can get to get on plane. Less cam is going to raise the DCR.

Last edited by Hipster_G; 08-22-2019 at 09:03 PM.
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Old 08-22-2019, 08:59 PM
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Tayto? You seem to be online? Why are you putting no effort into your thread?
Satrt posting both answers and questions!
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Old 08-22-2019, 09:35 PM
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boat is an '88 glastron cvx 18. mainly for cruising and watwr skiing. engine has 2 bolt mains but is studded. as i posted before original rods and hyper pistons, but rotating assembly was balanced. like mentioned, closed loop cooling but no external pump so not a "full" closed loop system. this might change if we can source a raw water pump. i will have to ask what geara are in the alpha leg, buddy has a collection of props as he has owned the boat since '92.

cam specs are:
218/228 @ .050"
.525/.525 lift w/1.6rr's
112* LSA
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Old 08-23-2019, 01:18 PM
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gear ratio in leg is 1.50. have a 23p and 19p props.
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Old 08-23-2019, 04:38 PM
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Quote:
closed loop cooling but no external pump so not a "full" closed loop system. this might change if we can source a raw water pump. i will have to ask what geara are in the alpha leg,
Please explain the coolin' system,..??
Even boat motors with full closed coolin' need a raw water pump to cool the heat exchanger,....
Most are raw water coolin', coolin' with the water yer boatin' in,....

What, 'n who's motor did all the Marine accessories come from, exhaust manifolds, ignition, all the electrical, etc....??

As far as a raw water pump, the Alpha drive has one on the top of the lower unit, under the upper gear case,.....
'bout in the middle of the outdrive,....

Vortec heads on a boat motor work Best, with d-dish pistons, the block decked to zero, 'n head gasket at .039/ .040 range,....
The size of the d-dish should be the ones that yield as close to 10:1 as possible, without goin' over that,....
Ya can't "Hear" detonation most times in boats, so holed pistons get more common over 10:1,.....

For a cam, look for one that builds peak power up to 4800 rpms,..... the redline of 99% of the boats out there,......
Race boats, some hot rodders, 'n jet drives sometimes run higher rpms,....
We do run at 3200 to 3500 rpms, cruising, as the higher the rpms, the shorter the driveline's life,....
The prop is the rev-limiter on a boat, so prop the motor to run as close to 4800 as you can get, runnin' Wot, light load, 'n trimmed neat,....

Last edited by bondo; 08-23-2019 at 04:43 PM.
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Old 08-25-2019, 11:48 AM
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Quote:
Originally Posted by vinniekq2 View Post
FIRST!: Marine engines are not cheap engines
Use nothing but premium parts
I would run an absolute maximum hi speed cruise at 3600 rpm. 3900 redline
after market forged rods
steel crank
best bottom end caps
marine air cleaner
roller rockers
roller cam
3:2 psru with adjustable pitch blades

350 cubes maybe too small?
choose engine based on power required and remember you want it by 3600 rpm

lets talk details and let Richard paper a great buils?
opening bid will be 25K
Gotta disagree a bit here. Marine engines used to be expensive. Back in the day, cast pistons, cast iron rings, and cast cranks were not up to the abuse of a marine application, but these days castings are so much better. Since about 1985, all of the mainstream boat power manufacturers (Mercruiser, Volvo, OMC, etc) use pretty much off-the-shelf crate motors from Chevy. No special parts. The 350 I built for my Baja started as an automotive Vortec longblock - hyp pistons, cast crank, PM rods. Even the cams are pretty interchangeable.

I also disagree with the 3600 rpms. My 350 ran at 4500 all day, and I had a 19p prop that would let it sing to 5000.

To the OP... you mentioned closed cooling, is that fully closed, or still wet exhaust? If it's wet exhaust, the LT4 hot cam is a no-go. Reversion will kill it. Even if it's a dry exhaust, the LT4 cam will put the torque peak way too high unless the boat is super light.

You need the torque peak to be below planing speed for two reasons: one, you don't want to be lugging the engine at a lower planing speed, and two, your ability to get to plane will be seriously limited. I have actually seen boats where the owner went too big on cam and the boat wouldn't plane. 400 hp is great, but if the torque peaks at 4000, you might never get to plane.

The one thing I will agree with vinnie, is engine size. In a prop-driven boat (also in a car, but more applicable in a boat) size is king. Displacement alone won't add hp, but it will shift the torque peak down. Subsequent mods to shift the torque peak up and make more power will just return the torque peak to where it was in the RPM band meaning more power without shifting the torque peak up in the RPMs. But the big thing... pick an engine displacement that gets you your HP goals AND keeps the torque peak below planing speed. 1 hp per cube is relatively easy in an I/O depending on hull design and weight.

Better yet, post the boat specs; weight, outdrive brand/series, hull type, deadrise, etc. They will all have a huge impact on what we're doing here. Building marine combos was my bag for a long time. I'm a little rusty, but we can get you going.

As far as the compression ratio, 10.5:1 might just be OK with 89 in Vortec heads. I ran 9.6:1 with .025" quench in my Vortec 350 and a much smaller cam, but admittedly I had fresh water cooling so the bypass always got some 70 degree water mixed in.
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Old 08-25-2019, 04:13 PM
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Pretty common practice to drop a GM crate 5.7 in for a marine engine around here. I've only been in a few. All looked like standard issue Vortec stuff even the head gaskets in a rwc cooled one.
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Old 08-25-2019, 09:44 PM
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dropping a crate sbc is cheap
The engine will last long enough,,,lol.
If you follow what I said about rpm Vs heat Vs sbc has a sh tty bottom end for bearing surface and end play bearing
anything using a PSRU is stronger and more reliable
slower rpm props are more efficient
What cools the crap valve springs on a crate 350
etc etc

ask Bogie about airplane engines and why they work the way they do
boat engines can have the same stress an an airplane engine.
If you boar around at 2200 rpm most of the day to save fuel/noise then drop anyones $2 box engine
The reason for me to use a vpp is for fuel economy.

and in your opinion, what is the best throttle position for best economy using a carb
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Old 08-26-2019, 02:21 AM
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Quote:
Originally Posted by vinniekq2 View Post
dropping a crate sbc is cheap
The engine will last long enough,,,lol.
If you follow what I said about rpm Vs heat Vs sbc has a sh tty bottom end for bearing surface and end play bearing
anything using a PSRU is stronger and more reliable
slower rpm props are more efficient
What cools the crap valve springs on a crate 350
etc etc

ask Bogie about airplane engines and why they work the way they do
boat engines can have the same stress an an airplane engine.
If you boar around at 2200 rpm most of the day to save fuel/noise then drop anyones $2 box engine
The reason for me to use a vpp is for fuel economy.

and in your opinion, what is the best throttle position for best economy using a carb
I normally enjoy reading your posts but the ones in this thread are bit out there. So you would put a $25k worth of drivetrain in a boat that's worth maybe $3500 on a good day so you can go skiing? Now that's LOL

Replacing it with a General Motors crate is the same damn thing it came with except for the truck cam versus the "395" marine cam and missing a few Mercruiser decals with the same life expectancy depending on care or abuse.. Lets get real.

No one disputes boat engines have a hard life but no time soon will boats be falling out of the sky. Bad analogy. You might want to check with Bogie before putting his name into your nonsense. He's a knowledgeable guy and if you need help understanding what a stock Mercruiser is he might be your go to guy.

Last edited by Hipster_G; 08-26-2019 at 02:28 AM.
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