SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: scotticq on August 12, 2016, 09:14:00 PM
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Are you suppose to have a larger piston cylinder clearance on jet boat motor compared to Street motor
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Short answer. Yes.
What are you building?
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So the engine knows it's in a jet boat vs a car ? I'm confused
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Yes!! V-drives are different as well... I'm not sure why mine hasn't blown up in the last 4 years yet!! Lol
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Gt's gonna hammer us right now with, cold inlet water temp and high cylinder head Temps effecting the expansion of pistons and cylinders thus effecting clearences just so you know.... lol I know it's comming! Educate us brother... ;)
I always rebuild my street cars engine every season so a cold rush of winter water don't blow it up!! Bahaha .. jk,, have at it..
Your friendly local v-drive owner.
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A boat runs at higher rpm for longer periods of time. Also with more load on the engine.
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Also cooling system open.
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My boats never seen 10.5 Rpm. My Car is 7600 on the Trans Brake. Now my boat runs 7200 on the bottle.
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Well the first thing I did when I swapped the 3.23 out of my car for 4.30's was rebuild my engine !!!! Not!!! It buzzes 4 grand all day on the freeway now at 65-70.. why hasn't it blown up???
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Gt jets, 460 I had a cam go flat with about 26 hour on a fresh motor, wiped out all the bearings. I'm taking the block, crank and rods to machine shop today. I was wondering how much clearance to hone it to
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to get the most accurate info. for your specific application call the piston mfg. tech dept. with the piston number and ask their recommendation for clearance. Make certain they know your building a jet boat motor and where you boat etc. (colder climates etc.)
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So the problem is you have a block that doesn't heat up as much due to cold water cooling so it doesn't expand at same rate as your rotating assembly so your crank is hot which when metal heats up it expands it needs more clearance than your closed cooling system which everything heats up evenly. Rod clearance is .0025-.003 mains .003 and Piston to wall for my JE forged pistons calls for .004-.005 + an extra .002-.003 for our cold blocks. I would call your piston manufacture to make sure what that piston needs for cold water pickup
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Thanks
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The biggest issue with a jet boat is the extreme temperature differential from the front two and rear two cylinder walls and the head temperature.
The actual finished bore size is a calculation based on the piston manufacturers recommendation and the expected running temperatures.
The warmer and less temperature differential you can achieve, the closer the tolerance can be to automotive spec. To a point. Unless you are running a closed cooling set up with a heat exchanger, never run the engine warmer than 160°F. Organic compounds (salt, calcium, lime, algae) start to cook at warmer temperatures and form on surfaces inside the cooling jackets, once this happens, they grab every particle that happens by. Virtually plugging up the cooling passages.
Do you know why the camshaft went flat?
Whether you decide to follow the machinists advice or not, just know that everything will have significant temperature differential and things will need room to grow.
Check the piston skirts for wall scuffing.
.0005" of additional wall and bearing clearance won't be the end of the world, even in a car.
Oil temperature and oil weight selection does become more important though.
Anybody who runs a hot rod engine full tilt while it's still cold is a fugging moron btw. (Yeah, I saw saw that).lol
Good luck bud, sorry for your frustration.
GT
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Add to that, the extreme differential between the piston and the cylinder.
Do you know how the water flows in a big block?
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The biggest issue with a jet boat is the extreme temperature differential from the front two and rear two cylinder walls and the head temperature.
The actual finished bore size is a calculation based on the piston manufacturers recommendation and the expected running temperatures.
The warmer and less temperature differential you can achieve, the closer the tolerance can be to automotive spec. To a point. Unless you are running a closed cooling set up with a heat exchanger, never run the engine warmer than 160°F. Organic compounds (salt, calcium, lime, algae) start to cook at warmer temperatures and form on surfaces inside the cooling jackets, once this happens, they grab every particle that happens by. Virtually plugging up the cooling passages.
Do you know why the camshaft went flat?
Whether you decide to follow the machinists advice or not, just know that everything will have significant temperature differential and things will need room to grow.
Check the piston skirts for wall scuffing.
.0005" of additional wall and bearing clearance won't be the end of the world, even in a car.
Oil temperature and oil weight selection does become more important though.
Anybody who runs a hot rod engine full tilt while it's still cold is a fugging moron btw. (Yeah, I saw saw that).lol
Good luck bud, sorry for your frustration.
GT
. Well said sir.
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. Well said sir.
Sent from my iPhone using SoCal Jet Boats
Thank you. Means a lot.
GT
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I love how detailed GT and FLUSHER get with their responses, takes awhile type all that chit out.... good job