SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: muffbro on April 19, 2014, 05:42:10 AM
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looks like I may have to RR my engine.
(depending on what I find) today.
i know these engines run different specs, than that of a street engine application..... anyone have a list of what these are?
thx
there are no Jetboat shops or mechanics within 300 miles of where I live.....
mobile
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The differences aren't so much "jet boat" related, but "open cooling" marine engine related. If you have a performance machine shop in your area that builds marine engines for I/O applications he should be able to get you set up properly.
I'll do some more looking around for the specifics you are looking for but I don't have them handy.
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Do you have any specifics on what you will be building and what you expect out of it?
That will actually help narrow down some of the information.
It will need additional piston to cylinder clearance (this will slightly vary based on what pistons are to be used)
What exhaust?, this will determine the best camshaft selection variables.
How hard do you plan on running it? this will help answer the crankshaft clearances needed.
I have a pretty good proven recipe for a jet motor build, but is really geared toward what I call "marathon motors".
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GT
key word: dependable:
I go out nearly every weekend and some weekdays during the summer. ( I hope)
lake running.... pulling the kids in the tube.... skiing....
it's got a Mk IV with 990 heads / a aluminum chevy intake #3963539 / Holly 3310-1 single pump
Dual feed 780 cfm / Glenwood logs
Berkeley 12 jg / 'A' impeller
it ran between 56-63 mph
I liked the way it ran, slight lope at idle, I don't know what the cam specs are...
mobile
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Here is a start for you.
I would go this route.
Forged crank (not necessary, but what could it hurt, this is theoretical :sly: ) When the machining is done on the crank, add .0002" to the final bearing clearances. Does not seem like much, but it will allow you to run a thicker weight oil.
On the cylinder to piston, this is where debate comes in. For forged pistons. We have always used 1.05 as a multiplier to the factory ring/piston manufacturer specifications, this allows for additional piston to cylinder clearances, the block is going to stay stupid cold while the pistons will really heat up, this gets close to the proper clearances in the conditions of a jet aplication. As soon as you have a closed system or are running a thermostat, this somewhat goes out the window, but it is the same spec GM used on the Corvette engines for decades. GM intentionally set them up "loose" because they knew they were going to get driven hard an put away wet. This is why they sometimes had a skirt slap at start up. (In Cliff Clavins's best voice "very little known fact").
Camshaft I would not settle for anything less than a hydraulic roller, use a wide lobe center, don't overdue the cam duration. (I may expand more on this later.).
The valve guides need to be a smidge loose as well, the cylinder heads run pretty darn cool. In most cases I would say iron guides add about .00025", ensure the stem seals are installed and the guide tops don't interfere with the rotators. You really should have twice the recommended clearance here. The reasons are a little lame, you may desire to increase the rocker ratio later to gain some lift, it don't cost nothin'... Doubt you will, but since your into it... :sly:
Gotta go stuff my face, more later.
GT