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Head Choice

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    Just Me snoc653's Avatar
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    Default Head Choice

    I picked up a 454 from a Baja I/O to put in one of the jet boats. It has stock chevy 3999295 pistons on simple rods with a forged crank and a 4 bolt main. The heads have crane roller rockers, guides and the large push rods. But they are 990 heads. I think the block is zero decked but I need to bring my depth Mic home to be sure. It should have 9:1 before decking. But will these heads work on a jet that won't see above 5000 RPM?

    Option 2 is a set of 3872702 closed chamber ovals off a Hardin marine 454. This should put it up around 10.7 before decking.

    The pistons are closed chamber pistons from a 71 LS6 (per the web) with a very small dome. I have the cam, lifters and intake from the Hardin Marine engine as well. While near the limit for pump has would the ovals be a better choice and save the 990s for a v drive engine build later? Thoughts would be appreciated.
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    steelcomp was here
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    Quote Originally Posted by snoc653 View Post
    I picked up a 454 from a Baja I/O to put in one of the jet boats. It has stock chevy 3999295 pistons on simple rods with a forged crank and a 4 bolt main. The heads have crane roller rockers, guides and the large push rods. But they are 990 heads. I think the block is zero decked but I need to bring my depth Mic home to be sure. It should have 9:1 before decking. But will these heads work on a jet that won't see above 5000 RPM?

    Option 2 is a set of 3872702 closed chamber ovals off a Hardin marine 454. This should put it up around 10.7 before decking.

    The pistons are closed chamber pistons from a 71 LS6 (per the web) with a very small dome. I have the cam, lifters and intake from the Hardin Marine engine as well. While near the limit for pump has would the ovals be a better choice and save the 990s for a v drive engine build later? Thoughts would be appreciated.
    10.7:1 will be really hard to manage with today's pump gas, carbureted and an iron head. The 990's can be made to work in a jet even at that rpm. Not ideal, but workable. Maybe try to bump the compression half a point if possible. The key will be getting a cam to address the port velocity issues. However, if you can get the compression down to 10:1 on the ovals they would be the better choice, but don't just do it with a thick head gasket. You need the quench to help fight off detonation. I don't know if the domes on those pistons can be milled or even if that's worthwhile for this build but it's a viable option and pretty simple to do. It's also not hard to open up the chambers a little on the heads. Few hours with a die grinder and cartridge roll will work wonders.
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    Just Me snoc653's Avatar
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    Thanks for the info. I'm leaning towards trying the 702s and saving the 990s for a 496 build.

    I checked deck height with a straight edge and feeler gauges at .015. That puts it at 10.6 with the .039 head gasket. What is the max thickness you would recommend?

    The cam from the Hardin engine is an Isky 286 hydraulic. Specs are: 510 lift, .050 duration of 224, Adv duration 286, and lobe center of 108. The intake and exhaust are the same. I've got a pair of solid lifters coming to check valve clearance, but I doubt the small valves in the 702s will have a clearance issue at 510 total lift.
    Last edited by snoc653; 08-04-2016 at 01:00 PM.
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    Option #3,, find some oval heads with a larger chamber. Those old closed chamber oval port with small valves aren't going to do a bunch for performance.

    Scott mentioned cutting pistons, pretty sure those old TRW/GM pistons are solid dome and can be cut, but obviously have to press off and pack onto the rods. Many years ago I did just that with a set when I was working at Gale Banks, or should say my buddy Ron the machinist cut them down for me.

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    steelcomp was here
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    Quote Originally Posted by snoc653 View Post
    Thanks for the info. I'm leaning towards trying the 702s and saving the 990s for a 496 build.

    I checked deck height with a straight edge and feeler gauges at .015. That puts it at 10.6 with the .039 head gasket. What is the max thickness you would recommend?

    The cam from the Hardin engine is an Isky 286 hydraulic. Specs are: 510 lift, .050 duration of 224, Adv duration 286, and lobe center of 108. The intake and exhaust are the same. I've got a pair of solid lifters coming to check valve clearance, but I doubt the small valves in the 702s will have a clearance issue at 510 total lift.
    That gives you .054 quench and I wouldn't go any more than that. Quench is your friend when trying to fight off detonation. If you try to lower the compression with a thicker head gasket you may well increase the chance of detonation. I'd like to see .045-.050".
    You're going to want a cam with some duration split favoring the exhaust for factory heads like those. I agree that valve/piston shouldn't be an issue.
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    Quote Originally Posted by obnoxious001 View Post
    Option #3,, find some oval heads with a larger chamber. Those old closed chamber oval port with small valves aren't going to do a bunch for performance.

    Scott mentioned cutting pistons, pretty sure those old TRW/GM pistons are solid dome and can be cut, but obviously have to press off and pack onto the rods. Many years ago I did just that with a set when I was working at Gale Banks, or should say my buddy Ron the machinist cut them down for me.
    I'm not looking to squeeze out every drop of performance, it's a mini-day jet boat after all. If it was mama's Litchfield that would be a different story. With about 425 HP Icould only turn 43 - 4500 rpm. If I can squeeze 5-600 out of this one, I'd be happy. That should still leave me under 5300 RPM so I was thinking they might be up to the task. And usually the boat doesn't stay full throttle for extended periods.

    Quote Originally Posted by scott foxwell View Post
    That gives you .054 quench and I wouldn't go any more than that. Quench is your friend when trying to fight off detonation. If you try to lower the compression with a thicker head gasket you may well increase the chance of detonation. I'd like to see .045-.050".
    You're going to want a cam with some duration split favoring the exhaust for factory heads like those. I agree that valve/piston shouldn't be an issue.
    Another set of heads might work if a set shows up around these parts. I stumbled into this engine as it was. I do see a fair number of peanut port heads on old trucks, but that wouldn't be any better really. I had been reading that .045 would be a good max target range for quench. So lets say pump gas is out and look at either a blend of 100LL avgas, or E85. 93 Octane is also available pretty regularly, but that might not be enough unless I add some 100 LL or other octane booster. If I change to a thin head gasket that will put me near 11:1 and a quench of .040 to .045. Not exactly the ideal E-85 range, but should be workable. Since I'm not turning big RPMs, would having the valves unshrouded and possibly going to the bigger exhaust valve (at least) bring this combination to life in the under 5500 RPM range or just leave it stock and do a good 3 angle valve job and call it done?

    We can also put the Isky cam back on the shelf and try to figure out what it needs to get the best results. Would a hydraulic roller be better or just a better stock hydraulic profile? The boat runs logs so I know it will have an exhaust limitation regardless of the heads. I'm just looking for the best it can be without going crazy. I pulled my blower motor back out of the cruiser to reinstall the blower, so I'm boatless again. This will hopefully be a workable solution that will last while I build the other projects. If I learn to quit letting the church kids drive the boat and run it up on sand bars, that is. I have a choice of intakes for it as well. I have the holley small runner street dominator or a Professional Products Power Plus Hurricane intake. I was thinking either the 800 or 850 Holley carb for it as well. I have a MSD ignition setup for it, or possibly a mag, but the boat isn't wired for the mag. It had points in it when the engine died.
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    Some guy obnoxious001's Avatar
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    Quote Originally Posted by snoc653 View Post
    I'm not looking to squeeze out every drop of performance, it's a mini-day jet boat after all.

    My point (and Scott's) is to build something that will not be too much compression for your stated pump gasoline engine.

    The closed chamber heads, while bumping your compression, are not going to do a bunch for performance, and not likely to get near 600 hp (you did say that after you said the above about performance) with your log exhaust and necessary small cam.

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    I see your point. I guess wishful thinking was getting carried away. I plan on using the 990s and just look for a good cam that will maximize that combination.
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    Quote Originally Posted by snoc653 View Post
    ...or possibly a mag, but the boat isn't wired for the mag. It had points in it when the engine died.
    If you want to run a mag, you can utilize the existing wiring/ignition switch and a Normally Closed SPST Relay or the normally closed portion of a SPDT Relay to ground out a mag. You can mount the relay under the dash, near the engine, or right at the mag... which ever seems easier or a cleaner install to you. Completely your preference.

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    steelcomp was here
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    The 990's on pump gas would probably be a better choice with the open chamber, even @ 9:1. The right cam can address that compression and still build some decent cylinder pressure. I would definitely go hyd. roller if you don't mind spending the money. Clay Smith has some "semi custom" shelf grinds that Straub designed. There's probably one that would suite that combination pretty well. The closed chamber heads might get you more compression but they don't breathe near as well as the open chambers.
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    One last option showed up today when I asked someone about a 113cc oval port head. If I switched the forged domed pistons for a set of KB flat tops I'd be in the mid 9s for compression with the 98cc heads. Do you think this combo might work for the mini day jet?
    Edit: the flat tops are new cast pistons instead of the used forged small domed pistons.

    I'd still like to save the 990s for something with more cubes and more sqeeze. Mama's Litchfield wood deck v drive will need a motor to go with the Crower stack injection I have squirilled away for it.

    My rule is you can build a mild cruiser engine, but the v drives need to go fast. (I hope)
    Last edited by snoc653; 08-08-2016 at 08:44 PM.
    So many projects, so little time

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