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another blown up jetboat motor sbc

General engine tech -- Drag Racing to Circle Track

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nwticerider
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another blown up jetboat motor sbc

Post by nwticerider » Tue Sep 10, 2019 4:37 pm

Hello everyone,

I read an old post about this same dropping valves from the jet boat motor in a 383 sbc and have experienced it twice now.
Just ripping along at around 3800rpm for twenty minutes and boom.

The old post was from 2011 and never did come up with a solution.

Dart SHP with a scat forged crank 5.7 rods wiseco dished pistons and a comp roller marine 276
Import heads with manley valves done by a reputable engine shop.

Timing 32 degrees. closed cooling. RPM intake with a street avenger.

could the dual pattern cam be holding the exhaust valve open to long? Overheating the seat ? and thus the valve.

just trying to figure this out. we have a stock marine 350 300hp that has been thrashed on for 8 years and no problems.

I will post more once I have it out and apart.

Regards,
John

CharlieB53
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Re: another blown up jetboat motor sbc

Post by CharlieB53 » Tue Sep 10, 2019 4:57 pm

I’d liked to have seen the valves from the prior failures, perhaps would know more if it was the valve head or keeper failed.

3800 cruising? What was the WOT rpm set by the pump clearance? To tight on the pump can overload the motor creating excessive exhaust valve heat.

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Re: another blown up jetboat motor sbc

Post by MadBill » Tue Sep 10, 2019 6:01 pm

Can you provide more specifics re ''dropping''i.e., detached valve head, broken stem, sheared collets, bent valve, etc.? How do the undamaged valves/pistons look; any sign of P/V contact or stem galling? One possibility is lean P/T AFR combined with tight stem clearances = hot valves sticking open.
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Re: another blown up jetboat motor sbc

Post by BillK » Tue Sep 10, 2019 7:22 pm

Intake or exhaust valve ? Which Manley valves ? If I was going to use Manley they would have to be the severe duty ones. If I had a choice I would use Ferrea I have seen them literally bent like pretzels but never broken.
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Re: another blown up jetboat motor sbc

Post by rebelrouser » Tue Sep 10, 2019 9:52 pm

I have built several 383 SBC jet boat engines, never had one drop valves, seen them blow head gaskets, bend rods from getting water in the cylinders, etc etc. One thing just to check, the guys around here like to go in 6 inches of water, at 60 mph, drinking beer and otherwise having a good time. Everyone of the engines I took apart that had damage from overheating had a lot of sand in the water jackets from pulling in river water, in the shallows. Have the exhaust manifolds checked if they are water cooled, lots of engines that have problems have internal cracks and water gets in the cylinders. I knew one guy that ruined three engines because he was too dumb to figure out that you can't run water in the exhaust ports. I am fairly close to the lake of the Ozarks and I have a marina that can pressure test the manifolds. Not sure about the jet boat guys in your area but the ones in my neck of the woods are pretty wild, and destroy some stuff.

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Re: another blown up jetboat motor sbc

Post by nwticerider » Wed Sep 11, 2019 1:49 am

Haven't torn the engine down yet this time. Removed it froom the boat this evening and the antifreeze was in the oil pan. not a good sign.

last one had detached exhaust valve head. They are the severe duty series not the extreme duty.
the rest of the cylinders looked a bit rich if anything. the rest of the valves were free in the guide and didn't show signs of galling, hitting, or seizing.

WOT will pull 4700 rpm and I am thinking the fast cruise rpm is just before the secondaries are opening. The pump is set up tight with minimal impellor to stator clearance. I could use slightly less pitch to have the pump spin up a bit to 5500 at WOT
Maybe it is overloading the engine with the current impellors.

Of course the engine gets some ( a lot ) wot and pushed pretty hard but both times it failed was during a long period of fast cruise.

I will post once I have it apart and learn how to post pictures.

Regards

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Re: another blown up jetboat motor sbc

Post by MELWAY » Wed Sep 11, 2019 6:02 am

Were is peak TQ and HP on these combos?

I have done some endurance boat racing and some advise that I was give was never prop and engine to run at pk TQ at WOT
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Re: another blown up jetboat motor sbc

Post by mag2555 » Wed Sep 11, 2019 6:12 am

Looks like you found the rpm where steady state Mid rpm loss of valve train control takes place!

I would change the heads right over to Beehive type springs

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Re: another blown up jetboat motor sbc

Post by CharlieB53 » Wed Sep 11, 2019 2:34 pm

WOT of 4700 is way too tight on the pump clearance, over loading the motor causing premature failures.

Loosen pump to get closer to 5500 and it should perform well and last much longer.

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Re: another blown up jetboat motor sbc

Post by nwticerider » Wed Sep 11, 2019 6:17 pm

Yep, That cam probably peaks at 37-3800 tq and 5500 hp.

The best fuel efficiency was at 36 to 3800 rpm.

Although a lot of the hamilton 212 pumps max the rpm at 4200 rpm .

After work I will start the teardown.

Regards

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Re: another blown up jetboat motor sbc

Post by rustbucket79 » Wed Sep 11, 2019 11:26 pm

On th dyno, way before I get to the power pulls, I spend time watching the AFR's and exhaust temps at the intended cruise RPM on a marine engine. Lean is murder, exhaust temperatures will happily peg the 1600 degree pyros in a very short time. Combustion temperatures must be controlled, fuel is your friend.

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Re: another blown up jetboat motor sbc

Post by rustbucket79 » Wed Sep 11, 2019 11:30 pm

Oh, and shoot for 75% of maximum RPM as your peak cruise RPM, a boat engine's life is rough enough.

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Re: another blown up jetboat motor sbc

Post by nwticerider » Tue Sep 17, 2019 12:14 am

I had a good look at the plugs from the cylinders that were not damaged and there was definitely lean signs.
Since the engine stopped right after running at this point it clearly showed lean. the photo is after I sprayed the oil off with brake clean
Previous plug checks were after a cool down or low speed to the dock,loading the boat, etc. They looked about right. Just a bit of color.

It turns out the street avenger is jetted lean at cruise as the holley guys were expecting a vacuum advance and less load on an automobile setup.

Over at the car sites there is a lot of guys using larger jets on both the primaries and secondaries of the street avengers.
I had never heard of anyone complaining that their holley was not using enough gas.

Feel free to read my plugs. I will have some pictures of the carnage later.
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nwticerider
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Re: another blown up jetboat motor sbc

Post by nwticerider » Tue Sep 17, 2019 12:24 am

Another couple of pictures.

The combustion chambers don't particularly look lean, but the may be getting carboned up from the choke and slow speed idling???

Wouldn't 20 minutes of running lean burn that off?
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Re: another blown up jetboat motor sbc

Post by gruntguru » Tue Sep 17, 2019 12:37 am

No vac advance? If you want to lean the cruise out, it will need more advance than the WOT setting.

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