SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: ralph on August 09, 2011, 05:12:56 PM
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Ok, My new to me 78 Mariah Jet has a 468 inch Chevy with a 6/71. I've been having a backfiring issue I thought was ignition related. The previous owner left the distributor (Vertex Magneto) a little loose. Timing kinda crept waaaay up on me before I figured that out. Finally did a compression test and found two cylinders with 0 compression! I'll be taking it apart over the next few days - I'll report back. Ouch!
Ralph
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that sucks!!!! sorry to hear about that!
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Ok, My new to me 78 Mariah Jet has a 468 inch Chevy with a 6/71. I've been having a backfiring issue I thought was ignition related. The previous owner left the distributor (Vertex Magneto) a little loose. Timing kinda crept waaaay up on me before I figured that out. Finally did a compression test and found two cylinders with 0 compression! I'll be taking it apart over the next few days - I'll report back. Ouch!
Ralph
Want to bet on burned pistons if the timing got too far advanced? Hope it's not two adjacent cylinders,, that could be ugly if the block and/or heads got burned.
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Well, chunks out of the ring lands on three cylinders. The cylinder walls still look good, and it might have a little valve damage. The cam lobes, though, seem like they're worn down to the nub. On the god side the block is only .030 over, so its got room for a rebore if necessary.
No wonder this thing didn't scream like it should. Seems like somebody built a 10.5 to 1 or so performance motor, then somebody else decided a blower would be fun without checking what parts were in there already. Recipe for detonating disaster!
I looked up the parts: the heads are 1966 only small chamber, the pistons list at 11 to 1 with those heads. Then somebody adds a blower!
Looks like i'm in the market for some blower pistons, maybe large chamber aftermarket heads, and a good blower cam. Or a blower capable crate long block.Opinions and suggestions welcomed.
Ralph
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So... any camshaft ideas? this thing (.030 454) has a Scat crank and Eagle rods, so hopefully they (and the block) are still good. I'll strip the block tonight. What I want is a low maintenance blower motor I can haul a tube around with, along with the occasional scream across the lake. not a max horsepower effort. I would think 650+ horsepower should be still tame enough with the blower, right? Help me out here. I've been poking around on the net and Edelbrock even has a decent set of Marine heads (#61559) with Inconel exhaust valves, no springs, $900 each. Theres probably something better out there. Ideas?
Oh, here's a pic of one of the pistons. I've got two more just like this one!
Ralph[attachimg=1]
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I would say the intake valve bitch slapped the piston more than once! ;D Don't think I've ever seen one actually wear a groove in like that......especially as fresh as it all looks....... :screwy:
Obviously, you're going to be going with dished pistons and not those Domed Speed Pros. Check out Diamond or Probe, personally. Cams, a lot of ways to go. On a mild build I would look at the old Crane Commander 292 or the Comp 301 Extreme Marine hydraulic grinds on a BBC (They still make them, under a different tag number nowadays, but work well with smallish blowers I'm told) Hopefully some of the expensive hardware in that engine are salvageable.
Good luck, keep us posted.
Ray
Edit, you should be able to find pistons to get the compression down to a more reasonable 7.5-8.5:1 (Low maintenance pump gas engine) even with your small chamber heads. Dunno if they're open or closed chamber, what you have. 990's are everywhere and reasonable compared to the $2k for aluminum if you don't have the change for them, but depends on what blower intake you have as well going from Oval to rectangle ports........but 650 HP should be easily attainable and on the low end of what you actually end up with, depending on blower condition, etc ......... ;)
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Thanks IRRebel
I took the rods and pistons out today. Now to send the block, crank, and rods off to see if they're salvageable. They look OK. The Comp Extreme Marine line looks interesting. #11-456-8 is a hydraulic roller with .566 lift, 296/302 duration. 2800-6200 range. I know squat about cams, so this will be tough for me. I usually research the hell out of every decision, so I could still be waffling on this next spring! What works good with a blower?
The Edelbrock heads? Again a subject I have no experience with. I'm just looking at off-the-shelf stuff and guessing.
Ralph
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I wouldn't have a problem at all with the Eddie heads on a blower engine. The trick with the cams is really going to be where your final compression ratio works out to be, compression height, etc. Cams for boats use, particularly jet boats, get a little complicated and more so with a blower.
Overdrive ratios, what impeller, pump condition, play into it in no small way either.
Once you figure out whats salvageable out of what you have, my first call would be to Barry at Obnoxious Racing Engines. (Obnoxious001 here on the boards). He's probably built more blown BBC jet boat engines than anyone I can think of and can make some very specific part recomendations for you based on a LOT of experience and what you have left to work with. I know on my blower engine build he has been very helpful, Paul Kane has been indispensable since this is a BBF build in my case. Tom/Josh at JBP could help as well. Can't remember the last time they built a boat without a blower on it.
Ray