We hurt the 496....
#21
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Guys,
It looks like we may be doing a "refresh" on our 496HO a bit earlier than we'd planned. All signs suggest we've dropped a cylinder. Don't really know what exactly is going on but I' thinking we took the top out of one of the pistons. Working on diagnosis; taking it to "Skinny" in Somerset as soon as he gets back from vacay. In the meantime, I'm examining options.
We have the stock Merc forged crank, confirmed through Merc tech build sheet, so we're good with that at the power level we're at. Don't plan on ramping that up at the moment.
Obviously, we'd replace with forged pistons. I'm thinking JE, but don't have any real details yet. I don't know if we will have to bore yet but would plan on at least cleaning up with honing. Bore may increase a bit.
Considering replacing Rods, as well, as they are the NEXT weakest link, being powdered metal. Don't know for sure. Thinking Molnar.
Might also address valve train improvements while it's apart. Definitely new springs at the very least.
This is a bit (at least) outside of my wheelhouse, though, so I'm seeking advice. I also understand Merc used something other than the typical rod length and compression height stack-up, so I really don't even know what to look for. Should I mimic Merc's specs, or are there better?
Suggestions? If I'm thinking way bigger on the budget than I need, tell me. I've got the cash to make it happen, but really didn't want to drop it on the boat at the moment.
Thanks. Brad.
It looks like we may be doing a "refresh" on our 496HO a bit earlier than we'd planned. All signs suggest we've dropped a cylinder. Don't really know what exactly is going on but I' thinking we took the top out of one of the pistons. Working on diagnosis; taking it to "Skinny" in Somerset as soon as he gets back from vacay. In the meantime, I'm examining options.
We have the stock Merc forged crank, confirmed through Merc tech build sheet, so we're good with that at the power level we're at. Don't plan on ramping that up at the moment.
Obviously, we'd replace with forged pistons. I'm thinking JE, but don't have any real details yet. I don't know if we will have to bore yet but would plan on at least cleaning up with honing. Bore may increase a bit.
Considering replacing Rods, as well, as they are the NEXT weakest link, being powdered metal. Don't know for sure. Thinking Molnar.
Might also address valve train improvements while it's apart. Definitely new springs at the very least.
This is a bit (at least) outside of my wheelhouse, though, so I'm seeking advice. I also understand Merc used something other than the typical rod length and compression height stack-up, so I really don't even know what to look for. Should I mimic Merc's specs, or are there better?
Suggestions? If I'm thinking way bigger on the budget than I need, tell me. I've got the cash to make it happen, but really didn't want to drop it on the boat at the moment.
Thanks. Brad.
Oh, but to dream....
If somebody's got a decent used set that will fit the 496 heads, cheap, I MIGHT be able to consider it. Otherwise, and additional $8K+ is NEVER going to make it past the finance dept. As of right now, she's still hoping this was just an overheat, and everything is going to be fiiiiiine....
Thanks. Brad.
Last edited by Brad Christy; 08-13-2024 at 03:16 PM.
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#22
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Guys,
Pretty sure we found the problem….
![](https://cimg0.ibsrv.net/gimg/www.offshoreonly.com-vbulletin/1920x1440/image_2024_08_17_16_24_50_330_34fe0dc73942cf9f215149d7208dee9665701ebb.jpeg)
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A thumb-sized hole in the top of the piston in cylinder #1.
Th bright side is we are gonna have a virtually brand new engine, with solid parts inside, come springtime. Probably a few more CI, too.
Thanks. Brad.
Pretty sure we found the problem….
![](https://cimg0.ibsrv.net/gimg/www.offshoreonly.com-vbulletin/1920x1440/image_2024_08_17_16_24_50_330_34fe0dc73942cf9f215149d7208dee9665701ebb.jpeg)
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A thumb-sized hole in the top of the piston in cylinder #1.
Th bright side is we are gonna have a virtually brand new engine, with solid parts inside, come springtime. Probably a few more CI, too.
Thanks. Brad.
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#23
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Brad, I was recently faced with some of the same decisions on parts. Since i was building two complete new engines I went larger. As far as pistons i went with J&E, Molnar PAD rods, and Manely forged crank. Whatever you think you will spend figure at least 1.5 times that amount. Also since you are already there make sure to go through your heads as well. i cannot believe the amount of worn valve springs I've heard and seen lately. good luck, i think you will like the Molnar stuff.
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Brad, I was recently faced with some of the same decisions on parts. Since i was building two complete new engines I went larger. As far as pistons i went with J&E, Molnar PAD rods, and Manely forged crank. Whatever you think you will spend figure at least 1.5 times that amount. Also since you are already there make sure to go through your heads as well. i cannot believe the amount of worn valve springs I've heard and seen lately. good luck, i think you will like the Molnar stuff.
Block and heads are getting the full machine shop treatment. Valve springs are already on the docket, per advice from 87Mirage. 130-140lb, as opposed to the stock 90lb. The builder is going to be doing a full valve job.
I haven’t settled on the pistons yet. Wiseco is a bit less expensive than JE, and I’m not sure I need to go premium on an engine that’s just a bit warmer than stock. Wiseco seems more than adequate, although there’s only about $300 difference between the two.
Pretty well settled on Molnar for the rods. I really like the proprietary specs on the bolts. As a toolmaker, the asymmetrical threads and larger controlled root radius are good selling points. I called them the other day and the guy (not Tom) recommended the PWR ADR rods, simply on the marine application aspect alone, let alone the mild boost.
I think we’re gonna keep the crank, unless we get in there are discover a need to replace it. It will likely be a Molnar if we get to that point.
Lots more talk about….
Thanks. Brad.
Last edited by Brad Christy; 08-17-2024 at 09:53 PM.
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What are your plans to keep this from happening again? That piston was severely lean burned. I see you are running Whipple's Stage 2 tune. Were you running 93 oct? Data logging I saw max timing advance of 34* with my Stage 2 tune. Stock HO tune was 24*. IMO too much timing with crap iron heads under boost.
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What are your plans to keep this from happening again? That piston was severely lean burned. I see you are running Whipple's Stage 2 tune. Were you running 93 oct? Data logging I saw max timing advance of 34* with my Stage 2 tune. Stock HO tune was 24*. IMO too much timing with crap iron heads under boost.
Gotta figure out where the lean condition originated. Check the fuel pump, lines, filters, etc. The injectors are going to Smitty for an eval. May replace them. I’m thinking we will replace the 2-stage fuel pressure regulator with a proportional regulator, get a good AFR table with O2 sensors and have a proper tune done by Boos or Whipple.
I’ve been running a 50-50 mix of 93 & 89, keeping it an average of 91 or better, as per the Whipple tune requirement. May have to stick with straight 93.
Thanks. Brad.
Last edited by Brad Christy; 08-17-2024 at 11:11 PM.
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Brand new plugs prior to the weekend this occurred.
Obviously #1 is oily. That was expected. 2 & 7 appear to be way cleaner than the rest. It could be we are still getting some debris in the injectors. The rust that was found when Smitty did the injectors in late ‘22 came from somewhere. I’m wondering about the fuel rail, being the OEM steel version. It may be that, once that rusting got started, it has continued. May have to replace with a stainless unit.
Thanks. Brad.
Last edited by Brad Christy; 08-17-2024 at 11:14 PM.
#28
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Eddie Young will tell you never ever run a projected plug in a boosted engine.
That electrode hangs out in the middle of the chamber like a glow plug.
That electrode hangs out in the middle of the chamber like a glow plug.
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^^^true.
And i’ll tell you don’t run huge gaps gm determined okay for epa guidelines car/truck engines in cars/trucks.
Stay with normal .035”-.040” gaps. No matter composition of plug and even if coil near plug systems.
And i’ll tell you don’t run huge gaps gm determined okay for epa guidelines car/truck engines in cars/trucks.
Stay with normal .035”-.040” gaps. No matter composition of plug and even if coil near plug systems.
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Padraig (08-18-2024)
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I have noticed a fair bit of carboned-up threads when I change plugs, suggesting they extended into the combustion chamber.
Those are ACDelco 41-983 plugs, the spec’d plug for the Merc/GM 496, gapped to .045”, as per an OSO respected opinion’s advice. I’ll check to see if they have changed during running. Do they do that?
Suggestions for an alternative?
Thanks. Brad.
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SB (08-18-2024)