Help with piston meltdown at 9 hours 454 sc
 

I am looking for ideas on what possibly happened. I burned a hole in the number 8 piston. My engines only have about 9 hours on them, and I am having a hard time trying to figure out what happened. The engines were dyno'd at 625 hp. they are 454's with 8 to 1 pistons running B&M 250 blowers @ 8 lbs of boost, 30 degrees of total timing w/dual 650 holley's. On the dyno we set the carbs up, and got everything dialed in. The exhaust temps I was told were right on, and everything looked good.
I put the motors in the boat and it was running good. I was running with about 10 other boats in a short 5 mile strecth. In this run there were two stops because of two no wake bridge passings. At the end of this five mile run I had an engine miss and oil sprayed all over the engines. WOT was at 5500 rpm and fuel pressure stayed at 6 psi. The gauges and senders are new so I have to believe that they are reading currectly. When I tore down the engines the other 7 cylinders all looked good, and did not see anything unusual with the engine or heads.
The part I am having trouble with if I leaned out a cylinder why did No. 8 burn down and not NO. 6 which is typically the lean cylinder on a BBC. Could I possibly got bad gas? I filled up at a marinia gas station only 20 minutes before with 93 octane.
Any ides or simular situations please let me know, I would appreciate it. I want to fix what ever happened, so it will not happen again.
Thanks,
Greg

Re: Help with piston meltdown at 9 hours 454 sc
 

I have close to the same set-up with 100 hrs on them. What size jets in the carbs, what power valves, how big is the fuel line, what upper pulley p/n on the blower, what camshaft, what spark plugs. One other question; on the dyno how long was the motor under a load at WOT, did the egt's stabilize?

Re: Help with piston meltdown at 9 hours 454 sc
 

Greg,
Sorry to hear of your troubles. As I am sure you know, there are only a few reasons a piston would burn. Either lean mixture or overheat. I have found that setting up an engine on a dyno could give you false readings when the engine is in the boat. Most dynos have huge fuel systems. Most of the tmes over kill. However, they do this for a reason...They dont want your motor to blow up on their dyno. What happens is that your fuel curve gets set up correctly with the dynos fuel and exhaust system. When your drop the engine in the boat, everything changes. Is your fuel system sufficient for your motors??

Re: Help with piston meltdown at 9 hours 454 sc
 

Did you dyno with the flame arrestors on? I just finished dynoing my 540 and after getting the a/f to 12.4-1 without the arrestors, we did one more pull with the arrestors on and the a/f went to 13.6-1! I had to go up 5 sizes on the jets to get it back down. Good Luck!

Re: Help with piston meltdown at 9 hours 454 sc
 

What was the difference in HP between flame arrestor & without.

Re: Help with piston meltdown at 9 hours 454 sc
 

What did all the sparkpulgs look like? Was the #8 loose. I am going to say fuel was/is the problem. Either jetting is wrong for in the boat or not enough fuel inside the carbs. Maybe enough for on the dyno but running a long time will suck the bowls dry.

Of all the motors I have destroyed in my life I can never remember tearing up number 6 or 8.

Re: Help with piston meltdown at 9 hours 454 sc
 

TJ: Peak torque dropped by 8lbft. at about 3600 rpm. I don't remember the hp.

Re: Help with piston meltdown at 9 hours 454 sc
 

just reread your post. I do not see where you say you have intercoolers. Do you? Forged pistons? I think without intercoolers you might be on the edge with 8 lbs of boost & 93 octane. Motor gets a little lean & parts start melting.

Re: Help with piston meltdown at 9 hours 454 sc
 

I just got back and saw all the post's. Thanks everyone. To try to answer the questions. My jets are 70 primary 76 secondary w/ 2.5 power valves in the primary only.
The pulls on the dyno were all short under load. Winding it up to 5500 and backing it down. In retrospect it probably would have been good to run it for a while under load to see if anything changed.
The pulley I am running is the B&M part for the 7 to 8 lbs of boost. I am not sure of the part number. When I bought the blowers I got two sets of pulleys. I was told one was 5 lbs and one was 7lbs. I have only run the smaller one so far, which turned out to be 8 lbs of boost.
I dyno'd the motors with my arrestors on. I read an article about how different arrestors made big changes in the fuel air mixture, so I wanted to make sure I didn't have any problems.
Spark plugs looked good. I was running NGK's Thats the weird part they all looked good. No.8 was the only one with problems. It was sprayed with aluminum from the heat, and was beat up from the broken pieces of the piston.
The pistons were forged blower pistons.
No intercooler. From everything I read on supercharged engines 12 to 1 effective compression is opptimual. 8 to 1 compression with 8 lbs is supposed to give me 12.4 effictive compression. I am thinking about trying my other upper pulley which would be 6 lbs and give me a effective compression ratio of 11.3 to 1.
My fuel lines are for the most part 1/2" but at the carbs and the fuel block they go down to 3/8". I don't know if that would be the problem. I bought these from a guy who originally had them on 600 hp Hawk motors, and then he built his engines up. He thought his engines were in the range of 700 to 750 Hp. I only bought the blowers, carbs, fuel lines, linkage, ect.
Does anyone know if bad gas would have caused this. I was talking to the marnia and they told me that they have all this equipment to check for debris, and for waterin their tanks, but I am wondering if the octane rating may have dropped from the gas not being fresh, because it was toward the end of the season.
Hopefully this information will help.
Any additional ideas would be apreciated.
Greg

Re: Help with piston meltdown at 9 hours 454 sc
 

sounds lean,they more boost,the more fuel you need!when your blowing more air,you need to match it!i had the same problem,what type of fuel pump?i had electrical pumps that didnt supply enough fuel at the higher rmps!they were only recieving 11.5 volts from the charging system so fuel pressure was weak and burning up the motors! another boat i had used edlebroke carbs and the float valve stuck shut and leaned out the motor and it also did the same as yours!

Re: Help with piston meltdown at 9 hours 454 sc
 

It sounds like you may be asking too much of your B&M 250 to make 8# of boost for any stretch of time with no chiller. It's got to be making some serious heat turning that hard. It does seem odd #8 got lean, but it may have just broke 1st. Look at other plugs with magnifying glass to see if any material present. If you plan on running it there going forward I would either put a chiller on, or get a bigger blower and turn it slower to make the same boost.

What heads and cam are you running?

Re: Help with piston meltdown at 9 hours 454 sc
 

I am starting to think that it possibly did run lean, but the weird thing is everything else looks really good, and why did No. 8 burn a hole. That is what's puzzling me. I will take a closer look at the plugs. I am running the stock mercruiser / Carter fuel pump. It is supposed to flow somewhere around 115 gph. I was told that it would be enough? One engine it stays at 9 psi and on the other engine, which melted down it stays at 6 psi. On that run I am pretty sure that there was not a drop in fuel pressure. I bought new Monster gauges and senders. They are mounted on the top of my dash right in my line of sight, and I was constantly checking the gauges because everything was so fresh.
My heads are Dart iron eagles, and my cam is a special grind Isky that the engine builder used before and had good results with on a similar set up.
Greg

Re: Help with piston meltdown at 9 hours 454 sc
 

JJones, when you leaned out your engines did it burn up just one piston or did you find damage on other pistons as well?

Re: Help with piston meltdown at 9 hours 454 sc
 

Just went through the same thing on similar setup, 502, intercooled, B&M 250. Burned the end pistons.
You ran lean, especially at 8psi, no intercooler. That's a lot on a marine engine.
Lower your boost, upgrade your fuel pump, filtering capacity, remove anti-siphon valve. BTW, It's not always a loss of pressure, it's the flow volume required to fill the carb bowls.
Gary

Re: Help with piston meltdown at 9 hours 454 sc
 

I would bet that your fuel pump does not flow nearly enough. Remember, pressure is not the same as volume. What size pick up do you have in the fuel tank? Does it still have the stock check valve? A fuel system is only as good as it's weakest link.

Re: Help with piston meltdown at 9 hours 454 sc
 

My holley 750 carbs are set up with 78 pri and 86 sec with 4.5 power valve in primary. This is about identical to the older merc 600sc. I am running the 155193 pulley which gives me 6 lbs of boost. 454 30over with 741 cam. Dyno at 625hp. The 155194 pulley gives me 575hp and 5 lbs boost.

Doesn't look like you are giving it enough fuel.