GS42fun

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

Wilks

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?

Vinny P

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??

cmcnitt

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!

Turbojack

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

Turbojack

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.

cmcnitt

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

Turbojack

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.

GS42fun

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

JJONES

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!