Supercharged BBC with dead hole.
 

I just bought a donor boat with a supercharged 496. The boat has been threw a few owners in the last while. Motor looks to be very low on hours and looks as if someone spent alot of time and money putting this together.
#4 cyl has no compression. The rest of the cyl. are fine. I plan on pulling the head tomorrow and seeing what I can find.
The guy I bought the boat from didn't seem to have much knowledge of how to tune the motor and had taken it to different altitudes. He thought he had a bad plug wire or something and was a little bummed when I pointed out the 0 compression in #4 cly.
While looking at things I took the distributor cap off and noticed a lot of play in the distributor as if the gear is half worn.
Now for my question. Even tough the previous owner set timing at 13 btc with a total of 30 btc, would it have gotten more advance at times causing a valve to burn? This looks like a new MSD distributor. Have I got a cam causing me troubles?

Things I know so far.
told it was a 496ci motor
Weiand 177 Supercharger
Big Holley double pumper
EMI Thunder Exhaust
MSD Ignition
Lots of fuel system upgrades including 1/2" lines big pump. Auto fuel shutoff system In line and up front gauges.
Large sump oil pan

I don't know what mods have been made inside, but I'm guessing some work had been done there also.
As far as the install, all the work done is very clean and looks as though someone knew what they were doing at one time.
I plan on putting this motor and the bravo drive in a very clean Formula 242ls I just acquired. I've owned many boats over the years. But this is the first performance boat for me.
Thanks Steve

There is really no way to tell anything until you tear it down. Total advance is total advance. It won't go any higher. That doesn't mean it didn't lean out.

Total advance can go higher if when it is set the distributor is set with the gear slop to one side, and when the backlash goes the other way could it not advance the timing more?

When there is increasing play in the timing chain or gears, it usually ends up retarding ignition and cam events. More likely bad gas or leaned out.

OK motors apart. Not sure how it even fired up. Perhaps I should of check compression on all the holes when looking at this boat. Might of not gotten it as cheap as I did.
Anyways. Head gasket blown between 3 and 5. head OK. Head gasket blown between 4 and 6. Head Cracked and has a 1/4" deep x 1/2 wide creator between the cylinders. No Burned valves. All pistons and cylinders look almost new.
I thing I found is this isn't a 496. It's a 454 .030 over. Other then roller rockers, everything looks fairly stock on the inside. Only have the heads off though. Maybe the motor is just full of bolt ons.
MSD distributor gear OK. Centrifugal advance missing springs inside.
How can I ID these heads? I've got a casting number of: 3964290. and a stamped number of: RR1 25515. Do these number tell me anything?

A little more resurch and I find that these heads are 69-70 Chevelle, Vette, Camero BBC. 101 cc chamber.
I have TRW L2453 Forged pistons. Pistons look to be low comp pistons. 7.81. R these head ok for the blown motor?

Compression is usually figured using 119cc for head volume. Looks like your compression is over 9 to 1. Not good for a blown motor in general.

Are those oval port or square port heads? That cc number sounds more like the closed chamber heads from the mid sixties. If so, they would only have value to a collector. Any pictures? How's the block? Deck should be resurfaced if the head was burnt that bad. Check it carefully for cracks. This engine was detonated bad.

My opinion is the 454 with a 177 blower needs higher compression to crutch the blower. It works well in fact. I run 9.5:1 with 177. Anyway, you certainly will have a low spot in the deck where the head cratered so decking is important. Then make sure you use cometic or equiv head gaskets and set the quench to .038-.040" to prevent future detonation. I just got done fixing a sbc crusader with the same head crater from a blown head gasket and it was from bad/old gas. Shame on me for running it with that gas :lolhit:

I notice you are from Ogden Utah. So am I, I live up in N. Ogden Lakeview Heights area. I have a 88 Formula 242LS with a 454SC Clone. Pretty much the same set up you have. I would like to see your boat. pm me.

Blue Thunder, while I'm sure it can be done, I'm not sure if it's a good idea to recommend 9.5 and iron heads and a blower without providing a lot of detail. I wouldn't attenpt that without an intercooler, good 93 octane perhaps with a splash of race gas, no thermostat to keep the head temps down, timing, fuel mixture, etc. When the 525SC came from Merc. it had 7.5 compression.

Change to an intercooled set up. Measure how far your pistons move down in the bore from TDC to determine your stroke. It may be a stroked 454 which will be i think a 489 when only bored over .030"

I agree patriyacht and in fact do run alum heads. I didn't do the calc on his compression but thought someone said it was 9:1 which would give an equivilant result (detonation wise) to mine with 9.5:1 and alum heads. That is if the quench is set tight and the ring end gaps loose along with the proper setup of the other items you mentioned. I don't use an intercooler.

It Isn't Stroked at all. Already measured cause I was told it was a 496.

I did all the measuring for the compression ratio. Heads were 101cc. Piston dish was -8.5cc. Piston was .025 below deck at TDC. Head gasket 4.375" x .035 thick. Cylinder diameter 4.280. Stroke 4.00" Ratio = 8.71 to 1

Ok, dish pistons. Then it was just bad tuning or bad gas. Looks like your block was never decked. Thats lucky. Toss that odd head and get a pair of more recent heads with 119cc chambers and a set of flat top pistons.