I would think that would be the upper limit. I have had 302 Ford street engines at around 9:1 CR in the 190 PSI range. These days a decently warmed over hot rod street engine runs about 200 PSI. You want to back off cranking compression on a heavier (over 3200ish LBS) car roughly 10-20 PSI and 10-15 if the engine is detonation sensitive which fatheads are, their are several more factors low octane gas, hot running ect but they don't really seem to be a factor in this case. So considering a 200PSI ideal and backing off 20 PSI for the cars weight and 15 PSI for detonation sensitivity you would be at 165 PSI and you would have some plus or minus wiggle room.

I have a couple questions and a couple possible answers. When you say 327 timing set I assume you are talking your timing gears and chain? In this engine you would ideally be somewhere in the area of 4 degrees advanced on the cam timing to be good (good performance that is) did you check the cam timing on the engine? It is possible you are one tooth off that would cause a backfire and/ or a miss.

With spark timing you can honestly make a timing pointer bring the engine to top dead center and make a tab that points to the 0 mark on the balancer or make one that points in a convenient location to check the timing and mark the balancer in that spot trust me it will work, race engines often have adjustable timing pointers.

When the head was repaired was it pressure tested? I have welded up then pressure tested many OHV aluminum racing heads only to find a pin hole or two that needed to be rewelded, that could cause a "miss" that feels similar to spark knock.

Have you run a compression test on the engine? This would tell you if you have a dead cylinder (blown head gasket or possible crack somewhere) and it will give you an idea if your compression is off the chart, 170ish PSI would be good anything over 200 PSI you probably have to much compression.

A bent valve will give you a miss and it it is slight it will still run slowly but when you put your foot into it it will get worse.

You don't want to modify the combustion chamber to much, the 54 head is the best designed chamber for the Packard line and one of the best combustion chambers in any flat head. You need some clearance over the valves to get good flow but the aluminum is not thick in that area.

A thick head gasket will reduce the quench on the engine and you will lose a lot of performance run stock thicknes. Flathead engines don't need a lot of spark lead so minimal if any spark advance.

You stated that the octane was around 109? This engine doesn't need anything that high. Remember higher the octane the more resistant the fuel is to igniting, that is why you can throw a lit match into a bucket of diesel and it will likely not ignite. I have friends with racing Ford flat heads that use 100 octane unleded aviation fuel, and that is with a 12V MSD ignition system with a lot of spark advance and compression.

Are you running a 6V ignition and points? There is a good chance that with the super high octane and a weak ignition system (say what you will but 6V ignitions are very weak) your flame is going out or firing inconsistently. Again good chance that this could cause your problem.

Did you use any sealer in the head gasket? With old aluminum and cast iron you get a lot of expansion rate difference and that puts a lot of strain on the head gasket their is this thick tar like sealer that is (was?) used in the aviation world, cant remember the name but might want to look into it.

The Hudson racers use to have head gasket problems with their aluminum heads back in the day and they would take a center punch and put a ridge around the combustion chamber, for them specifically around the valve area, it was like an early form of an O-ringed block/ head that you see today in race engines.

Are their any numbers internally or do you happen to have a picture of one or the other?

Has anyone had a L8 cam reground for performance? If so I would like to see your cam card or hear what lift ect you were able to get out of a stock core

Trying to ID my distributors, I know there was a slight cross over in the early 50s of the Delco and Autolite pieces, sadly my ID plates are corroded to the point I cant read them. One distributor is a 1953 Clipper 288 and the other is a 1952 250 club coup 327. any guesses?

That is a beautifully detailed engine.

Thanks for the info, I was thinking it needed some support on the other end I was considering using two stands or fabbing some kind of rotisserie setup but 2x4 sounds much easier. You say the V8 is heavier than the L8? what are the weights on each if you have the info available, I was told (don't remember where)that the L8 was close to a thousand pounds assembled, seemed high but the engine does look very heavy.

Has any one used a store bought engine stand for their 50's era L8? I have a harbor freight unit that has a hard time with a modern small block V8 and I know it will fail with the weight of an assembled L8 hanging off it but I really don't want to build a custom one if I don't have to, any tips?

It seems like a very simple straight forward build noting exotic, I like that and the car is gorgeous. Are you running the aluminum head or a cast iron piece? If you have it available would you be able to upload the cam card from the comp regrind?

IS the engine mostly stock with the mentioned port work or do you have other modifications as well?