My 8.55:1 static compression plus 8# of boost is a no go according to this chart Griff posted. Dyno chart says my blown 509ci made 855hp@6000 on 91 octane. This only makes sense if I use the dynamic cr which is 6.83:1 and that would allow the 8# boost figure. I have written down a final compression ratio 10.8 from one of my conversations with either BM or Jim Bersani. Am I correct in assuming that the dynamic cr is to be used or should I bore scope my pistons for damage? Engine has 8 dyno runs.


For reference:
https://www.gofastmath.com/Compressi...tio_Calculator

There`s many things to consider when it comes to Effective Compression ratio. It`s a general idea chart
I found the limits of my combo and if I had your set up I`d run boostane to be on the safe side .

I don’t know if I’d take that chart for gospel. Im running around 8-9lbs boost on my 525s with 8.75 static compression. Intercooler is constantly fed cool water but that wouldn’t drop two points of compression. I need to look up the calculation again.

Guess it depends if your filling your boat at marinas with 89 or with 93 on land. I build and dyno motors as part of my business, quite a few have whipple 2.3's and 3.3s where your going to run out of blower. On THOSE motors I ALWAYS shoot for 8.8-1 base, sometimes 9-1 base if its a 2.3 whipple. My own personal boat I used to have I ran 8.4-1 540 with procharger, thru the years each refresh I turned it up higher and higher. I was running 12-13 psi on 8.4-1 at 1000/1050 hp on 93, 13-14 psi on 80% 93/20& 110 at 1100 when I really wanted to push it. I freshened that motor every 3 to 4 years, around 100 to 120 hours. Never melted a piston, never tuned it into the 12s under boost, Used a basic efi , Mefi, nothing fancy. When I DID start doing my own tuning in 06 or 08, I found a certain dyno operator I had PREVIOUSLY paid to tune it left the ONLY safety it has, a knock sensor, turned OFF and was like that for a year.
Tune to a safe AFR #, watch for KR if its efi, I wouldnt hesitate to run 8 lbs boost on 8.55-1, dont turn timing to a crazy high number, dont lean it out into 12s under boost, Smitty

When I started playing with centrifugal blowers almost 30 yes ago, my old school machine shop looked up my 9:1 compression in the "chart". It was either 2 or 3lbs on 93 octane!!! I politely said ok, np. Picked up my parts and hit it with 12lbs for years!!! Lol.

I'd definitely fatten it up. Those BSFC and AFR numbers in my opinion are on the lean side for NA, but would not run them in a boosted boat.

Just looking for generalities here.....what is a 'safe' yet efficient target AFR number under boost (say 8-10psi)?

From an article in the RPM Outlet. According to this I'm on the money, just need to tame the AFR a bit. Something in my favor is a light boat, I'm thinking this engine will hardly notice there's a 25' cat attached to it.

For carbureted motors, the rules are slightly different. Carburetors deliver the vast majority of fuel in a liquid state, and as this raw fuel atomizes from liquid to gas, a chemical state change actually occurs. Due to this endothermic reaction, which draws heat and cools the incoming air, a carbureted motor can safely handle more boost than a comparable EFI/TPI motor. For carbureted engines with compression ratios of 9:1 or less and boost levels in the 8-14 psi range, pump gasoline works very well. Compression ratios of 10:1 and higher require lower boost levels, higher octane fuel, intercooling, or some combination of the above. Compression ratios in the 7or 8:1 range can usually handle 12-20 psi on pump gasoline.

I would be curious to know who developed the graph and if it was intended for boats or just automotive applications . I would find it hard to believe you could run 28 lbs of boost for 26 miles with a static compression ratio of 8 with out race fuel .