I'm not an expert here, but I have run a 454 with a small blower for a while now. What were the exhaust temps? The boost is lower than expected because the heads probably flow well.

Tom what kind of Torque numbers did it make? At what rpm did the Torque start coming alive?

I don't have the numbers in front of me to be exact, but I believe we were 1190-1200 across the board on the last pull.

Yes, the heads flow really well, but the blower was being overdriven about 2:1. It should have made a good bit more even with the big cylinder heads.

630 at 3600 was the peak. The spreadsheet was capturing from 3500-up, but we were well over 600 under 3K. Very flat too.

IMO, that small supercharger is not doing much but building heat spinning that fast and it still can't keep up with the heads along with cam timing mismatch for the setup. Put some timing in it and see if it helps.

Also, there is a reason Merc used a larger supercharger for 600sc.

I know it's not common practice these days but what did the plugs look like? Heat range? Did you ever try 93 and some timing?

It has been suggested that upping the timing some might help (thanks Joe). I might try that when I've got the boat back together in a week or two.

Tom, I certainly think with 2:1 overdrive, and only 5lb of boost range, the blower is severely tired, or something else is wrong, like valvetrain issues. How did the boost look at 3500rpm compared to 5500 rpm?

I can tell you, on my 468's when I had 250 B&M's, that 63% over, netted 5lbs. 80% over, netted 7lbs. 100% over, should have netted 9lbs of boost theoretically. The only reason I can see that blower you have having a 2:1 ratio, is if it was on a big ci engine, or had an intercooler and was running over 7psi in the manifold on a smaller ci .


I can tell you from some testing data I have here from when B&M developed the 250 blower, that increasing overdrive from 60%, to 110%, the manifold temperature at 6000RPM, increased from 140* at 60% over, to 230* at 110% over. Hot air kills power.

If you find no valvetrain issues, or any other issues, and get the blower rebuild back to spec, and make that 5lbs of boost with 60% overdrive, you'll find your horsepower you are missing.

Heres a thread you might like Tom.

http://www.offshoreonly.com/forums/g...0-refresh.html

Joe, no change in boost. Flat at 5 to 5.5. That's why my buddy asked me about where the blower came from.

Exact setup except for cubic inches (his 496) made over 700 HP with a larger pulley. Same heads, 250 B&M, 525 EFI cam, 30 degrees of timing.

Just out of curiosity, how much effort should be used to turnover a 250 by hand? This one is extremely easy to turn. Anyone have a rolling torque spec?