I helped a oso member out with similar builds. His was a 454 Mag to start with. He used a crane cam I suggested (flat tappet), weiand 177s, engine made 570HPand 600Ft lbs, with only 4.5psi of boost. So adding a blower and a cam swap, he picked up quite a bit! The 250 blower should make a tad more.

I would run twin 750 carbs on the 250's if possible. Upgrading to a hyd roller cam, a intercooler, 650hp shouldn't be a problem with 6-7psi. Aftermarket heads, 700+.

925hp a side with 420 blowers. Idles and runs like fuel injection!

Boat is a totally different animal, love when it starts getting into the boost (the whine is awesome too). I spoke to brett @bblades and he recommend I go with a labbed 28b1 from a beat up 26. I picked up 10mph so far (70) without really stretching out the legs and he said I have too much slip and should be upper 70's. Thanks again Joe.

Wayne272 how is it around the docks as far as starting and idling? How much more fuel does it use out of curiosity?

Anytime Wayne. Im sure you'll be picking up more with better props.

Another satisfied Mild Thunder customer

Attached is our data sheet representing a 548/420 mega blower build sheet for OSO member Carlton Johnson. The build was completed and tested in June 2013 and has just concluded its first complete season in a Daytona 25.

• 4.500 “/ 4.250” 548 CID

• 8.2:1 mechanical compression ratio

• AFR cylinder head package

• MarineKinetics/Morel .842” valvetrain

• 420 Mega blower w/ 2 x 750 CFM carburetors ( now in conversion to Holley EFI)

• Peak power boost 160.3 kPa (8.75 psi boost)

• Peak power 1001.8 @ 6200 RPM / Peak Trq 927.3 @ 4700 RPM

• Avg. Trq. 3500-6200 RPM pull = 96.9 % peak Trq.

The engine is a very docile, tractable piece, is demonstrated in the YouTube video.
http://youtu.be/C0xDoTxUItU I am confident the Holley EFI system will take it to the next level in tuning and engine management. It’s noteworthy that delta between the glory pull and the safe tune (boat) pull are only 11 hp apart.
I have attached the dyno test sheets validating the build.

Congratulations to Carlton on a well thought out piece displaying exceptional attention to detail.

Bob

bob,those are good numbers,what afr head was used on this build?

Stout!! Again, the 420's aren't given enough credit.

Bob, do you recall the drive ratio on that setup to achieve the 8.75psi with the chiller on that 548? Hopefully in the next couple seasons I will build some 540 shortblocks and reuse my 420 blowers.

It also looks like they were dyno'ed with imco manifolds??? That's impressive to say the least.