Very very impressive gains !!

Raylar is definitly on the right track with the 496's

Just switch the Basett headers to CMI's and you might be looking to another 3 to 4 MPH gain when right prop pitch has been found

Just to interject here again. The Bassett headers are good headers. In our testing we only saw about 10 to 15HP more with the CMI sport tubes, so unless 10-15HP is magic, I don't think you will see 3-4 mph using CMI's over the Bassetts. Don't get me wrong here the CMI's are great headers and we have seen about 565HP on our 525HP motors with CMI's kit and about 550Hp with the Bassetts. Pick your poison, both are good headers! If hatch clearance is an issue, the Bassetts win that battle by about 2". The CMI's are about 1.25" higher than stock 496 risers and the Bassetts are about an inch lower.

Ray @ Raylar

The BCK 103 Kit will add 150 HP to a Volvo 8,1 GS 375 HP or a 375 HP Merc 496 Magnum
You have to change or reprogramm the MEFI or PCM (Merc) to a 420(425) HP version
The BCK 103 kit will add at least 100 HP to a 420 HP Volvo or 425 HP Merc
We dynowed a Merc 496 HO with 542 HP @ 4800 RPM

Your units of measure for HP and TQ is different (N-M) and the curves will not cross at 5250 as we are used to seeing. Any conversion to lb-ft for TQ.

100 NM = 73.7561 lb-ft for TQ

806 NM = 594,5 lb-ft for TQ

the rev limiter is at 5150 - so 5250 is not possible