454 420 question...jetting and timing...
#12
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I have a lot of experiance with 420's
Just a few things you need to think about
Stock comp ratio is 8.8 with the stock 990 head which is 119cc
Most Merlin heads come 124cc or more
At 124cc comp ratio is now 8.4, that is assuming you used the same thickness head gasket
Next is the intake runner of the Merlin heads WAY toooo big for a 420
Single plane intake manifold will make less power under 5500 rpm than the stock dual plane high rise, more important the torque is MUCH better with the dual plane.
Last is the Jetting of the HP500 carb. It is really 830cfm but merc changes the jetting to stagger left and right in both the primary and secondary's call Holley and jet the stock jetting for that carb and leave it
Timing with that low compression 36
Just a few things you need to think about
Stock comp ratio is 8.8 with the stock 990 head which is 119cc
Most Merlin heads come 124cc or more
At 124cc comp ratio is now 8.4, that is assuming you used the same thickness head gasket
Next is the intake runner of the Merlin heads WAY toooo big for a 420
Single plane intake manifold will make less power under 5500 rpm than the stock dual plane high rise, more important the torque is MUCH better with the dual plane.
Last is the Jetting of the HP500 carb. It is really 830cfm but merc changes the jetting to stagger left and right in both the primary and secondary's call Holley and jet the stock jetting for that carb and leave it
Timing with that low compression 36
Last edited by Full Force; 05-09-2012 at 03:53 AM.
#13
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Yea, he's a good guy, I have redone the whole boat besides paint, thats coming soon.... saw him last summer at his shop and showed him pics of the boat flying... he just laughed and said he never ran it like that....poor boat..