454 420 question...jetting and timing...

Reply Subscribe

Thread Tools

05-08-2012 | 10:37 PM

#11

legend

Registered

Joined: Jan 2012

Posts: 316

Likes: 4

From: Madison Ohio

Quote:

Originally Posted by Full Force

what props??

23 mirage but plates where mounted high. That boat would blow out and would have to start over on take off.

05-09-2012 | 03:29 AM

#12

Full Force

Thread Starter

Gold Member

Joined: Oct 2005

Posts: 11,688

Likes: 216

From: Olmsted Falls,Ohio Marblehead,Oh

Quote:

Originally Posted by billy boats

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

From what I remember the Merlin heads I have were just a tad bigger then stock 420 heads, I know there were a couple different runner sizes, been 5 years so I cannot recall, I used the heads and intakes I got because I got them free with a huge pile of parts I bought, I would have to think the intakes may be more an issue then the heads? I thought about putting in 36 degrees this weekend and see how it runs..??

Last edited by Full Force; 05-09-2012 at 03:53 AM.

05-09-2012 | 03:30 AM

#13

Full Force

Thread Starter

Gold Member

Joined: Oct 2005

Posts: 11,688

Likes: 216

From: Olmsted Falls,Ohio Marblehead,Oh

Quote:

Originally Posted by legend

Known Lester for many moons. Good man. Cuts the five hairs that I have left on my head. He loved that boat. You would have a lot of fun with boost!

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..