WAGS382

Leathl_WS6,
The 850's work great, I know that when you do the math 850 cfm is too big, but it has excellent throttle response and with the larger primary's I can get a little more rpm before the secondary's open. I have read that you want a larger carb with a dual plane intake because the signal to the carb is weaker, I'm sure the 800 would be fine, that is what is on the HP 500. If I was to do it all over I would consider the 800 because the 850 marine carb was about $100 more mainly because of the electric choke, but I milled of the choke horn after all.
On the cfm note I built a 355 for on of my old boats approximately 350 HP put a 600 cfm on it and it just didn't perform the way I expected, then I put on a old 780 I had which is the same bore as an 800 and the motor really came alive. It seems like these marine engines like more cfm than the formula in the Holley book recommends.

Leathl_WS6

Thanks again everyone, there is a lot of great information here. It was very helpful.

Crane cams suggested the 731. They claimed with the 468 CI 9.5 comp and the fact I'm looking at 5300 rpm the 731 was my best choice. The 721 only makes power to 5200. He thought this might hinder my 5300 redline.

Looks like others are running this cam whith good results as well.

I decided to go with the 731.

WAGS thanks for that info. I have heard similar results as to what you claim. That was why I wasn't considering the 750. I will stick with a 800 or 830.

RM thats a great link thanks.

Rob

Vmax

My blower motor 454 has a 731 cam on 454 mark IV with 5 PSI boost. When I bought the boat had 850HP style double pump with no choke. Looking at old installed photos of the original carb it was also boost referenced. Went to HP500 800CFM with choke horn. and boost reference. Carb jetting ended up the same but throttle response, fuel mileage and, top RPM all improved for the better. Gained 400 RPM up top, throttle response is such you are no longer waiting for something to happen when you hear the secondaries begin to open up ever body needs to be seated and hanging onto something. Can cruise out of the secondaries up to 3500 RPM. I marked my throttle plate with boat off so I know exactly when the mechanical secondaries start to open. A day on the lake results in 10 to 15 dollars less fuel usage than what the 850CFM produced. Don't go to big on carb with the 454.

I know a guy with a modified LS6 Chevelle he tried a 850CFM Holley and motor didn' t pull strong in upper RPM above 6 grand went back to the original 780 CFM stagger jetted and he says it pulls strong all the way to 7300 RPM. Can't buy a 780 anymore from Holley but 800CFM convinced me it it is better for the 454.

Cord

iTrader: (2)

Vmax-check your fuel curve. Many marine carbs go big time lean just before the secondaries open.

Dyno-I you may be right.

24outlaw540

I am new to the forum...Recommended by sledge hammer. I blew up my 502/450hp crate motor. I am having it stroked and bored .030. I am considering either the 731 or 741 cam also. I guess I am leaning towards the 741, but don't want it to be unpractical as a liesure boat. What are your thoughts on this?

24outlaw540

I forgot to mention../ I am stroking to a 540. I also need a recommendation on headers for this application. I have also been told I need hydraulic steering. Any thoughts on this?