Witch carb to choose: 450 or 600 CFM???

[Pevito2]

I have a 20 footer V boat, with a mercury / Ford 351W engine, recently overhauled, and runs flawlessly. The drive is an OMC Cobra.
It has a Rochester 2bbl carb, 300 CFMs, very old an i am considering changing it to a new Holley 4bbl marine carburator. I intend to have more power and better fuel efficiency.
The newer models, OMC or Mercurys 5.8, have a Holley 4bbl 600 CFMs carb, but performing all those calculations with formulas, like on the link bellow :
http://www.buicks.net/shop/reference/carb_cfm.htm
It says that my engine will require no more than 420 CFMs air flow to run at 4600 RPM (max recommended) if i had a 90% volume efficiency.
The question is:
Wouldn´t a 450 CFMs carbutator be a better option for me, instead of the 600 CFM??
I think it will iddle better, run better on the midrange (2800-3800), and have a better fuel efficiency, without being a restrictor of the power developed by the engine.
Is there any specific intake manifold, for marine use i should order??
Dos anyone have experience with that?
Thanks in advance!!!!

[drewpster]

P, I have seen the tables before and I have experience with carb selection problems. Here is what I know. 99 percent of small block engines with 4 barrel aftermarket carbs are 600cfm and up. Here is why, it is true that by the math a 350+/- ci engine should only need 450 cfm carb. (In fact your boat builder thought it only needed 300cfm.) The reason guys put on the bigger carb is because it will mean more power. It is easier to lean the mixture from a bigger carb to tune it than it is to richen a carb that does not flow enough air. Assuming you stuck with the factory engine set up, ie. original intake, factory heads, and the original exhaust you have basically three choices.
1) You can stick with the factory set up, clean your intake out and get a replacement carberator. This will allow a simple bolt on and go approach.
2) You can get an aftermarket 2 barrel intake that has improved flow and a 450 Holley. This may take some simple tuning to get it right. And will provide a little more power. Or,
3) You can get an aftermarket 4 barrel intake and the 600 Holley. Generally 600cfm Holleys come out of the box with a set up that is supposed to work with small block engines. The consideration here is the fact that when Holley selects the jets, pump, squirters, power valve and so on, they are selecting parts based on the math of a small block engine flowing air/fuel on paper. More often than not you are not going to get a tuned carb out of the box because there are too many variables between Holley's set up and your engine. Also you are talking about using it on a MARINE engine in a boat. This complicates things even more! So to go this route and get all you can out of the bigger carb you will need to know that it will take some real world tuning to get the 600cfm working right. It is also very important to use a MARINE carb and parts. Marine stuff may look like car stuff on the outside but most often they are very different animals on the inside. A marine engine works under constant load where car engines work under varied loads. They are very different and should be considered so. If it were me I would go with the aftermarket 2 barrel intake and the 450cfm if I could find marine versions of them. This would provide just enough of a power upgrade without getting outside the design of the engine. ANd it would provide the least amount of tuning headaches.

[drewpster]

Carbs: http://www.holley.com/0-80364.asp
Intake: http://www.castheads.com/ford_289_302_351w.php
Drew

[GAWnCA]

Originally Posted by drewpster

Carbs: http://www.holley.com/0-80364.asp
Intake: http://www.castheads.com/ford_289_302_351w.php
Drew

Hey Drew....

I've got a 600 cfm Holley on my 454 and it seems to be just fine. I'm not out to race just have fun and it seems to me that I would save some gas with the smaller cfm rating.

Greg

[tuantom]

I talked to a tech @ holley about this setup last year. I was given a 450 cfm marine carb that was off a 302. He told me it would work on my 351; but it would run lean on the top end.
- Tom