tomcat

If a 540 with 100% V.E. @ 5500 RPM can only take in 860 CFM then it doesn't matter if the carb(s) are bigger. A marginal increase in CFM might occur due to the slightly lower pressure drop across the bigger carb(s) but compared to the total restriction of the intake pathway, that would not amount to much. You certainly cannot expect the engine to magically increase it's V. E. and use all the capacity of the larger carb(s).

I agree with Chris; you have to conclude that the bigger carb(s) or the tunnel ram are providing smoother(?) air flow and better fuel metering.

blue thunder

If you do the calculations on any marine production power package you will find I am correct with carb cfm upsizing in the 130% range. Maybe I am missing the boat when it comes to higher hp engnes.

Example using a 365mag with a 750cfm carb:

454 x 5000 x .85 / 3456 = 558cfm
750cfm / 558 cfm = 134 %


Also here is link that further refutes using theoretical cfm straight up to size a carb. Although I agree that increased cfm hurts throttle response. I am referring to marine WOT max speed applications here, which is what I am usually after.

http://www.edelbrock.com/automotive/index.html

Go to the technical information section on the left, then technical discussion section and scroll down to "CFM Rules".

BT

chromecat

Well.. here is another comparison... anyone know the specs on this cam... compared to the 651 crane that everyone seemed to choose here...

http://www.offshoreonly.com/forums/s...threadid=74464

Don

chromecat

what is the part number for the 651... i'm at cranes website and can't find it... closest thing i can find is the 168771 which is a 240/248 @ 50 and 621,632 lift

Don

PatriYacht

look in marine cams pt. #139651

PatriYacht

I have to wonder if this engine really makes 700 hp. Gm iron heads with a little exaust work? A flat tappet hyd. cam with ( I'm guessing because I can't find this cam on Cranes website) less duration than a HP420 cam. Low to mid 600's more likely, but also less maint.

chromecat

well since this thread has went so well... any debates on exhaust... I talked to CMI and they suggested sport tubes. dry to the transom.
what do you guys think

Don

PatriYacht

Great choice. Great looking. Big dollars.

cstraub69@comcast

When I "consult" on an engine package, it is from carb to pan. The whole engine is a working combination of parts, so the parts have to work together. The Crane cams referred to are good baseline camshafts that make good power on OEM style heads or heads with OEM flow numbers. With the new High Flow stuff available today, you can go with a smaller cam and make more power, better reliability, because the heads do the work, the cam doesn't have to.

BT,
Air is like a WC car on a short track, air can go fast as hell down the straights, but when it has to turn, it has to slow down. If you relate this to venturie size, I think you see what I mean 4150 to 4500 series carbs. I said in testing, the 4500 on both engines made more HP, but we lost torque. I don't care about HP, I look at torque. That was the reason for the 4150's 950's.

I'm know expert, just luck to have been involved and work in the HP enviroment as a Mfg. I'm glad I found this board.

Chris

chromecat

cstraub,
from what i have posted here, does this look like something you would recommend to someone or is there a direction that i am missing.. looking for good honest help.

Don