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Riley
05-29-2001, 08:24 AM
Hey guys, I've finally made the decision to upgrade the 2bbl set-up on my 1999 5.0 Mercruiser to an Edelbrock Performer intake and Edelbrock 600 cfm marine carb. I know the majority opinion on here has been to wait and upgrade to a 350 or 383, but I'm really curious as to what (if any) gains can be had by upgrading my present intake system.

My 305 has a flexible copper tube that runs from a threaded fitting on the starboard exhaust manifold to another threaded fitting on the intake manifold, right next to the 2bbl carb. What purpose does this tube serve? Is it a coolant line, or is it piping hot air to the intake manifold? I don't believe the Performer Vortec manifold I'm looking at has provisions for this fitting. Can I eliminate this cross-over tube?

I'd appreciate any advice or input you guys might have. Thanks, Chris.

Forrest
05-29-2001, 12:05 PM
It sounds like that copper tube is for the choke heater. Your new carb should have an electric choke. If you haven't purchased you carb yet, you may want to consider a 450 cfm four-barrel Holley marine carb. You will be over-carbureted with a 600 on a 307 cid engine.

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Forrest

Riley
05-29-2001, 01:34 PM
Forrest, thanks for the reply. The copper flex tube may be a heat supply for a choke, but the current 2bbl has an electric choke, too. The tube doens't go into the actual plenum area, but sits dead center on the starboard side of the intake manifold. Could it be some sort of "heat supply" to quickly heat up the intake manifold and reduce emissions on start-up? Or, for that matter, could it be heating the intake to better atomize the air/fuel charge?

As far as carb size goes, the 600 cfm carb is the one recommended by the tech guy at Edelbrock. I realize that I'd have to spin the 305 to about 6800 rpm to justify a 600 cfm carb, but I also know there are a lot of 350 and 383 owners running 750 cfm carbs, with good results.

Thanks again for your input.

Murphy
05-29-2001, 09:32 PM
Howdy,

Forrest is right on all counts. Crossovers that heat the manifold whether internal or external serve to actuate the heat sensitive spring that controls your choke. Cold engine, more choke, hot engine, less choke, etc. When you go to an electric choke you will gain high end performance by blocking off this crossover. Your thoughts about a heated manifold improving performance are incorrect. In fact, the cooler your manifold and carburetor, the better fuel atomization you get. When cold fuel hits a hot manifold it actually condenses into droplets rather than remaining vaporized like it should. Hard to imagine, but true. A cast iron manifold runs cooler than aluminum. The original GM high rise dual plane is still the best manifold for a marine engine in both fresh or salt water. Since aluminum and cast iron expand and contract at different rates, an aluminum manifold is always trying to blow it's own gaskets. Keep in mind, a thick spacer between your manifold and carb is designed to keep the carb cooler, i.e., running at it's best. Finally, a 600 cfm carb is more than you need. If you see a 750 on an engine that size, it's just to satisfy the owner's ego and prove that he doesn't have a clue about engine math. Also, a carb that pushes excess fuel into your engine is actually washing the oil out of your cylinders which will cause your rings to fail in about half the time. Thems the facts. Good luck.

Murph

Riley
05-30-2001, 07:51 AM
Hi Murph, thanks for the info. I appreciate a good sharing of ideas! That being said, I'm still not 100% sure the copper flex tube between the starboard exhuast manifold and the intake manifold is for choke heat, since the current 2bbl carb is equipped with an electric choke. There is no other obvious mechanism for activating the choke on the carb. Why would an engine with an electric choke need to heat the intake manifold?

I understand the theory behind a cooler, more dense intake charge providing better power. However, I wasn't aware that a hot intake manifold would cause the fuel/air charge to tend to fall out of atomization. If anything, I was under the impression that the hot intake charge would reduce power due to being less dense (containing fewer oxygen atoms for a given volume).

HOT ROD magazine had an excellent article several months ago regarding superheating of the air/fuel mixture. When superheated, the air/fuel mixture became thouroghly atomized. This atomization provided vastly reduced emissions, as well as vastly increased fuel economy. This is where I was coming from in reference to heating the intake manifold, doing it for emissions and economy, not for increased power.

Regarding carb size, the math is indisputable. However, it kind of makes you wonder why Mercruiser would put a 750 cfm Quadrajet on their 350 Magnum engine. Especially when one considers that a 750 cfm carb is sufficient up to 7400 rpm on a 350 cid engine! Or to put it another way, a 350 can only flow 506 cfm at 5000 rpm. So, why wouldn't Mercruiser put a 600 cfm carb on the 350? It would still flow enough to support 5900 rpm. I'll tell you, some of this carb size info can be confusing, to say the least!

Thanks again for your ideas and input.

Looped
05-30-2001, 08:02 AM
One way around keeping an oversized carb would be to down size the jets. I’m not sure what Mercruiser uses for jets but they may get a great discount price on the more common size carbs (larger cfm than needed) and just down size the jets so that you don’t get that old bog.

What the hey, just a thought,
Craig

PaulO
05-30-2001, 08:08 AM
I bet it's all economics. Mercruiser probably gets thousands of Q-Jet carbs for pennies a carb and would not want to incur the cost of a different brand carb. May be the same for Edelbrock. Their carbs only come in 600 and 750 CFM don't they? If that's true, they are not going to suggest a 450 Holley.
PaulO