Project cont's should I go 2bbl or 4bbl

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krisnowicki

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I bought a long block for my siezed block
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=140483406251

Boat was the merc 250hp 350 engine. It has a 2bbl carb

I am thinking of swapping to a 4bbl how much of a difference will it make? I will be using a marine carb. And the engine is fresh water cooled, so now worries about corrosion. Is it worth it or will it turn the engine into a gas hog? I know the mpi version of this was rated at 300hp
 
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dan t.

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Re: Project cont's should I go 2bbl or 4bbl

Go for the 4bbl, Q jet has smaller primarys than a 2bbl so better throttle responce and remember the secondarys are there if you need them, you dont have to use them:D
 
redjmp

redjmp

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Re: Project cont's should I go 2bbl or 4bbl

You will need a vortec manifold as well as the carb.
You have closed cooling, so you can use an aftermarket aluminum one from edelbrock or performance products, but if you were running raw water cooling, you would need the stock cast iron one from merc or an aluminum one that has had the water passages bronzed.
 
Bondo

Bondo

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Re: Project cont's should I go 2bbl or 4bbl

Ayuh,... About 30hp....
 
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krisnowicki

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Re: Project cont's should I go 2bbl or 4bbl

4bbl it is I like 30 hp more... so what cfm ? do i need 600 650 750? what do you think?
 
Bondo

Bondo

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Re: Project cont's should I go 2bbl or 4bbl

Ayuh,... I'm partial to the Edlebrock 1409,...
It's a carbon copy/ interchangeable with the Merc/ Weber carb...
600/650cfm...
 
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jkust

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Re: Project cont's should I go 2bbl or 4bbl

Hands down the 4bbl.
 
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John_S

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Re: Project cont's should I go 2bbl or 4bbl

4brl, and get the 1409 unless you have particular tuning experience with others. The edelbrock performer vortec eps should be a good intake for you.

Looks like a composite timing cover. You need the latter water pump to clear the cover.
 
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crewscontrol

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Re: Project cont's should I go 2bbl or 4bbl

I vote for the 4BBL too, but in order to get maximum benefit, you should change out the cam too. I know that is a PITA and I probably would not change the cam either, but in order to get max HP out of the 4BBL, the engine needs to be able to "breathe in" all the extra CFM's a 4BBL will provide.
 
Bondo

Bondo

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Re: Project cont's should I go 2bbl or 4bbl

crewscontrol said:
I vote for the 4BBL too, but in order to get maximum benefit, you should change out the cam too. I know that is a PITA and I probably would not change the cam either, but in order to get max HP out of the 4BBL, the engine needs to be able to "breathe in" all the extra CFM's a 4BBL will provide.
Click to expand...

Ayuh,... As with Any engine build, there's always horsepower left on the table...
But,...
It's an Honest 30hp with Just the carb/ manifold swap....

No reason to worry about screwing up a good package by changin' the Cam....
Besides,..
We all know the exhaust manifolds are the biggest inpediment to makin' HP with a SBC, in a boat....
 
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Aloysius

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Re: Project cont's should I go 2bbl or 4bbl

The only real horsepower difference will be found above 3500 rpms or so...the point at which the engine starts to get breathing restricted. There will be a marginal improvement in fuel economy across the board with a 4bbl, as long as you keep the secondaries closed. Reason..the smaller primaries do a better job of fuel atomization.

The MPI engine horsepower is rated at a higher rpm, that is why it has "more horsepower"...at cruise speed (3000-3500), all 3 induction systems produce just about the same power.
 
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fossill

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Re: Project cont's should I go 2bbl or 4bbl

I respectfully disagree with the fuel economy part. The 2 bbl will always do better. It doesn't take much for those secondaries to start opening on a 4 bbl. No such thing as free horsepower.
 
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John_S

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Re: Project cont's should I go 2bbl or 4bbl

Also, keep in mind that the old 2 barrel is not the same as the 2brl Merc used on the 350 vortec. I believe the flow numbers are different, old 425cfm, newer 475cfm. Upgrading that 2brl will probably be more costly than the 4brl.

That old hp rating of 250hp/2 brl, must be a at the crank number. It is more like 210-220 hp at prop. The 4brl was rated 250 at prop.
 
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Aloysius

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Re: Project cont's should I go 2bbl or 4bbl

fossill said:
I respectfully disagree with the fuel economy part. The 2 bbl will always do better. It doesn't take much for those secondaries to start opening on a 4 bbl. No such thing as free horsepower.
Click to expand...

The opening point for the secondaries is easily adjusted, and should never be open at cruise. In any case, fuel in equals power out. Sorry, that argument doesn't hold water.
 
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fossill

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Re: Project cont's should I go 2bbl or 4bbl

Yup, and to get the power out you got to put the fuel in.
 
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dan t.

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Re: Project cont's should I go 2bbl or 4bbl

Go for the q jet carb, far better than than the Carter/Webber/Edelbrock, it has much finer tuning,better venturi system and is an all around better carb that will when tuned properly give better fuel economy
 
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Aloysius

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Re: Project cont's should I go 2bbl or 4bbl

Just because the secondaries open doesn't mean you're getting more fuel. Fuel flow is directly related to the velocity of the air through the venturi. A 4bbl. has the POTENTIAL to flow more air and fuel, but at engines speeds below the capability of the carb, 2bbl or 4 bbl will flow the same air and fuel.

A 4bbl like a quadrajet will provide superior economy on the primaries because of the high velocity air flow, triple boosters and excellent fuel atomization.
 
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John_S

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Re: Project cont's should I go 2bbl or 4bbl

dan t. said:
Go for the q jet carb, far better than than the Carter/Webber/Edelbrock, it has much finer tuning,better venturi system and is an all around better carb that will when tuned properly give better fuel economy
Click to expand...

I agree that it is technically a better carb, but because it is long out of production, you are at the mercy of the quality and skill of the rebuilder.

My experience with the qjet from a rebuilder, was that they didn't have the stated tuning parts included. My tuning experience with it, seemed to need more tweaking from changes in temp, humidity, etc. The Edelbrock (1 season) need only the typical minor adjustments on install.
 
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