2bl vs 4bl

Khris Kraft

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Sep 13, 2010
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Hi. I'm looking for thoughts opinions experiences concerning switching from a 2bl to a 4bl carb. My 5.7 Volvo penta GS in a 21ft Chris Craft bow rider has a 600cfm 2 barrel on top of a factory 4 barrel manifold with an adaptor. Besides the regular answer of "using more fuel" is this worth doing? My calculations on the amount of air a stock 5.7 can process is about 650cfm. But I could be wrong. I wonder of the thoughts that volvo penta had by supplying the 4bl manifold.
 

Bondo

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Ayuh,..... The 4bbl carb will yield 'bout 20/ 25 hp over the 2bbl. especially in the mid to upper rpms,.....

Donno why Volvo does that, but it sure is Nice, not to have to replace the intake, to drop on a 4bbl,.....
 

mr 88

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With the right carb a 4 barrel will use LESS fuel than a large 2 barrel
Right up to the top speed of the 2 barrel , when you go past that point with the 4 barrel you will burn more fuel . That's the fun part for many, (mostly) new to boating.
 

Khris Kraft

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What 4 barrel would be best? Vacuum secondaries or mechanical... and what CFM. 650 750 ? Single or double pumper?
Right up to the top speed of the 2 barrel , when you go past that point with the 4 barrel you will burn more fuel . That's the fun part for many, (mostly) new to boating.
With that said... if I'm reaching max engine rpm with my 2 barrel and I never run it there. Where when and how will the 4 barrel improve. Will it just get me to the same top rpm faster?
 

mr 88

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You will see performance gains in your hole shot, cruising speed and top end as you are now producing a bit more HP. You may have to go to a higher pitch prop depending on how close to the max rated rpms you are with the 2 barrel. If your a couple hundred under it now you will be good , if your right on the redline you may have to go that route.
 

Scott Danforth

Grumpy old guy who plays with boats
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What 4 barrel would be best? Vacuum secondaries or mechanical... and what CFM. 650 750 ? Single or double pumper?

With that said... if I'm reaching max engine rpm with my 2 barrel and I never run it there. Where when and how will the 4 barrel improve. Will it just get me to the same top rpm faster?
for general boating, a 650-700 cfm vacuum secondary. (Volvo ran 715cfm vacuum secondary props for their 275hp and 310hp V8's for years). however those motors had a slightly different cam and heads than the typical mercruiser.

a spread-bore is slightly better than a square bore for the average boat with a stock motor for a better economy on low cruise stand-point.

you do not want a double pumper unless you are talking a hi performance application and you know how to tune it.


you will have more mid-range torque and you may be able to change props

then again, if you are not using the potential of your motor as it is now, dont worry about it. those of us that cruise at WOT for long periods want every ounce of power we can get.

If you are not
 

Khris Kraft

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Ok thank you. And thanks to all who responded. I was getting stuck on my calculations of the actual amount of air my engine could process.
 

Scott Danforth

Grumpy old guy who plays with boats
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plenty have swapped

Holley Marine or Edelbrock Marine carb

will need to tune it to your engine (no such thing as a carb perfectly adjusted out of the box)

Volvo set up the motors so they came in ready for either 2 barrel or 4 barrel. that way they could change as needed for the application.

if you want this in the volvo section. just flag your thread and the mods will move it.
 

Khris Kraft

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Sep 13, 2010
Messages
44
for general boating, a 650-700 cfm vacuum secondary. (Volvo ran 715cfm vacuum secondary props for their 275hp and 310hp V8's for years). however those motors had a slightly different cam and heads than the typical mercruiser.

a spread-bore is slightly better than a square bore for the average boat with a stock motor for a better economy on low cruise stand-point.

you do not want a double pumper unless you are talking a hi performance application and you know how to tune it.


you will have more mid-range torque and you may be able to change props

then again, if you are not using the potential of your motor as it is now, dont worry about it. those of us that cruise at WOT for long periods want every ounce of power we
 

Khris Kraft

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Ok thanks... the guy in the next slip to me says he runs is engine 5.7 Mercruiser for 2-3 hours at WOT. I thought he was nuts. But I guess many people do?
I would run mine for maybe a minute at WOT but never longer than that. Am I missing something or am I just overly protective. Been powerboating for about 40 years with a few different types of engines. My last was twin crusaders 454's in a 34 searay. I would sometimes crank them to WOT for maybe a minute then pull back to a 3400 rpm cruise. I just can't imagine running anything at WOT.. I even pull back on my JD lawn tractor😝😝😝😝😝
 

Scott Danforth

Grumpy old guy who plays with boats
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Am I missing something or am I just overly protective. Been powerboating for about 40 years with a few different types of engines. My last was twin crusaders 454's in a 34 searay. I would sometimes crank them to WOT for maybe a minute then pull back to a 3400 rpm cruise. I just can't imagine running anything at WOT.. I even pull back on my JD lawn tractor😝😝😝😝😝
you are overly protective. Marine motors are designed to be run at WOT for hours. I know many of the engineers at Mercruiser, Ilmor, Indmar, CAT and Cummins (my contacts at Volvo and John Deer are no longer in the marine group). They are tested at WOT fully loaded for weeks

same goes for your lawn tractor. regardless if its an older JD with the Onan engine, or one running a Kawasaki, Kohler, Suzuki, or Briggs, its designed and tested at WOT.

what kills motors is not high RPM operation. its lugging them at low speed and lack of maintenance.
 

Khris Kraft

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you are overly protective. Marine motors are designed to be run at WOT for hours. I know many of the engineers at Mercruiser, Ilmor, Indmar, CAT and Cummins (my contacts at Volvo and John Deer are no longer in the marine group). They are tested at WOT fully loaded for weeks

same goes for your lawn tractor. regardless if its an older JD with the Onan engine, or one running a Kawasaki, Kohler, Suzuki, or Briggs, its designed and tested at WOT.

what kills motors is not high RPM operation. its lugging them at low speed and lack of maintenance.
Wow...OK. Thanks . I still can't imagine doing it but at least I know now that it's ok.
 

Lou C

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Nov 10, 2002
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7,554
Just make sure the engine oil is full and your cooling system is good first!
The 4bbl is superior above 3500 rpm. When the huge secondaries on a Quadrajet open you will know it by the push in back and the HOWL....and running on the small primaries, it is actually more economical than a standard 2bbl.
I was always shocked how economical my 4.3 is with the Quadrajet.
You can see the size difference in the primaries vs the secondaries in this pic. The Holley and Edelbrock are both square bore carbs but Holley does make a spread bore 4bbl for marine use (4175) that Volvo used on some V8 applications.
 

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