4bbl carb swap

yjmechanic · **Joined:** Tue Jun 23, 2009 11:50 am **Posts:** 30

Hey guys, searched around and surprisingly haven't found much on this. I have an 05 5.0gl, and really have no problem with it but am just talking in theory right now. Would a 4bbl carb change much in the power department? If so would it make fuel consumption suffer more than the performance gain would be worth? I am only wondering cause when i looked at my carb it looks like a 2bbl sitting on a plate that is blocking the wholes for a 4bbl carb. I know how manufactures work and i would easily believe they used the same intake with a block off plate in order to only have one intake manifold to worry about. This is the main reason i am talking about this is that it looks like a 4bbl would bolt on and actually not be a big deal to accomplish. So does anyone have some info on power difference, if the intake will work, or if the whole thing is even worth doing? I would prob not be doing this right away because everything is working fine but I like to think about what I may do if something were to need changing. thanks guys

lmac2 · **Joined:** Sat Apr 04, 2009 8:25 pm **Posts:** 26

Well "yj" I hae kicked this topic around this forum, never quite getting the answer i had hoped for. It seems that very few people (if any) have tried the performance upgrade, altough if you search for the thread you will get some good, reasoned insight. All that being said, you probably have a two barrel carb sitting on top of a two barrel manifold. It appears that way because most carb flanges ( sqaure bore or spreadbore) have the same bolt configuration (two or four barrel). Try this link for some great ideas.

http://www.michiganmotorz.com/marine-parts-accessories-intake-manifolds-c-32_37.html

Good luck!

LouC · **Posted:** Tue May 18, 2010 9:34 pm

Well the 4.3 in my old 88 was rated at 175 with a 2bbl and 205 with a 4bbl so that's 30 hp due to the difference in carbs. I have the 4 bbl version and I think of all the power upgrades you can do this one is worth it because the surge of power you feel when the secondaries open up is NICE.....

Kvou812 · **Posted:** Tue May 18, 2010 10:46 pm

I noticed my 5.0 is a 2bbl too and wondered about upgrading to a 4bbl. Doesn't seem like anyone has done it... wonder what it would take and how much. I think it would make a noticible difference... my old boat with the 4.3 4bbl felt like it had more get up and go, although in all fareness that boat weighed significantly less.

boater · **Posted:** Wed May 19, 2010 8:25 am

My mechanic mentioned that my 2bbl carb was sitting on a 4bbl manifold a few years ago. We never did anything about it, but it's been in the back of my mind.
What boat do you have ? I think you should try it and let me know what happens.

Kvou812 · **Posted:** Thu May 20, 2010 5:14 pm

I've asked a few online carb specialists sites this question:

"Currently have a Volvo Penta 5.0 GL V8 rated at 220 hp with a Holley 2 Barrel
carb. Would like to know what performance increase I would
see if I converted to a 4 Barrel carb. How much would this cost and what
would be involved?"

This is what I've gotten so far:

"Converting to a 4bbl would require changing the intake manifold. Installing
a 4bbl carbureotr with an adaptor plate is not going to give you that much
of a performance increase."

datingle · **Joined:** Tue May 01, 2012 9:53 am **Posts:** 2

I have an 08 Reinell 200 LSE with a 5.0 Volvo GL I got last year. It's carbureted with a 2 bbl Holley 500 cfm. I just discovered its on a 4 bbl intake! So . . . carb swap time!! I've been researching. To confirm your intake, take the intake model # stamped on top of your intake and google it. My motor is essentially a Vortec 5.0 with a Vortec 4bbl spreadbore, dual-plane intake a/k/a Edelbrock Performer, best for low to med RPM applications (up to approx 5,000 rpm). Looking at a 4175 Holley 650 cfm, vacuum secondary spreadbore 4bbl carb. The primaries are smaller than the 2bbl 500 cfm that is on it currently. The beauty of the spreadbore is that the smaller primaries are quicker off the line/out of the hole (than the larger 2bbl throttle openings), and the larger secondaries will only open as long as there is sufficient engine vacuum to open them (usiually only at higher RPM's under WOT conditions), preventing bogging down at lower RPM. All that being said, I've not found any dyno results from this carb swap in a boat, but I'm estimating from past hotrod experience that this swap would add 30-40 hp on this motor, which gets it real close to the rating for the GX (injected) motor. I'm also going to upgrade from the oem 1.625" tall by 8" dia flame arrestor, to a 3.75" by 8" arrestor, to open up the breathing on the top end. A hotrodding buddy of mine swears by another gizmo that goes inside the flame arrestor (air cleaner on his car) that looks like a funnel that slides over the arrestor mounting stud that bends the airflow from horizontal coming in, to vertical going down into the carb throat. Lastly, I'm going to route some aluminum (don't use plastic!!!) flexible ducting for a dryer vent, that I will route from a fresh air source, right up to the flame arrestor to bring in cooler air instead of the 150+ degree air in the engine compartment. Free HP!

ric · **Posted:** Tue May 01, 2012 11:04 am

http://www.barrygrant.com/demon/default.aspx?page=21

It's a safe bet to say your 2bbl V8 has a 4bbl intake manifold. Always get a vacuum secondary 4 barrel. Installing a carb isn't just plug and play, it will take adjustments of the jet sizes, metering rods, etc to run right. You will need to take it to a marine mechanic with a test pool to get it tuned right.

ALSO BIG ALSO!

Get the carb from Holley, not Volvo Penta. Half the price.
This is calibrated for the 4.3 V6: http://www.summitracing.com/parts/HLY-0-80492
$426 shipped.

You can call up Holley or go on their website to get the part # for the V8 4bbl carbs (5.0 or 5.7)

boater · **Posted:** Tue May 01, 2012 4:21 pm

Datingle,
Care to share what kind of #'s you're getting now?
I think it's a great idea, and make sure you let us know how it goes!

ric · **Posted:** Tue May 01, 2012 4:39 pm

There's no way to tell if you improved anything without a stopwatch and a marked course with multiple before and after runs in EXACTLY the same weather conditions. That will let you know if you improved the low/mid range power. carbs are very sensitive to weather. If the conditions are not identical your results will vary wildly. Air temp, water temp, humidity level, sunny or cloudy, wind direction, surface condition, and air pressure.

In order to find out if you improved top end power, you'd have to install a prop that was too large for the boat that kept it 500-800 RPM's shy of redline. Then, with the new carb go out on the lake in the same conditions and see if you get more RPMS.

Boats only have one gear, so if you find out that yes you gained a second or 2 in standing still to whatever distance sprint, and in top end.. you gotta decide. Do you run the factory prop that keeps the same top end speed as 2bbl carb and don't overrevv it with the enjoyment of faster out of the hole, or do you prop it bigger to gain more top end and have the same out of the hole before you upgraded the carb.

All this of course, after you had the new carb (and ignition) professionally tuned in a test pool. Just bolting one out of the box will probably make it run pretty bad.

boater · **Posted:** Tue May 01, 2012 4:45 pm

I think he'll be able to figure out top speed without a marked course.
If he has to put a bigger prop on that's a good thing. :mrgreen:

LouC · **Posted:** Tue May 01, 2012 7:09 pm

I have never tried two comparable boats back to back with a 2bbl carb and a 4 bbl carb to compare, but if you look at a 4bbl with the primaries and secondaries open, it's obvious its going to flow much more air and since an engine is an air pump of sorts, it will produce more power but above certain rpms only. I am not sure when the secondaries on my Q-Jet open but for sure you feel a surge of power at about 4000 rpm and with the right prop it reaches max rpms very easily. I think the 4 bbl conversion is about the only way to get more hp out of these engines, if the rest is in optimal shape.
The nice thing about the Q-Jet is the small primaries let you cruise economically at about 6 gph if you keep it under 3500 rpm, but it gives you the power when you need it.
We had a 72 Chevy Impala 350 cu in with a 2bbl Rochester, and a 75 Olds Delta 88 350 cu in with the Rochester Q-Jet 4bbl. Comparable size and weight. Same engine displacement. The Olds was noticeably peppier at highway speeds.

datingle · **Joined:** Tue May 01, 2012 9:53 am **Posts:** 2

Thanks for the posts, esp appreciate the input about tuning the carb/ignition after installation. I'm certain the intake is a 4bbl as I referenced the part # and it came up online, complete with pictures. I"ve got parts on order, so give me a few weeks to receive them, install them, tune up a bit, and hit the water, and I will let you know the results. One other thing I will look into doing, is installing a distributor "recurve" kit, that has different springs and counterbalances that mechanically advances the timing a little quicker than normal. We used to put these on our vehicles that had GM small blocks. I had a 350 4bbl in a Chevy 4x4 and installing the recurve kit made a noticeable difference in seat of the pants and throttle feel. Not sure if this is do-able on the 5.0 but will look into it. On the few times that I did top speed, WOT, it consistently ran 57 mph according to the speedo, until we garfed up the aluminum prop just a bit, and it promptly lost 5 mph off the top. Bought a stainless but haven't put it on yet.

www.iFourWinns.com

4bbl carb swap

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