2 bbl carb vs 4 bbl carb

[john cole]

I currently have a 4.3 l engine 1985 with a 2 bbl (non vortex), I have the opportunity to pick up a 4bbl off of the same vintage 3.8l engine. Which brings up some questions, first being with the intake manifolds swap over? Next at say cruising speed (not with all 4 bbl going) will the 4bbl carb still use more fuel. Next is any real reason not to install it?

 

[MikDee]

No reason not to do the swap! As long as the secondarys are still closed at cruising speed, then it should have compatible fuel usage. The thing is, originally you're gonna be into the throttle! but this too shall pass. IMO, all V6's should have 4bbls on them at least!

 

[Howard Sterndrive]

intake will swap just fine. go for it.

 

[Boomyal]

.......As long as the secondarys are still closed at cruising speed, then it should have compatible fuel usage. The thing is, originally you're gonna be into the throttle! ........

Amen to that. That is what happened to me when I went from an 8:1 compression 2 bbl 302 to a 9:1 compression 4 bbl 5.0 roller motor. I was tickled pink until I watched the gas gauge sink like a lead weight. After that, I learned to stay out of the secondaries and the 4 bbl set up was as economical as the 2 bbl was.

One question for the poster, is the 4 bbl a marine carb?

 

[gottaboatnow]

Just my humble opinion, since the primary barrels of the 4bbl setup are actually smaller because you have the toilet flush capability of the secondaries, your economy will actually be better than with the 2bbl setup. As everyone else mentioned, you'll have a lot of fun with the extra power at first, but you'll get used to it. If you need to get rid of the 2bbl, I was wondering if it's a Rochester 2G. Since I swapped my 3.8 for a 4.3, it would be nice to have a carb that was tuned for that motor, although mine is a Vortec-headed 4.3. I passed on a 4V Edelbrock Intake with carb a while back and am kicking myself for doing it now.

 

[gottaboatnow]

What I meant to say is that if you keep the secondaries closed, you'll have better economy than what the 2 barrel gave you. Of course, that all goes out the window the moment you make that thing roar.

 

[Walt T]

just so there are no delusions, it will not go any faster with a 4bbl.

 

[midcarolina]

just so there are no delusions, it will not go any faster with a 4bbl.

Several of the posts prior to yours seem to disagree with yours........ I am not sure how in the world going from a 2 barrel to a 4 on a properly tuned motor can't increase performance.........I guess you are saying the additional HP is somehow poof lost........If this was the case why in the world would they put 4 barrels on OEM apllications, why wouldnt they just save the additional cost and just put 2 barrels on everything.

 

[Idlespeedonly]

Several of the posts prior to yours seem to disagree with yours........ I am not sure how in the world going from a 2 barrel to a 4 on a properly tuned motor can't increase performance.........I guess you are saying the additional HP is somehow poof lost........If this was the case why in the world would they put 4 barrels on OEM apllications, why wouldnt they just save the additional cost and just put 2 barrels on everything.

What Walt is saying, is as long as your WOT rpm is where it is supposed to be you will not gain any top speed. The only way you will gain any top end would be to put a bigger prop on it.

 

[midcarolina]

Two things..... one, you will sure as hell get to that WOT rpm quicker..... and two the addtional HP may well mean you need to change props........ I would think that would go with out saying......... That post leads people to believe that there is no difference from a 2 to a 4 barrel and that is just false information.

 

[Boomyal]

Two things..... one, you will sure as hell get to that WOT rpm quicker..... and two the addtional HP may well mean you need to change props........ I would think that would go with out saying......... That post leads people to believe that there is no difference from a 2 to a 4 barrel and that is just false information.

A big ditto! ....that assumes that you do not over carburete the motor with too big a 4bbl.

 

[bruceb58]

There have been a number of people on here that have done this mod and didn't see enough difference to make it worth the expense of changing since you need to change both the intake manifold and the carb. Do a search. You might get 20 more HP. A rule of thumb is 10 HP will get you 1 MPH.

If the parts are really free then I don't see a problem.

 

[john cole]

well, finally got around to changing the carb over to the 4bbl, and yes it is a marine one, it and the manifold came off a 3.8L motor. My 4.3l is listed with a 4 bbl option, so it shouldn't be over carbed. If I'm running my 17p prop I can get to my WOT pretty easy, but if I put on the 19p, the motor isn't making it to WOT. I just added some trim tabs to it and they made a whole world of difference to the perfomance. To answer another comment I got both the carb and intake for $51 dollars. I but another $50 approx into a carb kit and some misc and now getting ready to go get it wet today. As soon as I finish getting the idle smoothed out a little better. which brings up another question, is there and fine tuning tips on these Rochester quad jet carbs I should know about?
To answer another question about my 2 bbl carb I now have and letting it go, let me run this 4 bbl for a bit and see if I like the results and make my mind up.

thanks for the help.

 

[john cole]

Just got back from the sea trials with it. Talk about a huge performance difference. It does get to WOT faster and will actually over rev with my 17 pitch prop. With the 19 pitch prop it come right up to WOT rpm range and give me about 6 or 7 mph more top end speed, it is hard to tell today, between the 2 to 3 ft rollers and other boat waves, kind of tough to hold any real speed. I also ran it about 50 miles to day and it appears to have averaged 3mpg with the 17 pitch prop at cruising spped, I will assume the mileage will go up some with the 19 pitch on there. thanks for every ones advice.

 

[MikDee]

Sounds Good! Another Happy Boater!

 

[midcarolina]

So much for the" it won't go any faster" theory..............Thanks for the followup

 

[Howard Sterndrive]

nice!!
I had similar gains on my V6 going to the 4 bbl.

 

[Maclin]

I think, John, that you mentioned in another post you also reset the timing? Good job on making all that an improvement you can feel

 

[bruceb58]

I think, John, that you mentioned in another post you also reset the timing? Good job on making all that an improvement you can feel

I was wondering the same thing. If your timing was far off with the 2bbl carb, wonder what it would have done with it set correctly. That way you really know what the 4bbl vs 2bbl difference was.

 

[john cole]

I was wondering the same thing. If your timing was far off with the 2bbl carb, wonder what it would have done with it set correctly. That way you really know what the 4bbl vs 2bbl difference was.

I wonder some about that myself. But one thing for sure, I'm not about to switch it back to find out :faint2:. As far as where the timing was set before, i was trying to find the posts on that from before, but I can't seem to find it, so I could see who gave me the original specs I set it to. but either way it is running like a champ now, it even sounds "good", before it sounded "OK".