305 Intake 4bbl Carb Mod

[patrickk1]

Does anybody have any pictures of an intake modified for a 4bbl carb? I saw reference to it in the archived section of the forum and there's no photos there since its the archive. I see no reference to a 4bbl carb since then in the forums.

"I have converted several GMC V6 Magnum manifolds from 2bbl to a 4bbl, it's really a simple conversion that involves machining the base down flat then cutting out a square bore pattern followed by drilling & tapping 4 new mounting holes and bolting on a new 4bbl base plate which nothing more than a square bore 3/4 inch carb spacer. "No welding is required" in this operation, I can send you some pics tomorrow when I get home.
Pete Chronis Northern Cal."

Thanks

[bigblockv6]

Ok, here's the photo of the manifold.

[patrickk1]

Awesome! Thank you.

[bigblockv6]

One more pic from the top of the manifold.

[Clarke]

So, a while back I told you guys I took my truck to the local dyno shop and they said I could use larger jets on my stock 2bbl carb. I only bought the next oversized jets, because I read on this site that other members have actually installed smaller jets (or maybe one jet?). The dyno guys installed the larger jets, ran the truck on their machine, and said I could go up another size larger, but I didn't have anything larger.

Anyway, help me understand the differnce between putting larger jets in a 2bbl versus installing a 4bbl. I'm assuming the 4bbl allows more air to cycle through; therefore, you can mix more gas?

I don't have any desire to pursue the 4bbl mod, just curious what the difference is compared to rejetting the 2bbl. I may buy the next larger size jets if I take it back to the dyno shop.

Thanks,

Clarke

[patrickk1]

4bbl carbs have small primaries and large secondaries. If you keep your foot out of it, you can cruise on just the primaries. The secondaries kick in when you put your foot into it (assuming it's not a double pumper). The quadrajet 4bbl has even smaller primaries that most 4bbl carbs. To me, if tuned perfectly, an appropriate sized 4bbl would have a mpg advantage because the smaller primaries would allow a more precise air/fuel ratio at cruise speed but when you hit the gas, more fuel is going to get dumped down the carb from the large secondaries. Some people say 2bbl get better mileage, some people say 4bbls do. But it could be that some 2bbl just die off around 60mph and its a struggle to get to 70, so you rarely travel at that speed with a 2bbl, which would lower mpg due to wind resistance of the vehicle. I am speaking of carbs in general, not specific to the charateristics of how these v6's run.

More air/fuel would get to the engine from the larger secondaries, if they are open. Less air/fuel if you are going through just the primaries.

[turbobill]

Originally Posted by Clarke

Anyway, help me understand the differnce between putting larger jets in a 2bbl versus installing a 4bbl. I'm assuming the 4bbl allows more air to cycle through; therefore, you can mix more gas?

I don't have any desire to
Thanks,

Clarke

To understand, lets discuss the basics first.

An engine is an air pump. More air, more fuel, (mixed in the PROPER proportion) more power. The advantage of a 4 barrel carburetor over a two barrel is that the 4 barrel allows the engine to inhale more air at WOT than the two barrel, thus horsepower is increased.

Installing "larger" jets has nothing to do with what type of carburetor this is performed on. In my last paragraph, I mentioned "proper" proportion of air and fuel. This is commonly refered to as the air/fuel ratio.

This is a very technical subject, but instead, I will keep is simple to help you understand. If the air/fuel mixture is to "rich" (more fuel than is needed for proper combustion), fuel ecomony and power suffer, as well as more carbon deposits, oil contamination and spark plug fouling.

If the air fuel ratio is to "lean" (not enough fuel for proper combustion) power and driveability suffer, and the EGT (exhaust gas temperature) can become excessive.

Apparently, the dyno operator found the fuel/air mixture on your engine to be to lean. They may have measured it with an oxygen sensor or EGT probe in the exhaust, or just suspected it was lean by the way the engine behaved on the dyno. After the jet change, the figures probably showed "richening" the air/fuel ratio was improving the numbers (air fuel ratio, EGT, horsepower)the dyno operator was looking at.

[Clarke]

Thanks for the education guys...

Yes, I noticed the dyno guys cut a hole and welded a port into my exhaust Y pipe, assuming for their sensor as you described.