Carb CFM
 

Thought I'd bring this up, as today I was having this discussion with a friend, about carb sizing. For years, we have been taught to pick a carb, based on Cubic inch X RPM. Volumetric efficiency, was never brought up, nor is camshaft overlap.

You can take a 502, thats making say, 450hp, with a mild cam with minimal overlap, and maybe get by with a 800cfm carb. You then take a 502, that has ported heads, cam with a good amount of overlap, tuned exhaust, and maybe is making 650hp. The CFM demand, is now much greater.

I often see guys run an engine on the dyno, and the air turbine, might show the engine only used 790cfm, so their 800 carb, is good to go on all 502ci turning 6000rpm. You can simply swap the camshafts, to one with much more overlap, and now the CFM consumption, increases substantially, because some of that air, is being blown out the exhaust during overlap. It may, or may not, actually make more HP, but, the CFM demand, has now changed.

Then you have fuel atomizing. Just because a one carb may be rated at 950cfm, and the other at 800cfm, doesn't mean the smaller carb is going to atomize the fuel better. There is more too it. Booster style, venturi diameters, and so on.

A good example, is the old Quadrajet carbs. Some were rated at 800cfm if I recall, and they used them on some big CI Pontiacs back in the day, that never saw north of 5000 rpm, and were in very heavy vehicles, the engines barely made 200hp. Text book would tell you, that carburetor is grossly oversized, and would be a turd, poor fuel atomizing, sluggish, and terrible fuel economy. But they were anything but that. It was the carburetors design, that made them work.

Also, just because a carb may be a "800" cfm, doesn't mean that if you put it on an engine that requires say, 900cfm, the engine is gonna simply stop making power once it hits "800cfm" of air consumption on the turbine.

One thing that I am happy with when it comes to a carburetor, is the fuel curve. A good setup carburetor, on a dyno pull, should have a fairly consistant air fuel ratio report. Forget about the concept of "its ok that its at 13:1 at 3500, and 12:1 at 5500". The engine is at full throttle on the dyno, and theres no reason the air fuels should be all over the place. ESPECIALLY, when at 3500, the air fuels are 11:1, and by 5500-6000, they leaned out to 13:1 or higher. Thats simply not a good carb setup.

Part throttle tuning, is possible to do on most dynos. But, you really need to know the load percentage at specific RPM's, that are realistic to your setup. Most do not have that information, which makes tuning on the water, pretty much mandatory, even after a dyno session.

Just my personal thoughts. Im sure someone like SB can add some information to this topic.

Spent some hours when I was young searching for the secondary metering rods and the lower letter hangers to tune those Qjets to get more fuel

I remember on my dyno sheets my Quick fuels air fuel was very consistent the whole time, nice to see that..

Same here. I remember them being hard to find too .

intake manifold and of course heads/cam change how the carb see's the engine, or fuel signal. years ago I ran a super gas corvette, 439ci, this is mid-late 70's, there were only a couple of dominator intakes available at the time, C454 dual plane, and a offenhouser single, which I was running at the time, knowing they had not made much trick stuff since the flathead ! holley had a new single plane that was really good come out but was only std carb, tried it with a 850 car lost 3mph and few hundreds. long story short with a ton of time and marine tex I made it into a 4500, slapped it on, with no other changes..car slowed down, plugs kept showing lean..kept fattening it up, slowed down, still showed lean(no wide bands then) friend of mine came by as I was messing with it, we talked, told me they a same same on a hemi, but in fact was so rich was washing the plugs off showing lean. took a big swing at it going way leaner, car picked up 3 tenths on just the new intake, now ended up going more than 10 jet sizes down just from same carb on the offy intake

I bought a couple aerosol carbs that C&S did, they flowed 930, hope they perform well, I'll know in a couple months.

Summit has a decent carb calculator that at least takes performance mods into account. . It's not perfect, but for us neophytes it's helpful to get in the ballpark.

http://i622.photobucket.com/albums/t...0B3B91D20B.png

I don't think many do it my way. Here's a pic from the other year of me working on a Prosytems carb. :) :lolhit:

http://www.hardcoresledder.com/forum...1&d=1485958838

U never fail to humor me. That is way cool. Your actually better looking than I amagined. Lol... to funny.

What ya doin there? Changin from down leg to annular boosters?