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?

Plus just think of all the tight spots he can get to with those small hands, and beady eyes!! Also good for changing jets on those solenoids at either end of the carb!!! Just Sayin

So back to CFM stuff, when I first assembled my engines 17 years ago versions 1.0 in techie talk, we knew what Mr. Elkins did with the heads and pistons, we had a real good idea how the cam and headers would flow and scavenge, so Butch had me get in touch with a carb guy he knew in South Carolina to build me a couple carbs with the known engine specs 502 6k rpm. he built two wet flowed 1071 CFM carbs, they performed great but by reading the plugs I had to increase the Jets 2 sizes all the way around..(before we used AFRs to do this) so it really depends on your specific build!

gives a whole new meaning of throwing a monkey wrench in your stuff !

Here is my .02
Last year, we freshened up some 500 hp's. They were bored to 509, compression bumped to 9.5, Brodix BB2x heads (we CNC'd the chambers), decent size hyd roller cam, etc. I cut the choke horns from the stock 800 carbs that came on the 500 hp because I had a feeling that they would be too small. We only increased the engines by 7 ci. The heads were a little larger (but flow quite a bit more), quite a bit larger cam (but not crazy) and bumped the compression.

I put the first one on the dyno (full dress with wet exhaust) and after breaking it in, I made some power pulls. I threw everything at it that I could and it would not break 560 hp. Now, I'm looking for 610-625 hp. I tried all the timing in the world, leaned it down, etc. It wouldn't make more than 560 hp. I didn't have the manifold vacuum data on the printout, but I pulled it up. It showed the engine pulling over 4" vacuum at the top of the pull. This was enough to tell me that the carb was too small. Thinking ahead, I borrowed a 1050 (4150) Quickfuel from Mike Tkach. It was for a 2x4 blower setup, but we made it work. Even though it was pig rich on the first pull, it made over 610 hp. That's a 50+ hp jump from the carb. After leaning it some, it made around 625 hp. That's 65 hp over the 800. The vacuum came down to about 1.5" at the top of the pull.Needless to say, we changed the carbs. I ended up getting a 1050 Quickfuel that worked perfectly out the box. I put them on once the engines were off the dyno and in the boat. I figured that I would just tune it in the boat with the new carbs. I did not so much as have to lay a finger on them. Hell, I didn't have to even adjust the idle speed. They maintained a 12.5 AFR through the entire rpm range and 13 at idle. I never took a tool out of the tool bag. That was enough to sell me on Quickfuels. I've used several more since and only had to make some small changes to one of them. That was only because it was a fairly healthy piece.
Eddie

I remember that test Eddie. It was an eye opener!

Those quick fuels are really nice carbs , and they are reasonable.

I've had the displeasure of dealing with C&S 2 times. Horrible AFR swings thru the pulls on the last set. They were d!cked with for 2 days until a set of 950hp's were thrown on. Right out of box 50hp and about 50 torque. Better EGT's and flat AFR's... C&S = cry and scream. (I got that from another member here)

you saw how mine worked, very happy with them..

[QUOTE=Young Performance;4525542]Here is my .02
Last year, we freshened up some 500 hp's. They were bored to 509, compression bumped to 9.5, Brodix BB2x heads (we CNC'd the chambers), decent size hyd roller cam, etc. I cut the choke horns from the stock 800 carbs that came on the 500 hp because I had a feeling that they would be too small. We only increased the engines by 7 ci. The heads were a little larger (but flow quite a bit more), quite a bit larger cam (but not crazy) and bumped the compression.

I put the first one on the dyno (full dress with wet exhaust) and after breaking it in, I made some power pulls. I threw everything at it that I could and it would not break 560 hp. Now, I'm looking for 610

Now That's what I'm talking about!

Mark

Very cool.

Last go around I used AED. After getting them setup they're perfect. However, out of the box they were at least 4 jets too rich and after 3 pulls on the dyno being pig rich, I pulled a primary bowl off and both main jets were "LAYING" in the bowl... No wonder it was 10:1... Anyway, fixed that and began jetting after that and everything was good. They're still on the rich side at 11:4 but it's safe and I'm not breaking any speed records with this boat.

I don't know that they are as nice now that Holley owns them.

I purchased 4 of them last summer. I've had them apart and what not. The 850 marine series. Everything looks good, still using billet metering blocks, billet baseplates, threaded air bleeds, idle feed and powervalve restrictors, all 4 carbs matched setup wise.

A few guys I know have used the new Holley Ultra HP and Ultra gen 3 Dominators, and have been really happy with those too.

I'd personally take a new Quick fuel or holley carb ultra series, over some of the overpriced cast pot metal crap that has press in bleeds, that are still on the market.

Im curious, where those C&S carbs aerosol ??

I'm not saying they're bad, I just don't think the new aluminum main bodies are all that consistent. We're seeing quite a difference in A/F ratios between left and right sides of the motor.

Yes all of them. Which in itself is market crap... it doesn't "aerosol" the fuel. Functions just like any other carb, just sloppier. Ask Presicion on here his take on them... he knows first hand as well.