I was recently involved in a discussion at another site regarding carburetors and 292s. A comment was written that the Y's were noteworthy as poor breathers, with volumetric efficiencies in the neighborhood of 60%.
I did not think that was right, especially if big valve heads were being used, and would be interested in your comments. What are some estimates on efficiency of 292s generally, and mine in particular, say at 4500 rpm (113 heads with 1.92 valves, .440 lift cam, 305 cu in, blue thunder intake, no porting).
Mike

 

46: I'd say that the person who made such a comment would have expert flow potential when it comes to passing flatulence, but not in ay other obvious area. Consder that:

The 1954 EBU 239 Y Block came from the factory with 130 rated horsepower, vs the flathead 239's 95-100 hp.

Consider that the volumetric efficiency of the Flathead is about 75%, IIRC.

Check out the airflow measurements on the power page of my link below. You will see that the ECZ-G head was fully competitive with the 283 '57 bowtie head in its day.

Tell your web buddy to throw away his Hot Rod magazines when he grows up. Those old wives' tales he learns from them don't do him any credit. That magazine is only published as psychological help for chevy owners suffering from justifyable low self esteem.

Volumetric efficiency peaks at or near the torque peak rpm. That's when cylinders are most filled. The reason horsepower still goes up beyond that rpm is because even tho each cylinder is less filled and produces less power per stroke, higher rpm means that there are many more strokes of a little less power, so the result is more total power.

Given your setup the way you describe, I'd be guessing that your figure would be closer to 85%, but it could only be verified on a dyno.

Getting into a discussion with such "educated" people without data must inevitably turn into one similar to those of 50 years ago, when people reported the top speed of their cars by speedometer readings.... I'd justt let them go on with their blow, if I were you.

 

Wild bunch, you mentioned a link giving comparisons between the G head and 283 head. Where is it?
I have no interest in arguing with the person, there wouldn't be any point. But since I am messing around with changing to a different carburetor, the efficiency estimate is needed in the math calculating CFM consumption. Thanks, Mike

 

Sorry, thought I had it there, but forgot.

Try: http://m571.com/yblock/power.htm

I hardly think that those formulas will do you any good. For instance, how do those formulas explain that you may often need a smaller carb on a single plane manifold as opposed to a dual plane manifold? Or, how do they explain that 289 Fords in the 60s used to commonly be equipped with 4 Weber 2v carbs having a combined flow of almost 2000 cfm?

The carb and its restriction is a factor in volumetric efficiency itself, or the ability of a cylinder to ingest the same amount of air/fuel as its displacement indicates. Running a larger carb with less restriction will raise the volumetric efficiency (compared to a small carb).

The problem is that larger venturis are usually unable to meter fuel correctly at low air flows, as compared to a smaller carb of the same design. But, the design of carbs is not the same. Some are more efficient than others at low flow, even tho they both flow the same on the top end.

If you are running a 292 with a cam and Blue Thunder, you are probably looking for something in the 600 to 650 cfm range, either Holley or Edelbrock/Carter. If your engine is fairly mild, you can hardly go wrong with the Holley #1850 600 cfm. If you prefer to work on Carters, go that way.

The metering of either of these carbs is sufficient to work well with your motor at low speed, and either will offer plenty of capacity for wide-open performance.

 

Wild bunch, I have a Holley 1850 600CFM now. It has gone from 66, to 64, to 62 main jets, with a 4.5 power valve and 30cc acceleration pump, and is running extremely rich. Idle mixture screws are almost bottomed.
My motor has a particular problem that seems to make the Holley way too much carb. It has a compression of 8:1, due to very large deck height. I speculate that with that compression and the cam, there is not enough velocity going through the venturis for a good gas/air mix.
The deck height issue was something I discovered this summer, when I took off the old manifold, and heads. I think the machine shop screwed up using pistons with improper pin locations. The crank is 292. So, given all this I have pretty much put my dream of a "high performance" 292 on hold and am just trying to get it to run good.
My thought is that a restrictive carb below the calculated CFM rating would pretty much kill even normal performance levels, and that a carb significantly above the calculated CFM consumption would run into problems like I have now with the Holley.
I am considering an Autolite 2V 2100 with 1.21 venturis, which has a CFM rating of 351, until I can get to fixing the compression issue. How a 2 barrel will run on a Blue Thunder should be an interesting question, but it looks like I am going to find out. Mike

Edit Note: This is driving me nuts. The 1.21 2V venturi may be too large for good low end performance. That puts the Autolite 4V 4100 with 1.08 venturis up on the list of possibilities. I'm not sure.

 

I can't say just what the problem is over the web, but my experience has been that the 1850 should run pretty well out of the box, tho somewhat rich. They jet them a bit rich to cover every contingency. It's not surprising that you could go to 64s without a sweat, and maybe even to 62s if you live at high altitude, but it almost seems as if you are getting a fuel leak somewhere along the line.

You really aren't saying whether your richenss is at idle or at cruise or power, but if your problem is at idle, channging the main jets will have no effet on the problem. Similarly, if your problem is at cruise or pwer, the idle will have little effect.

The way the carb works is, as the rpm goes up, you will start by using the idle circuits, go to the transition circuit (the long slot just above the idle outlet) then to the main circuit, and finally, addint the power circuit to the main circuit. The first 3 of these will have overlapping operation.

As you go from idle to transition, the idle will gradually taper off and almost become a bleed for the transition circuit.

If you are getting richness all the way across the board, even while driving without idle, something is wrong. I would start by thinking about your float leavels being correct and then consider whehter a metering block gasket is leaking fuel somewhere.

Either of the 2v carbs you mention are not that much different from the primary side of your 1850, and that's what you are running until the airflow thru the carb is sufficient to open the secondaries. I really don't think that your problem is the size of the carb you are using, by any means.

When your Holley starts becoming too large, then you will start to see some of the crispness at the low end disappear.

Here's an example from my own experience:

I had a low compression brand x 454 in a truck, with automatic and 3.07 rear gears, along with tall tires. I had headers and "turbo" mufflers ending under the box. I had also recurved the ignition and was running an MSD 6A with a big Accell Yellow coil outside the HEI.

I went from the stock manifold to an Edelbrock Streetmaster, keeping the Q-Jet. Q-Jets have always been "soggy" performers for me, and they have always been hard to start, vx. Hollleys and AFBs, which tend to start right away. So, I put an 1850 on this rig. Mileage was the same (near 16 mpg) performance was about the same, except the low end was a lot crisper and the truck ran much better. (A Q Jet is a big carb, but the small primaries with their triple venturi configuration should have been a lot crisper than the Holley, according to theory...) Then, I put on a 780 Holley, a genuine 3310 (the GM factory stock one with the brand x part number and dog leg boosters). REcall, if you use one of those useless formulas, that at a max rpm of 5000, this carb is too big. IIRC, it is good to 6000 rpm, according to the formulas, on a 454. One of the bad things about those useless formulas is that they make no allowance for the difference between single plane manifolds (like my Streetmaster) and a dual plane manifold. In actuallity, the single plane manifold will make the carb seem "bigger", because each cylinder then "sees" all of the carb's flow capacity, rather than half of it. (Just straight flows relying on a constant pressure drop do not dimulate the pulsing that occurs in the manifold. this pulsing is an important issue and one rason why rod/stroke ratios can also determine what carb yo can run effectively, because the pulsing can make the fule systems in a carb more sensitive, i.e., allow a larger than normal carb to be used.)

With this combination of a larger carb, the low end was soggier, and had a flat spot, which was solved by going to a 50cc pump and .037 shooter. Later, I put an Isky torker cam in, and the bottom end was even less crisp. The lack of bottom end in this case was a help, since the light rear end would break free without much problem, and killing some low end made the thing easier for my wife to drive.

Mileage didn't change much with the bigger carb, but it dropped to about 13 mpg with the cam.

A few years later, I went back to the original cam, Q Jet, and manifold, since I had to drive the truck long distances to and from work. The Q Jet finally got a fuel-logged float, so I went to an Edelbrock performer (2 plane) with the 1850 Holley. I believe I was jetting the 1850 to 60s (stock 66) and the 3310 at 67 (stock 72), all of this at a mile high or more in elevation.

My point is that you can make many compinations work, and the effect of them isn't nearly as severe (if at all) as those formulas try to tell you.

Incidently, you can pick up books by experts, like David Vizard, that will tell you the same thing regarding those formulas, so it's not just me...

 

AMEN!
Ive been arguing with those formulas/experts for years.

One thing that opened eyes was when I did several weekends of dyno runs on a built 276 flathead. The best run was with a much modified 2 carb intake running a pair of Holley 2300's. At 600 cfm so much for the experts. The hp curve was still going up when the cam peaked.
That setup wasnt practical for the street so I wound up with the ubiquitous 600 Holley 4bbl; in my case it came off a 327 Chebby.
My dyno run was virtually dupicated in R&C about a year later by author Joe Abbin (Blown Flathead) using a 600 Holley. The "experts" say 390cfm is the absolute max.....

BTW, the Holleys use the same primary venturi size over 2 to 3 concurrent cfm models.

 

Right on, Carl! These old lies stick around worse that old politicians!

46: Remember that a carb's cfm rating only tells you how much air/fuel it will flow at a certain pressure differential. IT DOES NOT tell you whether its idle, transition, and main systems are sensitive enough to work at lesser flow rates. So, you can end up with two carbs: one that has a flow rating and works on an engine of a certian size within a given rpm range, and a second carb that is bigger, but more efficient, which works just as well as the smaller, less efficient carb at low speed, but flows more at high speed, giving more power. Those silly formulas don't tell you anything about that!

Also, remember that 2v carbs are rated at a 3 in pressure drop, while 4v carbs are rated at a 1.5 in pressure drop. What this means is that a 500cfm 2v is equal in flow to a 390cfm 4v. (to do this conversion, multiply or divide by the square root of 2, or 1.414)

 

Ok you guys, I will forget the math on this issue. I do have an Autolite 2100 I am going to try for the he-- of it. Then will go back to the task of making the 600 Holley work.
I wonder if it is possible there is a fuel pump issue. The pump is an electric Carter unit that puts out 8 psi. A Spector inline regulator is set at 4 psi. No overflow out of the carb is visible however. Will just keep working on it.
Thanks for the help. Mike

 

46: There is nothing at all wrong with the Ford Autolite 2100 or 4100 (4v) units. They are easy to work on, simple in design, and their annular discharge booster venturis were really ahead of their time. So, giving one a try is a good idea.

Note that higher fuel pressure will cause the fuel level in the float bowl to change -- higher pressure will give a higher level. Holleys usually work ok at 6 psi or so, but the old flathead era carbs liked pressures closer to 2 psi.

Higher than normal float/fuel levels in the bowl will mean that systems will begin earlier and require less signal to get them started, so richness will result, along with the possibility of fuel spilling over if the levels are high enough.

Somewhere along the line, if you keep having difficulty, even with a carb change, it might be a good idea to keep in mind to check your fuel pressure with a vacuum gage, most of which are also calibrated for that purpose.

Good luck.

 

Hey Wild, Just a quick note on the 289s using 4 45 IDA Webbers,
they aren't constant depression carbs, meaning they don't need to
see vacuum to operate, I have a 2.0L with 2 48 DCOEs, which will
flow a little over 1200cfm...

SteveL

 

Any carb that is used on an individual runner manifold (i.e., one carb throat is devoted solely to one port/cylinder) will operate in the manner you have described. The pulses get things going in these setups. There are models of the Holley 4500 Dominator that are set up to do the same thing. There was also a Holley 390 cfm (might have been list 8007, but I can't say for sure) that was meant to operate on Pinto engines -- it, too was set up to operate in a high pulse environment, even tho it was not an individual runner setup.

In these setups, sometimes air/fuel columns will stand above the carb bores, since the inertia of the airfuel mixture will "bounce back and forth thru the carb, and fuel will be metered into this airflow in both directions.

Webers can be used on a plenum manifold where there is a constant vacuum, also -- that's one of the beauties of them -- they can do it all, pretty much. Offenauser used to sell an internal cross ram for sbc that used three of them, where the wing carbs could be progressive, and they also used to sell tunnel ram tops for 3 or 4 of them, too, mainly for boats.