Hello friends. I've been searching high and low to find out the CFM rating on this particular carb and I can't find anything. If there's any fellow gear head who can give me some info I'd really appreciate it. Part # 7041114 Rochester 2 barrel originally off a 71 Chevelle with a 350, big base.
i THINK ABOUT 1970 LARRY, I would guess the large base carbs about 350 CFM..... they made an even larger 2bbl smog carb later in the 70s for Olds and Buicks
There is a partial chart in the book "Rochester Carburetors" by Doug Roe. If you acquire the book, and check the chart, you will note varying CFM's for different internal venturi sizes for the different size throttle bodies. There are some internal venturi sizes not listed. If a chart showing all Rochester carburetors and their internal venturi size exists, I have yet to see one. Remove the airhorn (top casting), remove the venturi cluster, measure the internal diameter of the main venturi, and then consult the chart in the book. Jon.
Found this information a while ago: 1 1/4" flange, 1 7/16" throttle bore, 1 3/32" venturi - 278 cfm 1 1/2" flange, 1 11/16" throttle bore, 1 3/16" venturi - 352 cfm 1 1/2" flange, 1 11/16" throttle bore, 1 1/4" venturi - 381 cfm 1 1/2" flange, 1 11/16" throttle bore, 1 5/16" venturi - 423 cfm 1 1/2" flange, 1 11/16" throttle bore, 1 3/8" venturi - 435 cfm
@THE FRENCHTOWN FLYER …………..Sorry about dragging this back up. I am in the middle of doing a 4 deuce setup on an older Edelbrock manifold. I bought a complete unit. I have never had any experience with 2G carbs. I found your comment very interesting,. I would have bet money that the CFM rating would not differ between 4 barrel and 2 barrel. CFM should be CFM. I sure hope you will see this and lend me a helping hand. Any information you can tell me would be greatly appreciated. I'm thinking I need to match the carbs exactly...……...same part number. Is that correct ? This unit will , when finished, be installed on the 355 SBC in the back seat of my ( ragged edge of HAMB time) 65 Corvair
The cfm is rated at different vacuum depressions between 2 bbl and 4 bbl. Divide the cfm of your two barrels by 1.414 to convert.
DAMN !! I have never heard that. Not even once. So does this equation go for any 2 barrel ? Stromberg or Edelbrock 2 barrels ? Thank you
Back in the days when I was racing limited Sportsman on oval dirt tracks, I hjad to run a 2 barrel at some tracks and did a lot of research on the subject and knew cfm wasa measured at a higher vacuum drop on 2bbls, but this is the first time I've ever read a formula to compare them to 4bbls. This was mid 60s and the biggest 2GC I found was off a '57 Pontiac. A little later I found a modified carb from the same folks that made the "Mana'Fre" 4X2 intake for Chevys that was faster yet. Then Holley came out with their 500cfm 2bbl and I tried one, but it wasn't as fast as the modified 2GC.
It is actually much worse than just a two-barrel rating and a four-barrel rating. There is also a "dry" rating and a "wet" rating; and then certain "sales" individuals/companies found that they could rate at additional vacuum ratings and sell more carburetors http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm The real expert on CFM ratings was Mark Twain He commented that "figures don't lie, but liars figure"! Jon
OK I was an engine development engineer at Ford. I did lots of dyno work and worked a lot with carbs too. Four barrel carbs are flow tested at a depression (i.e., vacuum) of 1.5 inches of mercury at WOT. That figure approximates how much manifold vacuum is present on an engine running at WOT at or near the horsepower peak. Imagine for a minute if you were to take that four barrel off and substitute it with a two barrel - with obviously less throttle and venturii area than a four barrel. If you ran it up to the same RPM at WOT the engine would obviously be trying to "suck" through a smaller opening. The vacuum in the intake will be significantly higher at any given RPM. Keeping this in mind carburetor manufacturers selected 3.0 inches of mercury for the CFM testing standard at WOT. As rq375 said to convert from 2 barrel CFM to 4 barrel CFM divide by 1.414 An easy way for me to remember is my 500 CFM 2V Holleys on my race cars convert to (approximately) 357 CFM 4V rating. As for wet vs dry flow testing the most sophisticated flow benches are capable of flowing fuel so that air/fuel ratios can also be measured. This is not an easy task for several reasons, a couple of biggies being the the liquid fuel when vaporized in the carb needs to be re-captured downstream of the test carb and re-condensed. The other biggie is that if fuel were to be used it could be inadvertently ignited. Imagine for a second atomized gasoline accidentally being ignited by an errant spark or even static electricity. That is a big safety factor. So our labs used a liquid called Stoddard, which is a non-explosive liquid with similar density and viscosity of gasoline. If you flow a carb "dry" it will flow a little more than a carb flowed "wet". That is because all the little passages meant to flow gas - jets, boosters, etc are now flowing air through them, and air being less dense than gas will flow easier through all these tiny passages, adding to the flow numbers CFM. As I recollect a Holley on an O/T Mustang GT 302 flowed about 670 cfm dry and about 630 cfm wet. One more fact of interest: Carbs need a partial vacuum to function due to the Bernoulii Principal. There will always have to be a vacuum below the carb for it to work. BUT! Fuel injection needs no vacuum. It needs no venturii restriction in the intake. So less restriction in the intake means more HP with fuel injection. Our later fuel injected Mustang 5.0 GTs had less than 1.0 " hg in the intake at maximum RPM. I hope I've answered your question hrm2k. Good luck with your project.
One other thing to consider about CFM ratings is WHO issued the rating. The engineers at Carter, Holley, etc. did NOT spend billions of hours of time figuring and testing venturii area to give 500 CFM or 625 CFM or 1100 CFM. They produced different size carburetors with a relationship of venturii area to throttle area with STANDARD sizes (after about 1935 you don't see any venturii of size 1 27/45). Once the carb was tested by the laboratory, figures were sent to the advertising department, and "round number figures" were then substituted and advertised. So a 612 CFM might be advertised as a 600 or a 625 or maybe something else. No truth in advertising. What was advertised was what was thought to sell the most carburetors. An example of this is one company that did not have a 600 to sell against Holley simply added a different part number to a 625 and advertised it as a 600. Guess which "600" would flow the best if some magazine ran a comparison test! Frenchtown - thanks for posting the name "Stoddard". I hadn't heard the name in maybe 20 years and had forgotten the name. My sources long ago suggested an approximate 8 percent differential from wet to dry which is about what you suggested with the 670/630. Jon.
Gentlemen, thank you so much for this education. One of the great things about the HAMB is the outstanding amount of information contained in this page. @carbking , I have saved your website for further reference. I really love the page...…...no glitz, no flash , just great information. @THE FRENCHTOWN FLYER , I'm not sure I could have gotten a better answer. Thank you for the information VIVA LA HAMB !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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