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Question about cfm

3561 Views 6 Replies 5 Participants Last post by  1937Austin
If you have one 600 cfm carb on an old tunnel ram, I know you have 600 cfm's. What about if you change the top to a dual carb & add another 600 cfm carb, is now 1200 cfm? Any help would be great. Thanks Gasserking
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Well yes and no. What you've done is allowed the engine to use what it needs. If your engine is built to run, lets say, 900 cfm, then it will only use 900 of the available 1200 CFM. On the one 600 it will be using almost or all of the 600, but once you add the second one , needing only 900, it will only use what it needs of the two. That is the simple answer. There are more involved issues as you add carburators too. Responce thru the throttle bores/venturies of a single carb (especially of smaller cfm than the engine wants) compared to having four more throttle bores to slow things down/lessened vacuum signal. Then you have the issue of the engine needing 1200 cfm but is the manifold of such design that it flows it smoothly, efficiently, is there reversion dilution of the intake charge because of cam timing versus runner length versus plenum size/design, versus throttle bore alignment with the runners under the plenum. Many different issues to look at but the first sentence is the most simple. Sorry if I got carried away there. Mark L
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Along the lines of what Mark said, it all depends on what your engine will take. Most "street" tunnel ram set ups will usually run a pair of 450's with vacuum secondaries. A mild street engine will only use about 600-750 of the possible 900.
Thank you

Thank you, to be honest it is a blown chevy 350. Weiand 6-71 supercharger. I had two 450's & it just wasn't enough cfm. So I now have a matched pair of custom 600's. I am also putting new 411's in the rear end. Probably a new 3000 stall for Christmas. Should do much better at the strip next year. But thank you for answering the question. Gasserking
I've found that blower motors need a lot of "different" tuning technichs as apposed to normal nonforced induction. Mine is 400ci with a 671. 2 750 holley hp blower carbs. Sounds like a lot of carburetion right? Wrong! Spark plugs look A little dark but dosn't load up, dosn't burn your eyballs out or anything like that. Timing locked out @ 36 degrees, it starts right up & runs damn near perfect. You may want to check the BDS or Blower Shop websites as they have a lot of helpful graphs & tuning recomendations. As for the converter, mines a 3200 & it could actually use a little more stall speed, maybe 3500 to 3600. Still wants to pull me though traffic lights when in gear @ idle
Like blownanglia said, carbs on top of big roots blowers, especially as they get bigger, also have to react to blower surge of the spinning rotors just beneath them. This can upset the carb signal just enough to cause erratic idle. This also shows up all the way up the rpm curve. I'm far from being any expert on carbs on blowers as I've never owned or tuned one of those set ups. But a carburator is still a pretty sophisticated device although we've come to think of them as simple compared to EFI. I think (and only my own feelings ) the bigger the carb the easier to get it over the rotor surge because the bigger carb is slower to react to the vacuum signal. A small carb like a pair of 600s tries to react too fast and it gets it all confused. Not saying a pair of 600s will not work, they will but they will be a touch more responsive to tuning and could possibly be too small depending on the entire combintion. Again I say I'm not an expert on tuning a blower/carb set up. Please, any one with good experience I would like to hear your thoughts on this so I can learn. I would love to set up to a big roots blower for my car someday and would like to do all tuning myself. I hope I'm not way off and confusing the issue. Mark L
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All normally aspirated engines are much different than supercharged engines. Reason being that a supercharger pushes so much more fuel and air into the cylinders that it creates a scenario that's much like adding both cubic inches and compreession to the engine. What would work well, or even over carburate a normal engine will be much less than a supercharged engine needs.
There's also much more to it than just adding cfm to a supercharged engine, although that's a good start. Jetting, nozzles, and (if it's a Holley) power valves, all need to be adjusted to make it work right. There's actually a formula that really good blower guys have to figure out the size blower, the pulley size, and carbs/tuning needed to make it all work well. I spoke with my friend Gale Plummer of Plumb Crazy Blowers, and when he starts talking about all the various numbers, etc. it makes my head hurt! ;)
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