Hi guys, got a question I've been pondering for a while now, and figured I'd ask the gurus to see the actual facts. I've been doing some research on the subject, but just haven't been able to locate anything that answers my question. So here it is: what's the CFM difference between the primaries and secondaries? I realize the main difference is the secondary butterflies, so I'm trying to discern where the CFM levels are at any given point when the secondaries are open. I've read about the 60*, 70*, 75*, and 80* limitations, but I'm not sure what the flow is at any given opening point. I could figure this out on my own, IF I knew what the CFM differences were between the primaries and secondaries. I know there are two basic castings for the Quadrajet, the 750 and 800. The only difference between the two is the primary venturi--the 800 is simply bored wider than the 750, with a 'dimple' visible on the 800 showing the most readily obvious difference between the two. So that leaves the secondaries as having the same CFM potential no matter which casting you had. At a glance, the spread bore holes appear to be at an approximate 3:1 ratio, so the 750 looks as if it may be 250 CFM for the primaries, and 500 for the secondaries. If this assumption is correct, then I can continue to calculate the secondary CFM at any given opening position ranging from 45* (closed) to 90* (wide open). I also understand that CFM can be affected by other factors other than butterfly position, so this is just to give me a general idea. So is this correct? 750 = 250 primaries 500 secondaries; 800 = 300 primaries 500 secondaries? If so, the 60* is at 50% opening potential, yielding 250 cfm, making the 60* limit Q-jet a '500' CFM carb. The 70* would be a '583' CFM, the 75* a 625 CFM, and the 80* a 666 CFM, and of course the full 90* wide open position gives the max 750 CFM. Tell me where I'm wrong, because I'm not so sure these figures look quite right.
Gary, you might find this link interesting. http://www.highperformancepontiac.com/tech/hppp_0612_rochester_quadrajet_carburetor/viewall.html
Rocky - great article and thanks for all the effort involved. Gary - I think you're on the right track, but from what I understand 80 degrees was the widest secondary opening offered by GM though all the years. I dont have the link but there's a Oldsmobile site out there somewhere that details the secondary air valve opening degrees by shaft part # - I'll paste the data here but I'm betting the columns get scrambled.... [TABLE] [TR] [TH="colspan: 6"][FONT=trebuchet ms, Arial, Helvetica]QUADRAJET AIR VALVE SHAFTS[/FONT][/TH] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PART #[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]ROTATION[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CAR MAKE[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]YEAR(S)[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ENG[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]NOTES[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](170)59924[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]NO DATA[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]BUICK[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]80-UP[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33766[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]BUICK[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]68-74[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]350[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 33767 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33767[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]BUICK[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]66-74[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL EXCEPT 350[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](170)61710[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]80-85[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]TRUCK[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]TRUCK WITH REAR VACUUM BREAK ONLY[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](170)63616[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]80-88[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]CAR[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33766[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]65-71[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]SB EXCEPT HP[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 33767 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33767[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]65-71[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL BB & HP SB[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)44888[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]72-74[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]BB[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]NO SLOT, HOLE ONLY, CHEVY ONLY[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)44889[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]72-74[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]SB EXCECPT HP[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]NO SLOT, HOLE ONLY, CHEVY ONLY[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)48838[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]CHEVY[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]75-79[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 48509 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](170)63617[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]60 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]OLDS[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]80-UP[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 63616 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33767[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]OLDS[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]66-67[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)38258[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]OLDS[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]68-74[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 33767 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)48838[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]OLDS[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]75-79[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 48509 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)32746[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]75 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]67-68[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL & 69 OHC6[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 33767 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)33767[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]69-70[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL EXCEPT 69 OHC6[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)40197[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]73[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL 73 MODELS[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)43995[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]71-72[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]MOST MODELS[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica] [/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)43996[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]75 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]71-72[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]FEW MODELS[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 43995 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)48509[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]80 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]74-79[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]ALL 74 MODELS[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USED ON A FEW HP 75-79 MODELS[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica](70)48838[/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica]70 DEG[/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica]PONTIAC[/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica]75-79[/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]MOST MODELS[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]USE 48509 FOR MORE OPENING[/FONT][/TD] [/TR] [TR] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica][/FONT][/TD] [TD="width: 10%"][FONT=trebuchet ms, Arial, Helvetica][/FONT][/TD] [TD="width: 11%"][FONT=trebuchet ms, Arial, Helvetica][/FONT][/TD] [TD="width: 13%"][FONT=trebuchet ms, Arial, Helvetica][/FONT][/TD] [TD="width: 16%"][FONT=trebuchet ms, Arial, Helvetica]GRZ SEP 2002[/FONT][/TD] [TD="width: 38%"][FONT=trebuchet ms, Arial, Helvetica]REVISED FOR WEBSITE MARCH 2004[/FONT][/TD] [/TR] [/TABLE] [FONT=trebuchet ms, Arial, Helvetica][/FONT] ---------- Post added at 09:07 PM ---------- Previous post was at 09:04 PM ---------- Hey! Not too bad! You'll notice that all olds Q-Jets past 68 had a 70 degree max..interesting.
It's been a while since I've checked this thread. Thanks for the great info! I've since learned a lot more about the Quadrajet since the date of this post, and much of this I already know, but it's always nice to review. This stuff is something most people don't seem to think about much, and I guess it's easy to discern how much CFM each pair gives on a square bore, eh? G
It's funny: After I posted that info, I went out to check what shaft my 66 Nailhead Q-Jet has...the 66 through 74 are listed as having a 33767 80 degree shaft, but I knew the 66 doesnt - it has a straight shaft without the pull off linkage plate. The damping is done by an arm and linkage attached to a dashpot sunk in the float well. And sure enough, it was only opening 70 degrees all this time.....I think that means I have to forfiet my guru status (like i ever was one)! A little grinding later and it's now opening 80 degrees...and makes an awesome noise.
I did that on my Q-jet many years ago. Loved the sound it made afterwards. Nothing matches the sound of a Rochester Quadrajet fully open. I thought though that the CFM rating was measured when it was fully open at 90*, but I guess not. It all depends on the demand of the engine too. If your engine can't use the extra CFM it won't open that far, so I figure it isn't hurting it one way or the other. If you grind it so it'll open further, or even do some grinding on the casing so it flows more, the engine won't open it unless it needs it (if it's tuned properly). Buick engines are very thirsty when the secondaries open though, so I say open'er up! G
If you read Cliff Ruggles book, there's a specific measurement for the air valve to open to for best performance....Something like 1.15 or so. But the measurement is hard to take, as it measures from the leading edge of the flap to the airhorn, so sometime just judging angle is easier....I thought I had it right on my 66 carb, but I obviously didnt.
While we're on the subject of Quadrajets, I have another thread going (probably should be here in this forum) about a 4-hole spacer that has swirls in each hole. Any feedback on that particular piece? It's in the smallblock tech forum. Hang on I'll get a link. Edit: here it is: http://www.v8buick.com/showthread.php?265056-Anyone-tried-this-spiral-spacer-yet G
