I'm in need of a carb for my Elky and was wondering whats the best way to go? It's a pretty mild build, .020" over 454, dual valve relif flat tops, medium combustion chambers and oval ports matched to Fel Pro gaskets, Magnum 282s cam(236/236 @.050" duration and .561/.561" lift with 110 lsa), and a Performer RPM intake. This is my first BBC so any info would be cool. Thanks abunch. -Jesse
It takes 1.3 cfm to make ! h.p. 750 divided by 1.3 =576.92307 850 =653.94615 Bigger ain't always better Gemini EFI
The best way to decide is do the math. Air flow required is c.i.d. x rpm / 3456. Or 454 x 5000 rpm = 2270000/3456 = 656.83. So at 5000 rpm, your 454 only needs 656 cfm at 100% volumetric efficiency. Short of a Pro Mod or Funny car, ther aren't many engines approaching 100% efficiency. 85% is a good estimate for most street motors, so 656 x .85 = 558 cfm. That's all you'll need to routinely pull 5000 rpm. Real world driving rarely gets that high. Unless you are drag racing a lot, find a nice 600 cfm with vac. secondaries. Have fun. -Scotty
That engine could go either way....with a 4 speed or with a loose torque converter I'd use an 850, otherwise a 750 would give better street manners.
Does this math apply to real life though?? It seems that the numbers are low for just about every calcuation. I have a 396 which is now 404cid - with that math @ 2500RPM Cruise Speed I would only need 248.50CFM of Air @ 85%V.E. ???? My motor is 10.25:1, Stock low flow oval port heads with some bowl work & 2.19/1.88 Valves, a 268 Duration cam with .515/.520 Lift (with 1.72:1 Rockers so it is slightly higher) I have a 3x2 set up on it with 3 small base Rochester set up progressively. A small base Rochester only Flows about 275-280CFM. If Im running @ 2500 RPM will the Center Carb be big enough to not lean out the motor? That is my concern (as I have never even Fired the motor yet. I will break in with a 4bbl then switch back to the tri-power) Sorry - not trying to Hijack the thread...similar question I guess
Size the carb for max rpm....not cruising, not 5000, but redline shift point. With a 236 degree cam this will be about 6000 rpm.
well 3x2 set up with 275CFM each would be 825CFM 6200RPM (Max) x 404 (Cid) / 3456 x .85 = 616CFM so the 3x2 would be plenty...since all three would be WO @ 6200RPM But if it is only using the Center Carb @ 2500-3000RPM of Cruising Speed will 275 CFM be enough or will it make the motor run super lean?
Real world, yep, you only need a couple hundred cfm. Remember that volumetric efficiency is less as the rpms increase and max flow the intake and exhaust pathways. At lower rpm, the engine is more efficient. Pick a good size for the majority of your driving. Even if you nail your setup for peak rpm in the quarter mile, how often will you use it and how will driveability suffer when you go cruisin?
I had a 454 with the exact same cam as yours, oval port heads & Edelbrock performer manifold. I ran a Holley 3310 750 with vac secondarys and a 1" spacer on the manifold. This worked fine on the street and also pretty good at the track. You can use the formulas and see what you come up with but I can tell ya for sure the 750 works out nicely.
Just from my experience I'd say a 750 would work good. I definitly wouldn't go any smaller than that. Personally I might go with the 850 cfm, but it might be over kill on that motor. Oh, and this is just me but I prefer a mechanical sec. double pumper to a vac sec. carb any day.
I have a 402 with a comp cam 218/224 spit pattern cam 9.5:1 with an edelbrock performer rpm. I run a 650 holley mechanical secondary carb. It has great throttle response. I've been really happy with it, but it took a lot of tunig to get it where it is now. If I were you and wanted a mechanical secondary carb I would run a 750, if you want vacuum secondaries you could run either.
I would run a 750 mech secondary, unless you are trying to build it for mileage. A mech sec carb can get mileage if you stay out of the secondaries while cruising. Don't be a pussy and put a small carb, you need to let it breathe. Your engine will run great with a 750. Don't go smaller. Those formulas tend to be too small for what actually runs good on the engine. If I listened to those formulas I would have not run a 1050 Dominator on my 454 at the drags. It ran better with that than a worked 850. This same engine is in my 38 right now with more restrictive exhaust and a 750 mech sec and it runs really nice on street. It does have less top end than if i would use a 850 for example, but the part throttle and street driving with the 750 is fine for me, it is not a race car anymore. My cam is 253 at .050, so bigger than yours, but it still runs fine on the 750, even if a little starved on top end.
Great info guys, thanks. I was thinking the 770 with the vac secondaires but was unsure. I recently bought the Horsepower Handbook from HP publications, it's basically abunch of Hot Rod mag articles in book form. It has a whole story/write up on the Holley 3310. -Jesse
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