Volumetric Efficiency
Volumetric Efficiency
(OP)
I am trying to size a carburator. The engine is a 12.5:1 naturally asspirated 331 ci engine. Single plane intake long tube headers. Air Flow Research 185cc aluminum cylinder heads. Mechanical roller cam, valve lift .633 In. .640 Ex., duration @ .050 250 In. 254 Ex. I have a safe RPM limit of 7400.
The big variable in the equation is volumetric efficiency. I would expect it to be higher than a stock type engine. I know anything I get from here would be a guess but I expect alot of you may have more experiance with this than I.
Tell me what you think. Let me know if I left out any information.
The big variable in the equation is volumetric efficiency. I would expect it to be higher than a stock type engine. I know anything I get from here would be a guess but I expect alot of you may have more experiance with this than I.
Tell me what you think. Let me know if I left out any information.





RE: Volumetric Efficiency
If you are racing i would put a 750 cfm
For street use id go with the 650 cfm
None of this based on scientific fact, just my personal experiance and opinion. I would like to here other opinions based on real testing.
RE: Volumetric Efficiency
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RE: Volumetric Efficiency
My gut tells me that its way too small. I know a too big carb hurts perf more than a too small carb but sure seems to me I would be better off with the 750.
RE: Volumetric Efficiency
For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can exceed 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components.
I might of answered my own question
RE: Volumetric Efficiency
RE: Volumetric Efficiency
Logical or not?
RE: Volumetric Efficiency
RE: Volumetric Efficiency
The rated capacity tells you how much the carburetor flows at a given pressure drop. I believe the pressure drop for rating 4BBL carbs is 1.5" Hg.
When you put the carburetor on your engine, it will flow as much as your engine draws through it, while imposing its own pressure drop that is a function of flow. If, coincidentally your engine happens flows 650cfm at max power with a 650 cfm carburetor attached the intake manifold, then the carburetor's pressure drop will be 1.5" Hg. Now if you replace that carburetor with a 750 cfm unit and run the engine at the same WOT rpm, the flow will be slightly more than 650 cfm, and the pressure drop will be slightly less than 1.5" Hg. Power will be increased roughly in proportion to the increased flow. Depending on the rest of the combo, I would also expect the rpm for peak power to increase somewhat, compounding the potential gain.
RE: Volumetric Efficiency
they were pioneers in NHRA Pro Stock... which intitially
used 331 ci motors.... they were getting 740 or so horsepower
from a 331 back in the late 70's.. Pro Stock is also the
zenith of naturally aspirated engine efficiency... achieving
over 2hp per cubic inch on a routine basis.
I think the 650 will be too small and cost you power on a 331 with AFR heads and the lift you are describing.
RE: Volumetric Efficiency
750
RE: Volumetric Efficiency