Volumetric Efficiency 1

[Scott367]

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.

 

[SteveHuck]

Even at 100 percent you are at 700 cfm.

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.

 

[patprimmer]

I agree, 650 to 750 CFM. Vacuum secondary is more forgiving, especially for a dual purpose engine.

Regards

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[Scott367]

This engine is a full time performance engine. I will be running 114 octane leaded fuel. I am definitly going mech secondary. My math tells me that at 90% VE a 650 is fine.

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.

 

[Scott367]

Just found this:

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

 

[Lou Scannon]

Keep in mind, it would be very unusual for your max VE (let alone VE>100%) to occur at peak power. Typically VE is on a downhill trend with rpm in the vicinity of peak power.

 

[Scott367]

The other variable I am thinking of is the carburators are rated/tested at a uniform level of vaccume. This engine will not generate as much vaccume as most. Therefore it is possible I will not be able to achieve the rated flows. That would justify over sizing. I was hoping for some number crunchers to be able to give some solid feedback. Don't want to spend $650 on a racing carburator on a guess.

Logical or not?

 

[MWPC]

My local performance shop uses 1.6 cfm/hp. Seems to work pretty well for them. Some time ago I checked it using appromimate densities of fuel and air, an assumed A/F ratio, and a BSCF of 0.5. as I recall it was pretty close depending on all of the assumptions.

 

[Lou Scannon]

It appears you are confusing engine vacuum at idle with the pressure drop used to rate carburetor capacity. There is no interaction there.
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.

 

[HighPoint]

You might give Reher Morrison in Arlington, Texas a call..
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.

 

[Fabrico]

750 ;-)

 

[FoMoCoMoFo]

if you are racing on a short track or tight road course where your having to accelerate alot from relativly low rpms there could be some advantage to the smaller carb otherwise 750