My point with the two less ports is that a 6 cylinder has to flow more per port to make the same hp as an 8.
Killer6 what's the formula for figuring carb cfm at different vacuum levels ??
Carb recommendations???
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My point with the two less ports is that a 6 cylinder has to flow more per port to make the same hp as an 8.
Killer6 what's the formula for figuring carb cfm at different vacuum levels ??
nm9stheham
Well-Known Member
Just be careful with the idea that a 2 BBL cfm rating is not as 'large' as the 4 BBL cfm rating. The conversion between the 2 BBL rating and 4 BBL rating to the same 1.5" vacuum level means is that the 2 BBL will flow less at the same 1.5" vacuum drop used to measure a 4 BBL flow, versus what it will flow at the 3" drop used to measure 2 BBL's. It does NOT mean that the 500 cfm 2 BBL won't flow 500 cfm; it will, but the drop across the carb will be 3" when it does.
How the engine/carb combination operates at that pressure drop is another matter. If the head flow numbers were measured with the flow bench at 28" drop, then the overall flows will be less with the 2 BBL 3" carb drop. Flow benching at 26.5" drop would better match the 2 BBL operation.
The formula to convert flow for pressure differentials is;
The orig. flow /by square root of (Orig. depression /by the New depression)
This can be used for head flow, BUT if the original test depression is too low,the
results will not be accurate.The poorer the design efficiency, the higher the error
will be,try keeping the conversion in the 20"h2o to 28"h2o range.Some testing
was done at very low #s, 5&6" range, way too low.Most have come around to
using 28" which Smokey Yunick determined decades ago to be the most accurate
correlation from bench to track.
As far as comparing 2bbl and 4bbl, I don't need to be careful at all. The ratings were
arbitrarily chosen strictly based on intended usage by those doing the choosing
at the time.If you are determining the flow and performance capabilities of any
prospective carburetor,they must be compared on common turf, and depression
is parameter #1.
The orig. flow /by square root of (Orig. depression /by the New depression)
This can be used for head flow, BUT if the original test depression is too low,the
results will not be accurate.The poorer the design efficiency, the higher the error
will be,try keeping the conversion in the 20"h2o to 28"h2o range.Some testing
was done at very low #s, 5&6" range, way too low.Most have come around to
using 28" which Smokey Yunick determined decades ago to be the most accurate
correlation from bench to track.
As far as comparing 2bbl and 4bbl, I don't need to be careful at all. The ratings were
arbitrarily chosen strictly based on intended usage by those doing the choosing
at the time.If you are determining the flow and performance capabilities of any
prospective carburetor,they must be compared on common turf, and depression
is parameter #1.
nm9stheham
Well-Known Member
I am sure you won't be confused LOL. Equalizing for comparison purposes is fine, but I don't want others to the think that somehow a 500 cfm 2 BBL won't actually do 500 cfm.
I think what people should realize the engine will pretty much pull the cfms needed with almost any carb of course there be problems at the extremes. Bigger carbs don't make an engine consume more air and fuel than the engines requires it's just if you go too big an engine loss too much velocity and vacuum to function properly and too small is just throwing away power.
nm9stheham
Well-Known Member
Speaking of which..... here is a decent article on a carb sizing experiment:
http://www.superchevy.com/how-to/148-0403-seven-holley-carbs-test/
http://www.superchevy.com/how-to/148-0403-seven-holley-carbs-test/
Just a heads up before I proceed, I corrected the formula I posted for you above,
I should know better than to fire off something like that when dog tired lol!! You'll find
the corrected one to work perfectly.
The primary reason to correct the depression. is to illustrate to the inexperienced
that a 500cfm holley 2bbl actually flows less air than a 390cfm holley 4bbl, so that they
choose wisely. 1bbl holleys were rated @3"Hg ,model 1940 came in three basic sizes,
170-180-&212cfm ratings. There were also versions 1945-203,and 1946-200. If you
were to put four of the model 1946s together they would have a combined rating of
800cfm@3"Hg, however in reality compared to a "real" 800 holley, they only flow566.
A 1946 could theoretically flow 516cfm using that mindset, but there would have
to be a 20"Hg drop across it, and at that velocity I doubt it would actually obtain that#.
Not to mention the engine would've quit climbing so far back you'd hafta to read about
it in a history book. Picking a carb for streeting around w/relatively avg. heads means
conservative choices,that would mean a 2bbl, or vac secondary390. Others have used
the 500avs with good results,I think that leaves room for stepping up w/o much down-
side.
I should know better than to fire off something like that when dog tired lol!! You'll find
the corrected one to work perfectly.
The primary reason to correct the depression. is to illustrate to the inexperienced
that a 500cfm holley 2bbl actually flows less air than a 390cfm holley 4bbl, so that they
choose wisely. 1bbl holleys were rated @3"Hg ,model 1940 came in three basic sizes,
170-180-&212cfm ratings. There were also versions 1945-203,and 1946-200. If you
were to put four of the model 1946s together they would have a combined rating of
800cfm@3"Hg, however in reality compared to a "real" 800 holley, they only flow566.
A 1946 could theoretically flow 516cfm using that mindset, but there would have
to be a 20"Hg drop across it, and at that velocity I doubt it would actually obtain that#.
Not to mention the engine would've quit climbing so far back you'd hafta to read about
it in a history book. Picking a carb for streeting around w/relatively avg. heads means
conservative choices,that would mean a 2bbl, or vac secondary390. Others have used
the 500avs with good results,I think that leaves room for stepping up w/o much down-
side.
You might want to read Doug Dutra's article in Mopar Action on Mr. Wilford Day
and his record setting/holding slanty at Bonneville, against factory backed 221 Furd V8s!
Article is titled "Six in a Row Do Go!", can't tell you which issue, have it here in
the "library" somewhere! Hows 145mph grab ya!!
nm9stheham
Well-Known Member
Roger all that, but again that is at 1.5" vacuum drop on a test bench. What the carb actually does on the actual engine is another matter entirely; 350 cfm 2 bbl's are being raced at flows pushing or greater than 350 cfm, and are tuned to match the different operating vacuum levels. And I am not arguing for either carb here..... just pointing out that picking a carb just based on one test flow spec (1.5" or 3") is not the full answer. Keeping the drop down to 1.5" or less just insures that the carb is not the biggest restriction in the intake flow, and at 3" drop it is still not the biggest flow restriction. The flow numbers do not mean that the engine won't produce more power even when the carb does become a bigger, or the biggest, restriction, nor that a carb is totally unusable or unsuitable.
I find this page to make an interesting point on carbs flowing at higher rates than the nominal cfm rating:
http://www.camcraftcams.com/index.php?page=cam-failures
I suspect none of this helps the OP much, however....Sorry for the diversion, guy!
First up, cylinder heads are flowed at inches of water to simulate their peak velocities,and
the carb is satisfying one peak at a time, plus preceding/succeeding events.28"h2o is 2"Hg
At no time if performance is desired,should the intake exceed 1.5"Hg depression.
Second, I got news for you, if you are up to 3"Hg in the intake, the carb is the biggest
restriction in the works.
Third, it does the OP no good to point out specialized racing applications, in which they
are by the very word "restricted" to running an undersized carb.Those engines, just like
NAPCAR restrictor plate engines, are cam timed w/laaate closings and as much chamber
squeeze as the rules and fuels allow. They are not driveable or useful for anything other
than the circle tracks and superspeedways the packages are engineered/developed for.
Yes, different engines use carbs somewhat differently, even within the same displac-
ment. For exaggeration, say 300cid six would demand 150cid per/rev,whereas a 300cid
single cyl would demand all 300cid in less than one full revolution. The demand on the
carb is obviously much higher per unit of time than the avg. in a one cyl. vs multiple cyl.
:coffee2:
the carb is satisfying one peak at a time, plus preceding/succeeding events.28"h2o is 2"Hg
At no time if performance is desired,should the intake exceed 1.5"Hg depression.
Second, I got news for you, if you are up to 3"Hg in the intake, the carb is the biggest
restriction in the works.
Third, it does the OP no good to point out specialized racing applications, in which they
are by the very word "restricted" to running an undersized carb.Those engines, just like
NAPCAR restrictor plate engines, are cam timed w/laaate closings and as much chamber
squeeze as the rules and fuels allow. They are not driveable or useful for anything other
than the circle tracks and superspeedways the packages are engineered/developed for.
Yes, different engines use carbs somewhat differently, even within the same displac-
ment. For exaggeration, say 300cid six would demand 150cid per/rev,whereas a 300cid
single cyl would demand all 300cid in less than one full revolution. The demand on the
carb is obviously much higher per unit of time than the avg. in a one cyl. vs multiple cyl.
:coffee2:
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