Formula for figuring carb size?
- Posted by Dan(The Old One) on 12/1/2002, 11:12 pm , in reply to "Re:
What size carb should I use?" on the MTS Messageboard..
The problem with the standard formula is that it doesn't figure in
the fact that a carb is tested with 1.5" vacuum. You DO NOT want any
manifold vacuum at wide open throttle. If you get a manifold vacuum
reading at WOT, your carb is to small.
I posted a link to Edelbrocks sight below. Click on "Technical
Information" (lower feft). Now 2nd one down, "Technical Discussion".
In here you will find many formulas, including the CFM= CID X RPM X
VE / 3456.
Using this formula, I get this: 500 X 5600 X .8 / 3456 = 648CFM.
This is the amount of air flowing into the motor. To find what size
carb, you would need to convert these numbers to "carb test
numbers". 1.5" for a 4bbl and 3" for a 2bbl. A 650CFM carb would be
to small. BY a bunch.
Near the bottom of the page, is a section titled "CFM Rules". In
this section they state that if running a single plane intake, a
carb that is 110% to 130% bigger is needed. With your dual plane,
120% to 150% bigger.
Now if we take the 648 CFM and figure the 120-150% in, we get: 778-
972 for a carb size. The 800 being right in there on the low side of
the scale. For street driving, better throttle response and fuel
mileage could be expected over, say a 950. The 950 would probably
edge the 800 out in total hp numbers though.
If the VE numbers where raised to 90%, a 5600 rpm 500 could in fact
use a 1050 Dominator on a dual plane intake. This would explain why
at least 2 are using a Dominator, and a bunch more an 850 DP.
My informed, or misinformed opinion is that a 800 Edelbrock carb
will work fine on a street strip car. If you are racing only, a HP
series 950 Holley would be nice. Dan
: Try this standard formula for choosing
: carb size: Multiply your cid X
: your maxrpm. Divide that by 3456.
: Then apply a chosen factor for
: "volumetric efficiency",
: which in most street applications,
: should be around an optimistic
: 0.8. Any good carb book (Holley,
: Carter, Rochester) will have this
: formula and elaborate on it in
: depth. If you go by that, it is
: hard to imagine that you could go
: wrong. All you have to deal with
: then, is informed or misinformed
: opinions. I would recommend
: getting one of those books, which
: are available at most any
: speedshop. Then you'll be armed
: with facts, for when you start
: getting bombarded with all the
: "personal facts".
: --Previous Message--
: I'm building a 500 cad with 10:1
: MT15 cam, 76cc heads w/ oversize
: valves, Edelbrock intake manifold,
: HEI dist. and headers. I was
: going to use a 800 Edelbrock but
: I'm being told this is too small.
: Is anyone running this combo w/ an
: 800 cfm carb? Does it work? Any
: help would be great.
- If you eliminate the ve from the formula you come up with 810 CFM.
I have used the CID x RPM / 3456 for years and it has always come
pretty close if the max rpm's are correct.
780 CFM w/ vacuum secondaries works good on 500 street engines.
- Yup, leaving out the ve works too. I won't argue with you on that
one. That formula should work for your average street rodder, like
--- In Cadillac_Performance_Association@y..., "C. Gregory Talbott"
> If you eliminate the ve from the formula you come up with 810 CFM.
> I have used the CID x RPM / 3456 for years and it has always come
> pretty close if the max rpm's are correct.
> 780 CFM w/ vacuum secondaries works good on 500 street engines.