what size carb? 500 cfm or 625 cfm?
Steve Barnhouse
What size carb do I need?
I'm getting my small block chevy 350 rebuilt (.030 over) and I want to
get a new carb to replace the old Quadrajet. I have an Edelbrock SP2P
intake manifold (older version of the Performer) and I'm geting a
relatively mild cam (don't have the specs right here). It's a '71 El
Camino 4 speed manual. I like low end pulling power and probably
won't be revving it too high. I've decided to get a Carter AFB 9000
carb, and my question is what size? The machine shop recommended 500
cfm; a tech-support guy at Summit suggested 600 cfm. Summit offers a
500 cfm and a 625 cfm with electric choke, and I'm trying to decide
between these two. Is 625 cfm too big? Will I find run out of power
with a 500 cfm? Better fuel economy? No difference?
Thanks,
--steve
Stephen D'Amelio
: What size carb do I need?
This question begets others, how do your heads flow? what size valves?
what compression are you running? Cam specs? If you can't answer any of
these, then get the lower cfm carburator. BTW, most people over carburate
their engines. A 350 can make 300+ HP with a 390cfm two-barrel or less
than 200hp with an 850cfm four barrel, it depends upon how it was built.
Too much cfm size can result in a slug off the line and possibly cylinder
wash-down (with gas). Although 600cfm doesn't seem too large for the
displacement, I would go with the engine builder's recommendation.
--
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Steve D'Amelio dam...@progress.com | 69 Mustang 351W
| 70 Mustang 351C
#7 & #28 | 88 Mustang 302
| - 1,025hp -
There's no replacement, for displacement.
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Dave Williams
-> I'm getting my small block chevy 350 rebuilt (.030 over) and I want
-> to get a new carb to replace the old Quadrajet. I have an Edelbrock
-> SP2P intake manifold (older version of the Performer)
RUN AWAY! RUN AWAY!
The SP2P is demonstrably inferior to the original Qjet manifold.
According to David Vizard's Chevy manifold dyno shootout in PHR, the
SP2P simply made less power than the original Chevy intake all the way
across the powerband. It didn't get better fuel economy either.
I'd check the Qjet for signs of the dreaded Throttle Shaft Slop, and if
it passed, I'd just stick it back on the stock intake and run it. Sell
the SP2P to someone who doesn't know any better.
Bruce F. Haley
I run a 600 CFM on my Vette 327 (which is a stock '65 350HP
build) and have no problem with it. I am not sure if any
differences exist with Carters (mine is a Holley), but 600 CFM
should be 600 CFM regardless. I would say your 350 CI should
be able to handle 600 CFM, and is usually the size recommended
for the 327/350 CI range.
Bruce
--
Bruce Haley
bz316
LEE LEWIS
>Thanks,
>--steve
With the intake and mild cam the engine is going to make bottom end and
depending on the cam mid-rang power.This dictates a smaller carb so I would
go with the 500 to keep airflow velocities up.
LEE
John Williams
>>What size carb do I need?
>>500 cfm and a 625 cfm with electric choke, and I'm trying to decide
>>between these two. Is 625 cfm too big? Will I find run out of power
>>with a 500 cfm? Better fuel economy? No difference?
>>--steve
>With the intake and mild cam the engine is going to make bottom end and
>depending on the cam mid-rang power.This dictates a smaller carb so I would
>go with the 500 to keep airflow velocities up.
>LEE
I'm glad somebody is actually thinking about what their doing, rather than the
old "bigger is better!" moto when it comes to carbs.
Here's a little math for you:
1 cubic foot = 1728 cubic inches
350 ci = 175 ci (per crankshaft revolution)
---
2
175 ci x 6000 RPM = 1,050,000 cubic inches (of air) per minute
1,050,000 x .85 VE = 892,500 cubic inches/min ( volumetric efficiency of a
"well" tuned engine )
892,500 / 1728 = 516 cubic feet/min
This all says that the most air the engine will draw is 516 CFM @ 6000 RPM.
If you are using this engine on the street, I doubt is will never see 6000 RPM.
A 500 CFM is idealy matched to the 350 you're building, and Lee is correct that
the velocity through the carb will be increased, giving you better throttle
response and probably better fuel milage.
Good luck,
John Willie
Dale Ulan
>le...@aci1.aci.ns.ca (LEE LEWIS) writes:
>>>500 cfm and a 625 cfm with electric choke, and I'm trying to decide
>>>between these two. Is 625 cfm too big? Will I find run out of power
>>>with a 500 cfm? Better fuel economy? No difference?
.... stuff deleted
>I'm glad somebody is actually thinking about what their doing, rather than the
>old "bigger is better!" moto when it comes to carbs.
Yes, I'm very glad, too.
The only thing the original poster forgot was whether the 500 cfm carb and
the 625 cfm carb have the same number of barrels... I assume so.
The four-bbl version of the same size gives better bottom-end than the deuce.
For example, a 500 cfm deuce is just that: a 500 cfm deuce. A 4-barrel
gives you good breathing at WOT (500 cfm), but gives you a carb
that's half the size for low-end. (250 cfm). Spread-bore carbs are
even better, where the two primaries are sized to around 150 cfm, with
secondaries making up the 350 cfm difference. These are just
approximate figures. I'm not in the carburetor manufacturing field, so...
To add to the confusion, vacuum-secondary four-barrels will try to
compensate for a mild bit of overcarburetion, where a double-pumper
will not. The double-pumper must be correctly sized for the application.
Accelerator pump tuning on a double-pumper is much more critical, too.
The four-barrel carb will usually give you better fuel economy if you
don't use the secondaries too often, because of the better metering
range (faster airspeed) the smaller primaries give you.
Holley actually has a book out on various aspects of carburetion, including
how to size the carburetor. They also stress being on the conservative
side, because buying too big of a jug will cause endless problems
in the long run.
73
-Dale