best cfm for fifth ave

[HectorVenezula]

I'm still running the same 318 in my 87 5th ave. I did a mild cam upgrade to roughly the same lift as the diplomat cop cars while still keeping the 302 heads and i purchased a summit 4 barrel 600cfm. I'm trying to find the right jet size to adjust the cfm. How much of a difference in cfm does each size decrease make and what would be the best cfm.

 

[89.Fifth]

That's not how carburetors work. The "cfm" rating is really just how much air the airhorn can flow. Different carburetors can possibly flow different amounts and matching a "cfm" to a particular engine is only a very basic rule that really doesn't apply all of the time.

The Jetting adjust the air fuel ratio and that happens inside the carburetor. It has nothing to do with the cfm rating. A 500cfm carb can run rich or lean and a 800cfm carb can run rich or lean. Either one can go on a 318 and as long as it's tuned correctly either one can be a good match.

If you want to tune your carb jetting you need to either learn how to read spark plugs (not something I trust), get a wide band O2 sensor in each exhaust pipe, do lots of jet changing and test drives, or be happy with an "off the shelf" tune that comes with your carb.

 

[Aspen500]

Without a wide band fuel ratio gauge it'll be mostly a seat of the pants thing and you'll never get it ideal.

The Summit carb, in addition to jets, also has idle air bleed orifices that can be changed. I won't go into the whole thing about how to determine which ones you need. There's instructions that come with the carb, as well as online at the Summit site.

In all honesty the Summit carb used on a 318 with a mild cam will probably run fine out of the box. Fine tuning for max power AND fuel economy is about impossible without the wide band A/F gauge as I mentioned. Even if you only monitor one side of the engine you'll, just put the same jets in each side. Unless your looking for that last horsepower, it'll be good.
FYI: The AEM A/F gauge works excellent and is one of the lowest priced units out there. (If you decide to take the plunge)
AEM Electronics Wideband Air/Fuel UEGO Gauge Kits 30-4110

 

[AJ/FormS]

Hector
I hate to rain on your parade, but
if you still have the 2.45 or less rear gear in it, both the 4bbl and the cam were a total waste of time. The secondaries when properly adjusted will not come in before 3500 rpm, which with a 2.74 low-gear is 41 mph. 41 mph,41 mph. With a 2.45 low this is 45 mph. The bit of a cam,may have stolen some of the torque you had, off the line.
But if you do bring the secondaries in earlier,and assuming the engine does not immediately detonate itself to death, then the secondaries will add nothing to your engine at all, cuz the 2bbl was good to to perhaps 4000 rpm all by itself. All the 4 bbl will do is share the airflow between four venturies instead of two, and now the airspeed thru those venturies has fallen to 50%, making jetting nearly impossible.
Your engine might be able to pull 414 cfm@ 4500@58 mph with 2.45x2.45x 26 inch tires at 100% VE. More likely is perhaps 310cfm @75% VE.........Your 600 is crazy big. Twice as big as the engine can use.

Lets say you put 4.10s in there.This brings the mph down to 4500=35mph with a 2.45 low. But your carb will still pull no more air than before, or about 310cfm
If you want performance
You have to get rid of those 2.45s or less.
Please say the 2.45s are gone.