Blof-fan wrote:jondee86 wrote:The 40's work well for the 4A-F. But the intake ports are maybe half the size of a bigport 4A-GE
so there will be a noticable difference in air velocity there, and that's why I was wondering if 45's
would be a better match.
I have never played with twin-choke carbs myself, so I'm no sort of expert. But I remember
riding with a friend who had a built 1600 Ford with 45's and he had to struggle to keep it idling
while waiting at the lights. Engines with cams generally don't have much vacuum at idle due
to reversion, and vacuum is needed to draw air in thru the carbs.
Carbs rely on airflow thru the venturi to create additional vacuum to suck fuel out of the jets,
and the choke size is adjusted to create the necessary vacuum. So when comparing 40's with
45's, you will end up making a choice between a 40 with a thin choke and a 45 with a thick choke.
Where (at what rpm) you want your engine to be most efficient is also a consideration. A 40DCOE
with a 30mm choke will outflow a 45DCOE with a 30mm choke up to around 160cfm per throat !!!
A 45mm carb has 26% greater cross section area at the throttle plate than a 40mm carb, so you
can expect the smaller carb to be more efficient at lower rpm, and promote slightly better
turbulence in the intake ports.
If you are planning to track the car (and my friend's car was his racecar) you could go for the
45's that will do their best work at high rpms. But for a lightly modded street car that usually
drives at lower rpm, it's likely you will get a better result with the 40's. For what it's worth,
when I put 42mm silvertop ITB's on my smallport with 270 deg cams, I actually incorporated
40mm tapered chokes into my bellmouths for improved bottom end performance. Of course
ITB's don't rely on venturi effect for fueling, so not a valid comparison.
Good luck... jondee86