Reply to "Need new carb"

> Just to add my 1-1/2 sense here (not cents), On a box stock cj type
> Cleveland, particularly if you are running the low compression version,
> you will probably be happiest with a 600 or 650 vacum secondary carb.

My low compression 351CJ is working great with a 735 Holley.
I would not be happy with 600 CFM on a 351C, even with low
compression. With proper tuning and booster selection, you
can have good throttle response without out choking off the
air flow.

> Since I'm going with the 750, but have the low compression heads, is it
> better to have the car running a little leaner?

Which specific 750 are you talking about? Regardless of carb CFM, you
want to run lean of stoichiometric at cruise for fuel economy and rich
of stoichiometric at wide open throttle (WOT) for fuel economy. 14.7:1
is the stoichiometric ratio. It's the "chemically ideal" ratio where
there is no excess fuel or oxygen left after combustion. Leaner means
there's excess oxygen left after combustion. Richer means there's excess
fuel left. Rich of stoichiometric at wide open throttle will make better
power and lean at cruise will yield better fuel economy. There's no single
ideal ratio that applies to all engines. Some engines make best power at
13:1, others closer to 12.5:1. 12.8:1 is a good WOT ratio to shoot for.
Note that the air fuel ratio is by weight. 13:1 means 13 pounds of air
are mixed with 1 pound of fuel. The usual target values for normally
aspirated 4 stroke engines are about 12.5 to 13 for WOT, 14.0-15.5 at
part-throttle cruise and 13.5-14.0 for part throttle acceleration (or
climbing a long hill, pulling a load, etc.). If you want to lean out the
mixture at cruise for best fuel economy, be aware that you'll also need to
adjust timing. Combustion gets much slower under lean conditions and if
you don't adjust spark timing, the combustion occurs much later and exhaust
temperature climbs. That's bad for the exhaust seats and valves. However,
if you adjust for MBT spark at each A/F ratio, exhaust temperature will
actually decrease relative to stoichimetric (rich will still be somewhat
cooler). For typical gasoline engines, the range or ratios is:

A/F Characteristics
Ratio
5 Rich burn limit. Combustion is weak and/or erratic.
6-9 Extremely rich. Black smoke and low power.
10-11 Very rich. Some supercharged engines run in this range at full power
as a means of controlling detonation.
12-13 Rich. Best power A/F for normally aspirated WOT.
14-15 Chemically ideal. At 14.6 the A/F is at the theoretical ideal ratio
with no excess fuel or oxygen after combustion. Good A/F target for
part throttle cruise and light to moderate acceleration.
16-17 Lean. Best fuel economy A/F ratio. Borderline for part throttle
drivability (worse than borderline if EGR is used).
18-19 Very lean. Usual lean limit (Driveability).
20-25 Lean burn limit. Varies with engine.

> I'm running a Holley 4150-735cmf from a late 60s early 70s 390-428 Ford.
> Works for me!

That's because Ford developed that carb. It has skirted Ford truck
boosters which are superior to the usual down leg and straight leg
Holley boosters and Ford's PVCR's are half the size of the main jets
which means it's possible to get good cruise mixture and good WOT
mixture.

> The mixture strength is read as the air/fuel ratio and isn't going to
> vary much according to the carb size.

The big discriminator is the ratio of the jets to PVCR's. Of course,
you don't know what your jets are going to be a priori but stay away
from carbs with large PVCR's. It's a lot easier to drill out a
too small PVCR than it is to close down one that is to large.
These days, I'd definately get a carb with replaceable PVCR's.

> What you are going to find is that a carb like 3310 which is a 780 vacuum,
> an 1850 which is a 600 vaccuum are both "street' carbs and are just about
> where you want them out of the box.

Not in my experience. Both the 3310 and 1850 are 4160's which means
you can't even change the secondary jets and both have the least
effective straight leg boosters. The PVCR's in the 3310 are likely
too large and the 1850 has side hung floats which aren't good for
cornering. Both of those carbs were designed to be cheap. If you
actually want to do some tuning, stick with a 4150 style carb,
preferably with center hung floats. Replaceable PVCR's would be
nice, too.

> Those carbs will smart your eyes at idle and can only be leaned out by
> enlarging the idle air bleed in the main body.

Carbs like those from QFT have replaceable bleeds and PVCR's which
make fine tuning much easier.

> If you are serious about getting the mixture strength correct you are
> going to need to do this all on a dyno or install an oxygen sensor in
> the exhaust system that you can read the a/f ratio with in real time.
> That involves a computer with red and green lines on a graph on your
> computer.

I'm using an Innovate LM1. You can use a computer for datalogging or
the hand helds built in display for real time data.

> For a street car with moderate performance I prefer the Edelbrock carbs.
> They seem to idle best and start better than holleys. A 600 works great
> on a 350/351 right out of the box. They also seem to have better part
> throttle response than the Holleys I have used.

AFB's definately win on ease of tuning. On the Pantera, what do you do
about the AFB idle mixture screws being up front? A 600 CFM AFB would
definately be too small on a 351C in a Pantera. I run a 500 CFM AFB on
a little Buick 215 (a.k.a. Rover 3.5L) and it works just fine. I've got
a 750 Edelbrock AFB to try on the dyno. A drag racer friend has tested
a 735 Holley from a 428CJ against a 750 AFB, said the Holley was worth
a bunch of power (0.35 seconds in the 1/4 mile on a 428CJ Torino).
Supposedly an 800 CFM AFB is on par with a 750 Holley, air flow wise.

Dan Jones