Speed tuning the small block Chevy
I am not a certified mechanic, the information in the following article is based
on my personal experience of building and tuning small block Chevys since 1986
and 12+ years in racing. There is no doubt in my mind that these tips work and
show a noticeable improvement in performance and reliability. Whether you agree
or use what is offered here is totally up to you and I do not offer any
It is not the intent to give specifics as to actual tuning numbers, rather think
of this as a generic guide to more consistent runs and smoother performance. The
speed tips can be applied to street performance and lower level racing classes
and are written for Small Block Chevy applications, although some of the tips
are non-specific to engine manufacturer.
A couple jet sizes could mean the difference between a winning run and total
meltdown. When working with small block Chevy stock cast iron heads and intake
manifolds, one thing you can be absolutely sure of is no two engines will have
the exact same fuel needs, although in many instances they will be close but
this is not to say you should not jet to your particular needs. For safety sake
it is always best to run a hair on the rich side, but it has been my experience
that the best overall performance will be when you run on the 'ragged edge', so
you'll have to test your comfort level. Camshaft profile, header design, cam
timing and ignition timing can all affect the jetting for your engine.
When you buy a new carburetor, the factory baseline will be enough to get you up
and running but don't assume that it is at it's optimum as the baseline is
intended to meet many different needs. Over the years I have noticed that Holley
4 barrel carbs with vacuum secondaries seem to be the most out of the ballpark.
The secondary vacuum spring and cam will need the most attention. Edelbrock 4
barrel have a better track record of being more closely tuned out of the box. If
your rules restrict you to 350 c.i. engines and the factory stock Holley 500 CFM
PN 4412, the 73 jet it comes with should be a good start point and has proved
for me to be the best jet for compression ratios up to 11:1 and most hydraulic
Probably the most overlooked and misunderstood of all carburetor adjustments.
Idle air mixture affects air/fuel delivery at all RPM's, not just at idle, so it
Never assume the idle mixture screws will be set to some magical number of turns
unless of course you buy your engines dynoed and turnkey. If your engines are
purchased this way then use the exact same carb settings as the engine was
delivered to you. If you're in the second group and are using home built or
generic speed shop engines then the idle mixtures need to be adjusted to the
needs of each bank of the engine. What does this mean? It means that it will be
super rare for the your mixture screws to be the exact same number of turns out
from side to side or any exact number at all for that matter. In fact it is much
more common for your air idle mixture adjustments to be set differently from
side to side of the carburetor. Don't just turns your screws out 1/1/4 turns and
be done with it. If you do this, all you will have done is turned your idle
mixture screws out 1 1/4 turns, you haven't actually done any tuning. I've
actually watched a certified mechanic 'tune' a carb this way and insist it was
right. The proper way to adjust air idle mixture is with a vacuum guage. Turn
the air mixture screws out until the highest engine vacuum is achieved and then
turn in a hair. I've found the vacuum gauge method to be a hassle and a good ear
can get the same results. Turn the air mixture screws out until the highest RPM
is achieved and then turn in a hair. If you've got a good ear, you'll get the
Adjusting the idle mixture on a four barrel would be the same except you need to
adjust both primary screws first and then move on to the sencondary screws.
One last note: the idle speed and throttle plate position must be correct
in order to adjust the air idle mixture. If your throttle plates are open too
far you won't hear or see any noticeable difference when adjusting the screws
and it will seem as if they have no affect. Actually the idle air screws are
still richening and leaning as you adjust but you just won't hear the affect,
thus making your adjustments useless and highly inaccurate. If your throttle
plates are positioned correctly you should be able to stall the engine by
turning any idle mixture screw all the way in.
How often should a race carb be cleaned? After every day/night of racing. I know
many will disagree with this one. Many racers have told me it's a waste of time.
Saying that every time they took the carb apart they didn't see or find
anything. Think about this, a piece of dirt big enough for you to plainly see
would be big enough to block an air bleed. Much smaller particles can get past
the naked eye test but still raise havoc inside a carb.
In my 5 years of working on and tuning a WISSOTA Super Stock, I can say without
any doubt that cleaning your carb after every night of racing will have a
dramatic affect on reliability and consistency. This was put to the test, racing
two tracks per week for one full season. Many races were won that year,
including a few features and we made it to the 'big show' at two invitationals.
Was our sparkly clean carb the sole factor of our success? Of course not, but it
was a definite contributing factor in our success and the driver/owner will
attest to that as well.
Back to the toolbox!