Car Craft – November 2019

➔HANDS ON

By Jeff Huneycutt / Photos: Jeff Huneycutt

FUEL SCHOOL

A 632 Chevy Schools Us on the “Booster Tip” Paradox—

Here are Two Radically Different Ways to Fix It

I

n the dark ages, a bunch of oddball

scientists calling themselves “alche-

mists” spent their lives coming up

with crazy experiments as they

attempted to turn lead into gold. The

things they tried were arcane, complex,

and only barely understandable after

committing your entire life to the

subject.

Carb tuners can relate.

We are currently living in the dawn

of easy EFI for everyone, but there are

still many thousands of cars running

carburetors out there—and many are

still in need of proper tuning. There is a

lot of truth to the old saying that a car-

buretor is merely a metered air leak. It

isn’t too hard to get a carburetor to

work well in a single situation, but get-

ting it to provide the proper fuel curve

in a wide variety of scenarios is much

more difficult.

Not too many years ago, carb tuners

spent hours on every carb drilling new

air passages and doing all types of

things to get a carburetor to work well

in a variety of situations. These days,

it’s a lot easier, thanks to modern carbs

like Holley’s Ultra XP and Ultra Domina-

tor carbs that come with a wealth of

tuning options built in. But you still

have to know how to take advantage

of all those tuning options to make

them work.

A common issue many car crafters

face is something lead engine builder,

Senior, at Prestige Motorsports in Con-

cord, North Carolina, calls “booster tip.”

The name for it may vary, but booster

tip is essentially when vacuum through

the carburetor’s venturis becomes

strong enough to pull too much fuel

through the main jets and causes the

engine to run far too rich.

In real-world applications, tip-in can

occur when you have a large (high-

flow) carburetor on an engine with

plenty of vacuum (typically by using a

camshaft with a shorter duration to

improve throttle response). The prob-

lem comes because if you size the jets

for crisp throttle response, the engine

runs dangerously lean on wide-open-

throttle (WOT) blasts. But if you go

with larger main jets to feed enough

fuel on high-rpm WOT blasts, the

engine won’t seem to run when driving

steady at part throttle.

That’s because at part throttle, the

partially closed throttle blades in the

carburetor create increased vacuum—

thus, more pull—on the main jets.

There’s an intersection where the vac-

uum created by both engine rpm and

throttle-blade position becomes so

strong that fuel starts dumping out of

the main jets and floods the engine.

Senior told us of running into this

issue recently with a 632ci big-block

Chevy build and showed us how he fixed

it. The engine is actually going into an

airboat and is tuned to produce maxi-

mum power around 5,200 rpm (where

the engine will spend hours running at

a steady state). So, while it isn’t a high-

rpm race engine, it created a perfect

storm of conditions for strong booster

tip (and a poorly running engine), which

many car crafters can relate to.

Senior could create tip-in on the

engine dyno by having the dyno hold

Prestige Motorsports’ 632ci big-block

Chevy is a power-boat engine with a

wide torque curve that runs out by

5,500 rpm. With tons of vacuum, it

exhibited the same issue that many

car crafters have experienced: The

carb wanted tons of main jet to

provide enough fuel at wide-open

throttle (WOT), but went way too rich

on a steady throttle at 2,500 rpm.

32 CAR CRAFT NOVEMBER 2019