While most of the nation is in a carb
cutting frenzy we've been living, breathing, and eating
carbs - at least of the four barrel variety. Our blow-through
supercharged 331 project is moving along better than expected.
When we dynoed the car last month (see Part
III: Tuning for Boost) the motor cranked out a conservative
425 horsepower to the wheels. We knew the engine was far from
done as this was merely a baseline dyno session with a carburetor
tuned for a naturally aspirated motor. This was no further than
evident in the air-fuel ratio which was dangerously lean despite
maximizing the jetting. Nonetheless,
we were on the right track and looked forward to more testing
once our carburetor was modified for forced induction.
it comes to carburetors it is easy to assume that a Summit catalog
is as far as you need to look. In reality, if you are serious
about extracting out every bit of available power from your
motor, you have to get away from the assembly line carbs and
seek out a qualified and experienced carb shop to custom calibrate
a carb for your engine. This becomes even more critical when
dealing with power adders, especially turbos and centrifugal
superchargers, where the progressive rise in pressure creates
unique demands on the carburetor's fuel curve. An out-of-the-box
carburetor can be outright dangerous for these type of applications.
We knew that the 750 cfm we'd been running on the naturally
aspirated 331 stroker was likely not going to have the proper
fuel curve for a blow-through environment. This proved itself
by means of very lean air-fuel ratios which did not respond
to increased jetting. While you can force a carb to deliver
more fuel by using huge primary and secondary jets and opening
up the power valve restriction channel, the downside is very
poor fuel economy and poor throttle response at cruise speeds.
A blow-through carburetor does not need to be a wide-open-throttle
only device, but it will take a carb rebuilder with plenty of
experience to properly calibrate the fuel metering.
We turned to TPC Racing, a performance carburetor shop in Southern
California, to assess our situation. TPC's president, Ty Lofstrom,
has recently formalized a blow-through carburetor program to
complement his shops long standing line of winning street, strip
and marine carbs. Ty challenged us to put their new blow-through
750 cfm 4150 to the test. The technicians at TPC went through
their normal process of obtaining our engine and vehicle specs,
including critical parameters such as compression ratio, boost
levels, and cam specs. TPC starts with a ProForm 750cfm casting,
however the similarities end there. TPC's techs tear down the
carburetor and begin an intricate process of modification and
calibration to the metering blocks, throttle plates and main
body. The carb is then reassembled and flow tested. With blow-through
carbs, TPC aims to to keep the carburetor A/F ratio around 13.5:1
at cruising speed and 11.5-12.2:1 during full boost. The carburetor
also receives physical modifications to withstand issues related
to high boost and fuel pressure. Solid nitrophyl floats replace
standard brass floats which collapse under pressure. A channel
is milled around the base plate with holes drilled to the throttle
shafts to reference them to boost pressure via a -3AN line.
This equalizes the pressure around the shaft and boost/fuel
leakage is prevented. This becomes important as boost pressures
approach or exceed 10 psi.
Back to the Dyno
the new carburetor squarely mounted on the motor we headed to
the dyno for some pulls. We weren't sure what sort of power
gains to expect with the carburetor change. Realistically we
were hoping to hit the 450 horsepower mark and similar numbers
for torque. This would be around a 10% improvement on the numbers
from the initial baseline dyno runs, and a safe place to be
at with our high compression motor. We also only measured 4
psi of boost during that dyno session. As a result we brought
along with us to this session a smaller blower pulley (2.77"
versus 3.33" stock Vortech S-strim diameter.) Our plan
was to dyno the car first with the stock 3.33" pulley and
TPC carburetor, and then, if boost levels were still only 4-psi,
we'd swap pulleys and try to hit 6-8-psi of boost pressure.
Once again, to keep things detonation free with our 10.5:1 compression,
we are running a 50-50 mix of 110 leaded race gasoline and 91
octane from the pump. Timing is set to 10 degrees initial, with
an additional 22 degrees at 3000 rpms via the centrifugal advance.
Using the MSD BTM we subtract out 3 degrees for every psi of
boost, giving us no more than 20 degrees advance under full
boost. We left the carburetor at TPC's baseline jetting of 76
primary / 86 secondary.
After strapping the car to the big red drums we fired it up
for the first pull. In fourth gear we slammed the pedal to the
floor and waited for the tach to hit our valve-float induced
redline of 6200 rpm. The engine seemed to take longer than usual
to climb though the rpms, and the final dyno peak of just 300
horsepower indicated something was amiss. Our dyno tech suspected
the throttle blades were not opening completely, a simple problem
and welcomed much more than the idea of a dead cylinder or leaking
head gasket. We checked for full primary and secondary blade
opening and as surmised they were far from straight-up vertical.
The culprit was a poor angle on the stock throttle rod, which
our dyno tech addressed by running to the hardware store and
fabricating a nice spherical rod-end linkage (see "Make
Your Own Custom Throttle Linkage").
An hour later we were back in action, this time with the throttle
blades opening completely. We made the next pull and the engine
was audibly a different beast. It roared through the 2500 -
6200 rpm window in a matter of seconds, and the thumbs up from
the dyno tech operating the Dynojet PC and brake was a sign
of success. The peak power hit a huge 516 horsepower (SAE corrected)
on that pull. Torque was at big-block like 465 lb.ft.
Dyno Results (RWHP):
Peak Torque (lb.ft)
516 @ 6100rpm
465 @ 4900rpm
424 @ 6100rpm
405 @ 5100rpm
Analyzing the Results Checking the recall on the boost gauge revealed a peak of
7psi. This was surprising because we were still running the
stock 3.33" pulley, and up to this point had never seen
beyond 4.5 psi. Other than the TPC carburetor there was no other
change. We can only speculate that the main body on the TPC
carb is less restrictive and does a better job of getting air
past the throttle blades than our milled choke-horn carb. Nevertheless
there was no desire on our part to throw on the smaller pulley
(rated to make 15psi) and push the engine into a dangerous state.
We opted not to make any more pulls that day because the power
numbers, while stellar, were beyond our threshold for the stock
fuel supply system. Our '67 Mustang is still equipped with the
factory in-tank pick up and 5/16" fuel line to the mechanical
(boost referenced) pump. Reviewing the air/fuel ratio chart
the TPC carb dips down to 12:1 around 5000 rpm, but then swings
back up to as high as 13.8:1 under full boost. We believe this
is a sign of the stock fuel pickup and lines hitting their limits.
After all those lines were never made to support a 600 horsepower
engine. The TPC carb certainly proved itself, making serious
power with significantly less jetting, and much better driveability.
After reviewing the A/F chart, TPC had some jetting and air
bleed change suggestions which they feel would flatten out the
curve, however we'll upgrade the fuel supply to 1/2" before
attempting to make any more power. In an upcoming article we'll
continue with tuning including some long awaited track runs.
In This Article:
Our Project '67 Mustang, running a Vortech supercharged
331 cid. engine, breaks the 500 rear wheel horsepower
mark with a custom tuned carburetor.
Blow-Through 750cfm 4150 TPC starts with a ProForm 750cfm body, tears
it down, and modifies it for forced induction. Note
the CNC billet annular boosters, used to keep the fuel
atomized and evenly distributed throughout the power
band Features such as screw-in air bleeds maximize tuneability
and room for growth.
Four pages of blueprints
detail the specifications for every passage, circuit,
slot, bleed and well in the carburetor.
TPC's goal is a fuel-curve that is efficient at cruise
while safe under full boost.
There is decades of experience put into these TPC carbs.
Not every secret is made available or listed on the
prints. Many of the passages in the main body have confidential
specs. We'll trust their expertise.
TPC's experience shows in their
quality workmanship, as seen here in the profiled throttle
shafts and modified base plate. Also note the machined
pressure channel in the baseplate to prevent pressure
leaks around the throttle shafts. An additional touch
is the ability to adjust secondary blade position from
the top side of the carb.