Back to cars again. I joined a TPI site on Facebook and was surprised to see how many enthusiasts there still are for this GM variant of port fuel injection, which only came on Camaro Z/28s/IROCs, Firebird Formula and Trans Ams, and Corvettes, and only between 1985 and 1992 on 5.0L and 5.7L V8s. With GM’s LS-series V8s being the current performance standard among the GM crowd, that tuned port injection still has many adherents is a remarkable surprise. My own L98 is still languishing in a machine shop in Fairfax, Virginia, rebuilt, balanced and blueprinted with a new camshaft and aluminum cylinder heads since the donor vehicle (intense weeping) burned up in 2012.
Carburetors. Before any form of fuel injection, there
were carburetors to mix air and fuel so an engine could make the car go. Carburetors use a venturi inside the barrel
to suck gas up and mix it with air. They’re
sized in “barrels” depending on the engine they go on. Four and six cylinder engines typically get
one barrel carburetors, six and eight cylinder engines get two barrels, and V8s
usually get four barrels. The Rochester
QuadraJet is GM’s most popular and common four barrel carburetor (and what’s on
my ’76 Firebird’s Olds 455 right now), the Carter AFB (Aluminum Four Barrel),
now made by Edelbrock, is a popular alternative, and Holley has traditionally
made aftermarket four barrels. The ’67-69
Firebird Sprint had an overhead cam straight six with a four barrel (QJet) – Delorean
was proud of that, but no one else seemed to care.
Generally a well-tuned four barrel is all a V8 needs, but
some have even more. The Pontiac Tri-Power
(1964-66 GTO) and Mopar Six-Pack set up had three two barrels. For idle and part throttle, the engine ran
off just the middle (inboard) two barrel carburetor, and at larger throttle
openings up to WOT (wide open throttle, pedal all the way down to the floor)
the outboard two barrels would open up, for a total of six barrels. Some engines went even further: the ’67-69 (first
generation) Camaro Z/28 had two four barrel carburetors on its fairly small 302
cubic inch V8, and the 426 Hemi had two Carter AFBs, a total of 8 barrels, aka
a “dual quad” setup. I haven’t seen a
dual quad setup with QuadraJets, though.
Listen to “409” by the Beach Boys, citing the “four-speed, dual-quad,
positraction 409”.
Mechanical Fuel Injection.
Engineers weren’t satisfied with carburetors and tried mechanical fuel
injection in the late 50s. The Corvette,
some Pontiac Bonnevilles and Chrysler 300s, got this deal, but mechanics were hard-pressed to
work on them due to their rarity.
Mercedes insisted on putting mechanical fuel injection on all their cars
from 1959 to the early 70s, at which point they switched to electronic fuel injection. And literally all diesel engines, by design,
have mechanical fuel injection. Diesels
inject the fuel directly into the cylinder on the firing stroke, which ignites immediately
upon hitting the highly compressed hot air in the cylinder. Mechanical fuel injection never managed to
effectively compete with carburetors.
Electronic Fuel Injection. Carburetors are actually fairly complex, as
they have to handle all the different fuel needs of the engine
mechanically. For cold start
conditions, there’s a choke. For idle, when
the throttle is closed (e.g. at a stoplight) there’s an idle circuit. For off-the-line boost, there’s an accelerator
pump.
But imagine if you simply had 4, 6, or 8 injectors,
connected to a computer, which pulse on and off to inject fuel. The computer could ascertain how much pulsewidth
to use, and simply pulse the injectors on and off. If the engine still doesn’t have enough fuel
for its needs with the injectors continuously open, simply upgrade to larger
injectors or higher fuel pressure. Moreover,
a performance makeover is as simple as replacing the EFI chip (PROM) or reprogramming
the ECM (electronic control module) with a unit designed for that purpose. And if any sensor is reporting back bizarre
readings outside its normal range, the computer can shoot back a trouble code, to
give the engine some self-diagnostic capability.
EFI got its start in the 1950s, but back then computers
ran on vacuum tubes, not nearly as small or fast to run an engine in real
time. Bendix, which had started the
whole thing, sold its rights to Bosch. When
transistors came around in the 1960s, Bosch was finally able to develop a
practical ECM, and thus K-Jetronic debuted on 1968 model year VWs. In 1972, Mercedes switched over to EFI on all
its cars, and in 1976 Cadillac and Lincoln started as well. In the early 80s, computers had gotten even
better, so the Big Three could finally put EFI on more mundane cars like Chevrolets,
Pontiacs, Fords, Dodges and Plymouths.
Throttle Body Injection.
Port fuel injection has an injector at each cylinder. When I bought my 1992 Pontiac Firebird (not
the Formula), it had a 5.0L V8. On top
of the engine was the familiar circular air cleaner with the snorkel. Huh?
This 1992 car has a carburetor?
Nope – spin the wing nut off the air cleaner cover and voila – two egg-like
things with wires pointing down in what looks like a carburetor but isn’t: the
throttle body supplying fuel to all 8 cylinders. TBI is a mid-range system in between carburetors
and port fuel injection.
TUNED PORT INJECTION.
Finally getting to my subject. By
1985 GM had developed a performance port fuel injection system for the Corvette,
Camaro and Firebird. Dropped onto the
small block Chevy V8 in 5.0L (LB9) and 5.7L (L98) size, the system made between
205 and 230 HP on the LB9 (the higher rating on T5 manual engines) and 225 and
245 HP on the L98. Most of those got the
700R4 automatic transmission, as GM knew only Corvette owners would be willing
to pay $3000 just to get a five speed manual ZF transmission.
The TPI setup has a baseplate, a pair of runners, a plenum, 8 injectors on a fuel rail with a fuel pressure regulator, and a throttle body. It has a distributor, HEI, either in-cap or external coil. Supposedly the
whole setup is optimized for 5.0L V8s and is a bit restrictive on 5.7L
V8s. Naturally the aftermarket has made
larger runners, a larger baseplate (apparently the plenum is not the choke
point) and larger throttle body plates.
My own L98 was upgraded with SLP parts: larger, siamesed runners, plus
headers, a larger throttle body, 3” exhaust, a performance PROM, etc. for a
total of 50 HP over the stock rating, thus 290 HP. On my watch I added a TPIS baseplate and
upgraded the transmission and rear end (3.73 rear ratio).
Again, 1992 was the last model year any GM vehicle had
this system, and the LS motors are GM’s current performance standard. But I’m still seeing strong interest in TPI engines,
often transplanted into trucks and other cars other than Camaros, Firebirds or Corvettes. Naturally those cars are also getting LS
swaps.
C4 (1984-96) Corvettes have a clamshell hood which hinges up
from the front, not only on top but around the sides, leaving the top of the
front tires exposed. In Camaros, the TPI
intake flows straight forward and splits into two inlets in front of the engine. On Firebirds, the TPI intake takes a 90
degree right turn and winds up taking its air from a box at the front right
corner of the engine compartment. Camaro
TPI air filters are small and square, while the Firebird TPI air filter is a
short cylinder.