Tuned Port Injection

 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. 

More power.  For those wanting more, there were nitrous setups for TPI engines, bolting on between the throttle body and the plenum.  Nitrous is really only practical for drag racing cars, though.  Paxton and Vortech both made supercharger setups which bolted to the front and pumped air into the throttle body.  Of course, nowadays the most often advice would be to swap in an LS motor.  Not having driven any late model GM cars (1998 or later) with those engines I can’t comment.  And I’m still trying to find an ’88-92 Firebird Formula to drop in that rebuilt L98 sitting in the machine shop….