Uncommon Q-Ship: Pontiac Grand Prix GTP
When thinking about a sleeper performance car, I am honest enough to admit that most people don’t immediately have a mid-ninties FWD Pontiac come to mind. Color me weird, but there is something tempting about paying a small amount of money for a comfortable car with a supercharged V6, great response to modifications, and enough torque to accidentally pan-fry Pirellis. Sure, it’s not the classiest car ever made, but you have to give the GTP some respect: It can show Mustang GT’s what’s what.
The Grand Prix is based on GM’s W-Body architecture, which underpins the GP along with the Buick Regal and Century, the Chevrolet Impala and Monte Carlo, and the Oldsmobile Intrigue (which really wasn’t that interesting. Sorry.) The platform originally debuted as a one-size-fits-all replacement for GM’s myriad selection of midsized passenger cars in 1988. The revised W-chassis cars (which we are discussing) debuted in 1997, as a Coupe and a Sedan.
When the revised Grand Prix came out in 1997, it was available with three engines. The base model SE had an awful 3.1L Pushrod V6 with 160 horsepower and a 4-speed automatic. The GT came with the Buick 231ci V6, which by then was simply called the 3800 Series II. This was an old-school design, but with tons of torque and reliability. Output on the Series II jumped to a more reasonable 195bhp, but this isn’t the model we’re concerned with either. Optional on either the Grand Prix GT Coupe or Sedan was the GTP package, which was basically an Eaton M90 Supercharger strapped to the top of the 3800 V6. This combination, internally referred to as the L67, made 240 horsepower and a considerable 280 lb-ft of torque - of course, still directed to the front wheels through an open differential.
Now, let’s be very up-front here: a Grand Prix GTP is NOT state of the art technology. It wasn’t even when it debuted in 1997. The Buick six has been around forever (which is why it’s very reliable) and the W-Body chassis has, like I said, been in production since 1988. Sure, it gained independent rear suspension along the way - but it’s still an old design. MacPherson struts in front, pushrod iron-block V6, 4-speed slushbox, etc etc.
So while it might sound like someone running a marathon with an iron lung and squish out gearchanges… eventually, it’s still fast. Those 280 torques propelled the original GTP to sixty miles an hour in 6.4 seconds, which is about as quick as a modern (multivalve, twin cam, VVT) Camry or Accord V6. This is more impressive when you consider how old this drivetrain is and how much power simply disappears through the 4L65E 4-speed automatic transmission. Stock GTP’s with a crank horsepower rating of 240 are lucky to put 190 to the front wheels. So that torque is the right-now-kick-in-the-pants that gets you down that 1320 respectably fast (budget about 15.8 seconds for that.)
What is great about the L67 (and later L32 engines, which were Series III with a newer blower and 260 horsepower) is that they respond alarmingly well to simple modifications. 300+ whp from this engine is just a matter of bolt-ons and some minor internal modifications. Stage 1 GTP’s (yes, I’m talking in arbitrary “stage” numbers. It’s my VW Heritage, get over it) usually have a 3.4″ Supercharger underdrive pulley, ported stock exhaust manifolds, a downpipe and an intake, exhaust work (U-bend delete) and colder spark plugs. This will net about a 20whp bump in output and enough torque to drop the 1/4 mile times by about a half-second.
More radical modifications of the L67 usually surround even smaller blower pulley sizes (usually down to 2.8″) and supporting modifications. For an example of an extremely fast GP with mostly stock internals, check out this entry on DragTimes: Click Here.
Past that, further supporting mods are required for smaller pulleys. A camshaft is necessary to retain some more top-end breathing ability, along with roller-rockers and pushrods - and valve guides. A built or ported stock set of heads is necessary to get proper flow, along with a set of real headers - not ported stock ones or P-Logs. Obviously, with more boost, bigger injectors and an auxiliary or replacement fuel pump is needed to keep up with demand. This type of setup (depending on specifics) can net anywhere from 300-350 horsepower, depending on boost levels and the tune. Thankfully the stock Eaton M90 blower is relatively reliable, although some of the blower ducting tends to develop leaks at high mileage.
Other things to look out for with your hot-rod GP include transmission failure. Despite being labeled as the “heavy duty” version of the 4L60E, the 4L65 still tends to have problems at high mileage, and these will obviously be exacerbated when you significantly increase the output. Some common problems include inconsistent torque converter clutch lockup in 4th gear, caused by a wear-induced fluid leak near the TCC valve body. A stuttering or jerky start (or consistent slip in 1st gear) can be caused by a leaking pump seal for the viscous Torque Converter pump. And many other issues. The good news is that GM made these transmissions for ages, in many different light-to-heavy-duty variants, with consistent updates and upgrades to fix known problems. This means that if you have an old-design light-duty trans and are trying to push 350 crank horsepower through it, you can use other 4L60/4L65 parts to replace, fix, and upgrade your ailing trans. There are multiple choices for aftermarket add-on trans coolers, which obviously help a lot.
With a little ingenuity, (relatively) little money, and some patience, it’s quite possible to turn this 3600lb American FWD sedan into a tire-melting rocket that runs 11’s. The path’s already been laid; it’s really a matter of following the signs. Grand Prixs retail for between 3,000 and 8,000 depending on year and condition. If you don’t require that new-car smell and don’t want to drive a new Camryolla anyway, then maybe a well cared-for GTP and a few choice modifications could be the thing for you.
I’ll finish things off with a video of WOT acceleration in a stock GTP. Mind you, this was uphill onto a steep on-ramp with three people in the car. But ohhh, listen to the noise! This car is owned by John Beerman, who was gracious enough to lend his car for photography purposes. Thanks, John!