DOHC GTP Project

crazyd's 1992 Grand Prix GTP 3.4 DOHC 5-speed1992 Grand Prix GTP 5-speed 3.4 DOHC Engine Replacement

Click here to go to the DOHC GTP project page.

The Story

This is a very rare version of an otherwise ubiquitous car. The GM W-body is a midsize, front-wheel drive platform that encompasses four different car lines, including the Pontiac Grand Prix, Chevrolet Lumina, Oldsmobile Cutlass Supreme, and the Buick Regal that went into production in 1988. It was an absurdly costly development cycle, costing seven billion dollars to develop these cars before even the first one was sold. In 1987’s money. To put that in context, the current C5/C6 Corvette ‘Y’ platform was developed for $500 million, a mere 1/14th as much. And that is in 1996 money. Accounting for inflation, this cost more than it took to develop the largest superjumbo passenger jet the world has ever seen, the Airbus A380. It was a hugely ambitious engineering project that got off to a rocky start – due to vastly overestimating their eroding market share, it had nearly bankrupted the company by 1992, and had effects on other cars as well – the new C5 Corvette was 4 years late due in large part to it. But it eventually remained in production through four generations and 26 years, until the last Impala rolled off the line in 2014.

In 1989-1990, Pontiac had teamed with ASC/McLaren to produce a limited-edition turbo 3.1 that made 205hp with a 4-speed automatic. These are the very rare Turbo Grand Prix (TGP) cars, but GM was working in-house on a new V6 that would produce as much or more power than the turbo 3.1 without a turbo, and would be available in the Chevy and Oldsmobile as well. In 1991 it was released: the optional 3.4-liter, dual-overhead-cam LQ1 V6 (VIN code X) that produces 200hp with the 4-speed automatic.

As standard equipment in 1992, these included a 3.1 liter OHV corporate GM V6 producing 160hp (VIN code M), with a 4-speed automatic transmission. An average of 94% of buyers went with the standard 3.1/auto configuration, and a short test drive in both makes it obvious why: the 3.1/auto feels more powerful off the line. It makes its torque down low, and as soon as you hit the gas you feel that torque pushing you forward with authority. It’ll even spin its tires without much effort. The 3.4 DOHC, by contrast, doesn’t spin its tires nor launch with that kind of low-torque pushrod authority – in fact it doesn’t really come alive until 3000rpm – and it feels less powerful around town than the 3.1. But a freeway onramp test or an overtake is what makes the DOHC engine shine. Push your right foot to the floor and hold it, and it revs. And pulls. And revs. And pulls. And revs – screaming to its 7000rpm redline with a smoothness and refinement unlike any of its pushrod 60 degree counterparts. It’s glorious, a technological marvel then and now.

The handful of remaining buyers went for this optional engine, between 5% and 8% in the years it was offered, from 1991-1996. Most of these engines were also mated to the excellent 4T60 automatic transaxle, like their 3.1 counterparts.

But from 1991-1993 a few were actually produced with a 5-speed manual, and it is the holy grail of manual transaxles: The Getrag model 284, RPO M27. The importance of this transaxle can’t be overstated. Previously, transaxles were rated for less than 200 foot-pound torque capacities, and this was the major limitation preventing more power from transverse configurations of the time. But the 284 didn’t just break through that barrier, it smashed it to pieces with a 400 foot-pound torque capacity. That’s how much torque the 2004 Z06 Corvette’s LS6 V8 produces. No other production transaxle before could handle that kind of torque.

In 1992, a mere 915 out of 119,319 Grand Prixs (only 0.7%) were built with the 284. When so optioned, horsepower was increased to 210 with short-deck heads that raised compression. This was intended to be a direct competitor to the enthusiast-minded Ford Taurus SHO without the fuddy-duddy blandness of Ford styling.

Obviously, GM had hoped that more people would go for this magnificent engine and transaxle combination. My guess is they figured 20-30% would splurge for the DOHC engine, and maybe half of those would go for the 5-speed manual. GM lost a lot of money on the W-body cars in their early years, and I’ll bet the high development costs and low production numbers of this engine had a lot to do with it. The LQ1 was never used in any other carline, and only in three of the four W-body models (Buick chose to use its 3800 90-degree pushrod V6 instead) so its development costs couldn’t be spread around on other platforms. If it had been just a little bit richer on low-RPM torque than the 3.1 around town while maintaining its awesome pull to redline, I have no doubt that more buyers would have opted for it. But because discovering its best qualities required more lunacy than most test-drive salesmen would allow, you rarely got to find out what made it so great before the sale, and ended up thinking you got a bargain on a 3.1 when you in fact really missed out on something special.

This car is a rust-free specimen I bought from a dealer in Phoenix, Arizona, and the only major thing wrong with it is the engine, which to me isn’t a big problem. It’s got a lot of miles, and it started rod-knocking a few weeks after I bought it – coincidentally when I did the first oil change. No big deal because I planned to replace it anyway. I kept driving it for a while, and when it got too bad I towed it up to Wisconsin in April 2005 with my ’88 Fiero GT – the same car, coincidentally, that was supposed to receive the engine that is going in this car instead.

I’d picked up the engine from a Milwaukee-area recycler back in 2003. Fresh from completing the blue V8 Fiero project (, I was already itching for another project and decided on a 3.4 DOHC in my silver one, which has had a pushrod 3.4 since 2001.

I found the motor in the back of an old trailer marked “Water in engine” and tagged as core, for parts only – since they didn’t know what was wrong with it. Nobody had raided it yet, and it was basically complete with injectors, plenum, intake, exhaust, and water pump. They wanted $150 for it. Since I planned to rebuild it anyway, it was exactly what I needed and the price was right. I loaded it into my U-haul trailer and towed it home with the blue V8 Fiero!

After I took apart my silver Fiero in the fall of 2003, I found that it just needed a clutch and exhaust, but there was plenty of life left in the engine. I just couldn’t bring myself to toss that motor yet, not to mention there wasn’t enough room to do an engine swap on it in that tiny 1-car garage anyway, so I threw it back together and got it running again in the summer of 2004 – in a rush, because I had to move out of the crappy apartment to my new house, with a 2.5-car garage.

I had that engine sitting in my garage and was wondering what to do with it. One day in September 2004 while I was browsing eBay looking for a 284 transaxle, I found this Grand Prix GTP with a 284 in it – that ran, sort of. Engine troubles. This would be the perfect place to put the engine in my garage! I bought the car and the rest you’ll see on the project pages ahead.

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