My friend Jeff just got his 2002 Trans-Am back – after much engine work – and is somewhat disappointed. The car sounds tough. Slightly choppy idle and booming voice under way, courtesy of a not-stock exhaust that still has cats but no mufflers. Chambered pipes being good for the soul.
But the car doesn’t feel as strong on the low end – coming off the line – as it did before all this work. The hot cam, the headers and head work and reprogrammed ECU. My Trans-Am – an old school TA from the ‘70s – feels stronger, even though I have no doubt Jeff’s TA makes more power.
I suspect two things account for this.
One, my TA has a bigger engine (7.4 liters vs. 5.7 in Jeff’s TA) and while his muscled-up Chevy LS V8 no doubt makes a lot of horsepower, I doubt it makes as much torque. The factory rating (Jeff’s TA) was 345 ft.-lbs. at 4,000 RPM.
Even though 1976 was a low ebb for both horsepower and torque, the Pontiac 455 still made 330 ft.-lbs. at a diesel-like 2,000 RPM. That’s 2,000 RPM sooner – a huge assist as far as getting a 3,700 pound machine rolling.
I have not dyno’d it, but for frame of reference, a 74 Trans-Am 455 (the regular 455, not the high-performance SD-455) produced 380 ft.-lbs. and it’s well-known that even a mildly hotted-up 455 can make 500-plus ft.-lbs. of torque. My 455 has a cam that’s hotter than the ’74 L75 (250 hp) 455 and so probably makes somewhere in the vicinity of 450 ft.-lbs. of torque. These big engines were all about torque. It’s what made them such great street performance engines. Their displacement (and long stroke) were – and still are – a big plus in a street-driven performance car. They also work exceptionally well with automatics (the transmission’s torque converter multiplying the already prodigious torque) and it is usually not necessary to rev them much to get neck-snapping, tire-chirping acceleration.
If you go back and check the 1/4 times of ’60 and ’70s-era stock muscle cars, you’ll discover that most of the very quickest ones were the big-engined ones.
Because big torque. Down low – and right now.
The cam he chose comes on strong at 4,000-plus RPM. Which makes it a great track-day cam. It still makes more than decent power at low-mid RPM, plenty to be street-drivable. But to really get something out of it, you’ve got to spin it. There’s plenty of horsepower up there, but you have to be willing to work for it. The low-mid-range is … disappointing.
Which brings me to the Other Thing.
Jeff’s TA originally came with a 3.42 final drive. My TA currently has a 3.90 final drive.
You know about leverage, right? In simple-speak, a 3.90 rearset gives you more of it. The car launches harder.
Jeff’s 3.42 rearset launches the car less hard.
Muscle cars – the old school ones – used to come with ratios like my car’s 3.90 (or even 4.11) but the downside was – in the days before overdrive transmissions – your cruise RPM would be unacceptably high and your top speed dramatically less. This was the sacrifice you made to burn rubber coming off the line. You also burned through gas as though there was a quarter-sized hole in the tank.
But with an overdrive transmission, you can have your cake and eat it, too. Neck-snapping off-the-line acceleration and moderate cruise RPM… even not-terrible gas mileage.
Which is why I put an overdrive transmission in my car. Even with the 3.90 rearset, with an overdrive transmission (2004R automatic with .67 overdrive in fourth) cruise RPM at 70-75 MPH is about 2,000 – well within acceptable range. And if I drive the car somewhat reasonably, it’ll only burn through gas as though there’s a dime-sized hole in the tank.
I think Jeff’s car would benefit a lot from a more aggressive set of rear gears, maybe a 3.73 or even a 3.90 like I have in mine. Particularly because he has a manual (six-speed) with an even steeper overdrive (sixth) than my four-speed automatic, which would keep his highway cruise RPMs at an even more reasonable level. The only reason his car didn’t come with a more aggressive (3.73 or 3.90) gearset when it was new was because Pontiac (GM) had to deal with the government’s Corporate Average Fuel Economy (CAFE) mandatory minimums and even though a 3.73 vs. a 3.42 axle would not have made much difference to the buyer (maybe 2 MPG) that slight difference was a big difference insofar as the government’s assessment of “fleet average” MPGs. Pontiac (GM) would get tagged with gas guzzler fines if the average dipped too low – which made a 3.73 or 3.90 rearset harder to justify as a factory option.
But that’s easily corrected.
A more aggressive rearset would get Jeff’s rear wheels spinning faster and sooner – which would make his TA accelerate more aggressively.
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