Saturday, May 07, 2005

 

'06 Silverado SS

PickupTruck.com drove the new Silverado SS, newly-available with 2WD and a lower sticker price (albeit one that's still a few grand higher than that of the departed Ford Lightning). This was interesting:

It still pulls hard with 345 horsepower coming from the 6.0-liter LQ9, but I found the aging 4L85-E 4-speed automatic transmission to be sluggish and slow to respond to the pedal. Also, GM went to a higher 3.73:1 rear axle ratio in the 2WD, compared to the 4.10:1 in the AWD version, which doesn’t help quickness when you stab the throttle.

They also mention elsewhere in the article that it's now using the 9.5" 14-bolt rear axle, from the pre-HD-era 2500-series trucks. This is all good for strength, as the 4L85E is far stronger than the 4L65E that it replaces in the SS application (I wonder how many warranty claims that GM has had with the LQ9/4L65E combination?!), and the 14-bolt should prove nearly indestructable on the street. But there's a couple of problems here that probably add up to the author's perspection of sluggishness.

First, the bigger rear axle will surely have greater frictional losses than the old 8.5" 10-bolt rear end. Additionally, the same is true of the bigger transmission. This is the same running gear that's in my K2500, and there's a big difference in perceived power loss between it and the 4L60E/10-bolt combo that was available in the same era half-ton pickups. Now certainly some of this additional frictional loss will be offset by the subtraction of the 4WD gear up front (although the weight loss that would occur with deleting the front axle hardward was likely gained back with the much heavier transmission and rear axle). But certainly that gear ratio change doesn't help, eh? The full story is even worse that what's indicated by the relatively mild 10% difference between the new 3.73 gear and the old 4.10 gear.

The 4L60/65 has a very low 3.06:1 first-gear ratio. It's not quite a creeper gear, but it's dang low (I never understood that, considering that it's a light-duty transmission). That combined with the 4.10:1 gearset out back resulted in a overall first-gear ratio greater than 12:1 - that's low. But the 4L80/85 has the same ratio as its TH400 parent - 2.48:1. Why not simply use the 4.10s with the 4L85E? Because the 65E has a 0.70:1 overdrive, while the 85E has a shorter 0.75:1 OD ratio. That little bit extra, combined with the greater frictional losses in the transmission and rear end, probably resulted in unacceptable fuel economy numbers with the 4.10s. So I'm guessing the taller gears were selected to drop the highway engine speeds back down, but in the process, the overall ratio in first-gear dropped down to 9.25:1 - a drop of nearly 25%, and surely enough to cause a very significant difference in feel at the seat of the pants.

What'll be interesting to see is if the new truck traps significantly faster in the 1/4-mile, due to the closer ratio spacing of the 85E. That's been known to happen when going from a 4L60E to a TH400. That won't satisfy those who want the best 0-30 MPH performance, of course. But for those that like to roll into the throttle from 30-40 MPH up to hyperlegal speeds, this new drivetrain gearing might keep them happy. More importantly, for those that want to unlock the potential of the GenIII V8 engine, they won't have to worry about replacing transmissions at the same frequency as they change their oil.

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