Last post on Mar 14, 2007 at 3:35 PM
You are in the Cadillac STS/STS-V
What is this discussion about?
Cadillac STS, Cadillac STS-V, Transmission, Sedan
May 23, 2006 (8:12 am)
I am not sure why GM wanted the six speed to have a 6:1 overall ratio of low gear to high gear. I assume that they expect some improvement in both fuel economy and performance.
Playing around with my spreadsheet and the equations for planetary gearsets suggest that one could get from two planetary gears the following six ratios (4:1, 2.8:1, 1.9:1, 1.33:1, 1:1, 0.75:1). This would have the top three gears a bit closer together than the bottom three. Compared to the 5 speed, this would have a lower first gear, so the axle ratio could be changed from 3.42 to 3.08. This would give a lower overall ratio in the first 5 gears and still provide an overdrive ratio that is more fuel efficent. A 2.73:1 axle ratio would be somewhat mixed, some lower, some higher.
I guess that my point is that I do not fully understand GM's choice of gears, or the sense in an overall ratio of the transmission to be 6:1. Based on the Covette's EPA results, I think the city mileage is not so good. The 2007 Saturn Aura is rated 18/27 with the 3.6 and six speed transmission, while the base Aura's 3.5 engine and 4 speed automatic are rated 20/30. This does not show that the six speed is resulting in better fuel economy. This may be a quirk in the EPA testing.
May 22, 2006 (6:11 am)
I am guessing that the final drive ratios for each version of the V8 will remain the same for 2007.
If so, that would mean that the overall first gear ratio in the six speed offers roughly 17.5% more torque multiplication than in the (now) ‘old’ five speed.
(Accounting in all numbers here for the roughly 2.33% diff. in rolling diameter between the STS-v’s 275 \ 40 x 19s and the STS’s optional 1SG \ QAF \ PCZ’s 255 \ 45 x 18s. I didn’t bother with looking at the 17” tires – this is left to the reader as an exercise – but I expect the result to be similar.)
In second gear there is a bit less than 7% more multiplication.
Third actually provides LESS multiplication – by over 4%. Odd.
Fourth offers roughly a 15% addition in multiplication.
Fifth offers roughly a 12% addition.
Sixth provides approx. 12% lower RPM at cruise.
Sounds interesting to me.
0 – 60 = 5.9 (R&T) and 6.0 (C+D)
Quarter = 14.3 97.5 (R&T) and 14.6 97 (C+D)
My guess (does not rise to the level of a ‘fearless prediction’) is that even with no increase in HP or TQ, the 2007 V8’s standing start acceleration will be improved. Perhaps by as much as a half second in both 0 – 60 and the Quarter. Rolling acceleration ought to also be improved. I’m also guessing that seat-of-the-pants acceleration feel will be somewhat improved. ( Did I mention that I am an acceleration junkie? )
And the reduced sixth gear cruising RPM ought to provide somewhat better ‘real world’ highway fuel economy.
And this last is a (purely psychological) cool feature for me. I happen to like looking down at the tachometer and seeing a relatively low number while traveling at 70 – 80 mph. That’s just me. I have seen no evidence to suggest that today’s internal combustion engines cannot survive for a long time turning higher RPM. Certainly Infiniti, (for example) in warranting the powertrain in their M45s with ‘only’ a five speed trans, a fifth gear ratio of 0.83, and ‘only’ 27.8 MPH \ 1,000 RPM (vs roughly 36 MPH \ 1,000 RPM for the new & improved STS V8 with six speed) for 72 mo. or 70,000 miles.
But it is still a valid selling point – for me.
Ready for my test drive . .
#12 of 19 Re: 2007 ratios [rayainsw]
May 22, 2006 (7:29 am)
I would guess that the axle ratios may continue as they were in 2006. However, the 1SG 3.42:1 ratio appeared to give far more torque at the wheels than could be used resulting in a 0-60 time that was no better than the 2.73:1 axle ratio. So, while I would guess that the 6 speed should improve the 0-60 time for the base axle ratio (2.73:1), the 3.42:1 performance ratio will probably not improve much. I am sure that the magazines will test the six speed automatic at some point.
The primary advantage of cruising at a lower engine speed is that fuel consumption is reduced. My guess is that performance would be optimal with an axle ratio somewhere between 3.10 to 3.20 :1, but I don't know. I would guess that the 3.23:1 axle might be better than the 3.42:1 with the six speed. The overdrive ratio with the 3.23:1 axle would provide about the same overall ratio as the 2.73:1 and the 5 speed overdrive.
#13 of 19 magazine tests Re: 2007 ratios [sls002]
May 22, 2006 (7:45 am)
“I am sure that the magazines will test the six speed automatic at some point.”
That’s actually something I was thinking about this weekend:
Will the magazines see enough different \ improved for 2007 to warrant publishing a(nother) test of the STS?
With the availability of a six speed (in place of the really old four speed) automatic on the 2006 Corvette being the only significant mechanical \ performance change compared to the 2005, it took over 6 months for any magazine to publish acceleration numbers. And (as far as I know) still no one has published a full test.
Unless there is a $60K – $65K Luxury Sedan Comparison test in the works, I doubt we’ll se a test of the 2007. The six speed and Brembo availability are just not enough different to justify re-test.
But I could be wrong.
Likely depending on my finely calibrated ‘butt dyno’ during test drives to evaluate the 2007’s acceleration.
#14 of 19 Re: magazine tests Re: 2007 ratios [rayainsw]
May 22, 2006 (7:52 am)
With respect to the Corvette, the 6 speed manual transmission is the first choice of serious sports car drivers. What I would expect is a comparision test sometime or perhaps a short take on the six speed. My guess is that there might be a comparison test, but a lot depends on how the STS's competition changes too. If the only change for 2007 in the STS class, is the STS's six speed transmission, then probably no test will be in the works.
#15 of 19 Re: Gearing [sls002]
Dec 27, 2006 (7:52 am)
I've measured fuel economy against final drive gearing (i.e. engine rpm in highest gear) in my last 8 cars and consistently found that, surprise surprise, the lower the engine rpm at any given speed -- the higher the fuel economy. I've have 3 GM vehicles with the 3.8L OHV V6, two with a 3.05 final drive ratio, and one with 2.86. The car with the 2.86 ('03 LeSabre) consistently deliver 10% or so better economy than did the two with the 3.05 ratio ('96 Riveria and '98 Intrigue). Had an '01 Aurora with the 3.5L OHC V6 in between -- with a 3.29 final gear, and it consistently delivered 10 - 15% poorer mileage than the cars with the 3.8L and numberically taller gearing. Interestingly, the seat-of-the-pants performance among these 4 cars differed little -- the Intrigue was actually the "sportiest" one of the bunch. So much for lower gearing translating to better apparent performance.
#16 of 19 '05 STS Cadillac Tranny Harsh Shifting
Mar 06, 2007 (1:19 pm)
Have '05 STS with 25K mi - with harsh shifting -
5L40-E tranny smooth as silk until 21K mi
shifting is now harsh in all shifts except 1-2
Any ideas ?
#17 of 19 Re: Gearing [lakerunner4h]
Mar 06, 2007 (3:02 pm)
Unless all the cars weigh the same it is still hard to figure difference in mileage. The Intrigue I bet was a whole lot lighter than the Aurora. You are comparing apples to oranges.
#18 of 19 FIVE OR SIX SPEED SWAP INTO '95 STS
Mar 14, 2007 (3:06 pm)
DOES ANYONE KNOW IF I CAN PUT A LATE MODEL 5 SPEED AUTO OR POSSIBLY THE NEW 6 SPEED AUTO IN PLACE OF MY CURRENT 4T80E IN MY 95 SEVILLE STS??. THE FINAL DRIVE IS CURRENTLY 3:73. I KNOW I CAN CHANGE TO THE SLS'S 3:11 AND WOULD LIKE TO KEEP THE SAME INITIAL OFF LINE PERFORMANCE WHILE GETTING THE EASIER RPM AT CRUISE.
#19 of 19 Re: Gearing [dmathews3]
Mar 14, 2007 (3:35 pm)
My '96 Riveria weighed about 700 lbs. more than my '98 Intrigue. Fuel mileage is virtually equal. The Aurora did weigh about 400 lbs. more than the Intrigue, and the LeSabre weighed virtually the same as the Aurora - and delivered 10 - 15% better fuel mileage.