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Discussion Starter #1 (Edited)
Calculating using the 0.73 5th gear ratio and the 3.91 rear end shows the Solstice will cruise at 70 mph at 2520 RPM. I think that is reasonable. Acceleration from a starting stop isn't as important to me as efficient highway cruising so I'm still thinking of going with a bit taller rear end.

So what off-the-shelf gears are available for the Cadillac CTS rear end. I think there is a 3.73 which would bring the 70 mph cruising down to about 2400 RPM. Something in a 3.50:1 would bring the 70mph cruise down to 2250 RPM. Does Caddy offer a 3.50?

If I do choose to swap rear gears, doesn't this affect the LSD as well? It may not be worth the swap if I have to replace the LSD. Or I may order without LSD and buy the corresponding LSD hardware for the rear end gears I buy for the swap.

I think the 3.91 rears yield an acceptable 2500 RPM cruising RPM at 70 mph. But since I plan on buyiing a turbo, stiffening the rear to 3.73 would give just a tad better fuel economy with virtually no loss of acceleration.

Any thoughts? Appreciate the help.
 

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jimbo said:
Calculating using the 0.73 5th gear ratio and the 3.91 rear end shows the Solstice will cruise at 70 mph at 2520 RPM. I think that is reasonable. Acceleration from a starting stop isn't as important to me as efficient highway cruising so I'm still thinking of going with a bit taller rear end.

So what off-the-shelf gears are available for the Cadillac CTS rear end. I think there is a 3.73 which would bring the 70 mph cruising down to about 2400 RPM. Something in a 3.50:1 would bring the 70mph cruise down to 2250 RPM. Does Caddy offer a 3.50?

If I do choose to swap rear gears, doesn't this affect the LSD as well? It may not be worth the swap if I have to replace the LSD. Or I may order without LSD and buy the corresponding LSD hardware for the rear end gears I buy for the swap.

Any thoughts? I would appreciate the help. I think the 3.91 rears yield an acceptable 2500 RPM cruising RPM at 70 mph. But since I plan on buyiing a turbo, stiffening the rear to 3.73 would give just a tad better fuel economy with virtually no loss of acceleration.

Any thoughts?
Well I don't think they make a 3.50 but a 3.42 is a popular ratio.

But I would rethink doing it, what may not seem like a lack of acceleration can dramatically change how the car behaves. Especially on lower torque cars, it can end up costing you more in fuel due to having to rev higher to get the same acceleration or speed. In my Trans am I can cruise around all day long shifting at 2500 RPM, but in my PT cruiser I can't even think about getting off a stoplight without shifitng before 3.5-4K.

Just saying that given the low torque of the engine, turbo lag (if/when you FI the engine) it would be a bad idea to make it harder for the engine to make power, which changing the gears would do (making gears longer, and acceleration slower) it would take longer for the engine to reach its powerband, thus meaning you would have to put in more fuel to reach the same acceleration, and thus making your car slower only to retain about average fuel economy.....unless you do 75% highway driving.

my .02
 

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WaitingForBoost said:
If you mean the GXP (as opposed to an aftermarket kit) I wouldn't worry about gear just yet. The GXP is rumored to have a 6-speed.
Note the emphasis. There have been a few rumors that turned out not to be true. This may be one of them. Among others:

-New Camaro coming
-Sunfire Kappa
-Sky gets a windshield or hidden antenna (recent photos show same mast antenna as Solstice)
-Chevy gets a Kappa
-GXP will get same Supercharged engine as Cobalt SS
-Solstice production has started (two months ago)
-Solstice production has started (6 weeks ago)
-Solstice production has started (7/11)
-....:glol:
no need to keep going on.

You can go ahead and wait based on rumor, or I can try and add a bit of rumor myself, if you get my drift.
 

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And that's why I wrote "rumored". :) The GXP itself is only rumored. GM hasn't made any official announcement that it will even make it to market. Leaks? Sure, but until there's a press release, GM can back out of that entire project and the only ones with egg on their faces are a few journalists.

Jimbo is making all kinds of plans, etc, that may be unnecessary. He didn't specify if he was thinking about an aftermarket turbo or a GXP. I just wanted to make sure he knew about the rumors, that it was possible that he might not need a different rear end ratio. I wasn't encouraging him to wait for a GXP if his intent was to buy an 06 and get an aftermarket kit.
 

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Discussion Starter #6
I would be waiting for a GXP rather than an aftermarket turbo, because I expect that GM would upgrade the quality of the components in the factory turbocharged engine over those for the base motor. I don't want to install an aftermarket turbocharger.

Nocturn brought up some great points, and it may be pointless to stiffen the rear end on a car that is not all that torquey to begin with. But I was expecting sufficient torque from the GXP to allow the swap.

I am expecting the GXP to include a 6-speed transmission, but I don't expect them to include a higher overdrive gear. I think gears 1-5 will be closer ratios to keep the turbo in the powerband better and I expect the top gear to still be 0.73:1. So I don't think top speed or cruising fuel economy will be any different.

Finally, I keep calling the turbocharged Solstice the "turbo" due to confusion over the upcoming name. Will it be the GXP or the GT? Or the GTP like the G6. Until they announce the car's monniker I'll continue to call it the "Turbo Solstice".

Furthermore, half the fun of this forum is getting to dream of the impractical mods I'll make to a car that is not even close to being in dealerships yet! :crazy:
 

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jimbo said:
I am expecting the GXP to include a 6-speed transmission, but I don't expect them to include a higher overdrive gear. I think gears 1-5 will be closer ratios to keep the turbo in the powerband better and I expect the top gear to still be 0.73:1. So I don't think top speed or cruising fuel economy will be any different.
We'll have to wait & see, as solsticeman alluded. I think the opposite, though. If there's a 6-speed it'll have a higher 6th for economy, since the car is on the cusp of the gas guzzler tax. Add the turbo'd engine, and it might need all the mpg help it can get. If I'm right, I think 5th would be lower than 5th in the 5-speed, but 6th would be higher. Heck, in a way, we both could be right: put less "space" between the lower ratios & still have a higher 6th.

Time will tell.

I'm doing the same thing as you, btw, waiting for the factory turbo. I figure that if it doesn't come, I'll buy a NA 2007 and will have avoided the craziness of the first year. Either way, I'm going to be jealous as hell in a couple of weeks when cars start being delivered!
 

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I envy your patience WaitingForBoost!
 

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Discussion Starter #9
WaitingForBoost said:
I'm doing the same thing as you, btw, waiting for the factory turbo. I figure that if it doesn't come, I'll buy a NA 2007 and will have avoided the craziness of the first year. Either way, I'm going to be jealous as hell in a couple of weeks when cars start being delivered!
I'm going to get antsy when I see these driving around town and I don't have mine. But I just know the 75 bump in HP will be worth the wait. Or so I will tell myself every time I see a Solstice float by and disappear into the Sunset.
 

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jimbo said:
Calculating using the 0.73 5th gear ratio and the 3.91 rear end shows the Solstice will cruise at 70 mph at 2520 RPM.

Any thoughts? Appreciate the help.
My calculation figured 2600 RPM @ 70 mph. Maybe we should start a pool. Closest to the real answer gets a T-shirt.

Note for lucky current test drivers: Check this out the next time you're out and let us know the answer.
 

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First time I saw this thread, I thought it was going to be swapping rears in the other direction. Putting in a 4.10 rear or something. Probably would help with merging into traffic.
 

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4.10s? Yeah, I bet we'll see people doing that, too. More likely 4.56s, though. We'll have to see what ratios are available for this rear. I wonder if there will be carrier issues.

One other thing: if you swap gears, you'll need to figure out how to recalibrate your speedometer.

It's going to be tough reading this Forum once cars come out, sure, but I'm trying to look at it as an exercise in personal growth. :) I think jealously will really hit the first time I actually see one in person, on the road. Hopefully I'll be driving my 93 Z28 and not my wife's Focus, or I'll probably never catch it. :)
 
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