New 3.91 or used 3.73? Not much money difference.
08-07-2015, 08:50 PM
#1
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New 3.91 or used 3.73? Not much money difference. Just being proactive, and I ain't plunkin' down the long green on an 8.8" or 9" ... at least not this go around. My current diff (3rd gen) whines like everyone else's, but is holding up just fine; I'm just tired of the whine.
As much as I like the idea of getting a 3.73 diff and not having to make changes in the PCM for the difference in ratios so the speed and mileage record properly, the lowest mileage used one I can find has about 70k on it, and it's always a gamble when you buy used. The money is about the same between the new 3.91 and a used 3.73 plus or minus.
The 3.91 is a new (
) SRX unit and would mean only about a 4.8% higher RPM reading at any given speed. It would provide a slight acceleration benefit and an ever-so-small fuel economy penalty at cruising speed. Another member local to me did it, and he loved it. Said it was "easy."
Either way, I'd have to transfer my posi unit from my current diff (about which I'd take lots of photos since I haven't run across any on the forums) which, again, is said to be easy.
If I do this, I'll pick up the pumpkin and sit on it till a better time. I'm not in a hurry to jump into it quite yet.
So, what do you think?
As much as I like the idea of getting a 3.73 diff and not having to make changes in the PCM for the difference in ratios so the speed and mileage record properly, the lowest mileage used one I can find has about 70k on it, and it's always a gamble when you buy used. The money is about the same between the new 3.91 and a used 3.73 plus or minus.
The 3.91 is a new (
) SRX unit and would mean only about a 4.8% higher RPM reading at any given speed. It would provide a slight acceleration benefit and an ever-so-small fuel economy penalty at cruising speed. Another member local to me did it, and he loved it. Said it was "easy."Either way, I'd have to transfer my posi unit from my current diff (about which I'd take lots of photos since I haven't run across any on the forums) which, again, is said to be easy.
If I do this, I'll pick up the pumpkin and sit on it till a better time. I'm not in a hurry to jump into it quite yet.
So, what do you think?
08-08-2015, 12:52 PM
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08-08-2015, 01:27 PM
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08-08-2015, 02:24 PM
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With the Maggie I bet those 3.91's would get old kind of fast in 1st and 2nd. N/A I think it would be a good move but your making 400ft lbs at less than 3k RPM.
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08-08-2015, 06:03 PM
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Our s/c cars don't need a gear. I think it hampers them.
08-08-2015, 08:17 PM
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Pretty sure I'm gonna buy that 3.91 on Monday. It's local (about 50 minute drive), so I'll pick it up this coming week. Wonder how much the darn thing weighs.
08-09-2015, 07:19 AM
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The question I don't have an answer for: Is it a direct bolt in with no modifications required?
08-09-2015, 09:26 AM
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Used or new?
08-09-2015, 11:04 AM
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08-09-2015, 11:49 AM
#15
New 3.91 or used 3.73? Not much money difference. I would go the other way with a performance engine unless your lookin for quicker hole shots...
I would love to increase my tahoe tires from31" to 35" and they recommend 3.91 crown and pinions for my upgrade to keep the rpms the same as a 3.48 with 31" tires
I would love to increase my tahoe tires from31" to 35" and they recommend 3.91 crown and pinions for my upgrade to keep the rpms the same as a 3.48 with 31" tires
08-10-2015, 02:32 AM
#16
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Is there anybody out there, who has swapped a Solstice/Saturn 3.91 diff into CTS-V, what must be done to match it, especially the front mounting/bushing looks different?!
Best regards,
Ronald
Best regards,
Ronald
08-10-2015, 07:01 AM
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The early solstice diffs will work because they use the same housing and just bolt an adapter through the front mount. The later model solstice have a revised case. they pretty much chop off the entire front mount and bolt on a different mount on the side of the case. I don't know what year they changed. The top diff will work with the mount removed, the bottom wont.
http://www.google.com/imgres?imgurl=...FU9BkgodtSMIEA
The other difference is the V supposedly has a HD clutch pack for the LSD vs the standard unit and the V also has a CV flange instead of the 3 bolt guibo flange. That's why the guide pin on the pinion needs to be cut off. The CV flange self locates, the guibo flange uses the guide pin to locate.
http://www.google.com/imgres?imgurl=...FU9BkgodtSMIEA
The other difference is the V supposedly has a HD clutch pack for the LSD vs the standard unit and the V also has a CV flange instead of the 3 bolt guibo flange. That's why the guide pin on the pinion needs to be cut off. The CV flange self locates, the guibo flange uses the guide pin to locate.
Last edited by raven154; 08-10-2015 at 07:12 AM.
08-10-2015, 08:55 AM
#19
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Just called them. I'm picking one up on Thursday.
BTW: They're new and unused, and they've got twenty of them in the warehouse.
EDIT: Thanks, Dennis!!!
