1. I've heard that the '97
and newer Grand Prix GT has a weaker 4T60e transmission, while the GTP and
Regal GS have the stronger 4T65e transmission. Is this true?
No, not exactly. The Grand Prix GT and SE with the normally
aspirated 3800 (that's without supercharger) L36 engines do have a weaker
transmission than the GTP with the L67 3800 SC, but it's not a 4T60e,
except in 1997, which is really an oddball year. All '98 and newer
L36 engines have a 4T65e, but they have the light duty version, RPO code
M15. All L67 engines have the 4T65e HD, meaning heavy duty (RPO code
MN7). The differences? The MN7 version have a larger
differential, and due to the larger differential, the output shaft on a
MN7 is about 1/4 inch longer. The MN7 also have a stronger input sun gear,
and have roller bearings on the differential housing whereas the light
duty 4T65e have a bushing instead. There may be some other minor
differences inside, but these are the major ones. Can you convert a
light duty trans to a MN7? Yes, but those pieces are a bit pricey.
By the way, the 4T60e (RPO code M13) is a much lighter duty transmission
than any 4T65e. These were found behind the Series 1 3800 SC engine
(the 225 HP version) found from '92-'96, and behind the 3100 and 3400
engines, and apparently behind the L36, normally aspirated 3800 in '97
(that's the '97 GT).
2. I've heard that the
'97 and newer Grand Prix GT has a weaker 4T60e transmission, while the GTP
and Regal GS have the stronger 4T65e transmission. Is this true?
How do you tell the difference between a 4T65e and a 4T60e? Look
under hood at the bell housing, the 4T65e has a large electrical connecter
at the top while the 4T60e doesn't. The shape of the top of the bell
housing is much different on a 4T60e also, and the 4T60e has a vacuum
modulator attached on it while the 4T65e doesn't.
||Here's a photo of a 4T65e clearly
showing this electrical connection where we have a device under test
plugged in. This is missing on the 4T60e.
3. Does the Grand Prix GT
have a different ratio differential vs the GTP? Doesn't the GT have
a 3.29 final drive ratio vs. the GTP's 2.93? What about the 2004
Grand Prix Comp G? It has a 3.29 also doesn't it?
You're correct on the final drive ratios, but all 4T65e transaxles
actually have the same 3.29:1 ratio differential. Yes, we know that the GT has a final drive of 3.29:1
and the GTP has a 2.93:1 final drive (in fact, all L67 3800 SC engines,
and hence MN7 transmissions, have this 2.93:1 final drive - the Comp G's
3800 SC engine is actually known as the L26). So how is this done?
It's all in the ratio of the chain sprockets, not in the differential.
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