th400 Long vs short - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 06-11-2018, 12:00 PM Thread Starter
 
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th400 Long vs short

I have finally have my engine sorted out. (famous last words) and turning my attention to the transmission. My 68 convertible has a th400 long tail. Is there any advantage to the sort vs the long. this is a street car with a stroked engine. would love to hear your ideas on this issue.
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post #2 of 7 (permalink) Old 06-11-2018, 04:28 PM
 
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Originally Posted by deanhickey View Post
I have finally have my engine sorted out. (famous last words) and turning my attention to the transmission. My 68 convertible has a th400 long tail. Is there any advantage to the sort vs the long. this is a street car with a stroked engine. would love to hear your ideas on this issue.
No advantages as far as I know. Long tailshafts were typically used in the full sized cars. If you have not been having any problems with it, driveshaft fits, no vibrations due to driveshaft/pinion angle mismatch, then keep it as is.

The GTO TH400 had a higher line pressure than other TH400's and could chirp tires on a wide open throttle shift from 1st to 2nd, so the long tailshaft most likely does not have the increased line pressure to give you faster/firmer shifting - they may have also had a different governor to increase the wide open throttle upshifts as well. You can get a kit, like B & M, that has the parts to modify your valve body to increase the line pressure and give you that GTO firm shift. Not to difficult to install.
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post #3 of 7 (permalink) Old 06-12-2018, 03:06 AM Thread Starter
 
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Thanks for the input Jim. I was just curious about the difference between the two. I don't think I will mess with the set up as it was squawking the tires going from first to second with a tiered untuned engine. I will just change the fluid and give it a good clean-up.
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post #4 of 7 (permalink) Old 06-12-2018, 08:48 PM
 
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Originally Posted by deanhickey View Post
Thanks for the input Jim. I was just curious about the difference between the two. I don't think I will mess with the set up as it was squawking the tires going from first to second with a tiered untuned engine. I will just change the fluid and give it a good clean-up.
As they say, If it ain't broke....... LOL
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post #5 of 7 (permalink) Old 06-13-2018, 02:09 PM
 
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Dean, IF car does not have the fan and fanshroud installed, you prob won't notice an issue.

On 3 separate occasions have ran into this exact deal/ 400 or 455 with long tail T400's that prev owner had rigged into a '69-72 Pontiac 2 door A-body (112" wheelbase). All were factory AC cars and had relatively good condition radiator core support bushings. One actually had new repro core support bushings. With the trans crossmember slid back & the bigcar long tail T400 installed, what it does is raise the rear of engine slightly in respect to the angle it originally would have with the correct length short tail auto trans (or a Muncie or 3spd manual). This in turn tilts the engine forward down in the front slightly and the 19 1/2" diam 7 blade fan gets into the lip of the bottom of the shroud. This condition is worsened today, by the inferior parts store trans mounts that are coming from overseas, they are slightly taller (thicker) and contribute slightly to the rise in the rear. Also, many higher mile or heavier engine/high torqued frames have experienced frame cradle sag. Frame cradle sag is very common on early A-body frames and causes the entire engine to set lower.
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post #6 of 7 (permalink) Old 06-15-2018, 03:47 PM Thread Starter
 
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Dean, IF car does not have the fan and fanshroud installed, you prob won't notice an issue.

On 3 separate occasions have ran into this exact deal/ 400 or 455 with long tail T400's that prev owner had rigged into a '69-72 Pontiac 2 door A-body (112" wheelbase). All were factory AC cars and had relatively good condition radiator core support bushings. One actually had new repro core support bushings. With the trans crossmember slid back & the bigcar long tail T400 installed, what it does is raise the rear of engine slightly in respect to the angle it originally would have with the correct length short tail auto trans (or a Muncie or 3spd manual). This in turn tilts the engine forward down in the front slightly and the 19 1/2" diam 7 blade fan gets into the lip of the bottom of the shroud. This condition is worsened today, by the inferior parts store trans mounts that are coming from overseas, they are slightly taller (thicker) and contribute slightly to the rise in the rear. Also, many higher mile or heavier engine/high torqued frames have experienced frame cradle sag. Frame cradle sag is very common on early A-body frames and causes the entire engine to set lower.
I have had no problems with the stock fan shroud, I see your point about the engine angle. I will make sure to measure the fan/shroud gap after installation.
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post #7 of 7 (permalink) Old 06-21-2018, 06:16 PM
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The only downside, besides the added bulk and weight of a long shaft TH400 would be incorrect driveline angles, which would result in vibration. If your car drives fine (and it sounds like it does) no need to worry about a thing.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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