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Old 01-21-2023, 07:45 PM
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If I were to replace the 326 HO 4 speed in my 66 LeMans would you go with a 389 or a 400? What advantages does the 400 have over the 389?


Last edited by arbys; 01-21-2023 at 08:06 PM.
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Old 01-21-2023, 07:58 PM
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I like 400 engines, but the 389 would be more "period correct"


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Old 01-21-2023, 08:22 PM
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Eleven cubic inches plus a better cylinder head and valvetrain design.

The availability of pistons that are reasonably priced is a big advantage for the 400 engines, good 389 pistons are scarce and expensive in comparison.

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Old 01-22-2023, 05:56 AM
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1963 and earlier 389 blocks wont hold the starter.
1964-66 389s may be bored out to 400 std.

400s is still plentiful, 389s is not.

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Old 01-22-2023, 12:53 PM
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65 and 66 389 have thin wall cast blocks, plus the connecting rods were strengthened in 1967 over 1966 by adding a rib on the big end next to the rod bolt.

Then there's what Bart said about the heads, and valvetrain being improved also. Unless you needed to have a 389 for class rules, I would go for a 400, hands down.

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Old 01-24-2023, 06:49 PM
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A '66 326 HO 4 speed LeMans is a rare enough bird (and way cool) that I would leave it alone. Put an 068 cam in it and bolt on a tri-power and you're all set.

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Old 01-24-2023, 10:58 PM
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I vote leave the 326 HO in and build that up some. But if you must, just go to a 400 stroker then.

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Old 01-25-2023, 12:43 PM
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https://www.youtube.com/watch?v=JTCZYSLh1rk

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Old 03-28-2023, 01:22 PM
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So its been over 2 months and no comments or feedback from the OP.

Whats the point of starting a thread and asking questions without even so much as acknowledging the respondents?

Internet forum etiquette.

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Old 03-28-2023, 09:40 PM
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Thanks for the input. I've had some medical problems, pinched nerve in back. I'm looking at some options if my engine repairs don't work out. I do appreciate the comments from experienced Pontiacers.

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Old 03-28-2023, 10:05 PM
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Differently Id go with a 400.

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Old 03-30-2023, 04:51 PM
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My 66 Tempest had a factory 326 in it. The nice thing I liked about it was 25 or so mpg it got out on the highway a 55mph. I don't know what the rearend ratio was, maybe a 336 or lower numbers, but I sure like the gas mileage it got.

Today I have a custom 400 in the 66 GTO I have and with the speed limits today, a 355 rear is not good at all for gas mileage.

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Old 03-30-2023, 10:25 PM
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Quote:
Originally Posted by GT182 View Post
My 66 Tempest had a factory 326 in it. The nice thing I liked about it was 25 or so mpg it got out on the highway a 55mph. I don't know what the rearend ratio was, maybe a 336 or lower numbers, but I sure like the gas mileage it got.

Today I have a custom 400 in the 66 GTO I have and with the speed limits today, a 355 rear is not good at all for gas mileage.
Likely a 2.56 axle in a 66 tempest with the turbine 300 transmission, 3.23 with a stick.

I had a friend that owned a 65 Le Mans that originally came with the 326, 2 bbl with the 2 speed auto that we did a 4 speed conversion to. That car (2.56 axle) got fantastic mileage after the conversion, but clutch life and acceleration from a dead stop wasn't anything to write home about. On a road trip from Erie PA, to Niagara Falls Canada, was only a 1/4 tank of gas, nearly 30 MPG. Same trip with the slushbox previously was around 20 MPG. Losing the torque convertor from the equation really bumped the mileage up.

I also had another friend that had a 66 Le Mans with a 326, 2bbl, 4 speed, that had a 3.23 axle in it. I owned 2, 67 326 Le Mans, both W/326 2 speeds, both had 2.56 axles in them.

I lost the 2 speed in one of the 67 Le Mans that I owned, and swapped a TH400 into it. Even with the 2.56 axle, the low 2.48 low gear with the convertor was a much better match than the 2 speed with a 1.76 low gear. The much needed torque multiplication, and an extra gear made for a much better driving car over the 2 speed that I yanked out. I did the same conversion to a 68, 350 Le Mans a couple years later.

The 67 car got converted into the clone 67 GTO dirt car in my signature pics complete with a 3.55 axle from a 70 GTO, same T 400 transmission, a 400, 2 bbl. engine from a 68 Executive, 068 cam, reworked Q jet, and a set of 48 heads, RA III exhaust manifolds. It made for a very strong dirt car that visited victory lane 13 times, in 2 years of campaigning it. The chevy guys hated getting beaten by it, they protested it constantly, with zero rules infractions.....


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Old 04-06-2023, 04:19 PM
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I pulled the 3.36 rear end out of my '67 GTO convertible with TH400 about 15+ years ago and installed a 2.56 posi because it was free. Went from 15 mpg at 65 mph to 21+ mpg at 75-80 mph. The 2-3 shift at WOT is 105-107 mph. The car just eats up the road while sipping gas and running cool. I have NO plans to remove it!
40+ years ago, a friend had a '65 GTO and was moving from CA to MO, and wanted to get better mileage so we pulled the 3.55 out and installed a 2.56 one legger. As Brad stated, not real easy on the clutch, but once gong , over 60 mph in first gear. Craig called us after he'd made the trip without issue and reported he averaged 23 mpg cross country.

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