If...............
 

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?

I like 400 engines, but the 389 would be more "period correct"

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.

1963 and earlier 389 blocks wont hold the starter.
1964-66 389´s may be bored out to 400 std.

400´s is still plentiful, 389´s is not.

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.

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.

I vote leave the 326 HO in and build that up some. But if you must, just go to a 400 stroker then.

So it’s been over 2 months and no comments or feedback from the OP. :rolleyes:

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

Internet forum etiquette.

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.

Differently I’d go with a 400.

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.....:D

:focus:

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.