66 lemans

[BARRY 17]

I have a 66 lemans with a 4 speed muncy trans. Can i use a 389 or 455 engine and what will i have to change. The car had a 326 and was changed to a chevy 350. Thanks for any advice

barry

[Brian Lohnes]

Barry, you should be able to just swap the bellhousing for the right Pontiac one (326, poncho 350,389,400,428,455 should all be the same if I am thinking correctly).

[BARRY 17]

Will existing motor mounts work on all and what about existing flywheel?

[silver_bullet]

You may want to make sure of the proper frame mount hole positions in the front crossmember...bolt your mount assemblies to a block for positioning...have had to unscramble one of these across the family GM swaps way back when...

[>>>>head]

FWIW

All Pontiac blocks have the same dimensions from the 287 in 1955 through the 455, there is NO Pontiac small block or big block. The slight exception is the racing only 303 motors and 265/301 engines from the late 70's/early '80s which had a shorter deck height

[BARRY 17]

All you guys are great. Thanks for the info

[Thumpin455]

The only thing on the chevy you can use on a Pontiac is the distributor cap and carb. Nothing interchanges between the two engines, and they are very different. Within Pontiac lots of stuff interchanges, but there have been some changes over the years that make things a bit more interesting. Most of it was early 60s and the majority of parts you will find will be later stuff. Every Pontiac head will physically bolt to any Pontiac block, there are issues with them but the bolt pattern and spacing is all the same from 54 to 82 when the last 301s were made. All the non 301 engines, meaning the 265 to 455 use the same 6.625" long connecting rods, and the rods are the weak link in factory engines. Its now super cheap to replace stock cast rods with forged aftermarket rods, its good insurance to put them in. You can get stock length forged rods for around $250 a set, if you build an engine, get good rods. Almost everything interchanges on these engines, so its easier to say what the differences are.

There is a .030 bore difference between a 400 (4.125) and a 455 (4.155). The difference is in the stroke, the 400 has a 3.75" stroke like the 389, 350, 326 and a few others. On the smaller displacement engines they used the bore size to increase displacement, so the 326 has tiny bores, the 350 a bit larger, and the 389 is .060 smaller than the 400. The 455 has a 4.21" stroke, but its considered to be with the 421 and 428 which both use a 4" crank because all three of those engines use a 3.25" main journal, and the 326-400 use 3" mains. You can turn down the 455 crank to fit in a 400 block but you have to weld up the thrust or use a special thrust bearing. Its easier to get a longer stroke crank with 3" mains. You can get up to 4.5" cranks in a 350-455 block, using the right pistons, and you can go as long as a 6.7" rod with those cranks. It is very common now to build a 400 with a 4.25" crank for 467ci. You can get a 4.50" - 4.25" or 4.21" crank cast, forged or billet. The cast cranks are more than strong enough for anything under 800hp, which is where the stock blocks start cracking. A .030 over 400 block with a 4.5" crank is 486ci, and they make incredible bottom end and mid range torque. A 428/455 is a tire shredder with the crank they come with too, and 400s have been known to be very traction limited very easily.

Intakes are either 64 earlier or 65 later, the flange angle changed in 65 so they wont fit without lots of work. Tri Power intakes matter in this case because they started making them quite early and you have to know which heads they fit. All the aftermarket intakes fit the 65 and later heads. Its easy to spot 67-72 intakes, which work better than most aftermarket intakes, they fit the Qjet and are iron. If you find an intake with a square bore flange for the Carter carb, then its 66 or earlier.

Its easier to use later heads on an early block unless you get the intake with it. The valve inclination angle changed in the heads in 67 when Pontiac went from a small valve 24 degree head to a 14 degree and larger 2.11/1.77 valves. The pistons require different valve reliefs for pre 67 heads, and if you put the early pistons with the later heads, or vice versa you will get bent valves, even with a stock cam unless you run the 455 heads with huge combustion chambers that would give a 389 or 400 around 7:1 SCR.

There are some engine mount issues you should be aware of since you have a 66. A later 400 from a 77-78 Firebird or Bonneville might need adapters to use the 66 mounts, some of them didnt have the holes you will need. You can see both bolt patterns on this 73 vintage 400 block, all the 455s will have both sets, some 400s will, and 428s and early engines will have the holes not used by this mount. Can be confusing, but it isnt that hard since all of this is from memory.



The starter is on the opposite side from chevy, as is the oil filter. The mounts are entirely different and the frame parts will probably need moved. What works in a chevy will make a Pontiac run slower, single plane intakes, huge cams, deep gears, spinning it over 6000 rpm, all isnt needed with the Pontiac. Most post 67 Pontiac heads come with factory screw in studs too, so no pulling them out or needing the screw in studs put in like chevy. Pontiac has a long intake port that creates high velocity so they fill the cylinders pretty well, and that translates into lots of grunt between idle and 5000 rpm. Most 455s dont need to be shifted any higher than 5800, and going higher than 6000 with stock rods usually ends up with a rod out the block.

There are some well proven recipes using iron heads making around 500 hp/tq, and they do it all under 6000 with mostly reworked stock parts. Its a different animal when you have almost 500ftlbs just off idle and it goes well over 500 with a nice flat curve. It means you can run a 2.73-3.55 gear even in heavy cars. If you run 3.73 or 4.10 you will be wasting the grunt the engine is making down low, and it will run slower in the 1/4. More than 4.10 is a serious waste of time and money unless you have a very different set of heads and a big cam, they just dont need the gear. Gears are usually the biggest mistake people make when going from chevy to Pontiac, and then they cant figure out why they run so much slower with a bigger engine.

Lastly, since Pontiac engines are great at making bottom end and midrange torque, and since you can get 13:1 compression out of a 455 with a head swap, they tend to run very well on ethanol. They make more power on E85 than they do on gas, and the bottom end and mid range power is quite impressive. I like running ethanol for a bunch of reasons, but the nice one is there is no worry about detonation or octane sensitivity. Usually on pump gas 9.5:1 is the max compression you can run with iron heads, so that means you are going to be running 71 and later heads, of which the 6X is the most available since they came on 350s, 400s, and 455s. Pump gas limits you more on what you can get out of your street engine, running ethanol opens up new doors and opportunities for compression and ignition/cam timing.

Got any pics of your LeMans? I have a 65 LeMans I am building over the winter, after the 65 GTO is done. The 70 GTO I have runs on E85 with a 455 using milled 1969 vintage 48 heads for just under 13:1 with a mild hydraulic roller cam, and a 700R4 trans behind it. Its silly fun driving that thing, I cant wait for spring. If you cant tell I love these engines.

[milkovich]

I always read through these posts from Thumpin455 and I usually learn something even though i'm supposed to be a "pontiac guy."

i didn't know the real difference between the 64 and earlier junk and the later stuff which is all I can afford to mess with.

[Thumpin455]

Thanks for the kind words milkovich. Been hunting for parts long enough that I had to learn what worked with what year stuff.

Almost forgot, since he has a 4 speed. Muncies dont live long behind 455s unless they are M22s, but they will still work. I just broke too many and went Th400.

The flywheels are different from chevy, and there are two different ones. You have a 10.5" clutch and an 11" clutch. Some flywheels are drilled for both, most are not. A 455 needs the 11" clutch, and its nice to have with a 400 too but not mandatory. If you want to retain the 4 speed, my best advice is to get an 11" aftermarket piece and run a quality diaphragm style pressure plate. The long style pressure plates tend to bend the clutch linkage.

The bellhousing is very different as well, so you will need one of those. The ST10, Saginaw, toploader 3 speed and any Muncie should bolt up to a stock Pontiac bellhousing.

headers can be a pain with a clutch. I had to dimple one set so the actuator rod didnt hit the header tube. The guy I got it from couldnt keep an 11" clutch in it, if you hold the clutch pedal down so it constantly slips, which in essence is what was happening, it is going to burn up clutches.

Headers are lots of fun on a Pontiac. You cant see the bolts and its rather tight getting in there. There are lots of tricks to getting them to seal up, but using the quality steel core gaskets works best. The paper gaskets last all of 10 minutes even putting them in wet, dont waste your time. I trim the gaskets so they arent connected to each other, they only go around the ports and bolt holes. Headers are the only thing that is really a pain in the ass about these engines.

[Deaf Bob]

Thump... Thank you!... Your comment about long vs diaphram clutches made me go crawl under the flatbed... Yup.. Linkage is slightly bowed!
As for headers.. I find the factory ones work best on out SBC in the derby cars.. They need removing sometimes and they can be reused.. Yeah, before anybody wisecracks about "it's just a derby car" Burned valves suck! ...
Son's motor prolly is better built than the majority or the SBC on the road and daughter's 327 is close but not as built..
Back to the gaskets... My 57 with the 327 was "knocking"! dammit to hell! Had a buddy who has ears for noises.. He was standing there listening.. Shrugged... Grinned, said that is sure sweet! I'm thinking WTH?? turned out that paper was missing on my header, making it sound to my deaf ears like a knock.. Always use metal embossed factory style..
Also collectors, if they neck down, the beveled gaskets for manifolds work better than the flatpaper too!

[silver_bullet]

If you build a GM family(small or big block Chevy(GASP)) and use Pontiac motorsports heads/valvecovers, you don't have to swap anything around... I have seen a few lifter valley blowouts in the back of the poncho blocks, plus the availability of the bowtie stuff makes it more budget friendly... sorry Pontiac gods, I'm just trying to give him as many options as possible...

[DanStokes]

The Tribe is lighting a huge bonfire and are sharpening their spears. If I were you, Patrick, I'd be hiding!

Dan

[Thumpin455]

The Tribe is lighting a huge bonfire and are sharpening their spears. If I were you, Patrick, I'd be hiding!

Dan

Indeed. To Dan you should listen with that heresy!

Never had a valley pan go, but I have replaced hundreds of intake gaskets on chevys, and seen more than a few pull studs out of the heads. Leaks abound, especially valve cover gaskets due to the as cast rail on the chevy vs machined for the Pontiac.

I'll stop now.. Gotta go eat with the wife and her family.

[oldsman496]

while Thump is gone i'll just say this..................


put an OLDSMOBILE motor in that thing!!!!!! its the only way to make a pontiac fast.......!

[OHC 6 Sprint]

Awesome information and advice. Just want to add one more thought to this:

The flywheels are different from chevy, and there are two different ones. You have a 10.5" clutch and an 11" clutch. Some flywheels are drilled for both, most are not. A 455 needs the 11" clutch, and its nice to have with a 400 too but not mandatory. If you want to retain the 4 speed, my best advice is to get an 11" aftermarket piece and run a quality diaphragm style pressure plate. The long style pressure plates tend to bend the clutch linkage.

There are two different crank registers on 350/400 cranks (early vs. later) and they do not interchange. All of the early cranks, up to about 1975 if memory serves, and all 455 cranks are the larger 2.75" register. You can get a dual-pattern (10.5" and 11") aftermarket type with the correct register hole. If you go with a 455, it's moot.