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Old 11-18-2022, 12:32 AM
Zooguy Zooguy is offline
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Default 1966 389 bottom end

I have a 1966 Star Chief executive it and it came with the two barrel 389 high compression engine. I know if I wanted to run a Roots blower lots of things would have to be changed. But how strong is the factory bottom end? I'm wanting to build a street cruiser so I'm probably going to be doing less than 5 lb of boost on whatever motor... Mainly I just want to cover the parasitic losses the blower creates so if I can do that with 3 lb of boost that's all I'm going to run.

I've been told the 1965 and 1966 389 engine blocks are substantially weaker than the other year 389s. Everybody keeps telling me go with a Pontiac 400 or 455 from the early 70s. I understand those in engine blocks are much stronger but if I'm just wanting to build a cruiser with minimal boost do I really need to have the strongest factory blocks the racers desire?

If I changed cylinder heads and ran a slightly thicker head gasket to help bring down compression with the factory bottom end last?

I already have an early '80s 454 Big block Chevy with peanut Port heads torn apart in the garage. If this 1966 389 is super fragile I might wind up swapping this big block into it.
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Old 11-18-2022, 01:39 AM
Dragncar Dragncar is offline
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66 389s do not have thick enough cylinder wall for boost, let alone the bottom end.
Running a thick enough head gasket to change compression much will screw it all up.
400 and 455 blocks do not have much of things "Pontiac racers desire".
Its just what we have.

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Old 11-18-2022, 12:16 PM
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Tom Vaught Tom Vaught is offline
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You can make a 1967 400 block live at 1600 HP but it would take a LOT of work (machining) and for your application a 421/428 4 bolt main block would IMO would be
a much better place to start.

The main webs are weak in the 65/67 blocks and the cast steel is not great.
A 67/68 4 bolt main block would be a much better starting point with some good rods and a "N" crankshaft.

No need for a set of high dollar heads or crank (with a engine built for "looks") BUT
the roots blowers do add a lot of stress to the #1 and #2 main area of the blocks and a cast crankshaft.

Tom V.

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  #4  
Old 11-18-2022, 12:30 PM
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https://forums.maxperformanceinc.com...d.php?t=859213

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  #5  
Old 11-18-2022, 12:47 PM
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25stevem 25stevem is offline
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A lot of things don't change much with even running 8 psi of boost, all the basics still apply.

The formost level of stress the block has to deal with in the main web area is still the G force load crated by the weight of the recipercating parts and the level of rpm.

These major stress loads are twice as high at 4000 rpm as they are at 2000 rpm, and they have doubled again at 6000 rpm..

Having the cylinder pressure to make 600 hp at 7000rpm is the same as making 600 hp at 5000 rpm, but the G loads on the bock web and main caps is reduced to 1/2 of the 7000 rpm level.

I would not Bore the 389 more then .030" over and I would do a 1/2 fill in the water jacket.

I don't know how your going to make only 3 psi of boost since I am not sure a underdrive pulley is made that big, or that small depending on where you bolt it on.

You will likely have to limit the amount the carb(s) open to keep tabs on the boost, yes the size of the carb(s) is also a big factor in the amount of boost made .



The bigger question here is getting the compression down to 9 to 1 which would work with even 5 psi of boost with the right Cam .

These early heads have badly restricted Exh ports that don't help with getting heat out of the motor when your making more heat from boost.

I would as a back up employ a water injection set up and run a mix of water and alky.

That's what I did on mine with 8.8 comp in a .030" over 455 which I had under driven 10%.

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Old 11-18-2022, 01:31 PM
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Your 389 vs your 454, I think the 454 wins every day of the week. There are downsides to putting a Chevy in a Pontiac but you don’t seem to be a “hard core” Pontiac guy. Do what makes sense to you and what works in your budget.
If you want to run a Pontiac then follow what Tom V mentioned above.

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