Pontiac 400 help

Hey guys, I am looking at a Pontiac 400 out of a 78 bonneville for my truck. My question is, how did they lower the compression in these motors? Was it bigger chambers in the heads, Dished pistons or both?

Thanks

 

It was pretty much all in the heads... Your best bet is to scrouge the junkyards in your area for a set of '67-'70 400 heads from a Grand Prix, Firebird, GTO, etc... Maybe some of the other guys here can add more info...............

 

Go to "PY on line Forums" for all your info on Pontiac. You are dealing with a different kind of animal and that forum has all info you need.

 

I have a running 63 389. I would like to get away from the slimjim trans. I found a killer deal on a running 400 with turbo 350 out of a 78 bonneville. I was thinking of running the heads and intake off of the 63 motor if it helps bump the compression. If it doesn't help then I am not going to mess with it.

 

It's possible to bolt on the 389 heads on the 400.. I think you'll need a special set of intake gaskets to make it work.

 

If it's a later 63 389 it may have the provision for a block mounted starter. Otherwise Butler performance sells the adapter to bolt the starter too than run a later trans. It's the same trans bolt pattern from 61 and up for Pontiacs.

That would be going backwards putting the 63 heads on the 78 400. Plus now it can run on pump gas all day long without pinging because of high compression.

 

I checked and it doesn't have starter mounts on the block. The 400/turbo350 combo is cheaper than the motor plate. As far as putting the 63 389 heads on, I will also be using the intake from the 63. I whole reason I am asking this is because the horsepower rating for the 63 389 is around 310, whereas the rating for the 400 is around 180. If I can achieve the 310 rating by changing heads, intake, and my edelbrock cam I have and only buying a few gaskets then I will be a happy camper.

 

Realize that factors in a drop from net HP to SAE (or do those initials go the other way around?) - around '72 they started figuring the HP at the rear with all the accessories on. Also, your '63 runs 10:1 or so compression, the 400 will be 8.6 or maybe even 7.6:1. The heads may be able to be swapped, but the valve angle changes and there's a chance with stock pistons they'll hit because they may not have the reliefs in the right spot - aftermarket pistons have two sets of reliefs for that reason.

Pontiac wasn't building 400s in '78, so make sure it's not an Olds 403. I have a '77 tucked away and I'm pretty sure anything after that was using up leftover motors and pretty scarce.

 

Back up the truck........the 303 hp figure of the 389 was gross horsepower computated with no drag from accessories, transmission, mufflers etc etc..Starting in 1971 the automakers were required by law to post their horsepower figures as net power. Engines had to be laden with all the stuff as if they were bolted in the car.
So, comparing horsepower figures of these two engines is like comparing apples and oranges.
There's more.....
The 389 has closed-chamber heads with a 10.25 to 1 compression ratio. Not suited to our shitty gas unless you buy 93 octane. Plus, the 389 isn't gonna have hardened valve seats. The 400 has hardened seats, cmpression to handle 87 octane and be happy about it.
There's also the problems with the starter that's always been discussed. And the accessories from the front of the 400 arent' gonna bolt up to the 389 without some massaging..
The heads will bolt-up ok but the rockers before 65 oiled through an oil galley in the heads [fed from the cam bearings] that went full length of the heads just below the rocker studs...those rocker studs [3/8ths inch] were hollow. Pressurized oil filled the galley and went up through the hollow studs....
65 and later V/8s oiled the rockers through the hollow pushrods like a small block chevy. I believe the earlier heads will be ok in this regard. The 65 and later head gaskets will cover this oil hole in the heads....
BTW, 389 head gaskets should be used.
In the end I don't think it's a good idea to use early heads on the later block. This coming from a guy who's using 1973 low compression [96cc] Pontiac heads on an early 1957 engine bored out .125" with domed pistons.

 

There was a valve angle change so the 389 heads will not work with the later pistons.

 

Thanks a million guys, I think I will leave the 400 alone. I will just stick my cam in there. I want this thing reliable. I forgot about the hardened valve seats and didn't know about the difference in oiling. The 389/slimjim will be up for sale soon, cheap too.

 

As mentioned, there can definitely be some gray areas when swapping earlier parts on to later motors as the difference is in the details, and there can be many.

However, there are a couple of "modern" head choices which will improve performance and still be dirt cheap. Any factory 400 head above '74 will yield about 7.6 compression. The '73-4 4X will bump it up to about 8.0, and can be had cheap or nearly free if you know where to shop. The 4X 4bbl heads will have screw in studs. There is a 455 version of the 4X which has a much larger chamber size, like 114CC so stay away from those.

'72 7K3 heads would add another couple of points to the CR and will be a little harder to find and a little bit more $. Also, the '71 96 casting would be a good head. These are a 400 head but are popular to put on a 455 since it has a 96CC (coincidence only) and will yield a friendly compression ratio. Priced accordingly to popularity, these will be the most expensive of the bunch but you can still find deals.

Ditch the EGR intake, add a cam that will compliment the torque curve of the motor and you final drive ratio, add whatever heads you decide and you'll have a strong and reliable motor for not a whole lot of jing.

 

I actually have a stock 68 400 with #16 heads. If I do a stock rebuild, will I be able to run premium? What is the real CR not advertised.

 

Assuming you go +0.030" on your '68 400, the actual CR will be just a shade over 10.0. vs. the factory advertised 10.75. The 16 heads have 72cc chambers. A lot of factors will determine the actual CR including how much the block and heads are decked, head gasket thickness, the actual CC of the relieved areas in the pistons you choose, etc., etc.,.

Can you run that on premium gas ? Maybe. I hear a lot of guys running 10.0 and higher on the street but my luck has never fared quite as well with iron heads, ayways. The quality of gas seems to be getting worse and depending on where you live, the higher octane levels may be limited. Also depends on your tolerance for doing frequent engine tunes. For an occasional cruiser it may be OK, but for a daily driver it may be a nuisance. You can always mix in a gallon or 2 or 4 of Cam 2 in each tank to help. Personally if I was doing a rebuild for a car that I was driving a lot, I'd look at dropping the CR to the low 9s. You could keep the 16s, which are good heads and use the pistons to alter the CR. If you're rebuilding anyways the only added cost will be the difference in standard and reverse dome of a given piston model, or about $30 a set.

 

I ran a '60 389 on cheap unleaded for a long time. The car wore out before the motor did. One of the valves did get messed up, I've never opened it to see how, I just assume the exhaust on #7 was burned or cracked because the compression was a tad lower on that cylinder and it would pop out the exhaust now and then. But it still ran like a raped ape.

Your '63 389 can be bolted into any 59-64 Pontiac, will need a trans swap to run in 59-60 but the short bell should work to bolt to a dual-coupling Hydramatic, if not they can be had.

 

do a little more homework and find out if you can buy a thinner head gasket for the 400. It won't make huge gains but should help some by bumping the comp. up a litlle. That with a good intake, carb, cam and headers through at least a 2 1/4 exhaust but not larger than probably 2 1/2 inch should help it run good.