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chevyelc81 06-06-2002 08:25 PM

Pontiac 400 vs. Pontiac 455
I just bought a 78' Pontiac trans am that has the 400 engine in it. I found that the block is cracked. The machine shop said they could repair it for $250 but I have a friend with a running pontiac 455 that i can pick up dirt cheap. Does the 455 perform better than the 400? Im not to familiar with pontiac and I was wondering if the 455 would just drop right in. Let me know if theres any kind of little things i need to change such as pullys or flywheels, or should I just spend the extra bucks on fixing the 400 block. The car is going to be built for street/strip. :rolleyes:

jeff of nc 06-07-2002 05:34 PM

should drop right in, only thing you might want to check on is your accesory bolt holes. if the 455 is older it may not have all the bolts holes on the heads. not sure about the flyweel or balancer.

madamstn 04-01-2006 03:51 AM

2 cents worth
jeffs right, many times the boss is there but not bored and tapped. use a block plate. More ci mean more power al else being the same. The common mistake is all else being the same. If for example your volumetric efficiency is 95 percent in one it has to be the same in the other. And that is a function of many things.NEVER SIMPLE ANSWERS. M

WDCreech 04-01-2006 11:45 PM

To make the installation easier, try to get the pulleys and accessory brackets that are on the 455. It may save you from looking for ones that will work, but most of the stuff from the 400 should work, if you use all of it.


Grusome Goat 04-01-2006 11:52 PM

One thing you may encounter is motor mounts as ones for a 455 are slightly different than the 400, these are readily available, get ones for a 71-76 as some were factory equipped with a 455. The 455 is a torque engine so keep the rpm's under 5500 with stock parts. Otherwise it's bolt in.

chevypower1 03-19-2011 05:19 PM

400 verus 455
the 455 is a much better engine that the 400 and the 421. You should be happy with it.

Thumpin455 03-19-2011 06:24 PM

It should be obvious which one I prefer, but I still like the 400. All the 455 really needs is a cam, some headers, and maybe a set of heads depending on which ones it has on it now. Get us a pic of the center exhaust ports, there should be a number cast into it like 6H, 6S, 6X, 4X or 7H1.. We can tell what heads it has then and if you should swap them or not.

Otherwise use the heads from the 400, they will increase the compression a little and bolt right on. You wont have any problems getting the pulleys and accessories to line up then. Or like Creech said if you can get the stuff that came with the 455 use that, because the water pump pulley is a different size to slow down the pump speed. It will still work with the 400 pulley if you cant get the 455 pulley, just a different diameter, and they should all have the same offsets unless someone swapped in a 67 earlier era front cover.

350, 400 and 455 use the same balancer and flexplate, so no worries there. They are internally balanced and most flex plates are drilled for both large and small pattern torque converters.

The F body mounts from 70-76 wont work on the 77-81 cars, but the 455 should have the bosses to use the brackets the 77-81 cars use to mount the engine. You cant find those mounts in stores, they are stamped steel and all you can get is the frame mount, so dont lose them. Just use the ones you have since most 455s have both mounting bosses. This is what the mounts look like on the 77-81 cars with Pontiac engines. I can get a pic of the 72 455 mounts if you like, they are different.

WDCreech 03-19-2011 07:31 PM


Originally Posted by chevypower1
the 455 is a much better engine that the 400 and the 421. You should be happy with it.

I've been running Pontiacs for almost fifty years and I've had all sizes. The 455 is no better than the 421/428 or 400. As a matter of fact, the 400 block is stronger because of it has 3 inch mains vs the 3.25" mains of the 421/428 and 455. I know because I've had one start to split. For the horsepower needed for a healthy street engine, the 455 would be the best choice because it has plenty of torque without replacing any internal components, even though I would change to forged rods and pistons.


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