[74gpman]

I am lookin to boost compression on my 455 from the stock 8.0 to somewhere in the 9's. I have been scanning pontiac head info and it looks like the 1975 350 5c heads will be awesome(also looking into 4x or 6x-4 heads). the 350 5c heads have 2.11 intake and 1.66 exhaust while having 89 cc combustion chambers. Is this a good way to go? Also, a question that has bothered me is whether or not machine work will need to be done on the 350 heads in order to fit the 455 cylinder bore. I am far from a head expert so im not sure if heads that are from smaller engines (ie 350/400's) need the combustion chambers machined to fit the larger 455. If the 350 heads dont work ill just pick up a set of 400 heads, but id really like the small 350 combustion chambers. thanks for any help.

[74gpman]

I am lookin to boost compression on my 455 from the stock 8.0 to somewhere in the 9's. I have been scanning pontiac head info and it looks like the 1975 350 5c heads will be awesome(also looking into 4x or 6x-4 heads). the 350 5c heads have 2.11 intake and 1.66 exhaust while having 89 cc combustion chambers. Is this a good way to go? Also, a question that has bothered me is whether or not machine work will need to be done on the 350 heads in order to fit the 455 cylinder bore. I am far from a head expert so im not sure if heads that are from smaller engines (ie 350/400's) need the combustion chambers machined to fit the larger 455. If the 350 heads dont work ill just pick up a set of 400 heads, but id really like the small 350 combustion chambers. thanks for any help.

[Scarebird]

You may want to rethink the 350 head idea. I run 71 400 #96's on my 455, and it used to ping under certain conditions. This head has around 95 cc chamber, so a 350 head would need avgas added (my solution) or run race fuel. Not worth it.

[74gpman]

ok that makes sense not to do it if it causes predetonation probs, but what causes this? If the head is only running a 96 cc combustion chamber volume, that would only put the head at around 9.4 compression, which should be low enough to prevent predetonation. If this is the case, 87 cc Edelbrock heads would be way TOO much.

[Thumpin455_GTO]

I have 5C heads on a 30 over 455. Its in my 4200lb GTO, they have been ported and if the timing istoo far advanced, its hot out, you have 87 octane gas, and you are near sea level (like in South Carolina) it will ping just a little bit. In Nebraska at 2400 ft above sea level it never pinged even on the worst gas I put in it. I retuned it and used a Demon 850 Carb, now it runs great on any gas I put in down here in the hot low south.

Get them ported, you will like the difference. 5C is almost identical to 6X, (2.11-1.66 valves, screw in studs etc.) stock they flow a few more CFM but its negligible. You wont have any trouble installing them on a 455. The 96's are good too, ran them a few years back on the same engine, then I put them back on the 400 I got them from and dropped it in my truck.

[Mr. P-Body]

Another piece of advice. Don't take published numbers for combustion chamber volume as accurate. They may or may not have been machined in the past. They may also have had larger (or smaller) chambers from the factory. Published numbers are "nominal". You MUST actually CC them to be certain what you have. A 92 CC chamber, on qa .030 over 455 will put you right at 9.5. This is considered the practical limit for an average streeter, using 93 octane.
The statements on the 5C and 6X being similar are accurate. A little port work, installing the larger exhaust valve, and a good valve job REALLY wake them up.

[PunchT37]

I run 6X-4`s on my 455 with no pinging problems.
They are 350 heads for the most part but are used on W-72 400`s as well.6X-8`s are the "real"
400 heads. My timing is a fast 36 degrees at
2000 rpm on 93 octane and NO problems. Couldn`t
say that about the 62`s i used to run.