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Switching from #15 72cc heads to #13(or other 72cc) heads/

  
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Switching from #15 72cc heads to #13(or other 72cc) heads/

 
CRM318 CRM318
New User | Posts: 5 | Joined: 02/14
Posted: 02/03/14
01:36 PM

I currently have a set of 1968 #15 72cc small valve heads on my 1970 400. It runs fine, however I was looking to get a head that has bigger valves and screw in studs. I have been told that a 400 cant use 72 cc heads and run on pump gas. I don't know what kind of pistons are currently in my engine, but It runs fine on pump gas and it has 72-75 cc heads. I have heard that the #13 would have too much compression. To me, it would be basically the same head with bigger valves. What do y'all think?  

gtojack1366 gtojack1366
Guru | Posts: 1257 | Joined: 11/09
Posted: 02/03/14
02:00 PM

Are you sure there 72 cc #15 heads Pontiac did make #15 87cc heads in 1970 ? Has the engine ever been rebuilt and the pistons changed ? If the heads have the same cc the compression should stay the same .  

CRM318 CRM318
New User | Posts: 5 | Joined: 02/14
Posted: 02/03/14
03:44 PM

The heads are date coded December 1967, which makes them 72cc chamber and 1.96 / 1.66 valves. I do not know anything about the engine.  

gtojack1366 gtojack1366
Guru | Posts: 1257 | Joined: 11/09
Posted: 02/03/14
04:12 PM

The engine has been apart you might have dish pistons in it .I would take the heads off cc the heads and check the piston type before buying new heads .Then just get heads with the same cc and bigger valves . There must of been a reason someone put 68 small valve heads on a 70 400 engine ? You don't know what's been done to that engine it might even have large valves installed in small valve heads ? You never know put probably not ! You have to open it up and see what's been done to it !  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 465 | Joined: 11/13
Posted: 02/03/14
04:32 PM

valve size does not affect head cc.

if it starts to ping, reduce the timing ir run corn gas like e15 or e85 and rejet accordingly.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 465 | Joined: 11/13
Posted: 02/03/14
04:33 PM

valve size does not affect head cc.

if it starts to ping, reduce the timing ir run corn gas like e15 or e85 and rejet accordingly.  

CRM318 CRM318
New User | Posts: 5 | Joined: 02/14
Posted: 02/03/14
05:03 PM

That's a good idea, I just cant stand having it off of the road. I just got it in October and I drive it anywhere and everywhere I can. I will do some more digging.  

CRM318 CRM318
New User | Posts: 5 | Joined: 02/14
Posted: 02/03/14
05:04 PM

Didn't the 350 hp GTO have the #13 heads from the factory? There seem to have been a lot of 72cc 400s factory that never had a problem.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 465 | Joined: 11/13
Posted: 02/03/14
05:48 PM

Didn't the 350 hp GTO have the #13 heads from the factory?

yes in 1970