# Piston question



## 66CHEEVO (Jan 24, 2021)

I currently have a low compression 8.00 1973 400ci X4 block ID #481988 with small valve 4X heads. I recently acquired some large valve 1968 #16 heads with screw in studs. I’m looking at swapping the cam, heads, and possibly intake. The motor currently has a 600 cam edelbrock and will more than likely look to go with an 800 Quadrajet. I was hoping not to have to pull the engine completely and use the existing pistons. Would a stock low compression 1973 400 engine have used the same pistons as a higher compression engine with #16 heads? Also any suggestions on a cam? I currently have a manual 4 speed transmission but still need to determine my gear ratio. Any feedback is greatly appreciated.


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## coyote595 (Dec 4, 2019)

OK, no one else replied, so here is my opinion of what is likely to be the case. Most, I said MOST pontiac V8s of that era used flat top pistons, including the higher compression 60's engines. When they needed to drop compression ratios, they generally increased the combustion chamber volume in the cylinder head. It is likely that your '73 engine has flat top pistons just like the 1968 engine would have had. If you pull the heads and the pistons are flat top, you are probably good to go for what you want to achieve. Your 4X heads probably have a chamber volume of about 98 cc. The #16 heads you bought should have a chamber volume of about 72 - 75 cc. Your compression ratio is going to increase significantly, so you will want to try to do everything you can to mitigate detonation.


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## PontiacJim (Dec 29, 2012)

As pointed out by* coyote595 *Pontiac uses flat top pistons and adjusts compression with head chamber volumes. You can install #16 heads, but this will raise compression and cylinder pressures.

Problem #1 is that if the rings are worn, you may experience blow-by and/or oil consumption due in part to the increase in cylinder pressure of the higher compression heads.

Problem #2 will be the need for higher octane. Generally, pump gas won't cut it and you will wind up with excessive detonation/pinging (unless you live at a high altitude which works with higher compression). So the solution will be to either blend in some racing gas to increase octane or use an octane booster.

You can of course try the #16 heads and see how it goes, but be aware that it may be counter productive.


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## 66CHEEVO (Jan 24, 2021)

Thank you guys for the response. I think I’m being a little hasty by trying to simply throw on the #16 heads and a new cam. I’m going to need to get my compression ration down a little as I’m planning on driving my car on pump gas and being that I live in CA the highest readily available octane is 91. That being said I’ll need to do my research on some dished pistons and proper cam selection. The car smokes a little which is obviously an indication of possibly needing some new rings. I might as well pull the motor and do a proper rebuild. I’ve only had my GTO for 5 months and was just hoping to slap on the upgrades and get some more enjoyment out of the car before I did a rebuild. I’ll probably just enjoy the car some more as is while I acquire some knowledge and needed parts for the time being. Thanks again for the responses 


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## PontiacJim (Dec 29, 2012)

Piratebates said:


> Thank you guys for the response. I think I’m being a little hasty by trying to simply throw on the #16 heads and a new cam. I’m going to need to get my compression ration down a little as I’m planning on driving my car on pump gas and being that I live in CA the highest readily available octane is 91. That being said I’ll need to do my research on some dished pistons and proper cam selection. The car smokes a little which is obviously an indication of possibly needing some new rings. I might as well pull the motor and do a proper rebuild. I’ve only had my GTO for 5 months and was just hoping to slap on the upgrades and get some more enjoyment out of the car before I did a rebuild. I’ll probably just enjoy the car some more as is while I acquire some knowledge and needed parts for the time being. Thanks again for the responses
> 
> 
> Sent from my iPhone using Tapatalk


Good idea, arm yourself with a good background of knowledge, and then you can put together a game plan for you build and then run it by us and we can help you fine tune the build if need be. Check out the listing of Pontiac books we have and purchase a couple on engine building and assembly. Even if you have a shop build your engine, you will know what he is talking about and not get taken for a ride.


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