[Omni]

Good Day

Looking for alternatives to dished pistons to lower the compression ratio on a .040 389 with 670 heads.

Trying to maintain stock appearance with the goal of lowering temperature/ability to survive on pump gas.

All thoughts welcome.

Thanks for your time

Omni

[ZeGermanHam]

If you're committed to keeping your pistons and 670 heads, you'd have to send the heads out to have the combustion chambers CNC'd out to a larger size. But it also depends on how much you'll need to enlarge them to achieve your desired compression ratio. You may run out of material on the 670 heads. Personally, I recommend just getting a different set of heads like 4X or 6X that provide a lower compression ratio and can be made to flow well.

[67gtospud]

If you’re main goal is to achieve stock appearance with the 670 heads then your best bet really is dished pistons. I know not your first choice, but still best bet.

[65 421 TODD]

As ZeGermanHam posted I had the combustion chambers enlarged. This is thinking back 20 years but pretty sure I remember what we did. I too wanted to use original and remain stock appearing but be able to run on 93 octane pump gas. I have 1966 093 heads which are the "closed chambers" design. The engine is 389 with .020 over stock flat top pistons that are set at approx 0.010 to 0.015 deck height, stock Pontiac original thickness head gaskets ( forget how thick they are ) and I'm using the current 067 high lift cam that's available. When the heads were rebuilt I had the chambers "opened" enlarged similar to the later style 1968 and up heads. After opening the chambers I calculated the compression ratio to be about 9.26. My motor runs excellent, timing set at 6 degrees btdc , idles at 580 rpm, NEVER gets over 180 degrees on cooler days or 190 on the hottest 90 degree days, but I am using an aluminum radiator. I have stock Carter AFB and stock exhaust manifolds and it's a stock st300 automatic transmission, 3.23 rear. I'm very pleased with the way the car runs smooth and reliable.

[Gary H]

Originally Posted by Omni

Good Day

Looking for alternatives to dished pistons to lower the compression ratio on a .040 389 with 670 heads.

Trying to maintain stock appearance with the goal of lowering temperature/ability to survive on pump gas.

All thoughts welcome.

Thanks for your time

Omni

If you're not committed to keep the 067 heads then a swap to different heads with a larger combustion chamber would be a great option. Some casting numbers to consider are: 71' 400 #96 heads, 72 400 #7k3's, both of which are around 96 cc's, 6x-4's @ ~92 cc's, 6X-8's @ ~100 cc's. Some other heads which will lower your compression less would 62's and 13's which are normally around 78 cc's. These compare to your 670's which have ~72 cc's.

[b-man]

First picture is a comparison of typical closed chamber heads and open chamber heads. 1964 #9770716 closed chamber top and 1970 #13 open chamber bottom.

Second picture is a 1964 head being opened up on the spark plug side to slightly increase combustion chamber volume to lower compression. The first modified chamber is one on the far right. Before starting the work all chambers were measured and came in between 73.6 and 74.2 ccs with the original valves installed. After the work was completed all chambers were checked again and came in a little under 78 ccs. The original factory specs for these heads would be 72 ccs.

Third picture is the finished heads with new stainless steel valves and hardened exhaust seats installed and a valve job, the finished volume checked out at 76.4 to 76.6 ccs. Heads received a minimal cut to clean up the surfaces as well.

The fourth picture is a Ross custom dished piston I used to get my compression down to a very safe 8.82:1. My car is a heavily optioned ‘64 Le Mans Convertible with a TH350 automatic, a tall highway gear (2.56} and factory air conditioning. I wanted to make sure my 421 engine would be happy running our substandard 91 octane ethanol-laden fuel under the worst conditions, hot summer days with the A/C blasting.

The #670 closed chamber heads are unique and able to be opened up similar to the factory open chamber heads. There’s only minimal material on the 389/421 heads that can be removed, but the #670 400/428 heads can be opened up more. I’m pretty sure they can be safely opened up to 82 ccs and perhaps even more without breaking through to the water jackets. This modification alone should net you somewhere around 9.25 to 9.5:1 compression depending on factors such as piston deck height and head gasket thickness.

[lust4speed]

While Dave at SD has a great CNC program for opening up 670 heads to the semi-open chambers, he is only able to obtain 85cc chambers on a virgin 670 head. Take the normal cleanup mill and 83cc's is the maximum you are going to get. I've done several heads and depending on how much was taken off in the cleanup mill(s), the final product ranged between 81cc's and 83cc's -- and things were getting thin. If you go for a .030" over and leave the piston .006" down in the hole you should be at 9.3:1 or zero deck will result in a little north of 9.4:1.

Now the big question - why are you against dished pistons? The combustion process doesn't really care how you get the required chamber size. Think I only have one Pontiac on the road now without dished pistons and it just happens to be the lowest powered engine in the fleet. Let's not forget that unless you are capable of doing the modifications to the 670 heads yourself, the small additional cost of a dished piston is much, much less than opening up chambers.

Before and after pics:

[geeteeohguy]

Since 670 heads never came on any 389, you are already not maintaining stock appearance. I would bolt on a set of 1970 #15 heads (455 heads) with 87cc chambers. I did just that with my own '67 400 which has the correct piston reliefs for the later '67-up heads, unlike your 389. It turned my engine from a rattle can into a smooth cruiser on pump gas.