Two ways to increase compression...which is better?

[speedo]

I picked up some 84mm heads a while back that had been "tweaked" at some time during their life. They went in a box and were basically ignored to recently when I started thinking about the next project as my R clone is a now a driver. Most of the time when one thinks of increasing compression, we default to a higher CR piston...with larger valve pockets. And there are simple ways to check for deck height and valve clearances during the rebuild process. I pulled these heads out recently and noticed that the traditional "open area" around the valves and the plug hole had been filled and remachined....very professionally executed. This basically removes some of the combustion volume, thereby increasing compression. Raises several questions for me. Was this done to run lower CR pistons? Maybe stock T pistons? How much piston can be paired with the "reduced volume" heads?
There are standard ways to measure head volume, do those methods work with these modified heads. I was told that this was a trick back in the 70s performed by some west coast race shops. Anybody familiar with this work...or ever seen a set of heads modified this way? What was the thinking that decided to focus the increase in CR on the heads vs simply replacing the pistons with higher CR pistons. Is there a downside to going after a higher CR this way aside from the tedium of massaging the heads (probably by hand)? I will post pictures of the heads for the thread in the next couple days when I dig these back out. I am curious now to know what sort of 2.2-2.4 engine can be built using these heads, and what combination of P and Cs and cams should end up together.
Old school comments and suggestions are welcome...Grady? John? Henry?

Speedo

[kenikh]

Search "peanut dome" on Pelican. Lots of info there.

[speedo]

I should have asked..."Old school comments and suggestions are welcome...Kenikh? Grady? John? Henry?

Speedo

[edmayo]

If you have what I think you do, I believe this was a method to allow high compression ratios on a single plug engine by forcing all the mixture towards the single plug so that all the gases would be burned by the one plug ignition. Very high compression ratios with symetrical domed pistons tend to leave end gases trapped away from the single spark plug which invites detonation. Based on an old hot rodders trick I had seen years ago I used to cut a fire slot on my E/P 356 11 to 1 pistons to allow the flame to get past the dome. Even carbon deposits on the piston dome proved it worked.

[kenikh]

Ed, I just sent Lars pics of what I think you think he has. If he does, it is pretty cool. "Fire slots"...interesting. I thought about doing this to my JEs. Cool to know it actually works!

[jpnovak]

Ed, This guy has been touting the advantages of flame slots in American Iron heads. Should work well for any non-symmetrical chamber where there is not ideal combustion shape. The 911 (or 356/912) heads are very non-symmetrical when you consider the offset spark plug location.

http://somender-singh.com/

Think about CIS pistons with the offset dome. This was done to help the burn pattern just as Ed described. The farther away or more isolated an area of combustion becomes the less it can contribute efficiently to the combustion process.

Shaping the heads is a great way to overcome the low compression with a small dome volume. In fact there are some advantages with leaving the dome small.

Check out the pictures of this head... is this what you have?



http://forums.pelicanparts.com/showt...ghlight=peanut

btw, this post also shows Henry's trick head spacers for shimming cam towers to compensate for chain length issues when shaving heads. I like it!

[speedo]

Thanks for chiming in Ed, JP. The link to the "grooving" site was definitely worth the read. As simple as this is, and as logical as it sounds, I had not heard of grooving before. I do understand that the CIS pistons are shaped to disperse the flame more effectively and avoid "hot spots". Makes me wonder yet again why our auto industry in the US hasn't embraced some of these "grass roots" technological advances...but I guess that is a thread for a different forum.
After stirring things up...now I have to pull the heads out and shoot some pictures. I have some dirty stock 2.7 heads that I will use for a pictoral comparison. I am still interested in knowing what pistons were used with these modified heads.
I guess a side question would be...has anyone played with "grooving" their 911 heads? Sounds like something you would do when you have a lot of time on your hands...or maybe for a thesis project. It would be very interesting to build up two identical engines side by side and "groove" the heads on one and leave the other stock. Then see what the dyno and the wide band O2 tell you.

Pics tomorrow...
Speedo

[edmayo]

Years ago we built a short stroke 66mm 2.7 for a customer since the rules only stated it had to be 2.7, ....didn't say how you had to get there. At that time we had a west coast head shop (Pacific Performance) put us on to the idea of welding up the non-plug side of the combustion chamber to force the gases to the spark plug. He said this was based on the 962 heads since the rules required a single plug set up. When I used to run SCCA in the sixties and seventies this would have illegal since the rules clearly stated NO metal could be added, only removed.

[jcge]

from a 962 (not mine)

[speedo]

The heads are date stamped 1970
http://www.early911sregistry.org/for...1&d=1247114696
The intake/exhaust ports as well as the valves are the same as a 2.7
http://www.early911sregistry.org/for...1&d=1247114832
http://www.early911sregistry.org/for...1&d=1247114879
http://www.early911sregistry.org/for...1&d=1247114918
Easy to see the additional material in the heads