[Fot] Six cylinder cams and compression

[Scott Janzen]

Compression - practically speaking my experience is that it is tough to attain more than 12:1 compression with flat top pistons and a GT6 head.  Best I have been able to achieve with zero deck height, flat top pistons and a payen gasket is about 11:7 to 1.  This is because the head inherently has some thin casting spots, where too much milling will start to intrude on the water jacket.  See the attached photos - this is a stock head that I had ultrasonically tested to see how much we could mill off of it - the thickness markings are indicated.  The thinnest spot is next to the #1 cylinder, which is where I had another head fail this year after it was milled too much.  The castings as manufactured vary - this one is not suitable as there is not much material left to take a cut off the head, and I think this one is uncut, so I need to find a better casting.  My good head, the one on the car right now with the 11:7 ratio, is about 140 thousandths thinner (as cut) than the one in the photos, and the thin spot is only 0.156 thick.  And yes, they are both circle B heads.  I would think you can safely cut to  about 0.10", but am not sure how far you can go below this.  The thin spot is right at the edge of the fire ring on the gasket, and a head I had excessively milled collapsed on a dyno run from pressure.

You might be able to go to pop-up pistons and get some more compression, as Chuck Gee has.  Piston to valve clearance is great in this motor, even with 1.55 rockers and high lift cams, so there is room to do it, but you will have to get custom made pistons.  The shape of the combustion chamber means there is not room for the whole piston to go beyond zero deck height and still have adequate clearance.  I don't understand the mechanics of squish and flame spread, so someone else can explain the pros and cons of pop-ups.

The experts I use for dyno time, engine parts and machine work tell me the difference between  11.5 and 12:1 compression is probably only about 4 hp in this motor, but I that's just anecdotal and I don't really know what the impact is. 

So, my answer is, try for 12.5 to one but you probably won't get there.    

On Oct 30, 2015, at 11:32 PM, McKearn McKearn wrote:

Hi All.
   Thanks to those who replied. Nice to know you all have an opinion.
   At this point I am in fact finding mode and trying to do some research on what will work best for me.  My ultimate goal is to have a free revving, good handling as light as possible GT6. Good looking is a bonus. I understand that compression ratio will not be the only factor in performance. I was simply trying to ascertain a consensus ( If that is possible) of what other GT6 racers and thrashers feel is an adequate compression to reach say---180+ HP. Yes I am aware that cam and headers and carburation and timing and gearing matter. And 180 HP is not a real number and always questionable until actually measured on a dyno. So, what I am trying to get across is  that I want an engine that has some power. Pump gas is also a plus but I will do what I have to do..
 Thanks.   P.J.


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