[Fot] Six cylinder cams and compression

[McKearn McKearn]

Thank you. That's what I'm looking for.....
                                        P.J.

On Sat, Oct 31, 2015 at 9:12 AM, Scott Janzen <sjanzen at me.com> wrote:

> 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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