[Fot] Six cylinder cams and compression

Sat Oct 31 10:19:07 MDT 2015

With Zero deck clearance it should be fairly easy for you to get over 12 to
one. (by then going after the head)
Basically, with the zero deck clearance as your first step you need to take
.125" to get to 12.75 to 1.
So if Scott's figure of the safety zone is .10" you would be at damn near
12 to 1 even with that.
I don't know if you have these #'s or not but for 10.75 to 1 C.R. with zero
deck clearance you will have a volume of 30 c.c.
For 12.75 to 1 the volume is 24.4 c.c.
Anything in that range should give you at least 135 HP at 10.75 to well
over 165 HP for the properly tuned 12.75.

Jason Ostrowski
Friendly Ghost Racing
1969 Triumph GT6+ Racecars

On Sat, Oct 31, 2015 at 10:30 AM, McKearn McKearn <mckearn2 at gmail.com>
wrote:

> 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.
>>
>>
>
> _______________________________________________
> fot at autox.team.net
>
> http://www.fot-racing.com
>
> Donate: http://www.team.net/donate.html
> Archive: http://www.team.net/archive
> Forums: http://www.team.net/forums
> Unsubscribe/Manage:
> http://autox.team.net/mailman/options/fot/jason@multivintage.com
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/fot/attachments/20151031/4ff56924/attachment-0001.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: IMG_0599.JPG
Type: image/jpeg
Size: 675528 bytes
Desc: not available
URL: <http://autox.team.net/pipermail/fot/attachments/20151031/4ff56924/attachment-0006.jpe>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: IMG_0600.JPG
Type: image/jpeg
Size: 602481 bytes
Desc: not available
URL: <http://autox.team.net/pipermail/fot/attachments/20151031/4ff56924/attachment-0007.jpe>