Shelby#18 wrote:It isn't costing me anything for the replacement. The restoration shop I'm using is paying for the new pistons. They have been really stand up guys with anything that has gone south on their end.

Sounds good.

Shelby#18 wrote:Our goal is 10.1 CR for the hp., seeing as the the Colony Park is not the lightest car around.

Yes, this makes sense. Good luck and keep us updated.

Does anyone know the effect on horspower between 9.625:1 and 10:1?

There are three possible reasons for the compression loss:

The size of the step.
The height of the deck of the piston
The size of the combustion chamber in the head, albeit small.

Chris Craft crazy wrote:Does anyone know the effect on horspower between 9.625:1 and 10:1?

That's a tough questtion that usually is left for computers.

Chris Craft crazy wrote:There are three possible reasons for the compression loss:

The size of the step.
The height of the deck of the piston

Are you refering to deck height or to piston height. Anyways, both variables must be taken into account (see illustration)

Chris Craft crazy wrote:The size of the combustion chamber in the head, albeit small.

No chamber in the head here. Since they are flatheads only the compressed thickness of the head gasket is a variable to calculate. That's nothing big that could have been messed up to produce such an erratic result like one full step of compression loss.
In Rick's case it's more likely that either the step (dome, pop up) and the piston height or both have been designed wrong.

Since they usually use computer templates to design the pistons my personal sneaking suspicion is that they just clicked the wrong button. Probably the most used popular button by habit.

My suspicion is piston height, since the step looks near identical. It is also the largest of the two surface areas, so a small difference in height would make the change.

Definitely. Piston height is a big factor. But we don't know if we can't take measurements. Hope the shop will get the right pistons on their next order.

Note by admin: This post has been extracted, edited and merged into this thread . The original thread is here
viewtopic.php?f=29&t=14
Please post further comments regarding pistons design and combustion chamber in this thread.

Text edited by admin:
Here they are. Note that one piston has been cut in two for better close up shots.

Awesome Rick, good info.

Shelby#18 wrote:Well we have poor results on the pistons. From the new pistons we received we only had a 9.625 compression. We are now going back and getting another set from a different supplier. Guys when this is done hopefully it will save some of you the time and trouble we have been going through. Delays, delays, delays... It looks like we'll have completion sometime this summer of the restoration.

Rick, do you know to which point the machine shop is referring to with their "deck height" numbers? Are they refering to the distance of the top of the piston step to the cylinder head?
If that's the case, the listed clearance numbers would not be satisfying from a racer's standpoint. Best air fuel propagandation and performance is achieved by tight quench areas. Optimum would be zero deck clearance.
I'm talking about this area marked yellow.
If the piston manufacturer knew the exact length of the intake side block height, then he could build a zero deck height piston. If you order it with a slightly wider step area then you would have enough meat to machine off according to the desired compression ratio.
It's always easier and less expensive to machine or grind off ,material from a piston than ending up w. a lesser than optimum compression. Oh, well trial and error.
Let's hope the other pistons will be closer to your desired compression ratio.

Thanks so much Theo! I had the restorer on the phone just now and had him look at your post. He agrees that that is what we want to achieve, and he is relaying that to the engine builder as we speak. As I have mentioned to him time and again the help received on this forum is amazing.

As previously mentioned I could possibly design the piston questioned. Thing is that with the MEL and it's canted deck we have a special situation. If I only had the dimensions of the dimensions listed below, then I could first calculate a few other things by hand and with some work around because of the MEL problematic I could finally feed it all into my little piston design program.

1. block height measured at the cylinder wall exhaust side.
2. block height measured at the cylinder wall intake side.
3. Gasket bore diameter
4. Gasket compressed thickness (this figure is usually listet on the outer packing)