Heres the situation. Stock a6 motor needed some help, so i picked up a y8 head and manifold. In preparing for the build, i ordered the stuff i needed including a d17a2 head gasket (it is thinner than the y8, thickness is .026) hoping to raise the compression a small amount more.
I get the head off to find out that i need rings pretty badly, so i order rings, and now im faced with a tough decision. As long as its torn apart, i should pick up some different oem pistions to raise the compression a little more and make it worth my time.

I figure with what i have right now, and the stock a6 pistons i should be at 10.6 : 1 static compression.
Im hoping for around 11:1 or so.

Ive tried 3 calculators now:
http://www.knology.net/~jediklc/D.htm
http://www.zealautowerks.com/dseries.html
http://www.c-speedracing.com/h...c.php

Each site has different values for the dome cc for the pm3 piston, and each is giving me slightly different compression values.

Does anyone know which calculator is the most reliable, or has anyone used pm3 pistons on an a6 block/y8 head setup before?

Thanks for any help.

 

Those calculators are meant to act as guides not the definitive answer.
If you need to know with absolute certainty then get the chambers cc'd.

Good luck

 

yeah, im not expecting 100% accuracy

but 11.1:1 compared to 12.67:1 is a pretty enourmous difference.
the difference between me being able to do the setup on my street driven car or not without pulling timing like crazy / risking detonation.

i just want to know which is probably more accurate of an estimate, or if anyone has actually tested the setup already before i waste quite a bit of time/money

 

I have more confidence in the zeal calculator, but if you are seeing differences as big as you reported then I would start looking into the assumptions behind the inputs.

 

zeal

 

yeah, its confusing because they all seem to give the same ratio for the setup with the pm6 pistons, but Zeal gives me the super high (12.67:1) estimate for the pm3 pistons while the other 2 sites are in the low 11's

 

I'm actually partial to the c-speed calculator. With the exception of the CTR piston calculations (which were re-done and in this forum if you search) I found the rest of it's calculations to be fairly accurate, certainly more than Zeal.

 

the diffrence between the three calc's is not only the dome height, but also the piston to deck height, remember the compression height of the two pistons is diffrent and the new piston to deck height is -.7 mm or -.028 inches. the zeal takes this into account, and the other two don't, you have to cahnge these numbers yourself. I trust the zeal calc personally.

 

all very nice compression calculators...good post

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00Red_SiR &raquo;</TD></TR><TR><TD CLASS="quote">I found the rest of it's calculations to be fairly accurate, certainly more than Zeal.</TD></TR></TABLE>

Ok, how did you come to this determination? Did you cc'd the chamber?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00Red_SiR &raquo;</TD></TR><TR><TD CLASS="quote">I'm actually partial to the c-speed calculator. With the exception of the CTR piston calculations (which were re-done and in this forum if you search) I found the rest of it's calculations to be fairly accurate, certainly more than Zeal.</TD></TR></TABLE>


ahhhhh i had ran these numbers several times last night, but i think due to the time of night i must have run the wrong numbers for the deck height/rod length/stroke.

Now i get the same numbers you've got, with a new piston to deck height of -.028

running new numbers, it looks like the only way to get the compression down to a reasonable range and still use the y8 head would be to run a thick headgasket (like .07" thick) which just sounds absolutely ridiculous

Should i just take the 10.6:1 estimate and be happy?


Modified by amthompson at 5:06 PM 3/11/2007

 

the other thing im kindof debating is to stay with the pm6 pistons and shave the head down 10-20 thousandths which would bring me to where i want to be.

 

I dont believe in any of those calculators, they come up 13.7 to 1 compression on my A6 motor yet it runs on 93 pump no problem.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mar778c &raquo;</TD></TR><TR><TD CLASS="quote">

Ok, how did you come to this determination? Did you cc'd the chamber?</TD></TR></TABLE>

Yes, that's exactly what I did and then I created a post about how to use these compression calculators properly. When I cc'd my pr3 head it came out to be 44cc which was greater than the 42.7 stated in the calculator. The fact is that a brand new pr3 head could very well be roughly 42.7cc but with production variations, wear and tear on the head, number of valve jobs, type of spark plug used, the actual number of cc's in a combustion chamber could vary by 1 or 2 easily.

The point is that for me, I find the c-speed calculator easy to use because I know HOW to use it and know what numbers to change and when. I also know that for the sake of accuracy I will take the time to properly measure the clearances so I can imput the most accurate numbers and get the most accurate answers.

Like any tool, it's only as good as the person using it and in this case, the information entered into it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00Red_SiR &raquo;</TD></TR><TR><TD CLASS="quote">Yes, that's exactly what I did and then I created a post about how to use these compression calculators properly. When I cc'd my pr3 head it came out to be 44cc which was greater than the 42.7 stated in the calculator. The fact is that a brand new pr3 head could very well be roughly 42.7cc but with production variations, wear and tear on the head, number of valve jobs, type of spark plug used, the actual number of cc's in a combustion chamber could vary by 1 or 2 easily.

The point is that for me, I find the c-speed calculator easy to use because I know HOW to use it and know what numbers to change and when. I also know that for the sake of accuracy I will take the time to properly measure the clearances so I can imput the most accurate numbers and get the most accurate answers.

Like any tool, it's only as good as the person using it and in this case, the information entered into it.</TD></TR></TABLE>

Thanks for the help, i should probably do that to have a more solid idea of what im working with.
What did it cost you to have your chambers cc'd?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by amthompson &raquo;</TD></TR><TR><TD CLASS="quote">

Thanks for the help, i should probably do that to have a more solid idea of what im working with.
What did it cost you to have your chambers cc'd?</TD></TR></TABLE>

It only cost me $30 to have my chambers cc'd but check with your local engine/machine shop for pricing as I'm sure it will vary.