Takumi Fujiwara

Alright, I'm about to rip all the hair out of my head. The online calculators are not matching up with the "correct" CR I received from a vendor of these pistons... and my calculations on sheets of paper are not matching up either!

So if you have the time, please calculate the compression for me and if it matches 11.4:1 then please post what formula you used and how you came about that answer.

Specs:
B18C1 Block & Crank + ITR Head + 3-layer OEM Headgasket:
81.00mm (Standard Bore)
42.70cc (Combustion Chamber Volume)
87.20cc (Stroke)
4.1cc (Piston Dome Displacement)
.006" (Piston to deck height)
.0290" (3-layer OEM headgasket thickness)

ludesleep

i got 9.96:1 let me double check my work

ludesleep

on the deck is it in the hole or out?

ludesleep

from the measurements you have listed it comes out to 9.967:1CR. Now this is assuming that your deck height is (+).

Takumi Fujiwara

I believe it's "IN" the hole.

Do cylinder specs usually not denote whether the piston to deck clearance is actually in the hole or not?

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">from the measurements you have listed it comes out to 9.967:1CR. Now this is assuming that your deck height is (+).</TD></TR></TABLE>

So now you've calculated a number that is different even from the other's I've got.
My calc = 12.7:1
1 builder = 10.7:1
CP vendor = 11.4:1 (Correct value)

Why is this compression calculation so complicated? It's just an equation... why so many different results?

ludesleep

what eguation are you using...

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">what eguation are you using...
</TD></TR></TABLE>

Static CR = Vol. @ BDC / Vol. @ TDC

BDC = Swept Vol. + Cyl. Head Vol. + Headgasket Vol. - Piston Dome Vol.
TDC = Cyl. Head Vol. + Headgasket Vol. - Piston Dome Vol.

Swept Vol. = [Pi (BORE^2) Stroke]/4000
Headgasket Volume = (.0290 + .006)25.4

What are you using?

ludesleep

ok you lied your piston is out of the hole not in...your sitting at 12.686:1 or 12.69:1 with 108.97 Cubic Inch Displacement

ludesleep

I use V1+V2+V3+V4+v5 / V2+V3+V4+V5
where as
V1=(Bore)square x stroke x .7854 x 16.39
V2=Combustion Chamber cc
V3=(Bore)square x Gasket x .7854 x 16.39
V4=(Bore)square x Deck(+/-) x .7854 x 16.39
V5=Piston dome(-) Dish (+)


Modified by ludesleep at 2:08 AM 4/11/2006

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">ok you lied your piston is out of the hole not in...your sitting at 12.686:1 or 12.69:1 with 108.97 Cubic Inch Displacement </TD></TR></TABLE>

With the formula I used, I got 12.4:1... which is closer to your number. Maybe I'll email CP with my specs and see what they give me.

The 11.4:1 is from Earl... he helped me pick out which pistons fit my criteria... I just can't seem to recreate it.

More importantly, because I'm having issues, I'll be overboring and installing new pistons... AGAIN... anyway, I can't seem to figure out what CR I will yield. I'm trying to get a couple more points to up it a little more so I can run a more aggressive cam.

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">I use V1+V2+V3+V4+v5 / V2+V3+V4+V5
where as
V1=(Bore)square x stroke x .7854 x 16.39
V2=Combustion Chamber cc
V3=(Bore)square x stroke x Gasket x .7854 x 16.39
V4=(Bore)square x stroke x Deck(+/-) x .7854 x 16.39
V5=Piston dome(-) Dish (+)</TD></TR></TABLE>

Are these all in inches? and is the gasket the thickness? volume? If there is a formal name for this formula, maybe I can google it.

BEER30

me and lude sleep went to the same school and the formula is in inches

V1=(Bore)^2 x stroke x .7854 x 16.39
V2=Combustion Chamber cc
V3=(Bore)^2 x Gasket crushed thickness x .7854 x 16.39
V4=(Bore)^2 x Deck(+/-) x .7854 x 16.39
V5=Piston dome(-) Dish (+)

V1+V2+V3+V4+v5 / V2+V3+V4+V5=compression ratio

V1 to V5 is your swept volume
V2 to V5 is your compressed volume

its pretty simple all you need to do is plug your numbers in
(by the way i got 11.735:1cr)

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SAMGrad &raquo;</TD></TR><TR><TD CLASS="quote">me and lude sleep went to the same school and the formula is in inches

V1=(Bore)^2 x stroke x .7854 x 16.39
V2=Combustion Chamber cc
V3=(Bore)^2 x Gasket crushed thickness x .7854 x 16.39
V4=(Bore)^2 x Deck(+/-) x .7854 x 16.39
V5=Piston dome(-) Dish (+)

V1+V2+V3+V4+v5 / V2+V3+V4+V5=compression ratio

V1 to V5 is your swept volume
V2 to V5 is your compressed volume

its pretty simple all you need to do is plug your numbers in
(by the way i got 11.735:1cr)</TD></TR></TABLE>

LoL, who did better in school? Your numbers don't match! hahaha... nor do anybody's that have done this... online calculators alike.

I'll try that formula and include my number that doesn't match!

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">I use V1+V2+V3+V4+v5 / V2+V3+V4+V5
where as
V1=(Bore)square x stroke x .7854 x 16.39
V2=Combustion Chamber cc
V3=(Bore)square x stroke x Gasket x .7854 x 16.39
V4=(Bore)square x stroke x Deck(+/-) x .7854 x 16.39
V5=Piston dome(-) Dish (+)</TD></TR></TABLE>

What are these numbers? .7854 x 16.39

BEER30

honestly....i dont remember, they drilled the formula into us but only explained the numbers to us a couple of times.............i think 16.39 is what converts it into cc's, as for .7854 i just dont remeber, lol i should have taken better notes

Takumi Fujiwara

OMG!!!

I came out with 11.4

This is what Earl verified as to what my compression should be with these pistons.

I can finally sleep.


Modified by Takumi Fujiwara at 3:21 AM 4/10/2006

Takumi Fujiwara

Trying to calculate compression for other piston options and I ran across the following issue. The above calculations were with a Compression Height of 1.195.

These other pistons I'm trying to calculate compression for have a CH of 1.181. I assume this will affect Piston to Deck Height. Anyone care to explain this a bit further so I don't add/subtract this value in the wrong direction.

One more thing, I used -4.1cc (negative value) Combustion Chamber Volume because these are domed pistons. The spec sheet for CP Pistons actually list this as a postive 4.1cc.

Is this correct that I did this? Domed pistons will actually reduce the chamber volume as dish pistons will increase chamber volume... which is why I did this.

I'm finally going to sleep now. Someone help me.

ludesleep

dish is (+) dome is (-)

Takumi Fujiwara

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ludesleep &raquo;</TD></TR><TR><TD CLASS="quote">dish is (+) dome is (-)</TD></TR></TABLE>

That's good to hear some verification on the subject.

Takumi Fujiwara

Quick question:

The numbers I came up with were with a compression height of 1.195" and .006" deck clearance (PTDH)....

Now, please let me know if this line of thinking is correct:
If the compression height is 1.181, it has 0.014" LESS compression height than the above with 1.195" CH. Is this a direct correlation to Piston to Deck Height?

Since .006" was the PTDH for the 1.195" CH pistons, is adding the .014 to .006 correct? My assumption is that for the 1.181CH pistons, the NEW PTDH that should be used is 0.020"

It seems logical to me, but please confirm. Other info that led me to believe this may be correct is the fact that the 1.195" CH pistons are for an ITR & these 1.181" CH pistons are for a GSR... which in stock form would make sense since the GSR head gives less room for the pistons.