If I put 12.5CR 81mm JE Pistons that are made for B18C1 into my B18C5 what CR will I have? 12.5? less? how much?

thx for answers

 

youll have just a tad less. prolly like 12.2-12.3 or so .

 

unless someone already knows the difference between the heads, you have to factor in the combustion chamber size (in CCs), headgasket thickness (or volume), and piston to deck clearance...you already know the bore and stroke. well, here, i'll give you the equation:

V1 + V2 +V3 + V4
_______________ = compression ratio

V2 + V3 + V4

V1 = cylinder volume ... it's bore^2 x stroke x.7854
V2 = deck clearance ... it's bore^2 x deck clearance x .7854
V3 = head gasket volume ... it's (the volume of whatever gasket you're using..should say on the packaging) x .061
V4 = chamber volume minus dome volume ... find out what the dome volume is (should also say on packaging, or call JE), subtract that from your combustion chamber volume, then x .061

that will give you the EXACT compression ratio your engine has. for fun, you could find out your displacement by taking V1 and multiplying by 4 (the number of cylinders you have). that will give you cubic inch displacement. convert to cubic centimeters (take CID and divide by .061) and that will tell you your exact displacement. but you already know its 1797cc it just might be fun to know how that figure is found.

without actually doing any math myself, i'm gonna guess (given your combustion chamber and head gasket and everything else has not be altered) that your CR should be about 13.1:1.

**EDIT: i should mention that that equation is figured for SAE, not metric. so before you mess with multiplying bore and stroke, you have to convert to metric. since you have the measurement in metric and not standard, take your 81mm bore and divide by 25.4. that should give you 3.1889 inches. by doing the same thing with the stroke, your stroke will be 3.4251 inches.


Modified by Train at 5:03 PM 4/1/2004

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by itr206 &raquo;</TD></TR><TR><TD CLASS="quote">youll have just a tad less. prolly like 12.2-12.3 or so . </TD></TR></TABLE>

he should have a higher CR since the ITR head has a smaller combustion chamber, right?

 

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

he should have a higher CR since the ITR head has a smaller combustion chamber, right?</TD></TR></TABLE>

other way around gsr has smaller combustion chamber

 

HMM???
oh, I thought I would be less than 12.5...
Maybe more answers? Some discussion
So... who is right who is wrong?

 

sigh...

- the GSR compression chamber is smaller
- Pistons designed to yield a 12.5:1 compression ratio with a GSR head will yield somewhere around a 12.19:1 compression ratio with a B16/ITR head.

 

I like your answer - thats what I thought

 

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

other way around gsr has smaller combustion chamber</TD></TR></TABLE>

shoot, you're right. my bad. i knew that in the back of my mind, but for some reason i wasn't thinking. k, so yeah....like they said.

but my formula is still right!