22 replies to this topic

[albo]

He has a 998

post jumpers who dont read clearly,then jump to the wrong conclusion,but i guess that what keeps this forum of interest

[Mrpeanut]

Can someone please help me on how to do the compression ratio calculation please! That last post has completly confused me, even better if you could just sum up what i need for flat top pistons, 0.60

What CR you need I can't help with, neither can I help with calculating.

Cooperman has posted on how to do it, and AC Dodd I believe. Perhaps pm them?

All I know (from expensive experience!) Is that 5cc in a head makes a massive difference and you have to get it right.

[timmy850]

OK a 998 bored out +060 has a bore of 66.088mm

Stroke is 76.2mm

 

Which makes the single cylinder capacity 261.36cc

 

Piston Dish 0 cc

Head volume 23cc

Head Gasket approx 4 cc including the ring land vol.

 Which makes the combustion chamber approximately 27cc in total

 

That'd give you a CR of about 10.7:1

 

But using such big valves, you'd need to mill out an area to accommodate the valves, which would increase your combustion chamber volume and reduce your CR. 

 

As Cooperman said, with such big valves, you won't need the extra lift from roller rockers.

 

Edit:

I've used this spreadsheet for working this out. You'll just need to remember the stroke of a 998 (76.2mm), and the bores are the same as 1098 engines

http://mk1-performan...web/cr_calc.xls

Edited by timmy850, 04 November 2014 - 11:25 PM.

[Mrpeanut]

He has a 998

post jumpers who dont read clearly,then jump to the wrong conclusion,but i guess that what keeps this forum of interest

??

Surely 5cc in a head matters in a 998?

[timmy850]

 

 

He has a 998

post jumpers who dont read clearly,then jump to the wrong conclusion,but i guess that what keeps this forum of interest

??

Surely 5cc in a head matters in a 998?

 

He is offsetting the 5cc in head volume with flat top or dished pistons. this would keep overall combustion chamber volume the same

[Mrpeanut]

 

 

He has a 998

post jumpers who dont read clearly,then jump to the wrong conclusion,but i guess that what keeps this forum of interest

??

Surely 5cc in a head matters in a 998?
 

He is offsetting the 5cc in head volume with flat top or dished pistons. this would keep overall combustion chamber volume the same

I shall now go to bed!

[kyle9832]

OK a 998 bored out +060 has a bore of 66.088mm

Stroke is 76.2mm

 

Which makes the single cylinder capacity 261.36cc

 

Piston Dish 0 cc

Head volume 23cc

Head Gasket approx 4 cc including the ring land vol.

 Which makes the combustion chamber approximately 27cc in total

 

That'd give you a CR of about 10.7:1

 

But using such big valves, you'd need to mill out an area to accommodate the valves, which would increase your combustion chamber volume and reduce your CR. 

 

As Cooperman said, with such big valves, you won't need the extra lift from roller rockers.

 

Edit:

I've used this spreadsheet for working this out. You'll just need to remember the stroke of a 998 (76.2mm), and the bores are the same as 1098 engines

http://mk1-performan...web/cr_calc.xls

brilliant help thankyou! im going with 33x29 valves now as they were too big, then flat pistons with 23cc chamber, from the calculation you gave me and from the excel document, this should work well then?

Edited by kyle9832, 05 November 2014 - 06:08 PM.

[Cooperman]

You still need to measure and finally machine after doing the calculations for CR.

The piston top will not come right to the block deck level at TDC and with a 998 each 0.010" down the bore is around 0.9 cc. I've seen pistons sit over 0.020" down the bores at TDC.

Do the calculation. If you are capable of doing the build you must be capable of doing the measuring and the maths. Then you will KNOW it is correct.