What Cr Ratio

[johnnyt]

hey guys im i have a rb26 that is being machined and im trying to figure out if im going to run into problems with CR ratios.

what i have is

supertech 87.5mm forged pistons

eagle h beam rods

about 12thou machined of the head

and the block is being decked (so we can run the cometic gasket)

1.2mm cometic mls gasket

info on the piston spec sheet is

bore size-3.4055/86.50mm

compression height -1.181 (30.00mm)

dome height/dish depth -+0.1378(+3.5mm)

dome/dish cc's - +15.5cc

is there a program i can use to suss out what cr ratio i will have once i find out how much is taken off the block?

and am i going to run into isssues with piston to head clearances?

thanks guys

johnny

[Adriano]

I would recommend dummy assembling the bottom end, and measureing the piston deck height, and cc the combustion chambers, its the only way to get accurate results. Then you can order the appropriate headgasket to suit your compression/squish.

[ovabooster]

whats the better option....to pick a gasket thickness to suit your desired comp ratio....or deck the head instead? Providing both options use the same pistions?

I have wiseco 40thou and 1.2mm gasket...going by their website, this will give comp in the 8.3 regoin....But i have a NEO which i heard have bigger combustion chambers.....giving even lower comp?? Shoudn't have any problems with decking the head?

[r33_racer]

I think its the other way round ovabooster, neo heads have smaller chambers. So that would result in a higher cr. Unless im getting mixed up.

cr calc http://www.csgnetwork.com/compcalc.html

Edited August 24, 2007 by r33_racer

[ovabooster]

lol mayb i am? that set aside.........regardless of if its smaller/larger.....whats the best option?

[r33_racer]

Depends on application. Alot of people use the headgasket as a way of achieving their desired comp ratio, which i dont think is the best way to go because you stuff up your squish. Really you should be using the pistons and machining to achieve what you want and keeping a similiar sized gasket to achieve a good squish of 40-50 thou. Its all based on everyones own personal opinion. Need to be careful how much you deck too, alot of builders dont like the piston protruding out of the bore and into the chamber/gasket a great deal. Plus piston to valve clearance and blah blah blah. More then meets the eye.

Find out roughly what your chamer cc's up to and put in your piston specs and whatnot into the calculator and work it out. But first what cr do you want to achieve?

[ovabooster]

safe and reliable is the goal....as with anything....its for a t04z with 1.x a/r.....Fully built and wana smash max 2bar...Lag is the factor so want it as high as possible to compensate...thinking 8.8??? options?

[r33_racer]

yeh i would be aiming for 9:1 but 8.8 would be close enough i guess. Alot of time when you work it out it could be out by .2 either way especially if the chambers are not cc'd and just guessing based off std info. If you have your piston spec sheet, you should be able to work it out with the calculator. Just play with the values until you get what you want and then you will know whether you wanna change gasket or machine.

[Spoolup]

There is no guessing when it come to compression ratios. There are so many variables involved. So many people buy pistons off the shelf and believe they will end up with the comp ratio that comes with the piston spec sheet.

There is only one way to calculate it ( it is time consuming and complicated ) and that is why most people and many engine builders evoid it.

Cc'ing piston dome / dish volumes, deck heights ( positive or negative ) , piston valve reliefs, compressed head gaskets ( not the stated thickness ), combustion chambers.

So you can guess, estimate and calculate - but unless you have all those precise measurements you are just walking and talking blind.

[Adriano]

Hi, spoolup, just wondering how you go about getting head gasket thickness