Jump to content
SAU Community

Rb30Det Compression Test


ZachTierney
 Share

Recommended Posts

i got 135ish across the lot which i thought was a little low, but it seems to be the norm, im running a fully built engine using cp flat tops, an a standard 26 head that has been fully reconditioned buy advan, only mods to perfomance side if the heas is adjustable cam gears

Link to comment
Share on other sites

i got 135ish across the lot which i thought was a little low, but it seems to be the norm, im running a fully built engine using cp flat tops, an a standard 26 head that has been fully reconditioned buy advan, only mods to perfomance side if the heas is adjustable cam gears

Well its reassuring to know I am not alone but in hindsight I would have preferred to get the 9:1 pistons but they were not available in standard size and would have involved a lot more expense. 9;1 pistons with a Neo head would be great and with a 26 head even better - maybe next time!

So what power are you making with your low compression and 26 head?

Link to comment
Share on other sites

the pistons dont make the compression ratio

bore x stroke = SV

piston to deck height + gasket bore x gasket thickness (+ piston dish/ valve releif) or (- piston dome) + chamber cc = CV

sv + cv ÷ vc = static compression ratio

Link to comment
Share on other sites

the pistons dont make the compression ratio

bore x stroke = SV

piston to deck height + gasket bore x gasket thickness (+ piston dish/ valve releif) or (- piston dome) + chamber cc = CV

sv + cv ÷ vc = static compression ratio

Either some piston suppliers are telling porkies or you're mistaken. If you have two pistons with generally the same dimensions and pin position etc then the one with a domed or raised part (various shapes are used sometimes with allowance made for the valves) crown will have a higher compression ratio - the swept volume will be the same but the volume of the chamber will be reduced by the amount the crown intrudes into it ergo higher static CR.
Link to comment
Share on other sites

in my trade i need to know how to do things properly

piston manufacturers will state "x:1" compression similar to cam manufaturers saying stage or type cams

they assume your measurements and specs will match theirs

they also assume the person who assembles will measure it first

here is the actual formula in full

Swept Volume

Bore x Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x stroke = Answer3

Answer3 ÷ 1000 = Swept Volume ( SV )

Compression Volume

Head CC +

Piston dome - / dish volume +

Deck height (piston below the deck)

Bore x Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x block deck to piston crown measurement = Answer3

Answer3 ÷ 1000 = deck height volume

Deck height (piston above the deck)

piston measurement above top ring land x piston measurement above top ring land = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x block deck to piston crown measurement = Answer3

Answer3 ÷ 1000 = deck height volume

Gasket

Gasket Bore x Gasket Bore = Answer1

Answer1 ÷ 4 = Answer2

Answer2 x π (3.142) x Gasket thickness = Answer3

Answer3 ÷ 1000 = Gasket volume

The Maths

Head CC + gasket Volume + Piston dish Volume(- Piston dome volume ) + deck height (-if piston is above the deck)= CV

SV + CV + Answer

Answer ÷ CV = Compression ratio

Example

my motor

87 x 87 = 7,569

÷ 4 = 1,892.25

x 3.142 x 85 = 505,363.2075

÷ 1000 = 505.36321 (sv)

Head CC 53.4

Piston Dome -10cc

deck height, piston above

0.005" ÷ 0.03837 = 0.127

86.1 x 86.1 = 7413.21

÷ 4 = 1853.3025

x 3.142 x 0.127 = 739.53071

÷ 1000 = -0.73953

head gasket

88 x 88 = 7,744

÷ 4 = 1,936

x 3.142 x 1.5 = 9,124.368

÷1000 = +9.12437

53.4 + 9.12437 - 10 - 0.73953 = 51.78484 CV

51.78484 + 505.36321 = 557.14805

÷ 51.78484= 10.7589 aka 10.75:1

Link to comment
Share on other sites

Just so you don't confuse the punters you will agree that you can use the tables supplied by the piston manufacturers/sellers to work out which will give the higher compression ratio e.g. Spool sell pistons for the RB30/26 - one labelled 8.2:1 CR and one labelled 9:1 CR and the latter will result in a higher static CR which is all that I was saying (I wish I had the higher compression i.e. 9:1 pistons).

You are quite correct to say that the resulting static CR will not necessarily be exactly 9:1 as there are many variables but you will agree that just changing the piston specs can increase the CR.

Link to comment
Share on other sites

i will NOT agree in saying that to buy a piston based on its stated compression ratio will ever be true,

I will also NOT agree to use their tables, that is guessing, i will actually use measurements that i personally measure myself, this eliminates variables, which when a company says that it has too many variables it means "they have no clue they just bought them from someone else" or "check everything is right cause we just told you how we are not liable for you purchasing the wrong pistons if you dont measure everything"

the formula is used with measurements taken from the piston, block, head gasket and cylinder head that those pistons are going into, not just 1 item, not someone elses parts or measurements.

This is the only right way, its only "too many variables" if there is a lazy engine builder or a company avoiding liability, why spend big money to guess?

sure the pistons with the higher comp ratio stated might be true in saying it will be higher, thats still a guess as to how much higher

can you garantee the head chamber isnt 61cc's where it should be 64? or is it 68 instead?

1mm head gasket not available but a 1.8mm is?

piston sitting 10 thou above before its been ground during a dummy assemble, was it ground and tunnel bored before?

you measure and calculate your comp. dont read what is on the sheet in the box

with engine building your either precisely spot on or your wrong and something will f**k up eventually

Link to comment
Share on other sites

very interesting guys, so many factors involved to build a quality engine, at the moment i am on a sort of run in tune. eg: ive done a bit of running in on a softer tune but now im doing some more with a slightly more aggressive tune, all though still a very soft tune... anyhow im makeing 317rwkw on 17/18 psi, as soon as i get some more cash im going to play around with my fuel system setup, prob get a larger surge tank an run a areomotive a1000 + change my fuel lines to teflon speedflo, after that im hoping to make at least 400kw

Link to comment
Share on other sites

This is getting tedious but I hate being misrepresented.

I never said that the stated CR on a box of pistons was 100% accurate. I just said that you could use the manufacturer's listed CRs as a guide.

Your impressively long formula will not give a 100% accurate CR either because of the irregular shape of most compression chambers and some piston crowns and the variations in the compressibility of the head gaskets etc. The correct measurement is actually quite simple if the spark plug hole is at the top centre.

With the engine fully assembled put the piston at TDC. Using a pipette measure the amount of liquid needed to fill the combustion chamber and subtract an amount equal to the volume of the part of the spark plug if any that protrudes into the combustion chamber (quite easy to determine). Call this volume x

Take the piston to BDC and measure the additional amount of liquid required to fill the cylinder to the top of the combustion chamber. Call this volume y.

The static CR is (x+y)divided by x

The product of this calculation will be sufficiently accurate for most purposes.

Link to comment
Share on other sites

very interesting guys, so many factors involved to build a quality engine, at the moment i am on a sort of run in tune. eg: ive done a bit of running in on a softer tune but now im doing some more with a slightly more aggressive tune, all though still a very soft tune... anyhow im makeing 317rwkw on 17/18 psi, as soon as i get some more cash im going to play around with my fuel system setup, prob get a larger surge tank an run a areomotive a1000 + change my fuel lines to teflon speedflo, after that im hoping to make at least 400kw

Well you're doing better than me - so far I only have 230awkw at 15psi. I have a GT3540 with .63AR. I will be getting it retuned soon to whatever it will take - probably around 25psi and I iwll be happy if I crack 300awkw. If you have a similar compression and turbo your extra power must be down to the RB26 head or do you have twins?
Link to comment
Share on other sites

using it as a guide is still wrong, you know how many people argue that i cant have 10:75:1 because someone else has a dome thats bigger then the one on my piston?

but yeas, i agree there, that is another more correct method and that would be just as accurate as measuring everything before it is put together, X is your CV and Y is your SV+CV, they do that with alot of the race cars to make sure they are within spec of the rule book

using a burrette you can also measure a combustion chamber or an eye dropper if you dont have one 1mm = 1cc

same with the piston, if its a dome, use play dough

and you can actually measure the squash of a head gasket with the right measuring tools, average it is about 10 thou, but that is a guess i still wont use

lol shall we go into dynamic compression and volumetric efficency?

sorry about whoring your thread zach

Link to comment
Share on other sites

yea havent comp teseted my 26/30 yet, havent had the need and cbf. but when i had a 25/30 comp was 160-165. standard pistons new rings.

and yea cams, 270s and up with a 3litre let em breath

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share



×
×
  • Create New...