Ideal compression ratio for my build??

[kemicalx]

Hey guys,

I'm at the point in my build where i have to order the copper head gasket..

Engine has the following,

Ross pistons,
Copper O-ringed block,
Standalone ignition/fuel control,
57 Trim running 16-18psi..

now the stock compression ratio is 8.4:1 so i presumed i should get as close as i can to that without exceeding 2.0mm in the gasket as any thicker and the o-rings will lose some of their effect as a fire ring. I've worked out my compression ratio as follows:

Basic measurements (Formulas used below)
Bore Diameter = 83.5mm
Stroke = 91mm
Bore Radius = 41.75mm
Piston Volume = 19cc
Deck Clearance = -2.8cc
Combustion Chamber Volume = 37.5cc
2.0mm x 84mm Head Gasket Volume = 11.08cc

Cylinder Volume
Bore size = 83.5mm
Stroke = 91mm
Bore Radius = 41.75mm
Volume of Cylinder = (pi*radius*radius*height)/1000
(3.1416*41.75*41.75*91)/1000 = 498.316cc
Total Bottom End Capacity = (498.316cc*6 = 2989.896cc)

Gasket Volume
Gasket thickness = 2.00mm
Bore size = 84mm
Volume of head gasket = (pi*radius*radius*height)/1000
(3.1416*42*42*2)/1000 = 11.08cc

Deck Clearance
Piston height = 0.5mm
Bore Size = 83.5mm
Volume of Exposed Piston = (pi*radius*radius*height)/1000
(3.1416*41.75*41.75*0.5)/1000 = 2.74cc

Compression Ratio
CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume

CR = (D + PV + DC + G + CC) / (PV + DC + G + CC)
8.685:1 = (498.316cc+19cc-2.74cc+11.08cc+37.5cc)/(19cc-2.74cc+11.08cc+37.5cc) = (563.156)/(64.84)

Now i was pretty happy with that.. a little higher than stock but not excessive. However i have been advised by another forum member to run a 1.2mm gasket and get the static compression ratio to about 9.2?

To quote him:

I personally wouldn't recommend a Head Gasket thickness of over 1.5mm at the extreme end of Boost builds......... Don't be surprised if the Engine doesn't make the power it should at the relative Boost pressure. Its okay if your never going to tune it on VP Import / C16 or even Shell 100 but if you are, then consider getting a 1.2mm Head gasket.

Also depending on your camshafts timing, your static CR is actually something not to worry about. You can seriously go as high as 10:1 on a 100Hp per cyl setup running 98 or 100 Ron Fuel.

Buy the 1.2mm Copper Head gasket and be happy.

What is boost ? Its the back-pressure in your Intake manifold, nothing more nothing less.

What Turbo you using ? If its not bigger then a GT35R then trust me when I say to run a 1.2mm Head gasket.

I couldn't care what others do with their own setups, what I'm telling you is that 10.1 static CR on a 100Hp per cyl using 98 Ron Fuel is considered an 'advanced level' build. So 9.## :1 CR is something that most tuners will have no problems dialling all knock out of it and then some just for fuel quality safety margin.

The only time a professional even considers a 2mm Head gasket is when they are restricted to a particular Fuel but not Boost level (ie 40+ psi).

This has thrown me a fair bit..
Anybody care to throw up what they have run after a build up, ideally when running completely tunable engine management and forged pistons.

Thanks heaps everyone, any help will be greatly appreciated.

 

[kemicalx]

hmm this should probably be in the technical section sorry.. admins feel free to move it.

 

[jdub]

Good luck getting the copper HG to seal...it is difficult to properly O-ring the 7M due to how close the cylinders are to each other. I wish you success.

Concerning compression, static CR is not as important as dynamic CR...this is affected by the cams you use and is what he is referring to. What's being left out is an accurate impact boost has on DCR. Here's a calculator:
http://www.rbracing-rsr.com/comprAdvHD.htm

It doesn't tell the whole story, but gives you an idea of what DCR is for a given SCR + Cam Timing + Boost.

I ran your numbers (8.6:1 SCR + 15 psi boost) using the specs for BC 264 cams (40 ABDC) and came up with 15.8:1 DCR

Up the boost to 18 psi and DCR jumps to 17.4:1

Assuming you used a 2mm HG in your calculations, personally, I would stick with that. It's a lot easier to tune for boost than it is to change CR once the motor is together.

 

[kemicalx]

thanks mate, yer I'm a little concerned with the copper/o-ring job.. to be honest I'm only really concerned about the oil/water gallery sealing.

I personally only really feel confident running close to the stock static CR.. I'm only going to be using the stock cams. And I found nothing wrong with how the engine performed before the build with lower boost levels (~12psi) Presumably it's only going to feel better with more boost and a slightly higher CR?

 

[Rennat]

Im shooting for anything below 9:1 as i have the probe pistons, and supposedly they up the compression a little, i would rather have a lower compression motor so in the end i can run more power on pump gas, i may not have a ton of off boost power, but whatever...

i would say just get the correct head gasket and be done with it... nobody on here o-rings the block, and they've been fine at elevated power levels... i dont even think IJ or Ron R are using o-ringed blocks... and they both have crazy builds.

 

[Buddafucco]

Pics of o-ring block?

 

[kemicalx]

To be honest I wouldn't have used o-rings and copper if the bottom end I'm using didn't already have the receiver grooves milled into the deck. A MLS gasket would have been much simpler, but unfortunately I've gotta work with what I've got.

 

[Buddafucco]

That was quick with the pics! Thanks

 

[kemicalx]

heheh pinched them from my build thread on toymods