Comp Calc.

[Cubes]

I posted this here out of PM's so all can see and possibly understand how to work out a comp ratio.

Before you read on have a quick look at this link those of you who don't understand how to work out a comp ratio, it will help you know what some of the measurements are. http://www.musclemotorparts.com/comp_ht.htm

One not mentioned in the link is deck clearance. This is measured when the piston is at TDC (as high as it will go in the bore) you then measure how far down the piston is sitting from the deck (top of the block)

Squish/quench is the deck clearance plus the thickness of the headgasket. This is not related to compression ratio however it is important as it prevents detonation, increases fuel economy, power and the general efficiency of the engine. Google a little on Quench and or Squish. Lots and lots of info.

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The Wiseco catalogue is a little strange in a few area's, which SK also pointed out. The head cc's are a little high (mine cc'd up to 62.2cc) & the final comp they state are .3 of a cr lower to what I work it out to be.

ok on to the specs.

In the catalogue the specified RB30DET pistons have a compression height of 1.260" which is the measurement from the center of the pin to the top deck of the piston. It states the deck clearance is .040" with that setup.

The RB25DET pistons have a compression height of 1.240" which means the piston sits over the rod 0.020". Hence lower in the bore by 0.020".

The builder decked the block by 0.020". Which theoretically means the piston will be sitting in the bore the same height from the deck as what the RB30DET piston would have (0.040"). Am I on the right track?

The head gasket I am using is 100% a .050" Cometic MLS HP. They do not compress once torqued.

When I punch these figures in to my Engine CR program it states I am going to have a 9.2:1 CR!. The builder measured the comp ratio by doing a dummy setup then measuring how far the piston went down the bore or something?!? May his tools are out a little. The squish/quench would obviously be .040" + .050" = .090".

The builder stated I will have an 8.2:1cr. The only way I could have an 8.2:1 cr is if the deck clearance was 0.1" which is 2.5mm down the bore.. Not good for performance & economy. I did a few little drawings. If the piston was sitting 2.5mm down the bore + the 1.25mm headgasket this would have the dome sitting basically level with the base of the head or level with the top of the head gasket. I've pulled a spark plug out and well it doesn't look that way at all.

The more I think about it the more I confuse my self.

Steve, If you read this could you post your details with regards to you headgasket thickness & expected CR? Is it a 1.6mm head gasket which gave u an 8.6:1cr?

[DoughBoy]

It's one thing to theorise about clearances and such, and another thing to what the tolerances are when the physical motor is built. With all the manufacturing allowances it's possible to get variations in parts that cannot be accounted for in pure theory. i.e. a deck height .05" more than it's "supposed" to be. This could be due to the the crank centerline being machined on a lower tolerance, the block being slightly larger, or the rods or pistons not being up to their spec etc.etc.

Those "RB30" pistons of Wiseco's are shocking, there is no way a dish of 11cc's can get you anywhere near a decent CR with a twin cam head.

[Cubes]

Still 2.5mm (0.10") in total is a hell of a lot when you are talking bearing clearances etc. But then what would I know, I don't build motors.

Yer they are shocker's having a 11cc dished piston.

Only way to find out for sure.

Slap it all down on paper and ask him.

He's easy to approach and makes time to explain things so you are happy.

[T0nyGTSt]

Hey so does anyone know about the CR with GT-R pistons in an RB25?

T.

[Cubes]

I spoke to Adelaide engine developments.

They did mention the comp ratio gets a little sloppy with GTR pistons in both a RB30 and a RB25.

Once again due to the compression height being a little lower.

The RB26 pistons sit lower down the bore by 0.059" which is around 1.475mm.

Unless you shaved around 1.475mm off the rb25 block with the gtr pistons in it you would have a quench that doesn't work. Hence the engine will make less power, be less responsive (especially down low), use more fuel etc..