Well my builder called me up today and told me a comp. test returned a result of around 16.0:1 (high 15's) The really stange thing is,all the hardware isnt that agressive. He thinks its the TODA pistons might be the reason,but they're only 87.5 mm. Other new parts include TODA valve springs,f22 retainers, darton sleeves and a cometic head gasket to match. (The head gasket is pretty much an OEM replacment)

He's gonna call TODA up and see what they've got to say. Just thought i'd ask incase anyone's got any ideas. Even with a thiker head gasket it would probably only lower to around 13.0:1 mark.

Any thoughts?

 

With that kind of ratio, the cylinders must be over 300 lbs per square inch.

 

Compression test measuurew the pressure in units ( psi etc) whereas compression ratio is the ratio of : (cylinder swept volume+combustion chamber volume) / combustion chamber volume , and is a number without units (ratio) . Your builder probably meassured the cranking compression of your engine using the bar scale thus getting high 15s , which is around 230 psi which is ok for a brand new high compression piston buld f20

 

The normal way of measuring the compression ratio is using liquid and a burette. The combustion chamber in the head is measured using a perspex plate with a hole in it. The space is filled with kero or similar liquid to determine the CC (cubic centimeters). If the pistons have a raised dome you move the crank so they are a known amount down the bore then measure the CC's and allow for how much they are down the bore in your calculations.

The compression ratio is calculated from this info also allowing for the bore/stroke and head gasket thickness.

An increase in bore will increase the compression ratio slightly. The main influence will be changes in stroke, piston crown volume and head gasket.

My psychic diagnosis is that the Toda pistons have a larger dome volume the standard, at that machining of both the top of the block during fitting of the sleeves, and underside of the head have contributed to the compression ratio.

I would check the piston to head clearance. 0.8mm is the minimum I would suggest. If its less than 0.5mm you have to get a thicker head gasket as the piston could hit the head on the over run when the rods stretch.

If the piston to head clearance is OK you can still use a thicker head gasket, or machine the piston crowns or open out the combustion chamber to get the compression you want.

Chris.


PS, you can also measure compression ratio with the head on by putting the piston at TDC on the firing stroke and filling the chamber till the liquid appears at the bottom of the spark plug thread. If you know the bore/stroke you can calculate the compression ratio.

 

I agree this is a test that measures the pressure in the combustion chamber not a compression ratio.

230-235 psi sounds fine for a compresion test.

The compresion ratio is the total volume of the cylinder and spherical portion of the head at BDC divided by the remaing volume in the cylinder including the spherical portion of the head at TDC.