Compression Ratio &how to burette ('cc') the head/pistons

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Compression Ratio &how to burette ('cc') the head/pistons

Post by Guy Croft » July 7th, 2006, 10:39 am

You use a burette to measure fluid volume of combustion chamber and piston dome/valve reliefs. So that you can calculate the compression ratio of the engine (High compression - HC for short might be 9/1, 10/1 or higher, low compression - LC might be 6.8/1, 7.2/1, 8/1 etc).

Any firm that supplies school laboratories can supply a burette, plastic or glass. I use turpentine substitute with a bit of auto trans fluid (ATF) to give colour or paraffin. The plastic plate for measuring is perspex with one fill hole and one air vent, you have to tilt the piston/head slightly to let the air out. The plate has to be stuck/sealed down with grease. If you don't use a plate over the head meniscus effect will spoil your reading, even assuming the fluid doesn't run all over the place.

A domed piston can be cc'd in the bore, just grease around the diameter. The dome is the volume of fluid to fill subtracted from the volume of the cylinder from block deck to piston crown, minus valve reliefs. With a piston with 'bowl' or 'dish' (usually but not always found on supercharged or turbocharged engine) you just stick the perspex plate on top of the piston crown.

The compression ratio (CR for short) is:

CR = 1 + (Vs/Vc)

where Vs is the swept volume of the cylinder
Vc is the clearance volume - the space in the chamber above the piston at TDC

Two Excel calclators are attached, simply put in in the yellow rows the desired CR and bore stroke, etc

Piston GC 131 16v cc.JPG
To assess this HC 16v piston dome volume, I taped card round the piston, sealed off with grease and filled with fluid.
Piston GC 131 16v cc.JPG (12.66 KiB) Viewed 14551 times
MH head cc.JPG
Buretting the combustion chamber, head is tilted slightly. Valve seats have a bit of grease on them and the spark plug is fitted. Volume on this TC 8v head is 53cc.
MH head cc.JPG (25.16 KiB) Viewed 14551 times
MH piston clearance_01.JPG
An HC piston intruder dome goes right into the combustion chamber, hence the name.
MH piston clearance_01.JPG (18.83 KiB) Viewed 14550 times
MH piston_head check.JPG
The volume between the 'top' of the piston (in fact down to the top ring) and the combustion chamber is called the 'clearance' volume and determines how high the CR will be.
MH piston_head check.JPG (17.68 KiB) Viewed 14547 times
CR basic master for website 8v.xls
(16.5 KiB) Downloaded 1285 times
CR basic master for website 16v.xls
(18 KiB) Downloaded 893 times


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