Checking compression ratio's...

[J1MS]

The cylinderhead pictured is sold as having a 9.5-1 compression ratio. When bolted to an engine & checked for combustion volume it was found to be much higher, set with a 1.5mm squish clearance it had a volume of 22.1cc with the piston at TDC. TS1 225
set with a 1.2mm squish clearance it was just over 21cc

This is a compression ratio of around 11-1 or 11.5-1 running on the 1.2mm clearance

I want this engine to run at a compression ratio of 9.5-1 & with the tighter squish clearance of 1.2mm, so it can be more easily run on unleaded fuel and use as much of that fuel to make power...
How the volume was checked;
I used a Burette, this is an accurate measuring device, marked in increments of tenths of a cc/ml. As can be seen in the picture below,

I use a mix of two stroke oil and petrol 60/40, then set the cylinder up so that the spark plug is at the highest point then open the valve on the burette to let the oil mix flow into the plug hole, so its filled the combustion chamber to the first thread of the spark plug. as in picture below...

To lower the compression ratio the combustion chamber needs to be deepened or enlarged but without reducing the squish area (Too much) as this is part of the cooling for the piston crown...
The best way (IMO) to remove material is by hand with a broad tungsten or HSS cutter then buff and polish I need a combustion volume when measured of 26.2. picture below after removing nearly 5cc of alloy..


Then buffed and polished a little to prevent carbon build up



bolted back to the engine squish set at 1.2mm then checked with the Burette again and a reading taken as in the picture below...
The reading is the amount that has been run out of the Burette not the remaining amount...

...

The same can be done with a syringe but you will not normally get as accurate a reading as with the burette but its much better than just believing what your told, as this is the fifth head I have fitted in the last two years with a claimed ratio of 9.5-1 that is way out...

Only posted this, as I have been asked many times how do you check the compression ratio and what do you do to alter it (lowered) if needed... I hope this explains how it can be checked and what can be done to alter it...

[sideswipe]

Excellent work J1MS, thats helped me out loads.

[Mel K]

So........what your saying is, J1ms, you spent yesterday in the shed polishing the head off it Beautiful work btw and that engine looks like it could get someone to Spain and back........C U in a bit

[oldbiker]

Some of the respected dealers will argue that you can`t always get the compression ratio and squish ratio that you want just because you ask for it.
I personaly think that a lot of the dealers simply either don`t know or don`t care what they are selling. Although a couple do

I think you will find that all the best tuners do this sort of work on their own engines, after all it`s all common sense and old hat, that has been written about since the days of roy bacon

and that head looks nice

[J1MS]

^^^Thanks^^^

[davidblythe]

that looks cracking

any chance of a pic of tool used

[RinB]

J1ms

Cracking post & Pics

Im always doing this myself as people bring me kits and heads to go on and when you measure there way out. I do the same as you shown

The head you show top pic is defo not a 9.5cr head the combustion chamber isnt the right size or shape from a known AF 9.5cr head. Is the head from AF if so they should know better.

Ive used loads of their 9.5CR heads and Ive never have had a wrong one yet.

Intresting