The International CBX Owners Association

Afternoon all,
About three years ago I was working on a friends 'special' that had low compression and some electrical issues. For those with a good memory the chassis was a Metisse Mk5 and the electrical problem turned out to be someone had extended the wiring from the ignition stator plate. I was using my compression tester a few days ago and got 130psi on all 4 cylinders of ZXR750, I thought this a bit odd so tested my FTR223 and also got 130psi. Thinking this a bit suspicious I borrowed a friend's tester and got much stronger readings.

My compression tester is a Draper model with a 14mm threaded brass fitting on the end of a flexible hose, identical testers are sold but with different brand names. I use the Draper 60mm adaptor, part number 02150, to give me the thread sizes for bike engines, and here lies the problem. The flexible hose has a non-return valve in the 14mm brass fitting, as the engine turns over the pressure builds up in the hose until it peaks at the same pressure as the cylinder. The adaptor doesn't have a non-return valve so it effectively adds to the combustion chamber volume resulting in lower compression.

My friend's tester uses a much shorter adaptor so the resulting increase in combustion chamber volume is much less. Looking at the 60mm adaptor I reckon it adds 2 - 3cc to the combustion chamber volume.

When you think about how much of the spark plug hole the spark plug occupies adding anything 'above' the spark plug hole adds to the combustion chamber volume and will lower the actual compression. The non-return valve needs to be as close to the head as possible. My friend's tester includes an adaptor tube with a rubber cone end that allows you to press against the plug hole, easy to use but the non-return valve in this case is in the gauge so the volume in the tube adds to the combustion chamber volume. Based on my tests using this arrangement lowers the compression reading by about 12psi.

Using the my friend's short adaptor the ZXR750 the readings were between 170 and 175psi for the 4 cylinders and this was verified with a completely different gauge. According to the Haynes manual the specification for a ZXR750K1 is 145 - 220psi.

Of course I could be talking rubbish........

Here's the adaptor
and here's the Metisse.
Regards,
Chris.

Good info, Chris. Thanks.
Old memory here trying to recall the head volume for the CBX -- thinking it is somewhere about 21cc. Add 2 cc to that and you have a 10% ish change. That's good for a 10 to 15 psi (approx) change depending upon temperatures.
Side note for CBX spec: The manual says 170 spec. That is for a cyl head/block at operating temp of approx 230F. If you are doing compression check with ambient (68F) temp, that equates to something in the lower 130 range.

BTW: Nice bike!!

But a few more revolutions of the motor would negate the additional volume, would it not? The length and diameter of the hose add volume too, thus the need for several revolutions.

Combustion chamber is 21.1cc. Bore = 64.5 Stroke =53.5. Total displacement 1046.36cc Thus compression ratio 9.27

EMS wrote: ↑

Mon Jan 23, 2023 11:13 am

Combustion chamber is 21.1cc. Bore = 64.5 Stroke =53.5. Total displacement 1046.36cc Thus compression ratio 9.27

Account for piston dome volume displacement and (minus) piston to head clearance
(deck height) ?

The 21.1cc is the volume of the combustion chamber with the piston at TDC, thus taking all that in account.

Hi,
I don't think extra strokes of the engine would compensate for the increased volume. Up until a few years ago I raced TZ250s and one of the changes made for different circuits was combustion chamber volume. This was achieved by by either increasing / reducing the squish or by using different head inserts if you use VHM type heads. The objective was to change the engine characteristics for twisty circuits where drive was critical compared to top end on circuits with long straights.

The bottom line is that combustion chamber volume is a key factor that determines the Compression Ratio. Adding any volume before the non-return valve increases volume.

Happy to be shot down in flames.

Regards,
Chris.

Chris:

You are totally correct. It is internal combustion engine 101. The compression ratio is (simplified) displacement volume/combustion chamber volume.
I say "simplified" because it is actually volume above the piston at BDC divided by volume above piston at TDC. This accounts for all the little knick-knacks, like piston domes, valve pockets and deck height as well as spark plug recess.

Great tech, fellas, I learned Stuff. Thanks!