BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: Matt.S on 23 July, 2017, 11:41:25

Title: Compression Test
Post by: Matt.S on 23 July, 2017, 11:41:25
Hi Folks

Can anyone tell me how many psi I should expected on compression testing my 1962 C15?

Title: Re: Compression Test
Post by: om15 on 23 July, 2017, 12:10:41
My service notes give the C15 compression ration of 8:1, (the C15S is 10:1), no mention of acceptable psi measurement, however I found this on wiki

Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.

If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
p = p 0    CR  γ     {\displaystyle p=p_{0}\times {\text{CR}}^{\gamma }}  p=p_{0}\times {\text{CR}}^{\gamma }
where p 0      {\displaystyle p_{0}\;}  p_{0}\; is the cylinder pressure at bottom dead center which is usually at 1 atm, CR    {\displaystyle {\text{CR}}}  {\text{CR}} is the compression ratio, and γ    {\displaystyle \gamma \;}  \gamma \; is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.

For example, if an engine running on gasoline has a compression ratio of 10:1, the cylinder pressure at top dead center is
p TDC   = 1  bar  10 1.4   = 25.1  bar    {\displaystyle p_{\text{TDC}}=1{\text{ bar}}\times 10^{1.4}=25.1{\text{ bar}}}  p_{\text{TDC}}=1{\text{ bar}}\times 10^{1.4}=25.1{\text{ bar}}
This figure, however, will also depend on cam (i.e. valve) timing. Generally, cylinder pressure for common automotive designs should at least equal 10 bar, or, roughly estimated in pounds per square inch (psi) as between 15 and 20 times the compression ratio, or in this case between 150 psi and 200 psi, depending on cam timing. Purpose-built racing engines, stationary engines etc. will return figures outside this range.

Title: Re: Compression Test
Post by: Matt.S on 23 July, 2017, 13:01:45

I seem to be getting a reading of about 100psi after kicking it over a few times, the bike pulls well in top gear and doesn't seem to smoke etc. So I guess this should be acceptable for the amount of miles I'll be doing.
Title: Re: Compression Test
Post by: STAR TWIN on 23 July, 2017, 14:09:42
Simplifying... Atmospheric pressure is approx 14.7 psi. So 8:1 compression gives 14.7 x 8 = 117.6 psi. However, the inlet valve closes after bdc so you have to reduce that by a factor depending on the cam. For rule of thumb, the expected reading would be approx 90% lower. so 117.6 x 90% = 106 psi. All assuming perfect ring and valve sealing. Pretty close to what you have recorded.   
Title: Re: Compression Test
Post by: Matt.S on 23 July, 2017, 15:03:04

Good news for now!