Dommie 99 compression figure

Previously wrote:

Hello all, my 99 died on me today - just got dropped at home by the AA man. done a few very basic checks while I was waiting for him to arrive - carb jets are clear + fuel getting through - good strong spark from the mag at both plugs, engine turns freely, no nasty noises etc.

Quickly had time to check the compression now I'm home and found LH cylinder on 75 PSI after about 4 or 5 kicks, RH will only make 50.

Obviously there is a bit of a difference here which needs to be resolved, and it looks like I've got some more investigating to do when I get the time.

The main reason for me posting is to try and find out a 'healthy' compression reading for a 1957 99 please? I've found commando figures, but am guessing these are going to be quite a bit more anyway.

Thanks Tom

Hello Tom well your Compression should be over a 100 psi with a warm engine so you may have some sticking valves or there not seating right or a cracked piston ring or rings .so in any event the top end needs a good looking at . I wish you all the best yours Anna J Dixon

Previously wrote:

Hello all, my 99 died on me today - just got dropped at home by the AA man. done a few very basic checks while I was waiting for him to arrive - carb jets are clear + fuel getting through - good strong spark from the mag at both plugs, engine turns freely, no nasty noises etc.

Quickly had time to check the compression now I'm home and found LH cylinder on 75 PSI after about 4 or 5 kicks, RH will only make 50.

Obviously there is a bit of a difference here which needs to be resolved, and it looks like I've got some more investigating to do when I get the time.

The main reason for me posting is to try and find out a 'healthy' compression reading for a 1957 99 please? I've found commando figures, but am guessing these are going to be quite a bit more anyway.

Thanks Tom

THe actual figure will vary dependant on how you do it, on my BMC 998 i used to get 120 to 180 on the same cylinder with different techniques. I have not tested a 99 but would hope to get 100 at least and would be more concearned about the pressures being within 10% to each other. A warm motor, a fast kick over with full throttle should give a good indication.

The compression pressures will obviously depend on the actual compression ratio. In a rebored engine this won't be the same ratio as the factory. The higher the c/r, the higher the pressure reading should be. Having said that, 100p.s.i. might be a ball park figure to start with and the most important thing, as Robert says, is that they should be within around 10%, or closer ,of each other - the closer the better.

Try the readings cold after leaving it overnight so that the oil has largely drained away. Disconnect the plug caps so it won't start and check it by just kicking it over. Then try it after putting a thimble full of thin oil down each bore (3-in-1 would be good). If there's a big increase in pressure then it's likely to be worn piston rings or bore. If there isn't then it's more likely to be valve and/or head related. Finally, check it after running it up to working temperature. I would expect that to give the highest readings as all the clearances would be taken up and everything lubricated.

Cheers, Lionel

This is something we checked a few weeks ago on our 99 and we recorded 10.25 bar and 10.50 bar which I think works out to 148psi and 152 psi.

Previously wrote:

This is something we checked a few weeks ago on our 99 and we recorded 10.25 bar and 10.50 bar which I think works out to 148psi and 152 psi.

I should have added, a 1958 model

I got something like 150psi on my 88SS last time I checked. After several brisk kicks, full throttle. I'm still not quite sure how a compression ratio of maybe 8 to 1 can deliver a pressure of 10 atmospheres. Doesn't seem to be possible to me...

As sold, the 88ss had higher than 8:1 compression. Owner's Manual says 9.45:1 for the 1962 model......

Previously wrote:

I got something like 150psi on my 88SS last time I checked. After several brisk kicks, full throttle. I'm still not quite sure how a compression ratio of maybe 8 to 1 can deliver a pressure of 10 atmospheres. Doesn't seem to be possible to me...

As sold, the 88ss had higher than 8:1 compression. Owner's Manual says 9.45:1 for the 1962 model......

Previously wrote:

I got something like 150psi on my 88SS last time I checked. After several brisk kicks, full throttle. I'm still not quite sure how a compression ratio of maybe 8 to 1 can deliver a pressure of 10 atmospheres. Doesn't seem to be possible to me...

When as an impetuous youth I fitted 10:1 pistons to my 99 (I know better now) I compression tested it and it gave 145 psi. Which you would expect really when atmospheric pressure is 14.7 psi or 1013 hectopascals for the metric fiends amongst us.

The only way you can know yourcompression ratio accuratelyis to measure it. It varysquite significantly, depending on what bore size the engine has as well as more obvious things like what pistons are fitted. Piston nominal C:R ratings must be taken with a large pinch of salt! After any rebore the C:R should increase - larger swept volume but same size combustion chamber. Obviously the engine capacity increases too - by quite a significant figure if you work it out. I think mine's at +.040" now.

The measurements need to be taken before refitting the head (after it has been re-worked and polished).I.e. the combustion chamber volume can be measured with the head "upside-down" (valve gear in position) and filled with fluid (oil or water if you're not worried!). Measure this amount of fluid as you need to know the hemisphere chamber volume to add it to the swept volume later. Next you would insert the piston crown to displacesome fluid, pour the remaining fluidinto a measuring flask and record this.(The kitchen jug is fine but not very accurate!). It willrepresent the combustion volume when pistons are at TDC. Measuring the actual bore or "swept" volume isn't quite so easy! Ideally you need to have the bottomend assembled complete with pistons, set them down the bottom at BDC, fill the bores to the brim with your choice of fluid (light oil would probably be best) using your beer or wine-brewing gauge to accurately record the amount of fluid you put in.

If we call the bore volume "A", the head hemisphere volume "B" and the combustion volume "C" (after piston crown inserted) the compression ratio will be (A+B)/C:1. E.g. bore vol. A=600cc;Hemisphere vol. B=100cc;combustion vol. C=80cc; C:R=8.75:1.

Compression pressure readings also vary depending on whether they are relatively static (just kickover) or dynamic (engine running).On cars with or 4 or more cylinders it's common to run the engine and obtain dynamic readings.

Cheers, Lionel

David

In a perfect engine you would expect to get more than 8 bar from a compression ration of 8:1 on a compression test, because it is adiabatic compression and the compressed gas gets a lot hotter, following the general gas equation instead of Boyle's law. It would go down to 8 bar if you had zero leakage and you could let it cool back down to ambient temperature (isothermal compression).

Of course at kicking-it-over engine speeds there won't be any compression at all until the inlet valve has closed, quite a long way in degrees past BDC, but only a few linear mm.

In theory you should get a higher than 8:1 effective ratio at high speeds where the tuned length of the inlet tract acts to ram a column of air in past the closing inlet valve.

Best

Steve