Compressions

[David2021]

Does anyone know what compression readings one should get on a healthy 12hp post war engine?

[HAD501]

My engine recorded 142,146,147 and 146 psi, after 240 miles running-in with the engine warm. I haven't got round to measurng it more recently!

[David2021]

Thank you....That will give me a benchmark figure
David

[luli]

What I have measured is 120 PSI.
See more here:
https://lulis.org/2014/08/07/%d7%a8%d7% ... %9b%d7%91/

[TonyG]

I suspect you have all the information you require. However, compressions are one thing I have pondered with regard to my Tourer in an effort to understand what the compression ratio is. I believe the standard c/r is around 7:1, probably a little higher on post War cars when the quality of fuel improved and with the introduction of lead as standard. My car has had the head skimmed by 3-4mm to improve performance (this was done in 1930s I believe) and the compression test shows two cylinders at 158 psi and the other two at 159 psi. By comparison, my Saloon had compression of around 108 psi. Based on these readings and air pressure at a nominal 15 psi would suggest around 10.5:1 and 7:1 c/r respectively. Luli’s at 120 psi would be 8:1, which is probably spot on for a post War car.

The higher c/r gives much improved performance but having the head skimmed brings with it other issues; spark plugs need to be very short reach, which are a challenge to source. The engine requires Super unleaded to run at it’s best and becomes temperamental to start as the fuel goes off. With the head height reduced to 90 mm the water pump fouls the cast lug on the front of the engine block, making torquing the head down difficult and necessitating either the pump to be modified or the lug to be ground down.

The important thing about compression testing is that all cylinders have similar readings and for our Rovers something in excess of 105psi. Worth bearing in mind that, when new, these cars went for a de-coke once a year so, while few of us do that now, any cylinders showing poor compression could be down to valves in need of a re-grind rather than anything more sinister.

We have discussed leaded and unleaded fuel on the forum before but worth mentioning that lead only arrived during the War with the need for high octane aero fuel. It was developed in the 1930s by GM in USA to achieve smoother running in its auto engines. The lead ensures a cleaner ignition and reduced knock; the lubricating effect was a secondary benefit.

I hope that helps a bit.

Tony.

[SHyslop]

That's a very interesting and detailed set of information about the compression ratios and I shall use these figures to give me a good starting point to determine the health, or otherwise, of my 47 12 engine.

There is only one point I would question and that concerns the date of the introduction of leaded fuel in the UK. I used to go in for pre war Austins and their 1934 handbooks all contained the same wording and in the handbook dated April 1934 for an Austin 12 it says:

"Leaded Fuels. The advent of 'leaded' fuels (petrol containing a small proportion of tetra-ethyl lead) on the British market has led to a number of queries concerning their use and effect in engines. Provided that the same reasonable attention is given to valves and adjustments as with ordinary petrols there will be no trouble when using these fuels. "

It then goes on to describe cleaning the deposits off the valves with a wire brush before grinding in. "The Motoring Encylcopedia" of 1933 makes no mention of lead in petrol at all so I've always reckoned that a car made from early 1934 onwards would have been used to petrol with lead in it.

[TonyG]

I suspect that lead in petrol preWar depended on the supplier, but the point is that it didn’t become standard until during/post War. Fuel was of varying quality back then, I suspect, hence the manual ignition advance capability of preWar Rovers and many other cars for use in achieving smooth running. I doubt anyone touches the advance/retard on their car now. The information in your Austin book is useful in that it confirms the arrival of leaded petrol and makes the point that not all fuels have it. With that in mind, the engines will most likely have been constructed to manage on unleaded.

Tony.