COMPRESSION RATIOS

Sorry, Howard, but fuel with Tetra-ethyl lead was available from the early thirties. The Shell petroleum handbook 1933 says it is used in aeroplanes and US gasoline, and the 1938 edition refers to it as a regular constituent of motor spirit. Such fuel was often called 'Ethyl' as distinct from Benzole (with benzene etc from coal) and Discol (with alcohol from the Distillers Company Ltd.

Cheers, Tim

PS The idea of a racing Harley Davidson always reminds me of an expression relating to sows ears.

Edited By Tim Stevens on 31/08/2020 15:35:03

Aeroplane fuel took time to "filter" through to be used as road fuel.

WW2 Aero engines used, and needed, 100 octane fuel, but it did not become commonplace on the road until much later.

"Pool" petrol persisted for some time after the war. Prewar car cars, mostly low compression sidevalves, ran quite happily on it.

"High Octane" petrol became available in the early 50s, since Compression Ratios were then rising above 7:1 in cars like the 803 cc A30, OHV Morris Minor, and Standard 8.

100 0ctane was needed for cars like the Rover P5, which carried a notice on the screen warning to use 100 octane.

Now, with fuel and engine development, engines with a C R of 10:1 can run happily on 95 Octane fuel. Although 95 Research Octane Number may not equate to the Octane numbers used in the 50s and 60s

Howard

Yes Pratt's Ethyl fuel became available in early 1930 and the 2.3 litre Talbot 90 was racing reliably with an unheard of 10:1 compression ratio. That enabled them to compete with the Bentleys with their 6 litre engines!

Russell

Ideally the pressure in the combustion chamber prior to ignition can be found by the relationship

PV^n = constant

so Pressure at top of stroke = Intake pressure x (compression ratio - 1) to the power of n

If there are no losses n = 1.4. However n = 1.3 to 1.35 makes more sense since there will be a small amount of heat and other losses.

In practice lot of other things will influence the pressure such as valve timing, exhaust pipe, resonance etc. However the above gives a good approximation.

Thomas Midgely found that lead prevented knock in engines in the mid 1920s. He then went on to introduce and develop CFCs for refrigerators. He ended the decade by drinking himself to death. Quite a man.

JA

Edited By JA on 31/08/2020 18:02:15

Edited By JA on 31/08/2020 18:03:42

PV^n = constant

That relationship is for adiabatic conditions, not isothermal.

Yes. I have used it frequently since analysing the performance of a petrol engine at tech.

The compression is rapid enough for there to be no major heat loss. For it to be truly adiabatic n would be 1.4 for air.

JA

Edited By JA on 31/08/2020 18:40:34

Only if you don't know much about Harley history but it did mostly happen on the other side of the pond and mostly on one mile dirt tracks. You haven't seen racing until you've experienced a dozen Harley XR750s sideways at the ton with straight pipes.

And they did get the older side valves going faster than any Manx or Goldie ever went at 149mph at Daytona.

They weren't all the Fatboys we see today. 😁

Doesn't seem that much to crow about, Harley at 1200cc being faster than Manx at 500cc

"Doesn't seem that much to crow about, Harley at 1200cc being faster than Manx at 500cc"

Duncan

the side valve Harleys in question were KR 750s not 1200s . The Manx was an overhead cam motor.

Cheers

Dave

Please don't forget that in the USA there was a rule that 750 side-valves could compete against 500cc ohv or ohc machines. Only in the USA, mind, I wonder why that was ...?

And any comparison of maximum speeds needs to take into account the fuel used. Was the super-quick 750 Harley on petrol, or methanol, or a nitro mix? In this context you might recall that Opel set maximum speed records in the 1930s without a cylinder or a piston. Comparisons which ignore relevant detail are vacuous.

Cheers, Tim

Petrol. With a compression ratio of 6 to 1 on the old side valve. It was their race bike in the Daytona 200.

500 OHV and 750 SV were found to be pretty evenly matched for competition originally based on road-going models. The Goldies won their fair share of races against the 750 side valves. As did many Brit 500 twins.

Then they changed the rules in 1969 and the Tridents and Rocket 3s wiped the floor with the old flatheads. Then the Yamaha TZ350 came along and all of a sudden we were living in the modern world.

But once Harley sorted the OHV 750 it went on to dominate the dirt flat tracks up to pretty much today.

Edited By Hopper on 02/09/2020 22:25:23

Edited By Hopper on 02/09/2020 22:26:09