COMPRESSION RATIOS

[not done it yetParticipant @notdoneityet]

PV^n = constant

That relationship is for adiabatic conditions, not isothermal.

[JAParticipant @ja]

Posted by not done it yet on 31/08/2020 18:05:55:

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

[HopperParticipant @hopper]

Posted by Tim Stevens on 31/08/2020 15:34:28:

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

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. 😁

[duncan webster 1Participant @duncanwebster1]

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

[David Davies 8Participant @daviddavies8]

"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

[Tim StevensParticipant @timstevens64731]

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

[HopperParticipant @hopper]

Posted by Tim Stevens on 02/09/2020 17:25:39:

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

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