View Full Version : Overall Compression Ratios of the Big Pistons
Mechta
28th Jul 2017, 21:09
The big piston engines such as the Merlin, Griffon, Napier Sabre, R-3350 & R-2800 are quoted as having compression ratios between 6:1 and 7:1. Clearly you wouldn't need fuel with the lead content that 115/145 avgas has if this were the whole story. The answer of course, is that the engines were either supercharged or turbocharged. What I haven't seen quoted is the effective compression ratio when the boost is taken into consideration as well. Have you any figures?
megan
29th Jul 2017, 02:32
Not seen any quoted anywhere. Since the compression ratio is given, and the max boost permitted, some back of the envelope calc should give you what you seek.
cyclic35
29th Jul 2017, 07:23
Try this link.
https://www.rbracing-rsr.com/compression.htm
chevvron
29th Jul 2017, 08:02
It's not the mechanical compression ratio that matters but the actual max pressure in the combustion chamber just before ignition.
Some years ago, an American motor engineer designed a camshaft which allowed the intake valve to close later than normal. By doing this he effectively lowered the peak pressure in the combustion chamber; this then allowed him to increase mechanical compression ratio to about 14:1 without pre-ignition using 'standard' unleaded fuel of about 86 octane thus making the engine more efficient.
Typically a 7 litre dragster engine which runs a high boost Roots type mechanical supercharger and burns a nitro methane/methanol mix of fuel produces about 3,000hp and has a compression ratio of 5 or 6 to 1.
Mechta
29th Jul 2017, 13:37
Thank you for the replies. Cyclic 35's link to a converter gave me what I was looking for.
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
Small correction on the Top Fuel Dragsters...that's more like 8000hp.
About a 1000hp per cylinder.
And it needs the equivalent of a plasma welding arc to ignite the fuel at those compression levels.
Hence the massive dual ignition systems.
https://frankyremtlaaaat.files.wordpress.com/2014/10/topfuel_withoutcanopy.jpg?w=560
pax britanica
29th Jul 2017, 16:57
B2N2
Impressive statistics there -as they only run over a quarter mile I suppose the recommended servicing interval is about every mile -tongue in cheek of course
Rory57
29th Jul 2017, 17:45
They are serviced after every run! Often that involves cylinder heads off. No pollen filter to change though.
cyclic35
30th Jul 2017, 10:53
Thank you for the replies. Cyclic 35's link to a converter gave me what I was looking for.
As an example, a RR Griffon 57 at 6:1 compression ratio with 25psi boost at sea level has an effective compression ratio of 16.2:1.
Now I can see why all that tetraethyl lead was required!
Thank you for acknowledgement Mechta. Pleased to be of assistance.
IcePaq
31st Jul 2017, 21:48
Top fuel is now around 10,000hp.
We're driving 2700hp on the street..........with air conditioning.
Top fuel is now around 10,000hpPlus a bit. ;)
NrsPfXIFzSA
aw ditor
5th Aug 2017, 19:30
Griffon 57 ran on 100/130 Avgas. Didn't like 115/145 due to plug fouling'.