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Thread: Did all the old warbirds have hi compression engines?

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    Mallory's Avatar
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    Did all the old warbirds have hi compression engines?

    Someone tried d to tell me the old P&W radials had Low compression. If so, why do many need 104 octane fuel ?

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    rwanttaja's Avatar
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    Quote Originally Posted by Mallory View Post
    Someone tried d to tell me the old P&W radials had Low compression. If so, why do many need 104 octane fuel ?
    Not an engine expert by any means, but it depends on the timing, doesn't it? Seems like the mechanics could back off on the spark advance a bit to make it compatible with lower-octane fuel. Also, ISTR they kept less of the high-octane fuel in the US during the war, and CONUS pilots were told to limit power levels.

    Ron Wanttaja

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    cwilliamrose's Avatar
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    A lot of those engines were supercharged.

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    If you mean WW2 engines then the normal compression ratio was between 6:1 and 7:1 - Merlin - 6:1, Sabre - 7:1 - PW R-1830 - 6.7:1, PW R-4360 - 6.7:1 - DB 601 - DB 601 - 6.9:1 - BMW 801 6.5:1. Power was increased by using water-meths injection and/or supercharger/turbocharger output, the Ta152H had increased power by using nitrous oxide and/or watermeths.

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    Mallory's Avatar
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    Yes WWII. I consider 7:1 getting up. Low compression I regard as 4:.1

    Shows what they known Y/A!

    How about the RR Merlin or V12 Allison? Or the P & W Wasp junior from the 'thirties (Non- super charged)?

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    Quick check - Merlin - 6:1, Allison V-1710 - 6.65:1, Wasp Junior - 6:1, Bristol Hecules (sleeve valve) - 7:1, Bristol Centaurus 7.2:1 (sleeve valve) Nakajima Sakae - 7:1, Even the RR Griffon was only 6:1, WW1 - Clerget - 4.56:1 or 5.3:1 - Hispano Suiza 8a - 4.7:1 or 5.3:1, Gnome Rhone - 4.85:1. I did work on the Centaurus for a short while but please don't ask me to explain sleeve valves systems and theory - they are a nightmare and will give you a thumping headache. If you are interested then the best man to read is Sir Harry Ricardo - he got a single cylinder sleeve valve engine to 8,000 rpm back in the 30s !!!

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    Engines with geared superchargers effectively have variable compression ratios. I spend a little time behind Pratt&Whitney's and the manual listed different operating limits when using different octane fuels. The lower the number on the fuel, the lower the published max MP. All of the engines listed above only can make their published max HP when using the highest octane fuel.

    My understanding is that Jimmy Doolittle was one of the major forces behind the development of 100+ octane aviation fuel in the late 1930's or early 1940's. The US military had it first which provided a performance advantage vs the Axis powers.

    Best of luck,

    Wes

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    A good book on the use of high octane fuels by the RAF as well as other military industries is "I kept no diary" by Air Commodore F.R. Banks - formerly Director of Engine Research and Develpment at the Air Ministry - ISBN code - 0950454397. A very good read.

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    Wes - You are quite right - there are so many factors that concern power output - octane rating - super/turbo charger efficiency - water meths injection - valve type, number and timing - ignition timing - plugs - carburettor/fuel injection - propellor efficiency etc., The Luftwaffe generally only had 87 octane hence the DB605 had to have some 35L (2136 cu ins) displacement and a relatively high CR of 7.5:1

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    I may be mistaken, but I think T-6/SNJ/Harvard original octane requirement was regular avgas, only 87 octane, which has not been available for some years, but I think if was back in the 70s/80s. Virtually every fighter engine and most of the bomber and tranwport ones are supercharged thus have lower compression ratios. By supercharged I also mean both gear driven as well as exhaust driven. On nice thing about gear driven is that boost is instant, not lag time for spool up.

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