Compression ratios

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Dec 16, 2009 | 03:38 PM

1lowscort

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From: Eugene, OR

Compression ratios

I am curious to know what compression ratio these engines have, and if they dropped compression on them in the 70's like they did on everything else. I am curious because I am now running a 75 engine converted to EFI in my 87. Just wondering if I have the same power as stock, or if a 75 engine has higher compression and I should be running higher octane gas.

Dec 16, 2009 | 05:28 PM

saga747

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Unless u hear detonatation...dnt worry about spending the xtra coin on higher octane...assuming u still hv all the electronics still hooked up and r using the fuel inj. the computer will retard timing to avoid havoc.
FYI the mid seventies were when all the compression ratios lowered to meet new EPA req's. If anything u hv less ponies than OEM.

Dec 18, 2009 | 09:25 PM

Ranger GT2

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The old chilton manual I have states that the horse power as follows.

1973 - 168 @ 3800 rpm
1974 - 168 @ 3800 rpm
1975 - 120 @ 3400 rpm
1976 - 120 @ 3400 rpm
1977 - 120 @ 3400 rpm
1978 - 132 @ 3600 rpm
1979 - 117 @ 3000 rpm
1980 - 115 @ 3200 rpm
1981 - 115 @ 3200 rpm
1982 - 115 @ 3200 rpm
1983 - 120 @ 3200 rpm
1984 - 120 @ 3200 rpm
1985 - 120 @ 3200 rpm
1986 - 120 @ 3200 rpm
1987 - 120 @ 3200 rpm
1988 - 120 @ 3200 rpm

I'm sure the compression ratio changed as the HP changed

Dec 18, 2009 | 10:00 PM

Harte3

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From: Spokane, WA

No way of knowing the actual power output without dyno pulls.

Dec 20, 2009 | 10:59 AM

Ranger GT2

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I didn't read over far enough to see that it actually gives the compression ratio also.

73 & 74 has 8.4 to 1
75 thru 82 has 8.0 to 1
83 thru 88 has 8.4 to 1

Dec 20, 2009 | 12:28 PM

1lowscort

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From: Eugene, OR

Awesome, thanks guys. I guess I really don't have to worry about running higher octane gas lol.

Dec 20, 2009 | 12:36 PM

Harte3

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From: Spokane, WA

None of these engines were built to run on premium gas so if one has a detonation/pinging problem, a bit of dizzy timing and advance tuning is in order.