Monte Posted February 1, 2006 Share Posted February 1, 2006 I'm looking at the FSM for the '86 GL wagon and find that the carb compression ratio is different than the spfi. A couple posters told me so, but I also read at a couple different places that they were the same. Now I know for sure that they're different. Anyway, what makes the difference in the 2 engines? Is it just the heads? Displacement, bore and stroke are the same for carb, spfi, mpfi (turbo and non-turbo). I'm thinking of doing a head job on the heads from the '88 donor car then tossing those and the spfi into the '86. Keep the '88 (short)block and rebuild it later. Good thinking, or bad? Monte Quote Link to comment Share on other sites More sharing options...
subaru_styles Posted February 1, 2006 Share Posted February 1, 2006 the pistons are what makes the CR difference.. .heads are the "same" but, will be different(due to intake manifold) on the turbo compared to the carb/spfi Quote Link to comment Share on other sites More sharing options...
Monte Posted February 1, 2006 Author Share Posted February 1, 2006 the pistons are what makes the CR difference. Oh bummer. Monte Quote Link to comment Share on other sites More sharing options...
subaru chuck Posted February 1, 2006 Share Posted February 1, 2006 In my 1988 XTowners manual it says the compression ratio is 9.5 to one. I dont believe it. The manual says to run 87 octane gas. Thats not logical. Years ago you couldnt run such crummy gas in a car with that ratio without it knocking badly. Who knows what the ACTUAL, not book compression ratio is? Many thanks, Subaru Chuck Quote Link to comment Share on other sites More sharing options...
84gl Posted February 1, 2006 Share Posted February 1, 2006 your lucky if you can run 87 in you rigs all of my cars ping on 87 only one stock though the other two aren't (decked heads delta cams) all of them get 93 Quote Link to comment Share on other sites More sharing options...
85Sub4WD Posted February 1, 2006 Share Posted February 1, 2006 In my 1988 XTowners manual it says the compression ratio is 9.5 to one. I dont believe it. The manual says to run 87 octane gas. Thats not logical. Years ago you couldnt run such crummy gas in a car with that ratio without it knocking badly. Who knows what the ACTUAL, not book compression ratio is? Many thanks, Subaru Chuck if you run the #'s yes, it is 9.5:1 CR - that question has been asked before - unless your block has been changed (then the lowest it could have is a 7.7:1 for turbo pistons) - and I doubt your timing is set right if you can run on 87 octane gas anyway - mine recommends 90 or higher octane, and if I run 87, I get problems - for that matter, engines today can run 87 octane with a 10:1 CR or higher and not get pinging due to the high quality engine control systems in them, so 9.5:1 is not far fetched at all, given the fact that timing and fuel advances on that car are determined by the computer try running 87 in a carbed engine with that CR from the 60's and you have a different story ..... Quote Link to comment Share on other sites More sharing options...
iamtheq Posted February 1, 2006 Share Posted February 1, 2006 Yup, I ran 87 octane in my 87 SPFI all the time and it never pinged a bit. Got great mileage, too (even with the 3AT). Quote Link to comment Share on other sites More sharing options...
Mr Fishums Posted February 1, 2006 Share Posted February 1, 2006 If your talking about EA-81 carbed, decking the heads works nicely Quote Link to comment Share on other sites More sharing options...
torxxx Posted February 2, 2006 Share Posted February 2, 2006 hell with gas prices the way they are, run 87 octane and if it pings, retard the timing a lil bit my car isnt fast enough to justify 2.50 a gallon. When the EJ22T is in there, I'll pay for premium Quote Link to comment Share on other sites More sharing options...
bgd73 Posted February 2, 2006 Share Posted February 2, 2006 the 87 pings at 9.5:1? its timing or air flow. I used to build V8 smallblock chevy with cr numbers unlikely to start (10:1 and +unfathomable) without high octane .Intakes, filters and cfm of injection/carb changed the hoax of "10:1 needs high octane". Todays fuel is even better (its been a few years).I stopped my soob from 87 octane pings just from changing amount of air easily accesible. Quote Link to comment Share on other sites More sharing options...
Mr Fishums Posted February 2, 2006 Share Posted February 2, 2006 Yeah BGD73 has a point there, 87 octane can be used with high compression engines if you can get more air to it. If you want to run 87 try making a free flow intake, it has a decent chance of working. If you retard your timing to much you can run it fine but you loose effiency. That lose will prolly cost you more in fuel than the extra 15 cents for premium gas. I run 87 octain on a SBC with a 10.75:1 compression.. it does not knock, it was dessigned for it :cool: naturally aspirated with an output of about 456 horsepower. So I wouldn't say it can't be done. There is always the cheap mans booster trick of taking moth balls (Methanol/Naphthalene) tieing them in a fish net and letting them soak around in your gas tank :cool: A great booster, lol not so consistent, or professional... But if you don't want to risk clogging your injectors/feul lines a higher octane gas for 15 cents more is the better option! Be warned though some of these new computer controlled cars will shizz the bed if you feed them octane that's to high! Quote Link to comment Share on other sites More sharing options...
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