idosubaru Posted September 6, 2005 Share Posted September 6, 2005 might be picking up an NA EA82 with a carbed block and turbo heads on it. he said it'll be around 11:1 compression ratio. would i have to run high test gasoline with that set up? i'll be running it NA. thanks, Quote Link to comment Share on other sites More sharing options...
Ross Posted September 6, 2005 Share Posted September 6, 2005 The turbo heads don't change compression ratio. Unless the heads have been planed, or something like that, the CR will still be 8.7:1. Quote Link to comment Share on other sites More sharing options...
Snowman Posted September 7, 2005 Share Posted September 7, 2005 The turbo heads don't change compression ratio. Unless the heads have been planed, or something like that, the CR will still be 8.7:1. Ditto. I saw mudrat's hi-comp EA82t at WCSS6, and that thing flat out screamed with stock boost and no intercooler. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 7, 2005 Author Share Posted September 7, 2005 the 8.7:1 pistons will get worse gas mileage than 9.0:1 or 9.5:1 compression ratio pistons right? any idea how much difference? Quote Link to comment Share on other sites More sharing options...
JonOfScio Posted September 7, 2005 Share Posted September 7, 2005 I've heard from one engine builder (mind you he was a hardcore domestic man) that lower compression gets better mileage (under the less power = more economy theory) and that higher compression means less mileage because it's taking in more air. Quote Link to comment Share on other sites More sharing options...
Ross Posted September 7, 2005 Share Posted September 7, 2005 No, the higher the compression ratio, the higher the thermal efficiency, so the 9.5:1 will get the best mileage. I could tell you the theoretical difference in efficiency, but that wont directly correlate to the difference in real world fuel economy. But you will notice an improvement with the higher cr pistons. Quote Link to comment Share on other sites More sharing options...
Snowman Posted September 7, 2005 Share Posted September 7, 2005 I would be really scared to go over 9:1 compression on an EA82T with the stock fuel system. Some sort of standalone would be mandatory in my book. Same with an intercooler. Quote Link to comment Share on other sites More sharing options...
Bobby_boucher Posted September 7, 2005 Share Posted September 7, 2005 sorry for being a dumb a$$ but what pistons would you use to raise the compression tha much? Quote Link to comment Share on other sites More sharing options...
Snowman Posted September 7, 2005 Share Posted September 7, 2005 Carbed ea82 is 8.7:1, SPFI EA82 is 9.5:1. Quote Link to comment Share on other sites More sharing options...
Bobby_boucher Posted September 7, 2005 Share Posted September 7, 2005 Carbed ea82 is 8.7:1, SPFI EA82 is 9.5:1. Thanks for the informantion, i really appriciate it. Quote Link to comment Share on other sites More sharing options...
Subarutex Posted September 7, 2005 Share Posted September 7, 2005 Gary... If you were to run this in a FI setup, I would be wary... however you indicated you would be running it NA. My question is how exactly? MPFI system from an XT? The turbo heads have a dual intake port, the normal carb'd/spfi heads have a single intake port on each head. So... turbo heads means you need to use a turbo intake manifold... or a spider manifold from a MPFI XT. Then what would you run for a computer? Best bet would be a MPFI XT ecu... but a normal turbo ecu may work ok. Now... the MPFI XT seems to go fairly well... but its at 9.5:1 or so. Dropping down in compression is only going to neuter the engine. Quote Link to comment Share on other sites More sharing options...
PoorManzImpreza Posted September 8, 2005 Share Posted September 8, 2005 just a correction.. carbed ea81 have 8.7:1 compression carbed ea82 are 9:1, MPFI GT spec ea82 (not MPFI XT, but fwd sadan with MPFI, 5 speed and disc brakes) have 9.7:1 and turbo mpfi ea82 have 7.7:1 compression... Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 8, 2005 Author Share Posted September 8, 2005 This will be an NA engine (NO TURBO). the car already has the turbo heads, spider intake and ECU, so i'm just trying to decide what block to put in it. someone has an 8.7 carbed block with turbo heads ready to install. i may install the 8.7 and see how it does while i rebuild the other block with 9.0 pistons. any idea if the gas mileage difference would be significant between 8.7, 9.0 and 9.5? are we talking like 0.3 mpg or like 5 mpg? thanks a ton for all the feedback fellers! Quote Link to comment Share on other sites More sharing options...
Ross Posted September 8, 2005 Share Posted September 8, 2005 This will be an NA engine (NO TURBO). the car already has the turbo heads, spider intake and ECU, so i'm just trying to decide what block to put in it. someone has an 8.7 carbed block with turbo heads ready to install. i may install the 8.7 and see how it does while i rebuild the other block with 9.0 pistons. any idea if the gas mileage difference would be significant between 8.7, 9.0 and 9.5? are we talking like 0.3 mpg or like 5 mpg? thanks a ton for all the feedback fellers! 0.3mpg is probably closer for the difference between 8.7 and 9. Could be more significant going to 9.5, but not 5mpg. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 8, 2005 Author Share Posted September 8, 2005 great! thanks ross! i've got a good source for a known good carbed block so i'd like to install that one but i didn't know anything about CR differences. i've always worked on XT6's, all of them have the same CR so this is all new to me. i'll try and post my results once it's installed. good news, thanks for the reply! Quote Link to comment Share on other sites More sharing options...
calebz Posted September 8, 2005 Share Posted September 8, 2005 86 FSM lists the carb'd motor at 9.0:1 NA - MPFI XT motor (spidered) is also 9.0:1 should run about the same either way. Quote Link to comment Share on other sites More sharing options...
archemitis Posted September 8, 2005 Share Posted September 8, 2005 i can attest to the 9.5:1 gettin the best mileage of all of the subarus i have ever owned. even the one with 200k miles on it got 30 mpg. i would not want to run any motor with less than 9:1 compression without a turbo. slow, and lower mpg. the only time high compression makes mileage worse, is when you put your foot into it. if you could stand to drive a 10.5:1 ea82 like a grandma, youd get crazy mileage. Quote Link to comment Share on other sites More sharing options...
archemitis Posted September 8, 2005 Share Posted September 8, 2005 0.3mpg is probably closer for the difference between 8.7 and 9. Could be more significant going to 9.5, but not 5mpg. i think 5 mpg is an under estimation. more like7 from carb block to spfi block. in my experience. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 8, 2005 Author Share Posted September 8, 2005 horsenuts, can't seem to get a consensus here. guess i'll run the carbed block for awhile and see how it does. i have another block and set of turbo heads that i can rebuild if it's a big difference. i'll definitley post results to clarify some. thanks all, Quote Link to comment Share on other sites More sharing options...
Ross Posted September 8, 2005 Share Posted September 8, 2005 horsenuts, can't seem to get a consensus here. guess i'll run the carbed block for awhile and see how it does. i have another block and set of turbo heads that i can rebuild if it's a big difference. i'll definitley post results to clarify some. thanks all, I was only guessing in my post, if archemitis has first hand experience between the two (by the way, same aspiration on both of them???) then listen to him. Quote Link to comment Share on other sites More sharing options...
BoostedBalls Posted September 8, 2005 Share Posted September 8, 2005 The lower compression in the turbo engine would allow higher boost levels for more power potential but the higher compression ratio would make it a little more drivable and possibly better on gas. Less residual gases left in chamber after exhaust stroke. The higher compression would increase volumetric efficiency, not thermal efficiency. Thermal efficiency would actually get worse, compression is the start of heat in an internal combustion engine. Turbo street bikes can get away with 12:1 compression on 92 octane for up to around 5psi but need water inection. They have a much better burning combustion chamber than we have and they have more RPM, RPM is safety when you are riding the knock limit of your head and fuel combo. Ever notice that head gaskets like to go at WOT at low RPM? I have run 25psi a few times with no engine damage on my ea82T with low compression, try that with the higher CR and KABLAM! I know a couple guys running 15psi in Acura engines but they also have an 8,000 RPM redline, and better burning heads. The high compression will work; just remember, don't bog the engine! Quote Link to comment Share on other sites More sharing options...
BoostedBalls Posted September 8, 2005 Share Posted September 8, 2005 OOPS, didn't see the part about you wanting to run it NA. Yeah, 11:1 should be ok on 92 octane but again, don't bog the engine. I wouldn't run the turbo cams on it, it'll run like crap. You need more overlap. At least go with the fuel injected non-turbo cams. And a grind would help wake that puppy up too. Good luck. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 8, 2005 Author Share Posted September 8, 2005 shat this gets confusing. compression...then turbo/non-turbo cams. someone else mentioned they thought the turbo cams installed on the NA block would offer better gas mileage. i'm just looking for gas mileage. i guess that's a weird request so that's why i'm getting conflicting information. i can always try more than one set up. the NA cams definitely work with the NA block so i guess there's no reason to try the turbo cams on an NA block built for gas mileage. anyone know if they are definitely different? NA block (start with carbed block, switch to SPFI if i don't like the mpg) NA cams turbo heads spider intake thanks kids, this will be fun. hoping to pick the block up this weekend and get started next week. Quote Link to comment Share on other sites More sharing options...
calebz Posted September 8, 2005 Share Posted September 8, 2005 shat this gets confusing. compression...then turbo/non-turbo cams. someone else mentioned they thought the turbo cams installed on the NA block would offer better gas mileage. i'm just looking for gas mileage. i guess that's a weird request so that's why i'm getting conflicting information. i can always try more than one set up. the NA cams definitely work with the NA block so i guess there's no reason to try the turbo cams on an NA block built for gas mileage. anyone know if they are definitely different? NA block (start with carbed block, switch to SPFI if i don't like the mpg) NA cams turbo heads spider intake thanks kids, this will be fun. hoping to pick the block up this weekend and get started next week. Just curious. Are you using a spider from a turbo XT? I have one from an NA XT. WOuld trade you straight across if you're interested Quote Link to comment Share on other sites More sharing options...
BoostedBalls Posted September 9, 2005 Share Posted September 9, 2005 Are the spider intakes different for turbo and non? How about the throttlebody? Quote Link to comment Share on other sites More sharing options...
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