Dirk Posted February 23, 2010 Share Posted February 23, 2010 I read in a thread that MPFI pistons can help to add a little hp in a carbed engine. Can anyone tell me why please? Does anyone have any pics to show any visual differences? Cheers Dirk. Link to comment Share on other sites More sharing options...
NinjaTech Posted February 23, 2010 Share Posted February 23, 2010 They would raise the compression ratio. Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 Ok. But why? Are you sure they raise compression? I can't figure it out. Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 Iv'e flicking around on the net and it looks as though MPFI pistons have indents in them. Is this a turbulence thing? As far as I can tell an indent in the piston is going to lower compression ratio. Is that why you would have to skim the heads? Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 23, 2010 Share Posted February 23, 2010 Carbed engine is 9.0:1 SPFI/MPFI are 9.5:1 The difference is the pistons. They are taller above the wrist pin's. Not worth the difference in HP for an N/A engine though. Too much work for too little gain without further mods. GD Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 23, 2010 Share Posted February 23, 2010 Iv'e flicking around on the net and it looks as though MPFI pistons have indents in them. Is this a turbulence thing? As far as I can tell an indent in the piston is going to lower compression ratio. Is that why you would have to skim the heads? Turbo pistons (which are also MPFI engines) have indents - they are 7.7:1 ratio pistons. N/A pistons have valve cut-out's but are not "dished" like the turbo pistons. GD Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 Carbed engine is 9.0:1 SPFI/MPFI are 9.5:1 The difference is the pistons. They are taller above the wrist pin's. GD Rick, I still don't get it. My pistons come up flush with the engine block. If they were taller, they would hit the head. Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 I think I'm starting to get it now. the difference between 9:1 and 9.5:1 is about half a milimeter on the piston length? Yes? Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 23, 2010 Share Posted February 23, 2010 Yes - it's very small. I don't know the exact dimentional difference with EA82 carb vs. EA82 SPFI, but I know that EA71 pistons are about .008 - .010" taller than EA81 pistons and give a ratio increase from 8.7:1 to around 9.3:1 when combined with .020" off the heads. That was off some calculations I ran years ago when I built up an EA81. GD Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 (edited) Sweet Thanks man. One more thing. Is it possible to calculate a compression ratio on an old engine that has had heads skimmed? I do not know how much was removed Oh yeah, The blocks were skimmed too. Edited February 23, 2010 by Dirk Link to comment Share on other sites More sharing options...
Txakura Posted February 23, 2010 Share Posted February 23, 2010 (edited) I think you answered your own question, you would first have to determine how much was removed from both by comparing dimensions with unmodified parts. Then it's simply a matter of mathematics to calculate the reduction in volume of the cylinder and the corresponding increase in compression resulting from it. But can anyone here divine through the internet the inner workings of your particular engine... not so much. And actually that would scare the hell out of me if they could. Edited February 23, 2010 by Txakura Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 But can anyone here divine through the internet the inner workings of your particular engine... not so much. And actually that would scare the hell out of me if they could. Ha ha. Let me put it this way: I am this 6th owner of this car and I'm pretty sure I'm not the only weekend mechanic to have had a go. The tell tale signs are all over. Which reminds me. I met a man at the fuel station before christmas who told me he used to own the car and he re-built the engine! That makes two of us! Link to comment Share on other sites More sharing options...
Txakura Posted February 23, 2010 Share Posted February 23, 2010 that's kind of cool to hear some of the history, some pics in another thread would be nice to see Link to comment Share on other sites More sharing options...
Dirk Posted February 23, 2010 Author Share Posted February 23, 2010 http://www.ultimatesubaru.org/forum/showthread.php?t=105569 http://www.ultimatesubaru.org/forum/showthread.php?t=105616 Link to comment Share on other sites More sharing options...
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