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Guest Message by DevFuse

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1800 pistons in a 1600?

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9 replies to this topic

#1 Caboobaroo


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Posted 26 February 2004 - 02:00 AM

So I know that some people that are building high compression EA81's use 1600 pistons in them. Well my question is would 1800 pistons fit in a 1600 and drop the compression? I'm thinking about modifying an EA81T manifold, putting injectors in it like an EA82T and mounting it on my 1600. I just had this idea earlier and I was just wondering it it would work? Then I would have an EA71T:D

EDIT: If I can't do this, how hard and which how much machining would it take to fit EA81T heads on an EA71?

#2 MilesFox


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Posted 26 February 2004 - 02:10 AM

why not use the later "fat case" ea71 heads(like in 85+hatches) and have them machined out to fit the turbo fittings and the injector bosses

from my comparisons, ea71 heads will line up at the combustion chambers, but the boly holes are closer. it almost looks you can slot the holes, if it wnt hurt the integrity of the heads

i also noticed that the ea71 heads have a smaller combustion chamber

#3 Caboobaroo


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Posted 26 February 2004 - 03:33 PM

Thanks for the info Miles I appreciate it:D If anyone else has any ideas lemme know.

#4 Ratty2Austin


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Posted 26 February 2004 - 08:17 PM

why not use a standard compression block, and use a turbo that has had the pressure reduced? (rather than modding the block down in compression, lower the input pressure)

for that matter, how do you intend on getting F/I stuff on a ea71? last time I checked, the ea81 and ea71 manifold length is slightly different... but like miles said, the fat case later modles might be the right size

#5 Snowman


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Posted 26 February 2004 - 08:24 PM

The reason for lower compression with a turbo is that you can make more power that way. Having a normal compression ratio severely limits your potential power gains because of the increased potential for detonation. More power can be gained by running low compression and higher boost.

#6 MilesFox


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Posted 26 February 2004 - 08:53 PM

the whole idea of having lower compression is you can squeeze more VOLUME of air/fuel charge, thus burning more per stroke, more power.

so i would go with the lower compression, rather than lower boost. simple physics. there was a topic about this, try searching compression+volume+turbo and see if you can find it

#7 Caboobaroo


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Posted 27 February 2004 - 12:24 AM

As for fuel injecting my 1600, I'd machine an EA81 intake and make it fit and put the injectors in the heads. *shrugs* I dunno, I just want the damn thing runnin first:rolleyes:

#8 MilesFox


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Posted 27 February 2004 - 04:00 AM

i like the injector in the manifold idea. if you have to have that made, why not cosider constant fuel injection? you could build up 2 manifolds, one for mpfi, one for cis, and thus the car would be modular, just leave the mpfi wires disconnected, plug them back in between swaps

>>>>>>>sidenote about the heads, the fat case 85+hatch motor heads have the same valves as the ea81, 83 and 84, and my comparisond prove that ea81T valves are the same size as ea81

#9 Skip


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Posted 27 February 2004 - 03:13 PM

Just a few words on turbo charging.
The turbo unit is an exhaust gas drive air pump.
You could call it a compressor of sort.

It pumps air into the engine and in doing so
increases the air pressure in the turbo's tract,
which includes the intake manifold and ultimately
the combustion chamber.

In doing so it increases the DENSITY of the air.
Now DENSITY in simple PHYSICS terms, is defined
by the mass divided by the VOLUME.

(unless some one has somehow "INVENTED" the variable displacement Subaru engine,
hey they are in developement.)
Repeat, there is no VOLUME change.

Simple math then tells us that the MASS
of air pumped in must increase.

Back to our simple PHYSICS.
Combustion is defined as rapid OXIDATION.
To "oxidize" something we need, yep you guessed it, oxygen.

Since air is made up of approx. 21 % 02, when we increase the mass of air entering,
we also increase the MASS of 02.

Add fuel and burn it rapidly. SIMPLE PHYSICS, ....
but now we find swirl factors, next thing you know
the head to block junction comes in to play as a "squish zone".
The spark plug position and ignition spark timing along with the head design all
must be examined when considering the flame front
PROPAGATION, but this is all just SIMPLE PHYSICS.

The higher compression a car starts with the higher
efficiency on and off boost.
Deal with the simple physics and have a more drive able car when not blowing the snot out of it.
And a more power car when you do.
But remember it's just simple physics.

>>>>>>>>side note on modifying the intake manifold,
as you now doubt know
the EA81T head is a single port intake.
The multiport fuel injection engines
have the injector positioned to fire on the back of the intake valve.
Thus the bosses on an EA81T head and the placement
of the injectors on the intake on an EA82T
(which is a twin port intake)
Just something to consider if you choose the manifold mounted injector that was suggested.
Must be something with the simple physics once again??

#10 CIS Subaru

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Posted 28 February 2004 - 05:51 PM

I've heard that EA71 heads fit on EA81 blocks, so I'd assume that the reverse would also be true. If that is the case, one should be able to put EA81T heads on an EA71 block in order to add fuel-injection and a turbo.

Why? Well, the EA71 has a shorter stroke and thus will rev up quicker than an EA81. I believe that this would be a nice quality for a turbo engine. As a bonus, this engine would likely get better gas mileage too.

Miles, you're just a little off about the "fat case" engines. There were actually 3 major versions of the EA71 in the USA. The '70s version has the funky external water pump. The "fat case" models look much more like an EA81. I believe that these were introduced in 1980 with the EA81, but they may have actually shown up in '78 or '79 (I'm not at home right now, so I'm working from memory). In 1984, a third EA71 variation appeared. These late models use the same bellhousing shape as the EA81, as well as the same valves. I've heard that there was yet another version available overseas that had hydraulic lifters.

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