I was reading through a chilton, when to my astonishment, I read up on some things.

 

80-82: 1800 - 72hp, 92 ft/lbs of torque, 8.7:1 CR

83-84: 1800 - 73hp, 94 ft/lbs of torque, 8.7:1 CR

85-87: 1800 - 82hp, 103 ft/lbs of torque, 9.5:1 CR

 

So, we know that the 85+ 1800s have bigger valves, and hydraulic lifters, as the earier versions have smaller valves and mechanical lifters. (there was once upon a time a rebuild kit for the mechanical lifter engines to convert them to hydraulic, genuine subaru even!)

 

So, with the difference of 9.5:1 compression pistons, hydraulic lifters, and bigger valves, my car (was) 9hp, and 9 ft/lbs of torque over the stock older 1800s.

 

 

NOTE: This also means that persons running pre '85 1800s only need change engines instead of adding 1600 pistons. Some say you get 9.5:1 CR with 1600 pistons, and I think I recall hearing from somewhere 9.7:1. -- So if 1600 pistons actually only get you away with 9.5:1 then there is no reason why someone couldn't grab an 85 or newer hatch motor instead of the hassle of swapping pistons!

Hhhmm - that's very interesting - I knew about the higher HP, and the larger valves / hydro lifters. I wasn't aware of the compression ratio difference tho. Curious - I wonder if the 85-89 engines actually already have a variant of the 1600 piston, or if adding the 1600 pistons would bring it up even higher. I know that decking the heads increases it a little.... that would be cool if I didn't have to change my pistons tho....

 

Now - another good question is does this apply to ALL hydro lifter engines? In other words would a hydro lifter engine from an 84 automatic have these same stats?

 

GD

I could be wrong but the 85-87 could be EA-82 not EA-81

The EA82 1800cc pistons fit in the EA81 1800cc engine. I'm running EA81T pistons in one of me EA82Ts.

do you get higher compression in a EA-82 if you use EA-81 pistons?

well my 89' ea81 hatch has who knows what in it.. no tag on the valve covers sayin don't adjust valve lash.. and from what i can tell it is the original motor..

 

all i can say is its a dog.. lol it'll be a ea82 turbo hatch this summer though..

torxxx,

 

Here is my understanding of the pistons we can run in our motors.

 

Stock ea82 carbed pison: 9:1 compression.

 

ea82 SPFI piston: 9.5:1 compression.

 

1600 piston 9.5:1 compression.

 

I'm trying to get ahold of some SPFI pistons to put in the motor I'm building right now, since they were originally designed to go in an ea82. 1600 pistons would probably be fine, but there could be some differences, so I want to play it on the safe side with pistons that are meant for that engine.

1600 pistons are only 9:1

Are the 1600 pistons 9:1 in a 1600, or in an 1800? I've heard that in the 1600 motor they only produce 9:1, but when put in an ea81 or ea82, they produce 9.5:1 due to the rod/piston/head relationships.

I've been told that at TDC a 1600 piston is the same distance from the deck of the block of both ea71 & ea81 engines; and that the heads for both have the same volume for the combustion chamber.

hmmm..... the mystery continues....

 

I think it was Qman that measured the 1600 pistons and 1800 pistons, and apparently the distance between the pin center and the top of the piston is greater on the 1600s.

um, I was incorrect in reading the stupid manual. It doesn't cover any of the EA81 engines past '84 at all... what I was reading was actually EA82 Carbureted engine stuff. It doesn't even come up with engine designation codes or OHV/OHC designations anywhere in the book...

 

And in '83 the Automatic EA81s had the '85+ hydraulic motors! It says that in there.

 

As for 1600 pistons, It says they are 9.0:1 in the 1600 motor. So I think we need someone to measure it up when they go about putting them in a EA81 block. It could be either way about the distance relationship of the rods/pistons and the heads.

 

But it's interesting to find that the manual covers the EA82 NA Carbed engine has 9.5:1 pistons. (which is what I was misreading as my engine)

 

There was only four engine listings for 85-87, and that was for SPFI, 2bbl, turbo, and the 1.2L Justy engine.

The EA71 pistons are not the same as EA81 pistons. Ask Qman - the difference is slight, but the piston pin is in a different location. CCR also knows about this - you can order an EA81 with EA71 pistons - it's common for them to do this for high performance engines they build.

 

GD

Fact.

 

The wristpin location is slightly different between 1.6 and 1.8. 1.6 is slightly lower on the piston. Maybe .010 but it is different. I showed this to a long time Subaru tech once and he didn't realize it until he saw it with his own eye's.

 

.010 across the surface of the piston could result in .5 CR?

well i cranked out some numbers. damn, why did I have to go to engineering school

 

okay, lets start out with what we know about the EA81.

Bore=92mm

stroke = 67mm

CR=8.7

 

from this we get a displacement/cylinder of 445.3898737 cm^3

 

Now to determine the CR with EA71 pistons you first need to determine what the Min volume and Max volume (min volume + displacement) is, of a EA81 cylinder and combustion chamber. Now we know that CR = Max/Min and that Min=Max-Dis. The 2 equations can be combined and be written as Min=(CR*Min)-Dis. Now this can now be rewritten as Min=Dis/(CR-1).

 

From here we get

Min Volume= 57.844 cm^3

Max Volume= 503.2338737 cm^3

CR=8.699845683

 

Now Ken say that the difference in the pistons is about .010in or .254mm which is a volume of 1.688492954cm^3 (Edit I had mm^3 there when it should be cm^3.)

 

So with EA71 Pistons

Min Volume= 56.15550705 cm^3

Max Volume= 501.5453807 cm^3

CR=8.9313659

 

So it would be safe to say that an EA81 with EA71 pistons would have a CR of 9.0:1

Is there a difference between 1600 and 1800 connecting rods?

well, either way, from math, we have figured out that it is at least probably a 9.0:1 CR for the 1600 piston swap.

 

But, the manual shows the the engine I thought was mine (and is actually the EA82 carb engine) that the compression ratio is 9.5:1, which also coincides with the SPFI pistons.

 

Now to get someone to verify this.

Here is a snapshot from the spec page in a 86 FSM.

 

I guess my chilton book must be wrong. I would think to trust the FSM over a chilton anyways.

A chiltons manual wrong?

Imagine that.

So would one benifit from SPFI EA82 pistons in an EA81 more than the EA71 pistons? If they produce 9.5:1 in an EA82, which is the same size engine, shouldn't they make an EA81 9.5:1 as well?

 

GD

When figuring CR, you have to account for the combustion chamber area in the head, also. That alone could account for upto 1.0 of the total CR. Ever hear of guys building engines and CCing the heads? That's what they are doing, checking the volume of the combustion chamber, and trying to equalize them all, but it is part of the overall CR of the engine.

GD

 

There's a good chance that an SPFI piston would rase the CR in a EA81 but someone would need to take some measurements of an SPFI piston in order to determine what the CR would end up being.

 

Tom

 

My earlier post dose include the combustion chamber volume when figuring CR.