Increasing the compression ratio on EA82?

[Alex White]

Hi all,

I'm in the middle of rebuilding an EA82 carb engine, and I'm
interested in increasing the compression ratio.

The engine as it stands has a 9.0:1 compression ratio, and the
handbook recommends fuel with an octane rating of at least 90.
The other 1.8 engine that the handbook discusses is the MPEFI version,
which has a compression ratio of 9.5:1, requiring an octane of at
least 95.

Pump fuel is ~94/95 octane (correct me if I'm deluded, I'm in Perth),
so I should easily be able to take advantage of this octane rating by
increasing the compression ratio to 9.4 or 9.5 : 1.

As well as the other obvious benefits: more power, better economy.

The pistons between these two engines are the same, as well as the
bore and stroke. So the only difference must be the combustion chamber
volume.

Therefore, to increase the compression ratio, I should have the head
milled.

So my question is, by how much?

I've found evidence on goooogle that people have previously used EA81
engines for aviation work, and raised the compression ratio from 8.5
to 9.2 with a
0.040" head milling. I don't have a comparision for the bore & stroke
& cc volume between the EA81 and EA82, so I can't exactly reuse these
figures. But using this as a rule of thumb, a 0.020" head milling
should produce about the right compression ratio I'm seeking.

Has anyone modified these engines in this manner before?

Cheers,
Alex.

 

[Charlie]

the engines normal compression ratio sounds like the most pump fuel will
handle. the increase in cr you're talking about would likely require
premium. unless perth is wildly different to the rest of aus, basic
unleaded is 91(ish) ron, premium is 96, and the 'special' premium blends
are generally 98.

Charlie

 

[Noddy]

Has anyone modified these engines in this manner before?

To a degree, but not extensively.

I've played around with the EA82's a bit, and the older EA81's as well, and
found them to be a pretty good little engine that are quite robust (if you
can stand the noise they make

I'd suggest that if you're going to go to the trouble of milling the heads
to raise the compression ratio half a point, you won't make any significant
gains in either power or economy, and the exercise may not be worth the
trouble. You'll no doubt get *some* gains, sure, but the net effect won't be
great, and on a bangs-per-buck scale it will be an expensive endeavour.

I remember looking into this when I did the engine in the wife's suby a
couple of years ago, and although I can't recall the specs now, I do know
that I concluded that it wasn't worth my trouble. You'd need more than 20
thou to pick up half a point I would expect, and Subaru engines *don't* lend
themselves well to lots of metal being removed from the head gasket face.

The aviation stuff (and other custom Suby engines) I've seen all used custom
made inlet manifolds, and in this instance you can get away with taking as
much metal off the heads as the engine will stand. However, if you're going
to use the *factory* inlet manifold, then you can run into serious trouble
in a big hurry with head machining, as the manifold bolt locations are
fairly tight straight out of the box.

Put very simply, take more than a *moderate* amount of metal off the heads
and you may very well find that you can't get the manifold to bolt on
anymore, and slotting the manifold bolt holes to accommodate a narrower bolt
spacing isn't a good idea in my humble opinion.

You'd be *much* better off with a pair of efi heads. As well as a different
combustion chamber shape, they also have a slightly different porting &
valve arrangement that makes them flow a tad better than the carburetted
ones.

Regards,
Noddy.

 

[Tony Hwang]

Hi,
Higher CR and premium gas may not be enough. ECU may complain.
It may need reprogramming.
Tony

 

[Charlie]

Hi,
Higher CR and premium gas may not be enough. ECU may complain.
It may need reprogramming.

how would the ecu tell?

Charlie

 

[Marco Spaccavento]

Hi,
Higher CR and premium gas may not be enough. ECU may complain.
It may need reprogramming.

Didn't he say it was a carb engine?

 

[Andrew Morris]

What ECU? the EA82 is a carb'd engine...

Hi,
Higher CR and premium gas may not be enough. ECU may complain.
It may need reprogramming.
Tony

 

[Bill Putney]

Hi all,

I'm in the middle of rebuilding an EA82 carb engine, and I'm
interested in increasing the compression ratio.

The engine as it stands has a 9.0:1 compression ratio, and the
handbook recommends fuel with an octane rating of at least 90.
The other 1.8 engine that the handbook discusses is the MPEFI version,
which has a compression ratio of 9.5:1, requiring an octane of at
least 95.

Pump fuel is ~94/95 octane (correct me if I'm deluded, I'm in Perth),
so I should easily be able to take advantage of this octane rating by
increasing the compression ratio to 9.4 or 9.5 : 1.

As well as the other obvious benefits: more power, better economy.

The pistons between these two engines are the same, as well as the
bore and stroke. So the only difference must be the combustion chamber
volume.

Therefore, to increase the compression ratio, I should have the head
milled.

So my question is, by how much?

I've found evidence on goooogle that people have previously used EA81
engines for aviation work, and raised the compression ratio from 8.5
to 9.2 with a
0.040" head milling. I don't have a comparision for the bore & stroke
& cc volume between the EA81 and EA82, so I can't exactly reuse these
figures. But using this as a rule of thumb, a 0.020" head milling
should produce about the right compression ratio I'm seeking.

Has anyone modified these engines in this manner before?

Cheers,
Alex.

One of the more popular mods to get some more power out of the EA81 is
to change the carb to a weber. Never done it myself, but they seem to
talk about that option a lot on the Ultimate Subaru board whenever
people ask about increasing power for reasonable cost.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

[Alex White]

Hi Noddy,

Put very simply, take more than a *moderate* amount of metal off the heads
and you may very well find that you can't get the manifold to bolt on
anymore, and slotting the manifold bolt holes to accommodate a narrower bolt
spacing isn't a good idea in my humble opinion.

That sounds like good advice. To be honest, I hadn't considered the
effect on the intake manifold.

You'd be *much* better off with a pair of efi heads. As well as a different
combustion chamber shape, they also have a slightly different porting &
valve arrangement that makes them flow a tad better than the carburetted
ones.

Can I use the efi heads with the carby intake setup? I'm not
interested in spending the $$$ and time it takes to convert the thing
to efi.

Thanks for your help,
Alex.

 

[Alex White]

Hi,

how would the ecu tell?

I guess you'd have to tell it, since it ain't plugged in to the carby

Ta for the octance advice, the MPEFI versions must have been tuned for Jap fuel.

Cheers,
Alex.

 

[S]

Hi All!

One of the more popular mods to get some more power out of the EA81 is
to change the carb to a weber. Never done it myself, but they seem to
talk about that option a lot on the Ultimate Subaru board whenever
people ask about increasing power for reasonable cost.

A bit off topic, but I will comment that I have had (and still have
for that matter) several mid '80s GLs with the EA81 power plant. One
of these, an '83 FWD 5spd wagon, came with an odd carb arrangement; a
Carter-Weber carb with a primitive fuel feedback loop that employs an
on-carb fuel enrichment (I assume) circuit, and an O2 sensor. I have
never seen any documentation on this arrangement, and dread the day
that it stops working, as I'm sure I'll never find parts for it, but
it makes _substantially_ more power than the other "normal" EA81s, and
gets better mileage as well. 240K miles, and still goin' strong, tho!
Don't suppose anyone can tell me anything more about this setup? AFAIK
it was bought new locally (Colorado Springs CO); I got it from one of
my grad students who was the original owner.

ByeBye! S.

Steve Jernigan KG0MB
Laboratory Manager
Microelectronics Research
University of Colorado
(719) 262-3101

 

[Decimal Cat]

One of the more popular mods to get some more power out of the EA81 is
to change the carb to a weber. Never done it myself, but they seem to
talk about that option a lot on the Ultimate Subaru board whenever
people ask about increasing power for reasonable cost.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

Bill,

Do they suggest anything for getting a little more power out of the
non-carbed non-turbo EA82? My Loyale needs all the help it can get. . *g*

Just curious. .

--Decimal Cat

 

[null_pointer]

One of the more popular mods to get some more power out of the EA81 is
to change the carb to a weber. Never done it myself, but they seem to
talk about that option a lot on the Ultimate Subaru board whenever
people ask about increasing power for reasonable cost.

Bill Putney

I know it's a totally different engine, but I put a Weber on my 1977
Toyota pickup (20R) and it made a world of difference.

 

[Noddy]

Can I use the efi heads with the carby intake setup? I'm not
interested in spending the $$$ and time it takes to convert the thing
to efi.

I'm pretty sure you can, but you'd have to check first as it's been a while
since I've done anything with them.

If they do actually fit, then they'd be a much better deal....

Regards,
Noddy.

 

[Tony Hwang]

Hi,
Knock sensor, timing, etc.
Tony

how would the ecu tell?

Charlie

 

[Charlie]

Hi,
Knock sensor, timing, etc.

timing can tell an engines had it's cr raised? crikees, they do make em
smart these days...

Charlie

 

[Bill Putney]

Do they suggest anything for getting a little more power out of the
non-carbed non-turbo EA82? My Loyale needs all the help it can get. . *g*

I don't recall - perhaps a weber can be fitted to that too. Or convert
to turbo, but many would recommend finding a car like you want rather
than radical upgrades (unless you have access to a donor car with the
harnesses, computers, engine, etc.).

You might try posting there yourself and getting a variety of options
with the pluses and minuses as far as benefits, cost, difficulty, etc.
Part of it depends on your use and type of driving.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

[Guest]

: A bit off topic, but I will comment that I have had (and still have
: for that matter) several mid '80s GLs with the EA81 power plant. One
: of these, an '83 FWD 5spd wagon, came with an odd carb arrangement; a
: Carter-Weber carb with a primitive fuel feedback loop that employs an
: on-carb fuel enrichment (I assume) circuit, and an O2 sensor. I have
: never seen any documentation on this arrangement, and dread the day
: that it stops working, as I'm sure I'll never find parts for it, but
: it makes _substantially_ more power than the other "normal" EA81s, and
: gets better mileage as well. 240K miles, and still goin' strong, tho!
: Don't suppose anyone can tell me anything more about this setup? AFAIK
: it was bought new locally (Colorado Springs CO); I got it from one of
: my grad students who was the original owner.

I've got an '85 GL Hatch with the EA-81, carb'd. What you are
describing is a "feedback carb." It was the predacessor to the SPFI, and
according to most people worked well when it worked. What it does it
sense the O2 and do a pulse-width modulation on a duty cycle solenoid that
allows a small (controlled) amount of ambient pressure air into the fuel
metering jet. That raises the absolute pressure (decreases the vacuum)
and reduces the amount of fuel that goes through the jet to the engine.
This leans the mixture for highway, steady-state, closed-loop mode, while
allowing for normal carb power enrichment and a relatively benign failure
mode. If the ECU or solenoid craps out, it runs a little rich, but still
goes. That's why many people didn't like them because they'd get *worst*
mileage than a stock carb if the system either broke or was intentionally
disabled. Remember that this was the time when U.S. folks were buying new
cars and ripping the emission stuff off them for performance and mileage.

I wish I had one of those, since I do most of my in-town driving
at 2100', and long-distance driving over the plains when I could pull the
(non-existent) big red knob back and go from 29-31 mpg to probably almost
40.
Oh well... maybe someday I'll buy an EFI car for my rolling
hangar...

-Cory

--
*************************************************************************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
*************************************************************************

 

[S]

Hi Cory!

On Fri, 19 Sep 2003 18:13:24 +0000 (UTC),

I've got an '85 GL Hatch with the EA-81, carb'd. What you are
describing is a "feedback carb." It was the predacessor to the SPFI, and
according to most people worked well when it worked. What it does it
sense the O2 and do a pulse-width modulation on a duty cycle solenoid that
allows a small (controlled) amount of ambient pressure air into the fuel
metering jet. That raises the absolute pressure (decreases the vacuum)
and reduces the amount of fuel that goes through the jet to the engine.
This leans the mixture for highway, steady-state, closed-loop mode, while
allowing for normal carb power enrichment and a relatively benign failure
mode. If the ECU or solenoid craps out, it runs a little rich, but still
goes. That's why many people didn't like them because they'd get *worst*
mileage than a stock carb if the system either broke or was intentionally
disabled. Remember that this was the time when U.S. folks were buying new
cars and ripping the emission stuff off them for performance and mileage.

Thanx for the nfo!
I'll print it out and stuff it in my Chiltons. Any idea if I can still
get service parts for it; gasket kit and such? The (more or less
worthless) local Subaru dealership denies that such a thing even
exists
Hope I decoded your email correctly!

ByeBye! S.

Steve Jernigan KG0MB
Laboratory Manager
Microelectronics Research
University of Colorado
(719) 262-3101