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Exhaust for Dual-Carb EA81?


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23 replies to this topic

#1 Dante

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Posted 01 September 2004 - 01:54 PM

Iunderstand the SPFI EA82 exhaust manifold is a good replacement for (flows better than) the stock EA81 exhaust manifold. Will it fit on my JDM dual-carb long block?

I'll have a fat custom intake, Weber and Pipercross filter on the intake side. I could probably use some more exhale, and an EA82 SPFI exhaust manifold would be way cheaper than having something fabricated...

#2 MorganM

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Posted 01 September 2004 - 02:36 PM

Can't really comment on the JDM aspect of your engine but maybe this will be another piece to the puzzel.

I have an EA81 exhuast manifold Y pipe on my EA82 SPFI engine. The funny little spacers go on the heads first then the Y pipe headers can bolt on. Kinda the opposite of what you are doing but maybe thats a good clue?

Hope this helps :)

#3 Dante

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Posted 01 September 2004 - 02:49 PM

Can't really comment on the JDM aspect of your engine but maybe this will be another piece to the puzzel.

I have an EA81 exhuast manifold Y pipe on my EA82 SPFI engine. The funny little spacers go on the heads first then the Y pipe headers can bolt on. Kinda the opposite of what you are doing but maybe thats a good clue?

Hope this helps :)


Thanks. MilesFox and others have posted about putting the EA82 SPFI Y-pipe on an EA81. The main difference between the US and JDM dual-carb EA81s is the space between the ports and studs. I have read about people having problems putting US EA81 intake manifolds on JDM dual-carb motors because the JDM blocks or heads are shaved so the ports and studs are closer together. The difference is small enough to resolve with a Dremel tool...

#4 MorganM

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Posted 01 September 2004 - 03:05 PM

Thanks. MilesFox and others have posted about putting the EA82 SPFI Y-pipe on an EA81. The main difference between the US and JDM dual-carb EA81s is the space between the ports and studs. I have read about people having problems putting US EA81 intake manifolds on JDM dual-carb motors because the JDM blocks or heads are shaved so the ports and studs are closer together. The difference is small enough to resolve with a Dremel tool...

I wonderd if there wasnt a minor difference in the JDM dual carb heads that would make it harder than a bolt on swap :(

Sounds like the difference is minor and you can maybe slot the holes on the header flanges to make it work? Or maybe an adapter plate if the difference is too much for slotting the holes?

#5 CIS Subaru

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Posted 01 September 2004 - 07:22 PM

I'm surprised that a standard Y-pipe can even be modified to fit. The pictures I've seen of JDM dual-carb EA81 heads show that the heads are almost completely different. The biggest difference between the dual-carb heads and the standard heads is that the dual carb heads have the intake valves directly under the intake port, just like an EA82. That part isn't why I questioned the exhaust fitting, though. I'm surprised that a standard Y-pipe will fit because the pics I've seen show the dual-carb heads each having an oval shaped exhaust port with a thin dividing wall splitting it down the middle. Somewhere I even have a picture of a true 4-into-1 custom exhaust header made for one of these engines. My understanding is that the original Y-pipe was just a normal 2-into-1 pipe that did not make use of the split ports though. Still, I would think you would get better performance if you had a proper pipe that matched the shape of your ports. I'd be afraid that a standard EA81 Y-pipe would partially cover the ends of the oval exaust ports.

#6 Dante

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Posted 02 September 2004 - 10:29 AM

...I'm surprised that a standard Y-pipe will fit because the pics I've seen show the dual-carb heads each having an oval shaped exhaust port with a thin dividing wall splitting it down the middle. Somewhere I even have a picture of a true 4-into-1 custom exhaust header made for one of these engines. My understanding is that the original Y-pipe was just a normal 2-into-1 pipe that did not make use of the split ports though. Still, I would think you would get better performance if you had a proper pipe that matched the shape of your ports. I'd be afraid that a standard EA81 Y-pipe would partially cover the ends of the oval exaust ports.


You are probably right. I posted this after reading about putting a EA82 Y-pipe on a US Spec. EA81. I'll have to take a look at my exhaust ports when I get home. Sounds like I'll have to have something fabricated.

#7 Dante

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Posted 02 September 2004 - 10:59 PM

Doh! You are right. There are two siamesed "D" shaped exhaust outlets on each side. I'll definitely be having some fabrication done after sending some e-mails to some aircraft guys (RAM, Stratus, Joe Souza, etc.)

#8 MorganM

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Posted 03 September 2004 - 07:56 AM

Sounds kinda like the exhuast ports on EJ22... maybe an EJ18 manifold would bolt up? Might be something to look into :D

#9 CIS Subaru

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Posted 04 September 2004 - 07:04 AM

No, the EJ ports are further apart.

I found the pic of the dual-carb port:

Attached Files



#10 Dante

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Posted 04 September 2004 - 11:59 AM

RAM will make flanges for me. How do I figure out how long to make the primaries to optimize performance? The intake has 10" x 1.5" runners and, as I said above, it will be sucking through a Weber 38 DGES (I know I may have to re-jet it to avoid overfueling) and Pipercross oiled foam filter.

#11 JWX

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Posted 04 September 2004 - 01:40 PM

what does the rest of the head look like? is it better than the stock ones?

#12 Dante

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Posted 04 September 2004 - 07:32 PM

what does the rest of the head look like? is it better than the stock ones?


The JDM dual carbs put out about 100HP stock (98 IIRC). The valves are arranged different so they breathe better. The cams are different, too.

You can still find them. I got my 30K Japanese longblock from an aircraft guy who had not gotten around to using it his project for $400. About the same time, I found a complete JDM dual-carb motor through an importer for about $1,300.

I should probably just get an EA81T and crossmember, but this will be a fun project...

#13 JWX

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Posted 04 September 2004 - 11:18 PM

whats the intake port look like?

#14 Dante

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Posted 05 September 2004 - 12:33 AM

whats the intake port look like?

Same as a regular EA81, I think. The port is round and about 1.5" in diameter with a rib on the block side of each intake port. The intake I've got is supposed to fit either head. The bolt holes in the intake match the JDM heads perfectly.

I havent had the intake off my US Spec head and have not seen any others first hand--only pictures--but the intake side looks the same as the photos I've seen.

#15 JWX

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Posted 05 September 2004 - 12:19 PM

really. this could be a head upgrade for us stock headed EA-81 guys

#16 Dante

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Posted 07 September 2004 - 09:50 AM

really. this could be a head upgrade for us stock headed EA-81 guys


The cams are different. There may be other idfferences as well. I don't know. Qman or someone on one of the EA81 aircraft boards could tell you.

#17 Dante

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Posted 10 September 2004 - 08:06 PM

Anyone have a good free or inexpensive resource to help calculate intake manifold and exhaust runner lengths to get torque and HP where you want them?

I've got a 1 1/2" aftermarket intake, but the calculator I have been playing with says just increasing the intake runner cross-section without increasing runner length increases peak torque rpm. It says my peak torque rpm will be around 4,900 RPM.

I'm thinking you could make a composite intake with runners the same size as the intake ports and angle them back toward the rear of the engine so you could make them longer to maximize low-end grunt. I've seen photos of carbon fiber and fiberglass intakes (built on steel stub tubes and flanges) on the airplane sites. I don't have a spare, so I have plenty of room.

I'll probably just try my aftermarket intake, but I am interested in learning more about intake and exhaust tuning.

#18 CIS Subaru

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Posted 18 September 2004 - 08:55 AM

The cams are different. There may be other idfferences as well. I don't know. Qman or someone on one of the EA81 aircraft boards could tell you.


These heads will NOT work on a standard EA81 because the intake and exhaust valves are in the opposite locations. That's why the cams are TOTALLY different. In other words, the 4 cam lobes of a standard EA81 cam go
Intake, Exhaust, Exhaust, Intake,
but the dual carb cam goes
Exhaust, Intake, Intake, Exhaust.
This puts the intake valves directly under the intake port of the head which makes for a much more direct path for the air/fuel mixture.

#19 Mike W

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Posted 18 September 2004 - 10:42 AM

Hey Dante, if you haven't already, a visit to NSI Aero up in Arlington might help. The company founder (Lance?) came to WCSS2 and seemed friendly toward us automotive folks. This guy knows pretty much all there is to know about EA engines since he used to buy JDM engines by the cargo container and mod them for aircraft use. Even though the engine you're building is very different from an aircraft engine, some insite from NSI might help a lot and who knows what treasures you might end up buying. A tour of NSI...oh man that'd be cool...I've wanting to go there for years.
http://www.nsiaero.com/aero_home.html

#20 Dante

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Posted 22 September 2004 - 07:11 PM

Thanks Noah and Mike,

Ron from RAM Performance told me basically the same thing as Noah--the heads AND CAM on the JDM dual-carb motor are different, but Ron told me everything else is the same. Ron is making me some flanges. I will have the rest of the exhaust fabricated locally.

I think I will have to give NSI a call. I did not realize they were local until I read Mike's post.

#21 waimaks

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Posted 13 April 2007 - 07:23 PM

Sorry for the whole zombie return from dead thing. But I have a lead on a dualcarb setup. And possibly the dualcarb heads, and maybe (just maybe) the rest of the block.
The less I have to get, the cheaper it'll be...
Soo I under stand the dualcarb heads flow better... But are different to ordinary ea81 heads. Can they work on an ordinary ea81 block? Do I need to get the dualcarb cam too? Can the dualcarb cam be used in an ordinary block?
Oh excitement! must have!

#22 Phizinza

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Posted 13 April 2007 - 08:05 PM

The heads have the valves arranged more like EA82's (look at some of the previous posts and you'll see what I'm talking about.)
Becuase of this, the cam has a completely different arrangment.. So you need the cam.
I'm 99% sure you can put the cam in a standard block, my dualy's engine number is from around the late 80's and I'm pretty sure dualies wern't made after the early 80's...
The dual carbs cam also uses solid lifters, so you might need to do something to your hydro lifter block.

I'm pretty sure all you need is:
Heads,
Intake manifold,
Exhaust manifold (or modded exhaust,)
Cam.

Or, you can just bolt the dual carb manifold onto a standard EA81. I've heard it is a good improvement.


As for the question in the subject line of this post:
They are different flanges to all other Subarus. The bolt spacing is wider. They need something made up, pretty easier really. 1.75" from heads to a 2 to 2.5" straight through will be enough..

#23 waimaks

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Posted 13 April 2007 - 08:14 PM

I have a few spare blocks around here. I'm pretty sure none of them are or at least one of them isn't hydro lifter.
Ok ok. so Cam, heads, and intake.. The exhaust will have to be custom as its not available.. Ok! Lets go shopping!

#24 Matty B

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Posted 13 April 2007 - 08:20 PM

I have a set of heads and a cam, make me a good offer.
And an exhaust manifold (custom)




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