Optimal Exhaust on 1800 motor

[Phizinza]

What would the optimal Y-pipe runners and Y-pipe back exhaust size pipe be on an EA81/82?

This is to have good power at 4000 to 5000 RPM and still have good low end pick up power.

 

Thoughts?

[Jerry DeMoss]

For power in that rpm range I would say 2.25" to 2.5" or 60-65mm pipe diameter. My brother and I put a full 2.5" exhaust from the turbo back on out ea81t coupe and that is about the right rpm level that we got as well.

[killshot_kustom]

(this is coming from diesel experience), I know that all of my turbo rigs, that after a certain point it did not matter how big of an exhaust I went, it did not increase, or decrease performance. It was explained to me buy the guys at banks, that the turbo alone creates enough back pressure, and the exhaust size is merly going to make the car/truck louder.

 

not shure how relevent this is for a gas powered subaru turbo though.

[Phizinza]

Cheers, but I was referring to N/A EA81/82 engines.

 

I guess there isn't really such thing as too big (although I have read that there is, but the difference in performance is so small you wouldn't notice it in an N/A car).

 

I guess I'm not asking what is optimal for power at 5000rpm, but more what is optimal for power up there and also good low end power.

 

Oh and if I may ask, about what is the smallest you can go without affecting the high end power by much?

 

Obviously because I'm talking in "abouts" and "much" etc I'm not wanting fact figures. Just near abouts figures.

[Durania]

For power in that rpm range I would say 2.25" to 2.5" or 60-65mm pipe diameter. My brother and I put a full 2.5" exhaust from the turbo back on out ea81t coupe and that is about the right rpm level that we got as well.

 

You don't have a video of this thing running do you? I would love to hear it.

[opus]

The bigger diameter the pipe, the less low end you will have.

[pwjm]

in my experience with conventional 4 cyl's, Exhaust velocity is more important than sheer volume. So a mandrel bent system with fewer bends in it that's a smaller diameter will out perform a larger crimp bent exhaust, in both the lower and the upper rpm ranges.

 

In other words, bigger is not always better, its just easier. Similar but less refined gains can be made. However A "tuned" mandrel bent exhaust will always outperform a larger diameter exhaust, providing good low end with excellent flow characteristics in the upper RPM ranges.

 

I don't totally understand all the aspects (equal or uneven length headers... intake runner sizes etc etc) But there's more of a science to an engine's breathing than bigger is better. Food for thought.

 

On my Mazda 1.8L I put a 2inch mandrel bent system on and that performed very well, still lost a little bit of bottom end though. Anything over 2.25 non mandrel would be overkill unless there's a turbo up front I would think.

Edited December 27, 2008 by pwjm

[Love_em_when_there_old]

Ive got planes for my EA81 one too, ill let you know how it goes.

 

Im keeping stock size, but where the two exhaust pipes meet the first cat that will be delete for a traditional X pipe than catless to the muffler.

[zukiru]

^^ that sounds cool.

 

what about going straight out from the first bend on each side?

[TomRhere]

I've read on here that 1-7/8" ID pipe is max for the lead pipes on a N/A engine, with various sizes mention for pipes after the cat.

 

I do know the EA-81 has smaller exhaust ports than the EA-82 does, and I made a 2" dual system for the '82 BRAT w/EA-82 engine in it. Can't say whether it was low on bottom end as I've never driven another EA-82 powered vehicle to compare it with.

 

I used to have a link to an airplane site, and in there they had a formula for determining the ID of lead pipes, length of said pipes before combining them into one, or changing pipe size. Yes it was for a non-Subaru aircraft engine, but the engine was of simular size, number of cylinders, and had 2 banks of cylinders, just like our engines. So, it seems to me it would work for us. Now, just finding it is the key...

[NoahDL88]

+1, I've done the math on an air bench.

 

1 7/8th inch diameter off the head. and 28-32 inches before they connect is best.

 

 

And to nip the back pressure argument in the bud.

 

Its not back pressure that you want, its optimal exhaust flow velocity If the pipe is too small you get back pressure, which is bad. and if its too big the air stagnates and you loose your scavenging effect. If it is the right diameter you get good flow, between 180-220 feet per second, which gives you the best possible power.

[TomRhere]

Think of back pressure like you were crossing some deep water.

 

You want to hit the water with enough speed to create a wave = exhaust valve opening= creation of pressure wave in pipe.

 

Now, you need to maintain your speed to stay in the trough behind the wave = scavaging the cylinder. IE, basically the wave is pulling the remaining exhaust gasses out of the cylinder.

 

Drive to fast, and you're in the wave = to small of pipe

 

Drive to slow = you're in the deep water = to big of pipe

 

To fast or to slow of wave front = exhaust gasses left in cylinder = lose power do to weaken/diluted cylinder charge.

 

That's why your engine will run poorly at idle if the EGR valve is open, to diluted of a charge. And to quench any arguement for EGR delete for more power; No EGR valve out there will flow enough exhaust gasses to make that much power difference at higher engine speeds over idle speed.

[cubastreet]

Yeah I've heard 1 3/4" I.D. up to the point where they join a few times.

after the join it doesn't matter so much - 2 1/2" or so is probably good.

[Phizinza]

Keep it coming guys. Now we're getting into the good detail. This info is for me to decide what to do about my exhaust but I'll leave it out of it just so it doesn't become the focus of attention.

 

So after the y-pipe it doesn't matter what size piping for the flow velocity? Bigger just means wasting pipe?

[carfreak85]

Its not back pressure that you want, its optimal exhaust flow velocity If the pipe is too small you get back pressure, which is bad. and if its too big the air stagnates and you loose your scavenging effect.

 

This guy knows his stuff, so listen to him.

 

I run a header with 1.5" primaries to a 1.75" up pipe and a 3" downpipe to a 2.5" exhaust and muffler on my EA81T and it works great. That same system on my N/A hatch would be suicide. Don't go any larger than 2.25" for your secondary pipe, not sure about the primaries on an N/A EA81.

[Phizinza]

My EA81 is a dual carb, so it is about 100hp vs std 78hp or EA82 86hp. So I'd say I need slightly larger pipe then most EA engines not including turbos.

 

"3" downpipe into 2.5" exhaust and muffler", When you go from 3" ot 2.5" this isn't an issue with the exhaust? Does it help over having 2.5" downpipe into 2.5" exhaust? What I'm trying to ask I guess is what is the reasoning behind having a larger pipe behind the turbo and then go smaller again?

[NoahDL88]

For the header, I wouldn't go any bigger than the 1 and 7/8th. For the rest, i think if you were to double the area you'd be OK, which would mean no bigger than 2.5.

 

And realistically, the intake has been and will always be the weak link in airflow, its like sucking a watermelon through a straw, added to the coolant heating the incoming air charge.

 

Basically, you want to figure out what rpm you want to make max power, do you want torque or HP? From there the fun math begins....

 

 

 

 

 

Helpful Article

 

 

Another Helpful Article

[Phizinza]

Personally I'd want it to run good between idle and 3000 rpm myself.

 

So getting away from the technical side of it. What would be a minimum size without really feeling the effects of power loss do you all reckon? Because as we know, smaller = quieter. And I'm past the loud obnoxious exhaust noise of a 2" straight through pipe.

[NoahDL88]

The mufflers will have more to do with quiet then the diameter of your pipes.

[Phizinza]

I was told by an old guy at a local exhaust shop and he said you can't make a 1.8 quiet 2" pipe, just too much flow no matter what muffler or something like that.

[carfreak85]

What I'm trying to ask I guess is what is the reasoning behind having a larger pipe behind the turbo and then go smaller again?

 

I had the exhaust made originally with the stock downpipe. Then I got a 3" downpipe from BoostedBalls and I was too cheap/lazy to buy a matching 3" muffler.

[joostvdw]

I don't know the maths behind it and I don't have any experience, but this is what I understand from exhaust dimensions. There are certain factors you have to weigh and they all have effect on each other.

 

If you're talking about the old subaru engines, you're talking about a 2 into 1 system, with both runners the same length. You want the flow in runner #1 to create a big vacuum in runner #2. This vacuum sucks out the burnt exhaust gasses. So ideally you want your exhaust gas velocity high, because this creates a large vacuum (due to the venturi effect) when it flies by in the collector.

 

If you want to speed up the gas velocity, you want to keep the runners small, but go too small and you'll restrict them too much. So there's your first consideration.

 

Then there's the length of the runner, make them too long and the exhaust gases in runner #1 haven't past the collector, which means no vacuum in runner #2. Make them too short and the gases have already passed the collector which means vacuum at the wrong time (exhaust valves shut in runner #2). Of course, when the engine is running at different RPMs the gases flow at different velocities, so a properly designed manifold works best at a certain RPM (and if you're smart, that's the same RPM at which the cams deliver most power).

 

As a rule of thumb I have read somewhere to make the volume of the runner the same as the volume of the cilinder that it is attached to. So on the subaru engines you will need a runner of about 450cc.

Quick example; when using a 50mm diameter runner, you will need 28cm or multiples of 28cm (56cm sounds more user friendly ) to give you a ballpark figure.

 

Like I said, I never tested anything of this, so it might be rubbish, please let me know if it is

[Quidam]

+1, I've done the math on an air bench.

 

1 7/8th inch diameter off the head. and 28-32 inches before they connect is best.

 

 

And to nip the back pressure argument in the bud.

 

Its not back pressure that you want, its optimal exhaust flow velocity If the pipe is too small you get back pressure, which is bad. and if its too big the air stagnates and you loose your scavenging effect. If it is the right diameter you get good flow, between 180-220 feet per second, which gives you the best possible power.

 

I'll add to this. A man who knows what he is doing with headers asked me a question. In this case, with siamese ports dumping into one pipe, that question would be:

 

How much horsepower does those two cylinders make? This is one consideration in choosing the diameter.

 

I've turned an EA-82 7000 rpm through 1 7/8" pipes. I ran out of camshaft, not exhaust.

 

hth

 

Doug

[Quidam]

Bigger just means wasting pipe?

 

Bigger will be louder and weigh more. Double negative for me.

 

Doug

[Quidam]

What would the optimal Y-pipe runners and Y-pipe back exhaust size pipe be on an EA81/82?

This is to have good power at 4000 to 5000 RPM and still have good low end pick up power.

 

Thoughts?

 

Snagged this on-line. Dan Lemons, Lemons Headers:

 

Primary Diameter

"I don't think there is an easy rule of thumb or formula for determining proper primary diameter, and it more often comes down to what works in the real world," says Lemons. "Engine builders and experienced header builders are usually a good source for input. When sizing a header, there are obvious things to take into consideration such as displacement, compression ratio, and horsepower. However, we must also consider the weight of the car. Heavy cars need more torque, as do cars with big tires. On a car with small tires, we may use a bigger tube to kill some of the torque and help manage the shock to the tires at launch, especially if there is nitrous or a blower. Converters with lower stall speeds will require more torque than a high-stall converter-hence smaller tubes-while nitrous and blower motors like bigger tube sizes.

 

To illustrate our point, let's suppose you have a 540ci big-block with 14.0:1 compression making 900 to 1,100 hp on motor plus another 300 to 500 hp on nitrous. In a 3,100-pound car with 10.5-inch tires, a 5,500-stall converter, and mufflers, the motor would typically need a stepped 21/4- to 23/8-inch header with 4- to 41/2-inch collectors."

 

To make it simple, 540 inch motor, max 1600 hp. 200 hp per cylinder. He would use a stepped 2 1/4' to 2 3/8" primary on each cylinder.

 

I know it ain't that simple, but hth.

 

Doug