Exhausting topics:single, dual, or dual with"X"

[raging squirrel]

A friend of mine who once owned several brats ( he also worked at a subie dealership as a young man) told me about the dual exhaust he ran one of his brats. He experienced very noticeable improvements in performance and picked up a few mpg's. I have read similar claims about single exhaust systems using 1 3/4"or 1 7/8" exhaust tubing. So this begs the question: which will scavenge better and yield the best torque? Single, dual, or dual with a crossover pipe? As I understand the theories, it seems that the single with 1 7/8" tubing would be the "sweet spot" for the EA81 engine with modest upgrades (no performance cam here). Any input from the experts? As cool as dual exhaust would seem, efficiency (both in terms of mpg and power per unit of fuel) is the ultimate goal here.

[nipper]

Crossover balances the exhaust pulses but can be more omplicated to hang.

 

When you change the cams on these engines, then you can upgrade the exhaust system to take advantage of it, but I would go with single pipe.

[MilesFox]

Based on the firing order, each cylinder bank fires almost simultaneously, as you will have 2 cylinder exhaust pulses at near the same time out of one port for that bank, alternately between the ports.

 

So if you think about it, you essentially have 2 inline twins on a common crank.

 

Each side of the header would be tuned to each bank, and both sides would collect at the end of the exhaust pulse, and after the collector, it's all muffler and tailpipe.

 

Making the exhaust in an X configuration, at the same lenght as the cat in the factory y pipe(collector), you would have the same dynamic velocity/pulse/scavenge effect either way. From there you will hae a larger dump pipe, or 2 smaller dual outlets.

 

And this, having 2 outlets gives you the same volume of flow, but the skinnier pipes keep the velocities up.

 

In a true dual setup, you can run individual runners with no balance pipe, if not for the consideration of cats and o2 sensors.

 

This is what i come to with my understanding of exhaust dynamics, and with subarus.

 

A cam will affect the engines performance curves, but the exhaust piping itself is just fluid dynamics all on its own, regardless of the cam

[monstaru]

i have had several exhausts on ea81 vehicles, and i have to say that a factory setup with gutted cat factory out,netted the same for me that a crossover dual setup at same tubing size.

i have also had gutted cat, 2" out from there, and had improvements....

had duals with crossover at 2", umm, had gutted cat, no cat, clogged cat:-p

 

i think it really depends on the condition of the engine as to what you should put on there.if you are rebuilding, go new crossover dual.an x is not really needed.

gonna stay carbed?or go spfi?there are a alot of factors i think....

complicated to hang is not true in my opinion, if you always base your crossover on the tranny hangerr that comes down.i have even made a few beefier mounts like that and bolted them to the tailhousing of the trans.there is room for some thin plate in there.

cheers, b

Edited January 25, 2011 by monstaru

[raging squirrel]

Pulse velocity is likely to be the most important factor with the goals I have. I will be starting with a fresh engine and am leaning toward a carb setup. Not the factory carb though. I understand that efi systems are best with a little more intake and exhaust volume, but that can always be changed later. I live in Wyoming, cats are something with four paws and wiskers. A small diameter dual system may be the best, probably 1 3/4". I can't help but wonder if a single would create a stronger vacuum at the cylinders. It should, but having to split that vacuum between two cylinders (the timing order thing) may negate that and then some.

[UraBUS09]

actually going to re-do my exhaust today. found this thread, interesting. i believe that the stock y pipe design is best for stock apps. and slightly modified soobs. got a custom/lifted rig, go custom exhaust. lol. im currently running duals with no cats, just cherry bombs. do not have much top end

(4.5-5k rpms max), of course motor isnt built for that, low torque! motor is runing a weber carb, delta torque gring cam, .75 over ea71 pistons, and big valve heads with work (ported and milled), and fresh everything to support it. but- going to a single exhaust, using a turbo maniflod off ea81/82?. but should give me more back pressure than i do now, maybe increasing the curve a bit. will post after drive it! sorry to ramble kinda...

[raging squirrel]

There are similarities with what you have and what I plan. I have a few questions, most of which depend on if you ran your car stock before modifying it. Before and after impression of cam change? Torque and mileage comparison? Any idea what your compression is with the O/S pistons? Big valve heads-before and after? I have pondered the big valve heads since I have 3 engines to choose from. I was wondering at what rpm level they become an improvement and what the trade off in low end, vacuum, and mileage will be.