Can I just replace midpipe and keep stock exhaust?

[bedabi]

I've done a search and this doesn't seem to have been covered.

I've been looking into replacing my exhaust, but would like the car to stay nearly as quiet as stock and retain sleeper looks. So I'm thinking that maybe the best solution would be to simply replace the midpie -- from the last cat to the exhaust -- with a 2.5", mandrel bent midpipe sans stock midpipe resonator would be the way to go. And cheaper.

So... would a 2.5 midpipe fit our stock exhaust? Has anyone done this? Would the stock exhaust totally negate any effect a 2.5" midpipe might have? I'm considering getting one by Brullen from GTBGuy.

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[JaMa]

I have heard of lots of people wanting to do this, and there are certainly performance advantages to thinking this way. Personally I am going 2.5 inch turbo back stainless with a single cat in the downpipe and a 3.5 inch stainless tip because I hate Subaru's corny bolt on tips for the WRX. Keep in mind the muffler on that system is very restrictive though and your real gains are going to be getting rid of it, depending on what other mods you have on your car.

I am almost positive that the midpipe and 3rd cat eliminator pipes from both Scoobysport and Stromung are direct bolt ups for this car and no other modifications are needed. You could eliminate that monster cat after the downpipe and the resonator before the rear muffler and leave the stock muffler in place with very little effort. I am not sure if the 3 inch systems are direct plug and play for the stock bolt locations. It would be a good bet BPM is not, MRT I am not sure of, and Brullen I am also not sure of. You could always PM GTBGUY who is the master of all things Brullen and see what he has to say.

JaMa

[GTBGUY]

bedabi - If you're keeping the stock muffler and upgrading to a 2.5" midpipe, it will not benefit you to any extent. Here's the reason why :

(This is more of an analogy)

Imagine you have a straw, in the center the diameter of the straw is bigger than either ends. If you starting to suck water through the straw, you'll notice that once it hits the middle section, it'll slow down, and past the middle section closer to the top, you'll notice that it'll be harder to suck water through than if it was all an uniform diameter.

It'd be better understood if I could use some fluid mechanics to illustrate this, but it gets extremely technical, so hopefully the above analogy will be able to give you an idea of what happens if you just upgrade the mid-pipe.

If you want to have a system that's sleeper, get the Quiet version of the cat-back, you can also add the street downpipe at a later date for more power.

FYI, the 3" ID WRX cat-back is directly bolt-on.

JaMa - I liked the

Master of all things Brullen

[JaMa]

OK, so I agree and disagree with the last post, but fundamentally I agree. It is best to have the same pipe diameter all the way from the turbo back. But, eliminating resonators and cats in the middle will still decrease backpressure by some margin.

GTBGUY, could you publish some dyno maps of your systems? I know everyone here is keen to see what your system makes from a baseline of WRX stock to cat back to turbo back to catless downpipe, etc. with various "quietness levels" and so forth. I for one would like to see some real numbers.

(here I go starting SH&& again... )

JaMa

[GTBGUY]

JaMa - Yes you are correct about decreasing resistance by removing the stock resonators and cats, but if you're doing that, replace it with same diameter pipe to keep exhaust gas velocity up. Otherwise, you will experience the same ordeal as the straw analogy.

I can see what I can do for #'s, but I guess for now G-tech times would be the closest thing as I can get to dyno numbers.

The different acoustic preferences will not hinder performance due to the design of the systems. It's only a matter of weight difference.

I will try to do some G-tech runs when I get my hands on the next WRX. One will be just cat-back alone, one will be cat-back and street version downpipe (w/ hi-flow cat), one will be with the new cat-less turbo up-pipe alone, and then all three together. Last but not least the stock set-up with nothing else on as the baseline.

I'm going to see if I can get Chris @ TRi Tuning to take a WRX down to Altered Atmosphere to do some dyno runs on the Race spec Brullen turbo-backs.

[bedabi]

Hey thanks for the responses JaMa and GTBGuy!

So, it looks like it won't be such a good idea after all.

As soon as I get my finances straightened out I'll be giving you a PM GTBGuy for a quiet dual tip with 2.5 inch midpipe. Then it's on to the Unichip. Damn this NYC rent!

See, my modification goals are much more modest than yours JaMa. In NYC, I rarely get a chance to open her up. What I need is quick torque response to pass monster SUV's wearing 20 inch chrome rims with 4" angled exhausts hogging the left lane and braking in all the fun curves!

[GTBGUY]

bedabi - We don't make a 2.5ID" system for the WRX, only a 3.0ID" system, but don't worry it's not going to be noisy with the Quiet set-up.

[bedabi]

WHAT!!!! No 2.5" ID pipe?!

All this time I thought Brullen had one!

GTBGuy, do you or JaMa or anyone think I would lose a lot of low end torque with a 3"? That's the real reason why I'm hesitating going to a 3". Those big-assed Caddy "bling bling" Escalades require some quick reponse, na'mean?

[hp12c]

Originally posted by GTBGUY
JaMa - I will try to do some G-tech runs when I get my hands on the next WRX. One will be just cat-back alone, one will be cat-back and street version downpipe (w/ hi-flow cat), one will be with the new cat-less turbo up-pipe alone, and then all three together. Last but not least the stock set-up with nothing else on as the baseline.

I'm going to see if I can get Chris @ TRi Tuning to take a WRX down to Altered Atmosphere to do some dyno runs on the Race spec Brullen turbo-backs.

Here's another quote from you back in 4/30/01
"The "off-road" versions will definately have a higher net power gain vs. the "street" version. We hopefully will be able to dyno all three soon. Otherwise, what we might end up doing is dyno'ing the Cat-back system as well as the "street" version, thus giving an indication of power gains."

GTBGUY:

I am not trying to flame...but I just want some proof of real world gains with ANY exhaust systems. If there are gains - how much exactly? I have $1,500 just burning a hole in my pocket, but like JaMa, I am not going to just rely on claims such as "spools much faster...more power all the way to redline...or sounds like a V8"...etc...

From reading the Autocar article that has been circulating recently, it stated that there are NO peak horsepower gains from 3" turbo back alone without ECU adjustments. Do you think that article is correct in its claims?

I do understand that regardless, opening up the exhaust would be beneficial for modifications further on down the road. But the real question among consumers is...if I only have $1,000 now what do I want to get first?

Sorry, no offense to you personally...but I am just frustrated that I can't find what I am looking for.

[GTBGUY]

bedabi - You have a WRX, unlike the 2.5RS, the less backpressure the better. It will help spool your turbo faster.


hp12c - Don't worry, I'm not taking it personally or anything. As I mentioned, there's no power differences between the acoustic preferences. There is however differences in between the "Race" mid-pipe and "Street" midpipe.

If you followed the previous post, you'll notice that the only AWD dyno in Canada that's available to public is in Quebec. I'm waiting on dyno results myself from Chris @ TRi Tuning.

As for the "no gains from cat-back w/o ECU tuning etc" that isn't the case. I've worked with turbo'd DSM's as well as SAAB's etc, the cat-back will give power gains, just not as much as both downpipe and cat-back together.

[wrxclnt]

FWIW, Stromung does make a 2.5 inch exhaust for the WRX. What's nice about it is the parts mate up to the stock exhaust sections. I just had the mid-pipe and rear muffler section installed and it sounds very nice - not too loud. You could just get the mid pipe and the 3rd cat eliminator pipe, which they call a test pipe, and keep the stock muffler section this way. They also make a down pipe too. Their stuff is built beautifully and also fits around the rear differential protector if you have one.

[bedabi]

Yeah, Stromung was the first one I was looking at (as I've said, took careful note of JaMa's experience with it, and I believe Ginseng has one too). But someone on these boards said the Brullen one was quieter.

As far as gains go with different exhausts, I'd personally like to see the gains (or lack of) made by different exhaust systems on a WRX with a chip installed on it. As was said on the Autocar article thread, it would've been even more helpful if the car was dynoed with the various mods AFTER the Unichip was installed. With all I've been reading here about boost controllers, I think I can pretty safely say that the WRX's ECU controls the turbo in nearly all respects, notwithstanding ambient conditions, much more so than prior turboed cars such as DSM. Chances are, if Mitsubishi were to offer a turbocharged model again, it'd be far more controlled by the ECU than before. I mean, I used to think I was the ***** when I got a 100 mHz Pentium computer in 1995!

[Samirr76]

What does the resonator look like? i didnt evne know the WRX had a exhaust resonator....

pics if anyone has em...

[JaMa]

I happen to know for a fact that the Stromung stuff bolts right into the stock location with no modifications. I think the Stromung stuff is fine as long as you fill the pipe. Just bolting it on a stock WRX, you may not do so. I heartily support Stromung’s decision to go with a 2.5 inch pipe diameter though as I think it is the best size for a mildly modified WRX.

As I have said before, I am not sure I buy this "your turbo will spool faster" stuff. Yes, fluid dynamics are one thing but there are also the principles of back pressure and exhaust scavenging.

So, when figuring out fluid dynamics for an exhaust system, you automatically know that a larger pipe is going to make the turbo spool more quickly because there is a lesser amount of pressure behind it to restrict the flow of the air back. But you also need to realize that there is an optimum amount of backpressure in any car, turbocharged or NA, which needs to be there for the engine to operate most efficiently. In a turbo car, even ours, you are still normally aspirated from 0-3500~ rpm. In that range, while the turbocharger is busy turning exhaust gases into pressurized air, you are still a normally aspirated engine pushing against a 15-20 foot long mass of air which you have to move by pushing exhaust gas down it. Everyone knows that a normally aspirated engine needs backpressure. And, while the turbo is not fully spooled, you don’t need all that pipe. Go out to a top fuel track sometime and watch the guys size exhaust pipes. They measure the estimated horsepower of the car, take into account the temperature and pressure, and cut the side pipes the appropriate length. This determines horsepower limits at the top, toque at the bottom and breathability everywhere. Do those guys just run 2 inch pipes off the headers? No! Why? Because they NEED some bloody backpressure. Plain and simple.

The thing is, a turbocharged engine should be tuned from exhaust port to the curb, just like a normally aspirated engine. The engineers at Subaru put a certain amount of backpressure in the WRX exhaust system to control emissions, power and sound control. If you change this backpressure in any way, you must completely re-engineer the power output of the engine to match the new exhaust system. The turbo, the 3 cats, the resonator, the muffler and the exhaust system pipe diameter all factor into this. Change any of these pieces and you must rethink the dynamics of the entire system.

Now, I am sure that if you re-engineered the intake-exhaust timing of the engine with ECU management or different cams, a bigger turbo and intercooler, the intake mod, and got exhaust really flowing through the pipe, you would probably need extra room to make your best power. But, a 3 inch system is going to run no better than a 2.5 inch system for most of the mods we are performing. This is undisputable and dyno proven in Europe (see nmyetis’ earlier post with dyno numbers) And, aside from ECU modifications, I have not heard of any WRX owner changing cams or making changes which would really require a larger exhaust system.

Vendors, if they really want to sell products, need to produce numbers. Baseline WRX vs WRX with exhaust vs WRX with turbo/chip and exhaust, etc. etc. etc. This is why I buy stuff from places like TurboXS – he dyno plots everything, no BS no speculation.

Ok, I shall now shut up

JaMa

PS – the resonator is a little kidney-shaped thing in the pipe a couple of feet behind the muffler. It is about 8x12 I would guess and is inline with the cat-back pipe on the stock system.

[bedabi]

Word to tha mutha.

[hp12c]

Originally posted by JaMa
Vendors, if they really want to sell products, need to produce numbers. Baseline WRX vs WRX with exhaust vs WRX with turbo/chip and exhaust, etc. etc. etc. This is why I buy stuff from places like TurboXS – he dyno plots everything, no BS no speculation.

JaMa...That's exactly what I'm talking about.

O.K. - I understand that AWD dynos are hard to come by. What's amazing to me however is that between all the vendors out there carrying all different types of turbo back exhaust systems...there's not ONE dyno plot done anywhere of the US spec WRX fitted with any exhaust mods. Why? If I am a vendor and truly believe that X type exhaust will offer my customer the most power gains, I'd do anything to get some real prove of the advantages of the system.

I guess what's even more disturbing is that owners are willing to spend upwards of $1,000+ for these exhaust systems not knowing what the exact real world performance gains are. What do you base your decisions on....sound? the exhaust tip?

I think we are all hanging on to notions that the WRX is just like any other turbo cars that preceded it. According to the Autocar article...it's not easy to get power out of the WRX with simple bolt-on mods. Again, I don't want to refer to it like the Bible...but this is the only source I've seen that provides real numbers.

Vendors...I promise that there will be many more buyers of your exhaust systems if you will just provide proofs...unless the truth is that...the article is correct - there are no real gains in just bolt-on exhaust mods.

[Samirr76]

Thanks for the explanation!