First Drive with New 3in Downpipe - OPEN!

[habitatguy187]

So I got my megan racing downpipe today, before I had full stock exhaust minus a straight pipe in place of the cat. I plan on bolting a muffler straight up to the downpipe, but I don't have the muffler yet, so I just got antsy and installed the downpipe and left it open to atmosphere.

I expected it to be loud, I've ran open dp before, but I wasn't quite sure how it'd sound on my car since a friend of mine ran open 3in dp on his evo 8 and it was semi-quiet (guess the bigger turbo silenced it a bit more than my 14b) But anyway I start it and it's loud, but not too loud for me, then I start to drive it. It was so loud and vibrating so hard that it was shaking the change in my coin holder. The turbo whistle was extreme too as I sounded like a semi truck everytime I let off the gas.

Performance-wise I can't say I'm impressed. I did feel a better off-boost response but I expected to feel a huge difference, it just wasn't there. I didn't get a chance to watch for spool-up differences since it's wet out and I'm fwd. I also didn't see any boost creep as I expected to, but I only revved to about 5200rpms so maybe that's why.

I have an internal wastegate o2 dump that I'll be putting on tomorrow (like I said I get antsy) I'm still on the stock one right now so maybe the better flow of the o2 housing will also help bring the dp to life.

Fitment-wise the megan racing downpipe was spot on. Looks like a quality piece and I didn't have any problems at all with installation.

 

[viprez586]

Spool up is a little bit faster, top-end really shines.
You need some back-pressure to get that initial pressure to give a quick spool-up.
With how much noise it makes you'd expect huge gains but it's not all that.

 

[JEFFREYCASTGSX]

I know what you mean. My buddy in his built GST has a dumped 3" downpipe and its TOO loud, everything vibrates.

 

[habitatguy187]

I need to take back what I said, I did a 3rd gear pull onto the highway on my way home, and noticed big improvement on the butt dyno. Especially from 4500-ish rpms all the way up. It's pulled like that in second but never in 3rd. Wonder how much the bigger o2 will help... (I'll post back here tomorrow after I put it on)

 

[nkosi]

I have a megan 3" down pipe with a 2.75" exhuast and i notice a huge difference on the top end pull ...my O2 housing i ported to hell and now no boost creep and the car pulls hard at top end ...still have to try it with nitrous though ....that will be later after i change timing belt and related parts this week....

 

[2DSMs]

I replaced my 2.5" cat and downpipe this weekend with a 3" Megan dp and 3" Vibrant cat. All I can say is WOW! Turbo spools quicker and it pulls noticeably harder. It was a pleasant surprise as I was not expecting that much improvement over the 2.5". The car is quieter to boot as I used truck style strap clamps in place of flanges. I say for $65 (ebay) you can't go wrong with this part and definitely a great bang for the buck!

 

[viperlp01]

YA I LOVE THE DESPOOL .... It is soo tight .... everyone is like WTF when i had my open DP... You best bet i am taking off the cat back when i go the track next ... open turbo on both sides..

 

[SpeedAddict62]

Spool up is a little bit faster, top-end really shines.
You need some back-pressure to get that initial pressure to give a quick spool-up.
With how much noise it makes you'd expect huge gains but it's not all that.

I don't have much to say about this post, other than "needing some back-pressure" is just plain wrong. No engine needs back pressure, it has and always will be a consequence of running a pipe that is a compromise between restriction and velocity. Putting a larger exhaust on a turbo car (which is after the turbo), will, if anything, make for quicker spool since there is a greater pressure drop across the turbine wheel.

Oh, and to keep this thread on track - I love running an open DP, the engine sounds pretty bad but hearing the turbo whistle is awesome. I cut the muffler off the stock exhaust once, it didn't do anything for power, and sounded like a tractor off boost, but sounded like a rally car at WOT.

 

[habitatguy187]

Alright so I got the o2 dump housing on today, and I can say that I'm impressed! Install was a bit of a pain (perfect fitment though) but got it done in just under an hour (lift and power tools were used)

I honestly wasn't expecting to notice a difference, but my spool-up and low-end power seemed muuuuch more responsive. Then came the boost.... 18psi and the wastegate opened, sounded like a pressure washer spraying the side of a aluminum trailer. Boost was extremely smooth and the power was strong.

I'm definitely satisfied with these upgrades and to anyone who still has the stock o2 but has a downpipe and catback, I HIGHLY recommend that you get a bigger, tubular exhaust mani (mine's ebay I think) as this upgrade seems to really bring the rest of the exhaust to life.


EDIT: If a mod would edit the title to include o2 dump that'd be helpful.

 

[viprez586]

I don't have much to say about this post, other than "needing some back-pressure" is just plain wrong. No engine needs back pressure, it has and always will be a consequence of running a pipe that is a compromise between restriction and velocity. Putting a larger exhaust on a turbo car (which is after the turbo), will, if anything, make for quicker spool since there is a greater pressure drop across the turbine wheel.

I stand corrected however this article couldn't explain back pressure better.
http://www.dsmtuners.com/forums/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.html

 

[pboglio]

That explanation in that faq about scavenging is 100% wrong. His explanation is essentially the "cadence" theory which was disproved about 50 years ago.

 

[habitatguy187]

I've read from a few wiseman (and a guy I know with a est. 700 somethin (dsmlink est.) dsm) that the turbo is all the backpressure required for scavaging.

 

[lacroixdp]

Here's a more accurate exhaust article I referenced a while back, although it wasn't in reference to a turbocharged engine... it's an interesting read:
Auto Exhaust Science

 

[SpeedAddict62]

I've read from a few wiseman (and a guy I know with a est. 700 somethin (dsmlink est.) dsm) that the turbo is all the backpressure required for scavaging.

Start reading up on exhaust theory and that link that was given above. Backpressure is not needed...period. Scavenging involves introducing a negative pressure wave at the exhaust valve on the exhaust stroke, helping to get rid of exhaust gas and pull in fresh charge. Backpressure would be a positive pressure, so you can see that would not be to our benefit. The perfect engine would have positive pressure in the intake (tuned length intakes and forced induction) and a vacuum in the exhaust. Obviously, this isn't possible...but that is what tuning is about.

If someone wants to discuss this more, another thread should be used, since this is sort of off topic.

Oh yeah, and one more thing, don't take everything you read for granted. The forums are an excellent learning tool but there is a lot of bullshit to sort through from people who say they know what they are talking about and really don't know a thing. Do your homework, research, and find reputable sources.

 

[jrohner]

I don't have much to say about this post, other than "needing some back-pressure" is just plain wrong. No engine needs back pressure, it has and always will be a consequence of running a pipe that is a compromise between restriction and velocity. Putting a larger exhaust on a turbo car (which is after the turbo), will, if anything, make for quicker spool since there is a greater pressure drop across the turbine wheel.

No turbo engine needs backpressure you mean, since the turbo creates a lot of it already. You can easily end up with more pressure in the exhaust manifold than in the intake manifold, especially running high boost on a small turbo.

 

[SpeedAddict62]

No turbo engine needs backpressure you mean, since the turbo creates a lot of it already. You can easily end up with more pressure in the exhaust manifold than in the intake manifold, especially running high boost on a small turbo.

No, any engine. It is purely a byproduct of a compromise on exhaust sizing.

Yap, you can definitely get more pressure in the exhaust than the intake, although its far from ideal.

 

[TurboATM]

Spool up is a little bit faster,.

...doesnt a bigger downpipe and such increase spool time, as in adds more lag...?

 

[SpeedAddict62]

...doesnt a bigger downpipe and such increase spool time, as in adds more lag...?

Read my post earlier in this thread as to why a bigger exhaust after the turbo should reduce lag.

 

[habitatguy187]

No, any engine. It is purely a byproduct of a compromise on exhaust sizing.

I saw a thing in a magazine not too long ago, they had a new civic si, put it on the dyno, showed the chart. Then they added a ram air intake, headers, downpipe and catback exhaust system.

The end result was obviously more power (not much though) after the upgrades, but the power on the dyno chart was changed dramatically.

There was NO low end power with the new mods, the torque was almost non-existent until until over 5k.

Non turbo cars do need backpressure to have some low end power, the dyno chart showed it all.

 

[SpeedAddict62]

I saw a thing in a magazine not too long ago, they had a new civic si, put it on the dyno, showed the chart. Then they added a ram air intake, headers, downpipe and catback exhaust system.

The end result was obviously more power (not much though) after the upgrades, but the power on the dyno chart was changed dramatically.

There was NO low end power with the new mods, the torque was almost non-existent until until over 5k.

Non turbo cars do need backpressure to have some low end power, the dyno chart showed it all.

No again. The reason is, backpressure didn't cause the loss in torque. Sure, it could have been lower with the mods, but that isn't looking at the full picture.

The intake/exhaust system must be tuned to a specific length and diameter. It is all about getting as much air as possible into the cylinder. The problem is, everything is a compromise. A system tuned for producing high rpm torque might fail at producing low end torque, while the opposite may also be true. Stock systems are and always will be a compromise to provide a wide powerband. That is good when you are racing too, but top end torque might be more important, so the engine is tuned as such.

The point is, that Honda didn't lose power because all of a sudden it didn't have "backpressure," it was because the intake/exhaust simply weren't efficient until high rpm. When the exhaust valve is opened, a high pressure pulse is generated. When this pulse hits a change in area (a large chamber, end of the pipe, etc.) some of the pulse is reflected back as a "negative" pressure. Exhaust tuning involves timing that pulse (by adjusting the length of the pipe) so that it reaches the port during the overlap period, providing a pressure difference across the intake/exhaust, thereby scavenging the rest of the exhaust out of the cylinder and bringing in fresh charge. That is great and wonderful, but one length of exhaust pipe that works at a certain rpm doesn't necessarily work at others, and so you must compromise. The same thing goes for pipe diameter - a large pipe flows well but velocity is poor, a small pipe has high velocity but flow will suffer.

So, a simple look at the dyno results and one might assume that the car lost low end torque because it doesn't have backpressure, but that simply isn't taking the full system into account.

 

[habitatguy187]

The point is, that Honda didn't lose power because all of a sudden it didn't have "backpressure," it was because the intake/exhaust simply weren't efficient until high rpm.

But the intake/exhaust was efficient before in the low rpms. So you can't blame it on the motor, it was in direct correlation to the exhaust modifications.

So yes, the Honda did lose power because it didn't have backpressure, you can't blame it on anything else.

 

[SpeedAddict62]

But the intake/exhaust was efficient before in the low rpms. So you can't blame it on the motor, it was in direct correlation to the exhaust modifications.

So yes, the Honda did lose power because it didn't have backpressure, you can't blame it on anything else.

Um, you said: "they added a ram air intake, headers, downpipe and catback exhaust system." So they changed ALL of the exhaust system, and half the intake. I'm pretty sure that means that whatever it was before, is not what it is now. Yeah, the stock intake/exhaust was efficient...how does that make the new one efficient too?

Yes, it made less power at low rpms. Yes, backpressure (the average pressure in the exhaust system) was probably lower, but there is no cause/effect relationship here.

So you know how I said, don't believe everything you hear? Well, don't believe what I say either, really, I mean it. Instead, go find some books on engine theory or talk to an engine designer. That doesn't mean listen to some internet guy who claims he knows it all, or some guy who made 700 hp because a computer said so, go find some real information. Here is a good book to start with: Scientific Design of Exhaust and Intake Systems.

After you do that, you can come back and explain to me, in scientific terms, how a higher pressure in the outlet (exhaust) of an air pump (engine) could possibly increase torque output.

 

[romeen]

But the intake/exhaust was efficient before in the low rpms. So you can't blame it on the motor, it was in direct correlation to the exhaust modifications.

So yes, the Honda did lose power because it didn't have backpressure, you can't blame it on anything else.

I had posted this link in a previous thread that you had started regarding exhaust back pressure.

Turbo Exhaust Theory

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

 

[viperlp01]

You dont need back pressure if your making full boost by 3k... you simply aren't going to create torque with pistons that have a comp ratio under 8.5.. If anything the 1g would have the worse torque numbers with 7.8:1.. The 2gs which are known for being torque "monsters" have the 8.5:1s....

 

[SpeedAddict62]

You dont need back pressure if your making full boost by 3k... you simply aren't going to create torque with pistons that have a comp ratio under 8.5.. If anything the 1g would have the worse torque numbers with 7.8:1.. The 2gs which are known for being torque "monsters" have the 8.5:1s....

I'm pretty sure what you mean is: you don't need scavenging if you have full boost by 3k rpm...

Yeah, scavenging isn't nearly important on a turbo car as it is on an NA car (thanks for the link romeen, thats a good page) since its more important to get the turbo spooled and fast.

And yes, I bet a 1G turbo block without a turbo would be an absolute slug to drive, haha.