btchnzcar

not...Ok the exhaust has been installed. It sounds good, and I noticed a slight increase in high end. However, it KILLED my low end. I knew I should have stuck with the Y pipe design.... Anyway I am now thinking of just going with a dyno proven, engineered cat back. I am thinking Stillen. Now, given that this is also a dual, would I notice the same affects as with my custom dual? I want to keep that low end pull that the VQ has. OR, what if I were to just fabricate a high flow Y-pipe and use a single inlet version of the muffler I have on there now?

Wired 24/7

iTrader: (2)

How about if you get some air/fuel ratios and see if you need a tune before you spend more money swapping exhausts?

P.S. did you change out the cats at all? They are the biggest restriction in my eyes, not the exhaust itself.

btchnzcar

I could try out a tune. But what I am thinking is because I went from a Y design to a dual, I lost too much back pressure, and that's why I lost torque. The cats stayed. The new exhaust started after the cats.

davidv

iTrader: (2)

Do you have a before and after dyno for your exhaust install?

btchnzcar

No I don't. I'm sort of broke at the moment...Do those of you who have true dual exhaust systems (Stillen, HKS, etc.) notice any decrease in low end torque? I'm wondering if I just need a tune to get my low end back, or if I need to have the Y-pipe and single pipe to keep my low end.

davidv

iTrader: (2)

Intake and exhaust mods alter the torque and horsepower curves. Feelings are very subjective. I have no dyno charts, but do my testing in the real world. Bolt-ons, such as the cat-back exhaust, have helped improve my ET.

charlie_rdstr_Z

iTrader: (2)

That is just intake and exhaust alone?

davidv

iTrader: (2)

May 2006 would have been the mods in my signature except the Motordyne spacer.

btchnzcar

I'm just wonderinig if the dual exhaust systems typically lack on the low end to give you high end, and if the single with Y-pipe systems help more on the low end.

btchnzcar

Crap, I just realized something. Because of the design of the Magnaflow muffler (basically an X-pipe design inside the muffler), I have two equalizers in my exhaust. So, on Monday I am going to remove the H-pipe and just run two separate pipes to the muffler. That should give me some of my back pressure.

Wired 24/7

iTrader: (2)

Honestly I'm not sure why you're spending so much time messing around with catback only. If you have money and time to play the game, why not approach this problem logically. Your stock exhaust from the cats back is nothing but hollow tubes. Unless you're maxing out the flow in those tubes, there's not much restriction.

The cats, however, are full of mesh that physically blocks flow. Whatever you do after the cats still can't change the fact that your cats are the biggest restriction.

My $0.02

btchnzcar

I was thinking about that earlier. But after feeling how much low end I lost with just the catback, I can't help but think that I will lose ALL low end power if I put in test pipes. Am I totally off?

Wired 24/7

iTrader: (2)

Exhaust systems are tricky beasts. It is not a constant flow system, which would be easy to design. It releases hundreds of pulses of gas every second, and more pulses per second at a high RPM.

In no way can backpressure actually help you. The exhaust gases want to come out of the cylinders, and backpressure does not help them come out.

You might think "I just put a huge exhaust on my car thus removing all backpressure. But I felt a loss in torque. Therefore backpressure must be important for making torque." This rationality is not correct...

What helps an exhaust pulse come is a fluid dynamic effect of the momentum of the previous exhaust pulse. Momentum is proportional to velocity. Velocity decreases as pipe diameter increases.

Consider this scenario:

At low RPM you have not much gas coming out of the exhaust. A "small pipe" is well suited for this and the velocity will be high.

Using this theoretical "small pipe" you increase the RPM. More flow is coming out, and the flow is starting to feel more friction from the walls of the pipe, bends, flanges, welds.... at this point your velocity has become so high that friction is becoming a factor. So you increase the pipe size again to a "medium pipe"

So now at lower RPMs, you have a lower exhaust gas velocity in the medium sized pipe. That means potentially lower torque at lower RPM, because the gases are not being whisked out of the cylinders as fast as before.

So you have basically tuned your engine for a "medium" RPM.

Same thing happens if you go to a "big" pipe, you are tuning your engine for a higher RPM.

So basically what davidv said,
is very true.

There is always an OPTIMAL exhaust design for a particular goal. I don't know much about the world of turbos...... but it seems that in the land of turbos bigger exhaust actually IS better. The reason for this is that you are trying to get the most out of your turbines as possible, and the easiest way to do this is to ensure that the pressure difference before and after the turbine is maximized. That is best accomplished by having a large and very free flowing exhaust. This is simply not the case for a N/A motor though.

It gets complicated when you add in H-pipes, y-pipes, or whatever.

But think about a stock catalytic converter... full of crap inside. This crap acts as an orifice which will briefly accelerate exhaust gases, at the cost of extreme friction. Not a good scenario for higher RPMs...

By using a modest sized testpipe or simply by going to a higher-flowing cat with the same pipe size as stock, you will definitely gain more power throughout the powerband than you will lose down low.

Your other exhaust ventures, I can't say. Maybe they're tuned for much higher RPM... or maybe they're just not a great design. Pictures are worth 1000 words (hint) and I don't believe you've even mentioned pipe diameters at all in this thread...

jondimellow

iTrader: (3)

nice post.... come on, bust out those navier stokes equations!

Wired 24/7

iTrader: (2)

I am a chemical engineer and I do know those equations, but I'm afraid a car exhaust system is WAY too complicated to model for me. I'd need Femlab and about 3 weeks

btchnzcar

Wow good information Wired 24/7. That all does make sense. Thanks for the post. All in all, this exhaust just feels a little too ghetto for me. I think what I am going to do is just have him put my factory exhaust back in, and replace the cats with test pipes. That should do the trick.

myZter

iTrader: (7)

nice post
i also just read top 100 questions.

99atlantic

iTrader: (1)

headres+hfcs = gains across the entire band, deeper/louder exhaust note, no losses anywhere