Back Pressure
 

Okay so forgive me if this is stupid or if it's been covered before. Couldn't find anything so here you go.

Basically, I'm learning about exhaust and everything. This is the first car I've modded so every mod I do is my very first experience with it. I've got an aftermarket intake, now I'm looking at the exhaust system.

I've picked out the headers, downpipe, and exhaust I want based on quite a bit of research. One thing that I always see conflicting answers for is back pressure. Basically I'm eyeing a 3" exhaust. I have two people (including the owner of the local tuner shop) telling me I need to wait until I install a turbo (which I do plan on) before I install a 3" exhaust because I'll "lose back pressure."

At the same time, I have several others (including a friend whose family has been into professional racing for generations) telling me that's stupid and that the only downside is that I may not see the full power gain until I have a turbo.

What detriments or dangers would there be if I put on a 3" exhaust? This is with stock headers and a stock downpipe. Will I just lose potential power, would it actually decrease power, or neither? Will it be bad for the engine in any way?

Most of this stuff I've picked up on pretty easily but I'm sick of hearing two opposite answers. I feel like there should be a pretty fine line here between "do it" and "don't do it."

I can only add my personal experience. When I put on my 3 inch header back I felt no decrease in power but a huge increase in noise. When I swapped to a catless header I felt a loss in low end torque. Unfortunately I have no dyno data to back any of this up so it could be viewed as a pointless post.

My view on all this is why would you buy your exhaust system twice? If you buy a 2.5 inch system for right now, and then buy a 3 inch for when you turbo that's just pissing money away.

People that say things like that are idiots. (The ops tuner shop) A 3 inch exhaust on a low hp na car will not "lose backpressure" it will lose exhaust charge velocity and scavenging potential which will increase backpressure slightly. 3 inch is a little big compared to the gains available.

Bigger isn't always optimal. http://blog.modbargains.com/exhaust-...ou-go-too-big/

I appreciate the info. I should have been more clear on my first post haha if It turns out to be better for me to wait til turbo I'm not gonna get a 2.5" exhaust, I'm just gonna buy the 3" and wait to install it with the turbo all at once.

If it maybe helps, the exhaust I'm looking at is the Tomei Type-80 3" titanium single exit exhaust.

So would you then recommend a 2.5" exhaust, even keeping in mind I plan for a turbo and about 300-350hp?

No. 3 inch is fine for a turbo car over 300whp. And so is a 2.5 overpipe. 3 inch is big for a 200hp na car.

Gotcha. Now would you happen to know if I'd actually lose power by installing this exhaust early? Would there be any possible damage or drawback?

Backpressure doesn't exist. As said, it's velocity and scavenging effects.


Now, 3" on an NA car is going to be extremely loud until you get a turbo to muffle that somewhat. Even on a turbo car, 3" is loud if you don't have a good system.


In reality though, because you have the stock header with the cats, you're not going to have any real issues as the true bottleneck in the system are the factory cats. Yes it will be loud, but performance really isn't going to change.

I read a David Vizard article years ago that suggested the myth about back pressure being good for lowend torque comes from using too much cam overlap and that backpressure stops airfuel mixture flowing straight out of the cylinder thus losing potential torque. So header and cam tuning can overcome this and still give you the free flowing top end from a larger exhaust.

I will see if the article comes up with some googling.

EDIT
I didn't read through it all but I think it was this article:
http://www.superchevy.com/how-to/exhaust/0505phr-exh/
or it may be in his cam selection article which is also worth a read.

Many years ago I attended an F1 race and had a pit-pass ticket. I managed to have a quick chat to one of the guys from Williams F1 team later in the day because I was wearing a Nismo Calsonic t-shirt and he struck up conversation saying he used to work with that team in Japan prior to getting into Williams.

Anyhow long story short, we talked about exhaust systems and he mentioned that with NA cars you generally don't want to go too big as you need to control the resonance (or something like that) that helps pull the gasses out of the cylinders. He mentioned that it can sometimes be tricky to get the right exhaust setup with NA and you'd need to try different diameter, length, and bends to get the right balance within the desired powerband.

FYI, he said that when it comes to turbo's, it pretty much a case of bigger is better to loose as much back pressure as you can.

I put a 2.75" catback on my car and will speak from my experience. I feel there is a slight lose at the low end and gain on the high end, with torque that is. I don't have a dyno to prove this but it is how it feels. It does not feel as peppy as it use to up to the torque dip, but it breaks free from the dip faster.

Also resonance and noise is unbelievably loud. If it were me, I'd only buy an exhaust once. and if I were going turbo it would be a 3". Deal with the noise and resonance for now. Unless you plan on getting a turbo a year or two down the road.

Yes, it's about velocity when it comes to low-end torque, but reducing back-pressure will also reduce MPG. I am waiting for parts to make a true anti-reversion header, not a stepped header. This should give both low RPM velocity, and high RPM flow, with much lower back-pressure than stock. I hope to install it next weekend, Nov. 15-16th. I will post my findings afterwards. I also want to pair this up with a vacuum operated back-pressure valve to give me better mileage on the freeway. Back-pressure is only good for increasing MPG, not for increasing torque or horsepower.

Keep us posted about the header!