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!

Here is the only FT86 anti-reversion header
 

Here are some pics before i install it. Remember this is untested technology, and it could totally suck! I wanted more low-end torque and these are the only anti-reversion things I could find to adapt. They are made for Harley's and need some trimming to fit in the after-market header flanges I bought. The pics with the gasket show that it will seal to the flange just outside the weld holding the cone in. The cones are shaped with a taper going in and a shorter taper going out, and will act as a venturi to speed up flow into the header. I wonder if a dyno will go down to 1000 rpm? It should have more low-end torque when I'm done, and with an exhaust back-pressure valve, should get better mileage too.

The sad part is that you don't know if you helped or hurt your performance. You should have done a dyno pull with the in modified headers, the one after modification. A few runs would not have killed the gaskets. Unless you got an extra header... :iono:

I've already tried 2 other headers. They both had the same thing in common. Less low-end torque than the factory header making it harder to drive in traffic. This is my DD and I like the power and acceleration of a header. I just don't like falling behind in traffic because I don't even have the low-end torque of the factory header. The venturi cone inserts should increase the velocity of flow giving better scavenging, better torque. I actually installed it today, but with a stripped bolt in the over-pipe, I didn't get it all put together. Tomorrow I'll post my first impressions.

Are the venturi cones completely sealed behind them? to the rest of the header? Like is there an open space between them and the header tubing

No, they do not touch the header tubes themselves. There is about 1/16 to 1/8" or more between the inside edge and the header tubing. They are only attached at the flanges. Once they heat up they should stay hot, since their heat can only travel out through the flange end, not through the header tubing. A hotter, narrower pipe and venturi will give greater velocity to the exhaust flow than the larger unflared transition from the exhaust port original header. I'm getting dressed for battle now, not only against the cold weather, but also against stubborn nuts and bolts. I'm also need to raise my coilovers 1 1/2" for winter mud and snow clearance. Long day ahead, but I'll let you know my initial impressions of the low-end torque capabilities once I get it back on the road this evening.

That's actually seems sick, the idea of arming the BRZ for winter lol

Yes! It's a success! World's first FT86 anti-reversion header
 

It's back together, tuned with Shiv's latest 2.06 91 Octane Equal Length header tune, and it is fun to drive. Low-end torque and power are definitely higher than with just an ordinary header. I have an A/T and both 5th and 6th are overdrive. I climbed a hill I usually shift down to 5th for, in 6th gear, and even accelerated! I know you flat-earthers won't believe a word I say without a dyno for proof, so I'll try to book some time in the weeks ahead. The important thing is now there is something to give automatic's more low-end torque for better drive-ability. I'll be driving in traffic tomorrow, so I can see how well it keeps up with the V6 and V8 trucks and minivans. PM me for a parts list if you are interested in making one of these for yourself.:burnrubber:

But what is there to compare it to?

Any normal header, stock exhaust, turbo, compare it to whatever you want. I've had stock, SBD turbo, and an Xs-power headers with stock over-pipe, mid, and rear pipes on it before, also stock, dual, and single with dual outlet mufflers. So far the stock setup had the most low-end torque and gave the best mileage. The power and acceleration was sadly lacking though. This setup with A/R header, 2.5" overpipe, 2.5" midpipe, stock rear pipe and top speed pro 1 single inlet dual outlet muffler has more low-end torque, more power, and acceleration you have to see the video to believe. Videos coming later....
[ame]http://youtu.be/sbo8axAwIWI[/ame]

http://www.gomog.com/allmorgan/exhaustbackpressure.html

Exhaust back pressure should be at its minimal extrema, exhaust velocity should be at its maximal extrema.

A good link but it doesn't cover using venturi's for faster flow with lower back-pressure. The after-market Anti-Reversion cones designed for Harley Davidson's(tm) are made to increase low-end torque for engines with larger, after-market exhaust tubing. The cones have an entrance taper and an exit taper and act with a venturi effect to increase velocity of flow and improve scavenging of the burnt fuel/air mixture. Come down and drive it if you like.