exhaust systems-bigger always better?

So, I have been thinking about playing around with the exhaust on my celica. The stock system uses a huge, and heavy primary cat on the stock down pipe, along with a secondary cat further down the road on the smallish stock piping. I already have an aftermarket 3" down pipe that is waiting to be installed, but I am wondering about the rest of the exhaust. Are their any benefits or downsides of running a straight exhaust without a muffler? I also plan on running a full 3" exhaust. If I was to do this I would incorporate a cat somewhere in the exhaust piping.

Also, recently I have hear lots of talk regarding exhaust back pressure. A friend swears that his sbc makes less power with open headers and more power with a muffler equipped exhaust. So what is the deal with back pressure? Does it vary from engine to engine? 2.0 4cyl turbo compared to a 350 v8?

In short Kinda, most turbo systems the more air you can move the better, N/A systems it usually just hurts it.

I am sure others will get more in depth with it.

Since you're turbo, things simplify greatly. The best exhaust for a turbo is no exhaust. It's more a question of fitting into the chassis and weight. The lightest way to get the exhaust safely out and away from the cabin while meeting all applicable rules, regulations, and/or laws while still being large enough to flow well.

Ballpark? 3" is usually about right for a turbo 2.0L, if millions of Honda guys are any indication.

The simple way I've heard it explained is that the turbo makes all the backpressure the engine needs, anything behind that just makes life difficult for the turbo...

so on turbo engines bigger is better...

what about v8's?

Usually, unless you care about noise. Bigger pipe = bigger noise.

For n/a, you want zero "back pressure", but good exhaust scavenging...

backpressure is always a bad thing (i'm sure everyone's always heard about how an engine is an air pump and blah blahblah) but it is possible to over scavenge the exhaust and literally pull the air/fuel mixture right thru the cylinder without burning all of it by going too big with the pipe. this isn't an issue with a turbo- the less resistance after the turbo, the better.

novaderrik wrote: it is possible to over scavenge the exhaust and literally pull the air/fuel mixture right thru the cylinder without burning all of it by going too big with the pipe. this isn't an issue with a turbo- the less resistance after the turbo, the better.

I do not think this is accurate. The bigger pipe would not draw as much vacuum as a smaller pipe owing to gas velocity in the pipes. This is the reason for retaining a smaller size, higher exhaust gas velocity to a point allows for greater scavenging effect.

Besides, valve timing will negate the effect that you are referencing.

I would think for a turbo (post turbo) the sizing would not matter as much as there wouldnt be timed pulses from each cylinder firing as it would be smoothed out by the turbo backpressure/turbine/etc.

It may be possible to get close with catch-alls, but it would always depend on the particular system. You would either have to do some fun fluid dynamics calculations, and/or lots of dyno testing to really perfect a system.

Wanting back pressure on an engine is like wanting constipation in your bowels. You can't eat more if it ain't getting out. The right sized exhaust pipe is like the right kind of regular; not too little, but not too much either.

Backpressure is a word that should be saved for Chiropractors.

Exhaust is much like intake - you want adequate flow to make HP, but adequate velocity to help scavenging.

But... In a turbo car there is no need for scavenging since you have pressurized intake, nor is there very much cam overlap to create time to scavenge anyway. Plus, too much overlap just means you'd be pushing the intake charge right out the exhaust.

Diesels (both turbo and not) often have zero cam overlap, so the best exhaust is the biggest... or none.

We did an experiment a few years ago, and put a very low restriction turbo exhaust on an other-wise stock Miata.

Dyno chart (PDF). In case you didn't guess, the red line is the big exhaust.

It's been my understanding that once you get past the header collector, you're pretty much looking for as little backpressure as possible. All the magic happens in the primaries and secondaries. The dyno chart would back that up. For those who don't know, 1995 Miatas (such as the one on the dyno) have a 4-1 header from the factory.

interesting. so for an engine as primitive as a sbc(n/a), back pressure beyond the header collectors should not be an issue?

I'm going to install this on my car soon. Test results coming.

smog7 wrote: interesting. so for an engine as primitive as a sbc(n/a), back pressure beyond the header collectors should not be an issue?

I don't think that's what they're saying. More that once you get past the collector you no longer care about velocity. You just want flow.

That said, I would still go with as small as you can without hurting power. A 2 inch exhaust is significantly lighter than a 4 incher.

Backpressure would not be a benefit. It's still an issue in that it can cost you power, but it doesn't help.

curtis73 wrote: Backpressure is a word that should be saved for Chiropractors.

Thank you.

smog7 wrote: So, I have been thinking about playing around with the exhaust on my celica. The stock system uses a huge, and heavy primary cat on the stock down pipe, along with a secondary cat further down the road on the smallish stock piping. I already have an aftermarket 3" down pipe that is waiting to be installed, but I am wondering about the rest of the exhaust. Are their any benefits or downsides of running a straight exhaust without a muffler? I also plan on running a full 3" exhaust. If I was to do this I would incorporate a cat somewhere in the exhaust piping. Also, recently I have hear lots of talk regarding exhaust back pressure. A friend swears that his sbc makes less power with open headers and more power with a muffler equipped exhaust. So what is the deal with back pressure? Does it vary from engine to engine? 2.0 4cyl turbo compared to a 350 v8?

After you fix everything else, i would go no smaller than 3" full exhaust on that car. No muffler might raise some eyebrows, but it's not that bad on a 3sgte. Looks a little funny, though. If it were MY car, i'd do a full 3" to fart can of my choice.

anybody have any experience with how the location of a muffler in a system would affect sound/performance? like if you ran a straight pipe with a muffler under the middle vs. right at the rear bumper?

WilberM3 wrote: anybody have any experience with how the location of a muffler in a system would affect sound/performance? like if you ran a straight pipe with a muffler under the middle vs. right at the rear bumper?

I also need this answer

I went from a 2.25" under-axle fart-can on my Corolla, to a 2" over-axle with resonator AND muffler, and had a noticeable improvement in throttle response and power. And a much less annoying sound.

As for muffler placement, it's not quite so cut and dried, because different engines and exhaust lengths and sizes will have different frequencies and flow rates.

--sarge

Pretty sure SCC disproved all this backpressure scavenging mumbo-jumbo about 15 years ago. Bigger pipes make more power everywhere, or at the very least does not reduce power anywhere.

i think i remember hearing about a Muscle Mustangs or equivalent ford mag do a comparo between stock, full aftermarket x-pipe, h-pipe, and open header systems and the open header didnt make the most power and lost a bunch under the curve. anyone else recall this article?

Either Burns stainless or Headers by Ed has some really good reading on the exhaust comparos. Basically most headers are crap and have worthless collectors so the section immediately after the collector is very important to the exhaust systems behavior.

curtis73 wrote: Backpressure is a word that should be saved for Chiropractors. Exhaust is much like intake - you want adequate flow to make HP, but adequate velocity to help scavenging. But... In a turbo car there is no need for scavenging since you have pressurized intake, nor is there very much cam overlap to create time to scavenge anyway. Plus, too much overlap just means you'd be pushing the intake charge right out the exhaust. Diesels (both turbo and not) often have zero cam overlap, so the best exhaust is the biggest... or none.

Right!

I am just going to follow you around and tell you you're right all of the time, apparently.