The Most Ideal/Best Flowing Exhaust dimensions?

[Spiff325iS]

Yes, there are shitloads of threads on what people want for a ccertain sound etc and there are many answers..but sound aside, what do you think the most ideal/best flowing pipe dimensions would be for a stock M20 motor with just a TMS chip?

I've got a MF 14815 muffler waiting to go on when the E30 gets out of the body-shop, and I don't want to just get it welded on(unless stock piping is good, which I have heard from a few people).

A few words I've heard tossed around are 'turbulent' having to do with what i'm assuming is interrupted or distorted flow due to possibly EXCESSIVE airflow, regarding single 2.5'' piping..if that makes any sense.

Would it be ideal to run single 2.5'' from the headers back through a HFC and then back to a single 2.5'' then to the muffler(I am still trying to stay somewhat smog legal in CA). Or should I only be modifying piping after the cat? Should I even do a HFC, I've heard they 'burn through' really quickly..

Any info would be appreciated. I know a lot of people have asked very similiar questions, but I couldn't find any specifics on ideal flow(regardless of the sound). Not that I want it to sound like shit, but I want to hear opinions on the BEST FLOWING exhaust PIPING, not just the sound(but sound evidence helps also).

-Dan

[Jordan]

Stock diameter is pretty ideal IMO.

[backtrail69]

Leave it alone. You won't see any real gains other than more noise on a stock motor...

[Spiff325iS]

Alright sweet. I've heard from a few that the stock piping is a good dimension and has good flow, I only considered going single 2.5'' because of less weight, but if the flow is relatively the same, then I will just get the muffler welded on.

Thanks!

[MR 325]

If you do re pipe it, single 2.5 is plenty.

[Spiff325iS]

So would you say 2.5'' from the headers back through stock cat or what?

[MR 325]

If you're going custom, just do a Magnaflow high flow cat or something.

[Spiff325iS]

So if the price is right I'm going to do a single 2.5'' all the way through with a magna flow HFC. Justs to give you guys an update(because I know you care so much). I'll have quality exhaust vids up this week.

[AndrewBird]

What about two pipes from the headers into an x-pipe then 2 into the muffler?

I suppose it would be overkill on a mostly stock engine however.

[StereoInstaller1]

I have a "High Flow Cat" and single 2.5" all ghetto rigged (NMF, PO did it) and it definitely sounds aggressive and way too damn loud. The "Y" is just after the manifolds. I lose a bit of torque, but definitely pull harder up to redline than stock. I am not happy with it, sounds way too "ricey", I want a meatier tone, quieter.

All the kids in town thinks it sounds incredible. So do the cops.

In terms of airflow, instead of CFM, think more along the lines of resonant frequency affecting how the exhaust pulses combine and at what RPM you will have more scavenging. Smaller pipes will give you better torque due to a lower resonant frequency.

Luke

[Spiff325iS]

Interesting...hmmm. Would you think that keeping the pipes after the headers stock, then going to a HFC and then single 2.5'' to the new muffler would be good? What might you suggest then?

[MR 325]

You'd probably want a nice muffler out back and a resonator in the center section to go along with the cat.

[Spiff325iS]

I have a quality muffler waiting to go on, so I think I will go ahead with a HFC and single 2.5" piping ONLY after the cat to the muffler.

[eugeman]

Stock!

[JGood]

In theory, pipe sizing makes a difference. In e30 practicality (with a fairly stock engine), it doesn't.

I switched from all stock, with a 260k mile clogged up cat...

To stainless long tube headers, into Y pipe, to a Magnaflow high flow cat, to a Borla "straight through" muffler, with all single 2.5" stainless piping. Probably dropped weight down 75%. Sounds mean.

Didn't notice a single difference in power.