LocostUSA.com

Hi Guys,

I'm building my miata exhaust now. It's a non turboed setup. Do the exhaust runners have to be equal length?

Also

The miata runners are 1.5 inch diameter to a 3 inch cat to a 2 inch exhaust. I plan to do something where its 2 inch pipe to 2.5 inch collector to a 2.5 exhaust. Would the o2 sensor be okay with this configuration?

There is absolutely no requirement for them to be equal length, as most factory systems are not, but it is a generally desirable characteristic for performance. So figure the closer the better, but it quickly gets into the realm of diminishing returns too.

Unless you're building something pretty wild engine wise, the 2" primaries will be too large. I believe the most popular aftermarket 1.8L Miata headers are 1.5" primaries. If it were me with a mild 1.8 Miata engine, I'd run 1.5" or 1.625" primaries for 32-34 inches, dump them in sequential circular cylinder pulsing order into a nice long merge collector with a 1.75" or 1.875" (respectively) outlet, then have a gentile transition cone bring it up to 2.25 or 2.5" for the rest of the exhaust.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

^This. And the O2 sensor will not care what diameter tubing it is in.

Hi Zig!

Thanks for the information. I really appreciate. The reason why I said 2 inch runners is that im using 1 1/2 inch pipe. I ordered this header flange from PRL and the price was really good. For 1 1/2 inch pipe, the ID will be 1.6 ish, which is what you recommend. The OD of the pipe is 1.9

https://prlmotorsports.com/miata18headf ... 5wQAvD_BwE

With a 1.9 OD, I plan to have it all go to this merge collector.

https://www.summitracing.com/parts/stw- ... /overview/

Since it seems most stuff is 2.5, it will go ta flex connector,

https://www.amazon.com/gp/product/B017A ... UTF8&psc=1

Then to my muffler

https://www.amazon.com/gp/product/B00TD ... UTF8&psc=1

Then to my exhaust tip

https://www.summitracing.com/parts/SUM-691200

I know its not the most efficient exhaust, but if it will work decently well, I will be happy.

The schedule 10/40 pipe is usually used to build a turbo header. You could build a full N/A header with it, but even schedule 10 is nearly 1/8" thick and weighs 2 lbs/ft. If you have four 32" runners going to your collector, that's 23lbs in runners alone, plus collector and flanges. With a massive 1/2" thick header flange at probably 8 lbs and your collector being another couple pounds, you just made a 35lb header.

For reference, each of these has a 5/16" header flange, 4 x 33" 1-5/8" mandrel-bent primary, slip-fit collector with an 18" long 3" secondary and a 3-bolt flange. Each complete header weighs 15 lbs.



Also, you picked a mild steel flange, unknown pipe (probably mild?) then a stainless collector. Do you have the equipment available to weld mild up to stainless?

Your head flange and piping are turbo parts and are way overkill for an N/A header. A 5/16" or 3/8" head flange is more than enough for a simple N/A build. You can buy a set of mandrel bends for cheap. I built those headers for less than $300 for the pair.

_________________
Ford 5.0 into an M3
mikaelvroom.com | @MikaelVroom

Hi Laminar,

Thanks for your reply. I just realize what you say is true. The material is mostly made for turbo, and may be overkill for my applications.

I think I can save weight, since I dont plan to have 32" inch runners. I plan to have the collector inside the engine bay, and the runners as short as possible. The runner ipiing will also be schedule 10 SS.

I have the migwire and gas for doign stainless steel. I plan to do a SS gas tank eventually to replace my plastic one, so the investment on that side isnt bad.

Even though I make the header lighter, is there a negative affect of having too heavy of a header? I Am just guessing its extra weight and maybe more stress on the engine mounts.

Primary and secondary diameter and length have a large effect on N/A powerband. If you try and make the primaries "as short as possible," you may end up killing your powerband.

https://www.hotrod.com/articles/ford-35 ... e-headers/

They went from shorties to long tubes and gained 5% throughout the whole powerband.

Since you're making the headers yourself now, there's not really any extra cost to optimizing them for your desired powerband.

Obviously packaging is a concern but if someone offered you 5% more power for free, wouldn't you take it??

_________________
Ford 5.0 into an M3
mikaelvroom.com | @MikaelVroom