Everyone knows that stainless exhausts last much longer than mild steel ones. So how come the stock exhaust is not stainless? It seems to last OK. And I was taught in Materials Engineering class that carbon steel can withstand high temperatures better than steel so thats why exhaust manifolds are typically carbon steel and not stainless. So are there realiable downpipes and other exhaust components out there that are made of mild steel but that are durable?

 

Mils steel is cheaper, so with the exception of the Corvette Z06 (titanium)that's what ya get.

HKS makes a mild steel DP and it is heat coated.

 

Stainless steels main advantages as an exhaust are that it rejects heat 215% per foot better than mild steel and has higher melting point (Higher than Titanium, actually- but lower than Inconel).

This means Stainless systems keep the exhaust hotter, thus keeping it less dense and easier to "push" out of the exhsust system and can be made much thinner and lighter than mild steel due to its heat resistance and strength.

The rust protection is simply a nice side benifit- and somewhat false, as when stainless is heated carbon precipitates from the matrix and oxidization occurs. Check a stainless downpipe where it first bends and passes close to the firewall and you will see mild surface oxidization.

Now, factory cars in the future are likely to feature more stainless "headers" (especially of e-pipe- or double walled design) as cold start emmissions standards force manufacturers to keep the exhaust as hot as possible going into close coupled catylitic converters. Another alternative is for them to cast the entire exhaust manifold integrally into the head casting and have the catylitic right off that like Honda's new Accord engine- lets hope this is NOT the norm as it makes aftermarket performance exhaust difficult...

Ian

 

Well, I think I understand what you saying but it still leaves me wondering:

1.) If the stock exhaust is mild steel (right?), why can't a downpipe be made of mild steel and last?

2.) Is there a mild steel downpipe, or, midpipe, or cat-back that is durable and made of mild steel?

 

1. It can last, it is all relative to the conditions you live in. Someone in Chicago may have a lot more trouble with rust than say someone in Arizona.. it's not as if these aftermarket mild steel components won't last as long as the stock system, they are just as prone to problems though. Most people opt for a SS system because it is only a tad more expensive most times, and will most likely last the life of the car and beyond.

2. Yes, it's not as if mild steel is totally inferior to SS.. you can get a DP, MP, or catback in mild steel from a lot of places.

 

For instance Racing Beat uses very thick mild steel for its exhausts and then coats them w/ chrome like ceramic coating. Very heavy!

But, having a thick walled exhaust system really quiets down the car as less sound energy escapes out the piping walls. I have a ~1.5mm thick SS 3 1/2" turbo back exhaust and it is very loud- even before the muffler. You can hear every exhaust pulse leave the turbo and dump into the DP. Before this I had a RB 2.5" downpipe and it was much quieter.

Mild steel is fine, but Stainless is better because it will provide more HP and be lighter. Titanium is great since even though it has to be a little thicker (melts at lower temp) it is 60% lighter than steel. Yes, they can all last for years and years- My point was corrosion resistance is just a side benifit of stainless systems, not the main reason for its use in performance exhausts!

Ian

 

so which DP would be best then? I mean, I was originally thinking of going with a stainless steel one, but as far as fittment would go, is it better to replace the stock studs? Or just to keep them, as I've read that they can be a pain to get out, as they crack, strip or just dont come out.

 

As w/ everything, there is no best. There is best fot this purpose and best for that purpose.

Only you can determine what is best for your purposes...

 

ahh the fortune cookie response.... Im kidding.

I guess you are right though, as there is no real best. However, I meant best in the sense of: ease of installation, price, and value or rather, bang for the buck. Whats gonna last the longest, be the toughest or durable to the extreme heat, and of course perform.

 

any DP will outlast your car [period].

HKS (MS) and M2 (SS) both fit perfectly & allow you to keep the stock studs (you should, however, replace the nuts as they are only meant to be used once - they crush themselves into place - $4 each from Mazda). There are a few out there that don't fit perfectly.

You should make your choice based on availability, price, and ease of installation, as your butt dyno won't be able to tell the difference in the material you use.

 

OK, I think I get it. In a mild climate (I'm in Houston) and from a practical standpoint stainless is not much more durable than mild steel. However, it is going to be lighter and (because no coatings are required) it will have a greater bore allowing for superior horsepower gains.

I think the majority of people out there believe mild steel will disintegrate in a few months and so is just not worth buying.

 

There is no way that SS is going to provide more HP than MS.

Larger bore does not neccesarily mean performance gains either, on ANY motor. Many times larger bores are detrimental.

The wall thickness of a metal pipe has absolutely nothing to do with decible ratings, unless you are talking about one made out of aluminum foil vs. heavy gauge steel. Sound travels with air, not through solid materials.

A downpipe is a tube, plain and simple. The most important issue is ease of fitment and the quality of the welds, every other aspect is academic. Any name brand downpipe will work just fine - get a good used one for $100 to $150 and put your cash toward other upgrades.

Oh yeah, use NEW studs, nuts and gaskets and don't forget the high temperature anti-sieze.

 

i have a japenese engine in my car. it came with the stock japanese DP. i am guessing that it is mild steel because it came from the factory....i dont mean to get off subject but what is the difference between a stock Japanese DP and an aftermarket one performance wise?

 

Quote-There is no way that SS is going to provide more HP than MS.

Then why are ceramic coatings and thermal wrap used to keep exhaust gasses in the highest energy state as possible? Yes, more HP. Noticeable, no- but in racing, every bit counts.

Quote-Larger bore does not neccesarily mean performance gains either, on ANY motor. Many times larger bores are detrimental.

On a turbo vehicle you want to maximize the pressure differential between the two sides of the turbo exhaust wheel. Bigger is better on a turbo vehicle EVERY TIME as it allows the turbo to spool more quickly!

Quote-The wall thickness of a metal pipe has absolutely nothing to do with decible ratings, unless you are talking about one made out of aluminum foil vs. heavy gauge steel. Sound travels with air, not through solid materials.

Sound easily travels through the downpipe walls. I am comparing ~1mm thick SS Spec 90 to 4mm thick RB downpipe. It is just the same as when you remove the insulation from the car interior- you hear the downpipe more.
You have never noticed you can hear the engine running right next to the engine and not just from the tailpipe? Where is that sound comming from? OK...

Quote-A downpipe is a tube, plain and simple. The most important issue is ease of fitment and the quality of the welds, every other aspect is academic. Any name brand downpipe will work just fine - get a good used one for $100 to $150 and put your cash toward other upgrades.

So you think a 2.5" mild steel DP w/ crush bent construction and standard turbo flange- as long as it fits and will hold up well will make as much power and be as good as a 3.5" SS DP w/ mandrel bends and flange made to divide the WG flow?

Quote-Oh yeah, use NEW studs, nuts and gaskets and don't forget the high temperature anti-sieze.

Agreed! Once you use anti-sieze and non-corroding nuts you can crack the nuts loose and take them off w/ your fingers! I have taken my DP off about 6 times...

Ian

 

Q... what is the difference between a stock Japanese DP and an aftermarket one performance wise?

A.... I think the stock Japanese downpipe has to be constricted to get around the right hand steering column. US based units don't have the constriction and so are freer flowing. The materials used in the Japanese unit should not be significantly different to the US counterpart.

 

Maximizing the pressure drop across the turbo (exhaust inlet pressure - exhaust outlet pressure) increases horsepower because the turbo spins quicker. That seems reasonable.

A wider exit pipe allows the engine to eliminate the hot exhaust at a lower exit pressure by providing lots of room for it to flow.

An insulating pipe will keep the exhaust hotter, which keeps the density down, again reducing back pressure. Stainless or coated mild steel tend to be best for this (but mild steel must be more heavily constructed).

Sound definitely travels through 'materials' because it is simply a pressure wave that vibrate molecules in a specific direction. It usually travels more quickly through dense medium. For example it travels 5 times faster in water than in air.

By opening up the exhaust do you increase the torque and horsepower range all the way up and down the RPM range?

 

Well, I could argue the fine points forever, and spending money on my car has never been an issue, but here's my experience:

I first put on a Bonez ss downpipe ($365.00) that was mandrel bent and completely round, left on the main cat, and had an N1 single cat back ($450.00). The torque band had a pronounced shift to higher revs and the "sound" was horrible - loud and "tinney". Just like all the Honda ricers. So I bought the silencer ($55.00) and now it was quieter and tinney. There was no question I was gettting more "bump" at higher revs, but at the expense of driveability and mid range response. I also hated the "single tip" fart tube look. I also had invested $870.00 at this point and was not happy. So I sold all this stuff. Oh yeah, I had boost spikes to 17.

I then bought a used "cobra head" ms HKS dp for $150.00 and wrapped it with header tape, a DynoMax Ultra Flow muffler for $79.00 minus a $20.00 mail in rebate, and twin stainless steel tips for $75.00. Oh, and some pipe and clamps. Grand total:
$294.00. I got out the welder and slammed it all together.

I now have MUCH more pronounced mid-range torque,
(thanks in part to the "cobra head" and SLIGHTLY more restrictive DynoMax exhaust). I have lowered the decibel rating dramatically to where it still has an authoritative "sound" - and the best part is the deep bass element to it - have a better looking *** end as well as an extra $576.00 in my pocket. I am now happy.
My boost spikes are now 12.

I think either set up "might" be good for an extra 20 hp max over stock. I sure as hell would not spend the money for a dyno run to find out. Mid range torque is what is important to me, especially on an inherently peaky engine.

The moral of the story - you can "tune" an engine with exhaust components, and you don't need to spend big bucks to do it.

 

So why do we lose low/mid end torque when we open up the exhaust? The turbos are spooling very quickly at the lower RPMs and the pressure drop across the turbos is maximized (especially if you are hitting 17 psi)?

 

This would require comparing dyno runs. I wouldn't doubt a pronounced power increase at higher rpms on the less restrictive exhaust would make it "feel" like you had less midrange power.

-edit- didn't need sig...

 

Back when I was a wee laddie in the 70's I raced two stroke bikes in motocross, shortrack, trials and a even a bit of roadracing (and illegal drag racing). I worked after school as a mechanic at the Harley shop, and specialized in Aermachhi two strokes imported and branded by Harley. I raced every weekend for 6 years. My best friend's dad owned the shop, so we pretty much had free everything at our disposal. For a teenage kid it was a dream come true.

I pretty much built each bike from stock. The most important part of each bike was the expansion chambers. We would spend hours and hours discussing how the pressure waves would bounce around inside there before exiting. We were all going after maximum scavenging in order for the engine to "spool up" faster.

Depending on the shape, diameter and length of the pipe we could literally shift the torque around the powerband, depending where it was most useful.

For motocross and road racing mid-range was the best, for shortrack mid-high was best, for trials low was best and for dragging ultra high was best. Each pipe was welded together individually, and I'll bet I built a couple of hundred pipes for other guys as well. It was the "holy grail" of tuning. It was a lot guessing, but we developed a very sensitive "butt dyno". Rotaries are a lot like two strokes, and with the advent of sophisticated dynos it is now possible to measure these shifts.

Generally speaking, ultimate horsepower ruled, but even though my two stroke bikes would stomp equivalent four strokes in drag racing, I frequently got my lunch handed to me in motocross, road racing and short track by much lower hp four strokes - why? They had more torque in the middle of the powerband, and the longer, slower power impulses meant better traction and more "thrust".
They could literally accelerate quicker at the speeds we were racing (70 to 100 mph) than I could, even though I had 30 to 40% more horsepower.

Most, if not all of the guys on this forum want the biggest dp's they can find, and the biggest pipes and tips because they think "bigger is better".

If you want to build an all out dragster I would have to agree, since you are going to spend 100% of your time in the top of the powerband. But if you want a useable "track" car (and that's what the RX7 really is) an exhaust system that shifts the torque back toward the middle of the powerband is much more satisfying and easier to drive, and will be quicker around a roadracing circuit than an equal horsepower "peaky" engine.

It all depends what you are after. I could not find what I wanted (since all the FD3S aftermarket systems are pretty much all the same straight through "big" pipes in order to pander to the popular marketing misconception) so I built my own.

If you want a sluggish, quiet, balky, hard to drive car that overheats ALL the time and will shorten the life of your motor and turbos, leave the complete stock system in place.

If you want a responsive, reasonably quiet, cooler running and very driveable car that will not automatically grenade you will need to have a dp and some restriction in the exhaust, whether by pipe size or muffler.

If you want an all out horsepower monster, that will be louder than hell, a bitch to drive around town, that will require a beefier clutch, larger injectors and sophisticated electronics to keep things in check, and a major polluter, go straight through with the biggest pipes you can find.

But back to the original question: ss or ms? It doesn't really matter...

 

Very interesting...

I am not personally interested in a dragster, although I can see why some find them fun. I would rather have the road race car primarily because it plays to the RX7's strengths and true purpose. When I decided to buy my first 3rd gen the image I had in mind was more of a fast, alround performer like the Ferrari or Lotus and not simply another version of the Camaro or Mustang.

So, from what I have heard..... for a better road racing car I need mid range torque. To get that I want a downpipe and some kind of downstream restriction... maybe the RB dual or M2 dual catback (or maybe just stock?).

The choice of stainless downpipe versus mild steel is not very significant except that the mild steel unit may be somewhat heavier. The durability of either style is similar.

A downpipe will tend to increase the car's noise level although it shouldn't rival the neighbor's dog and you won't be embarrassed take someone out.

 

Well, I think you've got it pretty well figured out, but dump the stock exhaust, it is truly a pos, and is a major restriction from the internal guts rotting as well as the severe convoluted bend it takes to clear the rear axle.
It is a turkey invented to satisfy one client: the EPA.

I think either one of the catbacks you proposed are good, and the Petit is highly regarded but damned expensive. The N1 dual is a little too loud for my tastes.
ANY downpipe is a 100% improvement over the pre-cat.

Straight lines in exhaust systems are good.

Good luck and happy rotoring!

 

Here's a question and speaking of bikes... does Supertrapp make an exhaust for the FD? I've seen them on S2ks. Supertrapps have removeable disks on the tips that allow you to adjust for noise and backpressure. Could you tune the car for low,mid, or high range peak power at will? Or would boost psi make it difficult to dial in?

 

Is there a Supertrapp for an FD?

Yes I think there is. Mike Ancas refers to one in his book, 'Mazda RX-7 Performance Handbook', MBI Publishing, 2000 pg 66. Looks kind of heavy.

 

MS vs SS dp/mp?

SS is lighter but costs a bit more. We drive performance cars so lighter is better, period. Flame away.

MS is typically thicker and therefore no issue with longevity even in houston (where I live and have run ms dp and mp for years by Mindtrain, never any corrosion ... SS dp and mp were not available back then).

If you want to insulate for reduced underhood temps which you should btw (where most of the neg hp loss is found, flame away), use the wrap or ceramic coat it. Header wrap is cheaper, but may reduce the life of MS a little. I'd buy SS and wrap it.

Look for used dp and mp in the for sale section.

I enjoyed the want-a-be metallurgist comments. IF you don't know metallurgy try to refrain from parroting back stuff you have read or heard, only confusing these poor folks in here. After half the stuff I have read about metallurgy on this forum I get confused and have to resort to talking with Metallurgists at work and even (gulp) read some tech references.