My buddy is looking for an fc to replace his s13. He emailed someone local about his 91 n/a, and got back a huge email describing the rx7 and rotary engine in detail. this was part of it:

"The biggest misconception on the Rx-7 is that the engines don't last and need rebuilds... usually true for one reason only.. people don't know the cars the 13b engine has half the moving parts of a conventional piston driven engine and will last and last and last if maintained and the back pressure on the engine is correct... most Rx-7's mufflers last for five years or so then they go out... to replace them with factory mufflers or designed to spec for the 13b was over $900 in 2003?(not sure the year i replaced them). I put on mufflers that were more expensive than the factory but designed to above factory spec with a lifetime warranty on them cause I was never going to sell my baby. Most people when hit with the cost buy what any muffler shop will put on cheap like 100-300 bucks with a warranty on the muffler But The back pressure is INCORRECT and it causes the Seals in the Engine to Fail and require an engine rebuild.. so most websites and people think the Rx-7's engine is only good for 60-120k then it needs a rebuild but that's only because they screwed the cars engine with the wrong back pressure from cheap mufflers. The year I replaced mine my best friend had an 90 Rx and his mufflers failed he put an a set for $150 with a two year warranty on the muffler i warned him against it but he couldn't swing the $900 six months latter the seals on his engine blew and he had to buy a a cheap used car as he couldn't afford the rebuild and another beautiful rxended up in the wrecking yard."


edit- this is part of a second email: "As for rebuild's mine has never had one and should never need one. they can last forever like toyota's with 600,000 miles not km with out rebuilds. But you have to realize that 99% of these cars end up with the wrong mufflers and that ruins the engine and requires a rebuild then the reworked engine with the wrong back pressure can only last somewhere from 50,000-150,000km before needing another rebuild depending on how hard it's being driven. so all the websites and almost all the owners all the mechanics will say the engines need rebuilds and they are speaking from real world experiance, it's like people dying in hospitals in the 1700's because doctors were ignorant of the concept of invisible germs that can spread with out hand washing... they just don't realize how incredbily tight these engines were designed and how such a small thing as a slight, small variation in back pressure over time will destroy the seals in the rotary...but don't take my word for it if you like the car and agree on a price have the car inspected by BCAA they charge around $150 but they will meet you at the my mothers test drive it give it a 120 or so point inspection including a compression check on the engine if it needed a rebuild it would show up there."

so.... fact or fiction? i'm running a 2.5" exhaust from my manifold to a presilencer which replaces the cat, and then the 3" corksport catback.

 

No, they don't need exhaust pressure, and CERTAINLY not to keep the seals from failing. The NA stock setup for the auxiliary ports needs exhaust pressure to open them, but some aftermarket systems account for that or instead you can rig them to open via RPM switches and little motors or an air pump.

 

It usually has LESS than half the number of internal parts...

Bullshit on the back pressure comment.


Bullshit.


Bullshit.


Story might be true, but it has nothing to do with backpressure.

Bullshit.


Bullshit.


Bullshit.

So this guy is...well, BULLSHIT.
I would stay away from such cons.


-Ted

 

Thanks for the elaboration, Ted.

 

thank you for confirming my thoughts ted

 

Fiction
too much backpressure( like from a shop who thinks your car has a 4 banger and throws on a tiny muffler) can cause a performance drop.
and too little will cause you Aux. Ports from opening and you loose top end power.
but as for backpressure causing the seals to go out? pa-lease
how ever the people that usually go aftermarket( or STR8 pipes) are more likely then not to be revving this **** out of the motor alot more. as a result the engines go quicker( or all at once like a friend who though he could rev his 12a and hold it at 8k like i did on my 88 NA( but mine didnt blow, although i was trying!), but his wasnt warm yet, so pop!

 

Sup man. Rotaries obviously do have alot of backpressure and obviously that wouldn't have any thing to do with apex seals going. Looks like he thinks he knows what hes talking about though.

 

Wow, where did you get this info from?

 

i have no clue where he got it from. and nice sig btw travis lol

 

Rotary engines are very hard on mufflers, and cheapie mufflers will get blown out quickly. I think the originator of the email is just confused by the REASON our cars need good mufflers.

It is true that a compression check is a very good indicator if the engine needs a rebuild. If you are considering buying an RX-7, I recommend paying the $150 for a professional inspection, as stated in the email. An engine normally lasts about 200,000 to 250,000 km before it needs a rebuld.

As for the 2.5" - 3" exhaust, that is a little large for a NA RX-7, but it will not hurt anything but performance, and maybe the noise level, lol.

 

back pressure is good for torque and bad for gas mileage. 3" mandrel with a aftermarket cat is the best setup IMO.

NA: Pacesetter header - 3" highflow cat - RB turbo catback.
T2: RB Rev II - chopped downpipe with add in 3" highflow cat. (I am so loving this setup on my S5 right now. Fast spool and horsepower upgrade, passes emissions, no need for FCD, and is whistle quiet )

that guy is an idiot or thinks you're a sucker.

 

I'd tell your buddy to have the car check out by someone who knows FCs cause obviously the owner has no idea

 

backpressure is merely a by-product of an exhaust setup; it does not directly translate into better torque. It's not like saying, you should aim for as much backpressure as possible to gain torque.

It's all about properly sized exhaust systems. Size it for the rpm range that you want your power for.

 

I feel like I am on a personal crusade against this line of thinking.


Backpressure ISNT good for torque. Backpressure isnt good for making power at all. It is a restriction in the exhuast that requires energy from the combustion cycle to move. In other words the engine spends power to move the exhaust.

The idea that it is good for torque comes from V8 guys 'losing' low end power when they put headers or large exhausts on their cars. They didnt LOSE anything. What happened was the power band shifted up int he rpm range. They produced more power under the curve, but their torque curve shifted, so off idle it felt like they lost power. It wasnt lost, just moved.

The fact that this was happening had to do with scavenging and peak flow of the exhuast system. The stock system on most cars reaches its peak flow at lower rpms. At higher rpms it chokes up requiring power to push the exhaust out. When you open that exhaust system up the peak flow point moves higher and continues making past the point of the stock system. The effect is that it makes it peak power higher. So instead of it making peak power at 3000-3500 rpms it makes power at 4k and above.

The ultimate effect was that people thought the backpressure from the stock exhaust "made" torque, when in reality the larger exhaust just moved the torque away from the low end.


Having an ehxuast that is too large has the efffect of inducing back pressure all over as the exhaust loses velocity and requires the engine to push it out. Typically a 3" exhaust is too large for a naturally aspirated engine. A 13b is no different....


BC

 

I believe 2.25" ID on a n/a is large enough. Any bigger and I think you could start loosing performance. Go smaller and the velocity will be higher along with the pressure. Need to have a balance.

I used to love hearing the v8 guys say they burn valves because they put a high flow exhaust on and this was because there was no backpressure......

 

Holy ****... 99% of these cars ends up in the hands of people who think they know what they are talking about... and are too proud to research. that is why they end up in the junkyard.

 

Backpressure is good for NOTHING! You never want backpressure ever. It only hurts power. It never helps. What you do want is the proper flow amount at the max usable exhaust velocity for the rpm and load you are looking to make the most power at. What flows the best at a lower rpm is a restriction at higher rpm hence the use of the term backpressure. Above it's peak efficiency zone, it is indeed restrictive. What flows best and high rpms has too little exhaust velocity down low and hence makes less power down low. True it has no backpressure but that's not the issue. When people talk about backpressure making low end and no backpressure making poor low end power, they are incorrect. It's all about exhaust velocity and nothing else. Anyone that says that backpressure is important to power anywhere on any engine clearly doesn't know how exhaust gasses work. 2 strokes are a bit different but that's not the topic of this thread.

 

new to this forum and everything, but i've heard this debated several times on the mx6 forums since ive been there for a long time. its always between people who say you need back pressure for an engine to work right and people who say no back pressure is need. as said above, the correct answer has to do with exhaust velocity. imagine trying to blow through a a straw that's (for a general example) 1 inch wide, its easy to blow through but there is no velocity at all. conversely, if you blow in a very small diameter straw, the air coming out of it is faster, but there is not as much of it coming out at once. the key is balance. this guy sounds like a tool.

 

Lol, BS articles like this were made for RETed. Not a word of truth in the article.

 

It sure sounded convincing though. The amount of BS flung around was impressive in two paragraphs.

Here is something for the experts, I'm looking at you rotarygod, the RX7 that took the runoffs win this year has 4" exhaust, as do a few of the other top 5 that were all RX7's.
I've done a bunch of testing and understand gas flow a little more than most(own a Superflow110) is this overkill or do you think they are gaining anything? Figure they are never below 4500rpm, probably closer to 8K most of the time.

 

wow 4" , i'm sure the gains over a 3 or 3.5" are small, or maybe a 4" muffler flows like a 3" pipe or something?

 

There is a bit more to it but I suspect a race engine probably had a proper merge collector with a divergent cone (megaphone) that transitioned gently to a 4" pipe. That changes the rules a bit as you will widen the powerband this way but at the expense of extreme peak power. It's average that wins races though.

On a collected exhaust system you really need to think of it as 2 parts. Part 1 is anything before the collector. This is where most of the initial tuning will be and where pipe size and length is the most critical for velocity to determine where your power peak is. The collector merge itself is also critical as far as smoothness and angle of convergence. What happens in the collector itself is a bit interesting. When the exhaust gasses collect, their phase reverses and now a negative wave is a positive wave and vice versa. That means if you have a negative wave moving towards the engine from the collector, the same negative wave is also moving out the exhaust pipe at the same speed. This also applies to the positive wave. Keep in mind I'm not physically talking about the airflow itself but rather the acoustic tuning standpoint of it which does also have an effect on internal pipe pressure.

The size of the collector must not be sized too small or too large as this will effect tuning. For example let's say we have 2-2" exhaust pipes coming off of the engine. We don't want to just dump them into a 3" or 4" single pipe. We want to dump them smoothly into a collector that has the area of a 2-1/4" or 2-1/2" pipe and then gently transition it to a larger pipe such as a 3" or 4" pipe. What this has done is allowed the two waves to combine smoothly with no harsh velocity change. After this point is where we want the divergent cone. This cone can be thought of just like a megaphone.

A megaphone will make your voice louder but it will also make your voice sound different. You know a person who is talking through a megaphone when you hear it. It's hard for me to describe. The megaphone in the exhaust is oding the same thing. The reason a megaphone makes your voice louder is because it is amplifying your voice through acoustic leverage. The reason your voice sounds different than normal is because it can't amplify your entire vocal range. This is the same in an exhaust system. It amplifies the waves over a range. In the case of an engine this means over a certain area of the powerband. It widens and strengthens the powerband. A larger megaphone will amplify a larger range. However don't think that larger is always better. One thing about megaphones (or horns) is that the larger they get, the lower they are effective. This also means that they cut off the top end. One thing about a horn is that it has a high end and a low end that it tunes to (and everything in between). Above and below these points, it falls off very sharply. If you had a megaphone on your exhaust that was designed with a top limit of 7000 hp, above this point it's benefit will not only fall off quickly but it will actually fall off at a rate much steeper than if you didn't have a magaphone. The same is true below it's lowest tuning point.

If you look at a Formula 1 car, take a look at their headers. There is no megaphone. These cars have a very narrow powerband too. They have no need for a megaphone as they have extremely close gear ratios that maximize such a narrow powerband. While average and not peak power is what wins races, their average is over such a narrow rpm range that a megaphone's benefits can't be realized.

The rate at which the megaphone diverges also affects powerband. The longer it takes to get to the max dia, typically the better but space constraints are also potentially an issue. There are other things that can be done with these that I've never seen anyone try yet. A straight megaphone divergent cone mathematically is referred to as a conical horn. There are other ways to flare it out though and each has their strengths and weaknesses in different frequency ranges. There is the exponential flare, the tractrix, the hyperbolic, the parabolic flare and others I'm probably leaving out. There are also combinations of these. I'm not getting into all of that here as you'd be better off studying a textbook on acoustics.

It's very possible that there are race engines running a 4" pipe somewhere after the collector. After the collector it's all about acoustics. Before the collector it's about acoustics and velocity. Keep in mind that exhaust designs like this are trying to get that last extra horsepower out of the engine. A very basic exhaust system will get you most of the way there. For racing though it's all about that last little bit.

 

Awesome reading once again.

 

The gods have enlightened us

 

you need back pressure to open the 6PI on a s4 or pumpless s5 n/a. You throw in a cat to add this back pressure and now you have functioning valves. Having that cat there also improves your low rpm torque and this is dyno proven. I've ran many exhaust setups and the two I posted are what I feel as city-perfect.