jjwalker

Exhaust Gas theory for the RX7. More applied to the NA RX7 vs. turbo.

The facts...

1) Smaller exhaust is restrictive to a point and if you add the catylitic converter and cheap/exotic exhaust systems, this can be an issue.
2)Too big of an exhaust can cause issues as well, especially with restrictive mufflers.
3)Exhaust heat and retention of exhaust velocity can greatly improve performance.

The stock, unmodified exaust starts off with a 2.5 inch pipe through the cats, and divides into two 2 inch pipes going to the mufflers. You can leave this configuration and get some power gains, and it's cheap! This is more of an article on how to maximize your HP gains via exhaust improvements on the cheap.
Let me clarify a bit of more information before I continue, The sooner exhaust gas enters the atmospheric conditions, the shorter your exhaust system is. There are many ways to do this. There are also many ways NOT to do this, and I will explain.

2.5 inch straight pipes are ideal for an NA RX7 from the engine, so if you can get some 2.25 inch splits, you are doing good. Just because you are splitting, you want to make the exit velocity of the exhaust as fast as possible. We need velocity right? Giant pipes out the rear do not provide this. Also remember that this smaller pipe size increases velocity. You have one large pipe evacuating to two smaller pipes of equal size, you have to keep in mind the restriction at the "Y" pipe.
This does not apply to single outlet systems, and I am not covering them here. Only Dual exhaust.
if you want to keep the factory Y pipe, weld on 2.5 inch straight through mufflers closest to the factory y pipe bolting points. You just decreased your length to atmospheric pressure by about 24 inches. It will be loud, but your exhaust will find atmospheric pressure much faster as you have essentially shortened your exhaust length. The longer your exhaust, you more you engine has to work. Intakes work differently, but I'll cover that in another thread.
You can weld on a 2.25 inch muffler and decrease the sound, but you aren't opening to atmosphere as fast.

Evil Aviator

Ideal pipe size from the engine is about 1.75" inside diameter for street engines, plus or minus about 1/16" depending on where in the rpm range you would like to maximize the torque. Smaller pipe moves the sweet spot lower in rpm, larger pipe moves it higher in rpm. For example, high-rpm peripheral ported engines have about 2" ID exhaust piping out of the engine.

Too much velocity (smaller pipe) results in drag, which is otherwise known as a "restriction" on internet forums. Too little velocity (larger pipe) results in a loss of momentum, reducing the effectiveness of the exhaust system. There is a happy medium. You do NOT want maximum velocity on a non-turbo street engine.

A Y-pipe isn't much of a restriction if the Y is kept to a 30 degree included angle (15 degree bends). These are available pre-made from several vendors, such as Burns Stainless. However, there is obviously more drag with a Y-pipe than there would be with a single straight pipe, even if it is a small amount. The other drag problem with dual piping is that there is more pipe area exposed to the gas flow, and more area means more drag. The big advantage of a Y-pipe is that the exhaust is split parallel into two mufflers, reducing the sound, which is a good thing for non-turbo rotary engines, lol.

The exhaust system works best if the length is tuned to the engine. A professional race tuner will "cc" the exhaust system to make sure the pipe volume is optimal. Shortening the length to the atmosphere does not necessarily make it any better. For a basic guideline, see the Racing Beat guidelines for exhaust system lengths.
Rotary Tech Tips: Exhaust System Configurations

Chuckie Hustle

I was wondering about this myself. I just picked up an FC and the exhaust bends are pretty jacked and rub all over the place. It atleast has a nice set of headers. But it needs cleaning up.

Also I love how dude tried to post and eeeeeveryones like uuuummmmm aaaaccctttually hahahahah

wthdidusay82

iTrader: (15)

2.5" should be good for most street cars, 3" or bigger is more for people running alot of boost and have alot of mods, its overkill if you don't need it in my opinion

Rotary > Pistons

jjwalker

The thread was meant for discussion.

j9fd3s

iTrader: (3)

the hardest part of the exhaust selection, is that there are a lot of theories, and its difficult to know which ones, and when to apply them.

for instance, a turbo car has no exhaust tuning after the turbo, the only things we care about are restriction (we can't have too little), and noise. this makes selecting an exhaust size really simple, and 3" works really well.

the NA is more complex because we get a third variable, which is the resonance tuning a header offers.

after the header collector/manifold the NA is the same as the turbo, except since we're at a different power level, we pick a different size. the factory used a 50mm ID pipe on the FC, and this actually seems to be pretty ideal. going bigger does gain on top, but the gains are very small.

the second consideration on an NA is noise, and the bigger the pipe the louder the car seems to be*

the third is the header, and its really easy to get lost in all the ideas with these, when evidence suggests that the rotary really doesn't care much about the header design.


ok so what to look for when building the exhaust?!?!?@#?

1. turbo, just buy an RB system and call it good. the diameters are right, its not louder than a jet plane... performance is fine.

NA, i will offer two, and after the collector these are basically the same.

simple street system; use the stock exhaust manifold. the stock manifold is cleverly designed to act as a muffling chamber and it also acts as an exhaust box. so not only is it quiet, it removes the tuned effects from the rest of the system.

i'd recommend 50mm ID pipe (going up to 60mm ID is fine), the OD depends on the wall thickness you choose, i'd suggest thicker will be quieter, so either .065 or .120 wall is good, or you can buy the RB downpipe.

since we have no tuned effect in the exhaust, this means all we care about for muffler(s) and or a cat is the flow, so you can run anything you want as long as it flows enough.

the header system; this changes things. the header turns the whole exhaust into a tuned piece, so everything works together, this makes things harder. the other frustration is that the off the shelf headers don't match up to the engine very well. the engine outlet is 48mm, and RB headers are 44mm...

the real problem is that the engine outlet is too big, but its easier to port the header. 44mm ID headers are the correct size.

the length is a bit odd, as the wave tuning works in multiples, so a 32" long header tunes to the same RPM as a 120" header. so you have to do some cut and pasting to see what works.

after the header i still like 50mm ID piping. muffler/cat selection should be about the same as the manifold system

so that's all i know...

*noise is weird! the human ear doesn't hear linearly, so there are some frequencies that we notice more than others, so if you're measuring, a change in pitch can seem quieter to the ear, but not register on the db meter.

wthdidusay82

iTrader: (15)

how much of a.difference in power/spool etc would I see if I switch to 3" from my current mostly 2.5" ?

Rotary > Pistons

j9fd3s

iTrader: (3)

a lot.

rx7 SE

iTrader: (5)

^ +1

3" is kind of the sweet spot for a typical turbo rotary. With that said I went down from a loud *** 3.5" JIC straight thru exhaust to a smooth purring 3" RB Rev II and the midrange loss didn't feel like more than 5-10 hp.

wthdidusay82

iTrader: (15)

Id like to upgrade , just curious so I thought I'd ask.

3" on stock Turbo won't cause too much boost creep? I do have my waste gate ported a.decent amount so I think ill be okay

Rotary > Pistons

Chuckie Hustle

After a little research? Looks like the stock exhaust has a y pipe? I definitely have 2 straight back from the headers. No y pipe?

GuiltySoul

iTrader: (7)

Now does the air pump effect the flow of exhaust since it injects air in to the cat, I have a rb header with rb mid pipe(spilt air tube on) with trust cat back

j9fd3s

iTrader: (3)

makes no difference at all. the amount of air that the air pump moves is teeny compared to the engine

fc323

i keep reading about port matching the headers. can someone elaborate on this.
and also what about single out exhaust?

j9fd3s

iTrader: (3)

the inlet of the RB headers are 44mm, and the outlet of the engine is 48mm, so there is a big step there. the older engines are a better match, so really the FC is too big, but there is enough meat in the RB header to go from a step to a bevel

fc323

is it worth it to bevel the edge?
i mean, i can see how big of a difference it is since the exhaust leaves a soot residue that shows the size difference

rx7 SE

iTrader: (5)

On my stock ported turbo I would creep to 12 psi on a warm day and 13-15 psi on a cold on stock 5.5 wastegate spring pressure. Only way I could get it to stay at 5 was to keep around 1/4 throttle...we all know how hard that is

wthdidusay82

iTrader: (15)

with a ported waste gate or just a ported compressor housing?

Rotary > Pistons

rx7 SE

iTrader: (5)

IIRC, it was a s4 wastegate ported large enough where I had a larger washer welded onto the wastegate port lever to still cover the port. It wasn't pretty so might not be a fair comparison?

j9fd3s

iTrader: (3)

yes, the match between the header pipe and the exhaust port is bad! actually the stock manifold inlet is 50mm, so its larger than the exhaust port, and not smaller, which is why headers don't do much on an FC

Whizbang

RotaryGod has covered this topic quite well

https://www.rx7club.com/naturally-as...-tunes-883371/

https://www.rx7club.com/general-rota...length-199788/

https://www.rx7club.com/2nd-generati...icient-820601/

j9fd3s

iTrader: (3)

the exhaust thread is pretty good, Fred is pushing the exhaust box thing, and this is actually exactly what the stock NA manifold is, which is why it works. it lets the engine run with a relatively high backpressure.

the intake thread has an error in the calculation, if you actually read the thread you'd see at the end, he tells you not to use it

jjwalker

The exhaust box theory is interesting, I'll have to do some research on that.

I didn't state this in my original post, but removing the cat and replacing it with a presilencer amde the engine feel way more powerful when VDI kicks in. There is a very pronounced change in power when the VDI kicks in, which I couldn't feel with the cat installed (the cat was less than a year old, so not clogged). I don't know if that has to do with tuning or flow capacity.

As far as CC'ing the exhaust, that's way out of the realm of most car owners. You got to just play with the system until you get it right.

RotaryEvolution

iTrader: (30)

most people won't ever need over 2.25" for an n/a engine in the upper range and over 3" for turbo engines.

but for more power you need more volume, some of the bigger turbos require 4-5" piping to make the best use of the turbo...

Chuckie Hustle

I gotta say after a few days of riding around in this car I gotta say "true dual" is disgusting. The rattle is horrible! It's not bad off throttle. Completely dies down. But on throttle it is hhhhhorrible.