sosonic

I'm a bit confused about this and was hoping some experts could enlighten me. For the an NA RX-8, would you be sure to lose power by changing to a larger exhaust pipe diameter?

With turbo, it makes sense to have a larger exhaust diameter to help expel exhaust gas, but that is not the case with NA. I've read the concept of exhaust velocity and back pressure. I don't think back pressure applies with the NA RX-8, but exhaust velocity seems to. The exhaust gas velocity theory is something like a smaller pipe speeds up the exhaust gas and a bigger pipe would slow it down.

But what if you modified your air duct and air intake (thus getting a bit more air)? Would a slightly larger exhaust pipe diameter help or hurt?

The default exhaust pipe size is 60mm (2.3 inches). Many aftermarket makers have 65mm (2.5 inches), 70mm (2.75in), etc... pipe diameters with 90mm (3.5in) to 100mm (~4in) tips.

Anybody do a before and after dyno with different diameter exhausts and have info.?

Brettus

iTrader: (2)

Interesting topic .
from what I have read here it seems a 3" system will give you some gains & backpressure is not an issue with the renesis .

sosonic

3" pipes (not talking tips, but before the muffler), which would be in the 80mm range in metric , would be good for turbo, but not NA RX-8. From what I've read and heard, its seems that people with 70mm pipes and above were losing power on NA RX-8s. The "fat pipes" appear only good for turbo. Now superchargers would be a different argument and I'm not clear on that one.

Big tips 90mm/100mm or around 3.5" tips are fine and look good, so that is OK. I'm talking fat piping (70mm/2.75") and above) before the muffler seems bad unless your turbo.

65mm (~2.5") like with the R-Magic exhaust or weird size changes like 76.3mm to 60.5mm on the Fujitsubo exhaust (before the muffler) are question marks, but it seems they might be just OK (R-Magic).

At 65mm (~2.5") range seems that you will lose a bit of low end hp/torque, but gain it back and some more above 5,000 rpm (depending on the exhaust design).

So overall 65mm/~2.5in. may give you HP, but it would be a low end vs high end issue for NA RX-8s. The same thing appears to happen with 70mm (2.75") piping and up (loss on low end and maybe something back or more on the high end). That is if the exhaust is the type that saves weight vs a heavier than/as stock exhaust.

Edit- The Mazdaspeed single muffler type was 65mm (before muffler). Single muffler size can be slightly larger than the dual muffler design. The dual muffler design appears to me more efficient, thus slight smaller pipe size is OK.

2.3" to 2.4" (~60mm) exhaust piping before the muffler (dual muffler type) seems about right for NA.

Edit- Example would be the Autoexe exhaust (60.5mm....which has some odd manufacturing relationship/partnership with Mazda Japan)

Now why this is so, is something I would like the experts to comment on. I think its important, because it seems a number of RX-8 owners have bought the wrong size exhaust pipes for their setup. So it would be nice if we could agree on and state approximate exhaust pipe size for different setups and mods.

Moostafa29

3in pipes made my 8 a lot faster than stock. Yes the difference was much greater with turbo, but was still significantly faster when I had it NA.

sosonic

What exhaust do you have?

Also, if your exhaust is light weight (or Ti) than there may be some gains from that (its not so cut and dry). It would seem that lightweight 3in pipes might also give some hp over 5,000 rpm gains, but I'm hearing about low end hp loss issues.

N10S

Although its obviously not the same as the RX8/Rennesis, in the old days when 2-stroke dirtbikes were the deal, the manufacturers introduced a power valve system that restricted the exhaust at lower RPM's to gain more power and torque. Once the rpm's increased the "power valve" would open fully letting the exhaust flow wide open for maximum full throttle power. This was not a variable valve timing situation, but a simply a variable exhaust restrictor to allow more power at the low-end.

I think the premise is that porting your exaust into a larger, smoother pipe/pipes promotes enhance velocity and flow. Get too big though, and the amount of gasses being expelled through an exhaust port is overwhelmed by an over-sized exhaust pipe resulting in gas expansion, loss of velocity and cooling, which results in denser slower flowing exhaust gases. The bottom line would be that based on the size of the engine and output of exhaust gases, the exhaust sizing and flow capabilties could be optimized for the engine, but exceeding its capabilites would result in less power.

With a turbo set-up this is less of an issue as the exhaust of a turbocharged car has a lot more output potential and varies with the amount of boost being run. Exhaust sizing is still an issue with turbo cars though, and in particular you have to watch the affect a catless DP or overly large exhaust will have on a turbochargers internal wastegates abilitity to keep up with the newfound flow. The result can be boost creep which is definitely not a fun thing to deal with.

Anyway, I absolutely believe that an overly large exhaust can cause problems for an NA car.

O'Renesis

Exhaust theory and practice for 2 stroke engines does not apply to a rotary engine. So basically the rotary engine does not need back pressure or any form of purging to help with torque/horsepower at whatever revs.

Moostafa29

I had Turbo XS exhaust and racepipe. Although the gains were great, it was a little too loud for continued daily driver street use.

N10S

I think I said that the 2-stroke engine situation was not the same as the RX8 Rennesis, but thanks for re-stating it. The fact that oversizing an exhaust for an NA car or a turbo car can be a problem is the point. Regardless of whether its NA or FI the best exhaust solution is always going to be one that is tuned to the engines needs. Blindly thinking bigger is better is a fools game.

O'Renesis

So I need to restate what I meant, a larger diameter exhaust pipe, ie 3" instead of 2" or 2.5", from my personal experience was beneficial to my engine for both torque and horsepower at all revs. I have just recently reinstalled the stock exhaust system and I am very sure that the drop in torque is very noticeable especially at lower revs. This manifests itself as having to rev the engine harder/higher to get a quick and smooth take off from a standing start. Also throttle response is much better with the 3" exhaust than with stock system. So bigger is better, but I don't know when an exhaust system for the Renesis is too big? That is another question entirely.

N10S

Yep, I understand what you are saying. The "right size" for the exhaust scenario is the point in question. Maybe 3" is the right size, I don't know the answer to that. It would be cool to take a stock car and do a baseline on the dyno, then add a typical 2.5" free flow, then go to a 3" to see if it makes a noticeable difference one way or the other. Considering no exhaust seems to make a great deal of difference I suppose the delta between the two would be marginal regardless, but it would be interesting to see nonetheless.

I can remember reading info on the aftermarket headers for the Rennesis and how they were actually resulting in losing hp, so it would be interesting to confirm. I remember a guy that recently installed an Agency 3" exhaust and commenting that his car fell slower..? Obviously there are lots of guys running the 3" Borla and TXS exhaust that really seem to like them and don't complain about the car feeling less powerful.

O'Renesis

Further to this discussion I have measured the total cross sectional area of the stock exhaust header ports, 3 in total, which measures approx 5,165 Sq Millimetres. The cross sectional area of a 75mm ID exhaust is 4,717 Sq Millimetres. (I hope my maths holds up, please correct me if I am wrong). This would lead me to believe that even a 75mm/3" exhaust pipe could cause some back pressure? assuming the header is also a 3" design. I could only measure the stock header but would have preferred to have been able to measure the cross sectional area of the ports on the Renesis engine. But I still think this shows that the exhaust pipe could be even larger in diameter than 3". As to whether this is of any real benefit to a NA Renesis engine is open to debate. After all you gotta get it in before you can get it out, and that's where FI comes in.

sosonic

That's what I'm trying to understand. I talked to the R-Magic guys and they said they have dyno charts showing a slight HP loss at lower RPMs and HP gains at higher RPMs in NA RX-8s.

Their exhaust has 65mm/2.5 in pipe before the muffler and 90mm/3.5 in tips. Their exhaust is lighter than the stock exhaust (and lighter than Racing Beat's exhaust) and they claim a 10hp+ overall gain. R-Magic is no joke in Japan and have been doing rotaries for a long time.

Then there are guys I e-mailed with the HKS exhaust with 70mm pipe before the muffler claiming they might have lost some power. HKS never claims to gain power on the low end but to "maintain it" and give power on the high end.

I've noticed that people with the larger exhaust diameters before the muffler, seemed to be losing power on the NA at the low end.

I'm wondering if we have the info. from dyno charts and user input to start matching the right exhaust pipe size and the right exhaust maker for the type of RX-8 people have. What's best for AT RX-8s, MT RX-8s, NA RX-8s, Supercharged, Turbo, etc...

Just to make all this even crazier, if the exhaust pipe is too small for what you have than it appears you will get the "pop" sound with the really large tips. I don't know what is going on with the stock muffler that prevents pops. Is it the stock muffler or is it the smaller stock tips that are helping?

sosonic

N10S


From what else I've read and talking to racing/mechanic types, N10S appears to have been on point...

The larger exhaust pipes can slow gas velocity down because of gas expansion and cooling. Thus the exhaust gas becomes harder to expel and you may lose some power at the low end.

Smaller exhaust pipes maintain exhaust gas velocity, which can create a vacuum and better scavenging.

Too small, though, would cause back pressure (which is bad) and cause the engine to work harder to expel the exhaust gas.

Therefore the exhaust diameter should match your engine size and type of exhaust (single or dual).

For an NA 2.6 L (piston) engine at around 200 HP to 300 HP, that size appears to be 2.3 to 2.5 in. (60mm to 65mm). Interestingly, the stock RX-8 muffler and mazdaspeed pipe size before the muffler is in that range. Of course their problem is the too small tips and the single muffler does not appear to be as efficient as a dual muffler (dual muffler pipe sizes can be a little smaller than a single muffler).

It appears the exception would be turbo, so you would be at least 2.5 in (65mm) up to 3 in (76mm). If you don't have turbo, than such large pipe sizes can be hurting your low end power.

TeamRX8

iTrader: (25)

believe whatever you want ...

sosonic

Well no exhaust and no cat may produce some gains. It would just be as loud as hell, anger your neighbors, and add to the pollution.

Feel free to elaborate on your point of view.

maxxdamigz

iTrader: (1)

Exhaust tuning is like voodoo engineering. You can make a case for lots of things but until you bust out the hot wires and pressure taps, it's tough to have an idea of what is going on. I like to use a combination of 8 inch PVC and stirring straws myself.

TeamRX8

iTrader: (25)

I already have, regurgitated book theory never outguns actual experience:


https://www.rx8club.com/showpost.php...0&postcount=51

https://www.rx8club.com/showpost.php...&postcount=406

https://www.rx8club.com/showpost.php...&postcount=159


Mazda provided a tuned system, just because you change one part and see a negative outcome doesn't mean it doesn't work, it may mean that you simply have more work to do to fully realize the potential

sosonic

TeamRX8, first off, I truly respect your work as a racer. But...

You did not disprove the theory of exhaust pipe size should match your engine, HP, and setup. Which is why I agree with Maxx and Charles, and would love to see more scientific testing in this area. RG, seems to have played with this a bit, but there are no guidelines out there.

TeamRX8, your custom exhaust was also dropping a large amount of weight. So you might be getting gains in this area.

You also have been doing other mods too. So, the dyno is not clear if its a before and after of just the exhaust setup. Which by the way, has dropped a lot of weight in comparison to the stock one.

Dropping weight, should almost always work. The problem is that you usually have to go Titanium for major weight saving and this cost major dollars... Unless they have your type of skills to come up with something custom.

Then there are issues like design, material, and if the pipe or muffler is obstructing flow in anyway. Your custom design may allow exhaust gas to flow more efficiently, despite the size of the pipe. The pickup in the efficiency of expelling the exhaust gas may compensate for the cooling and slowing effect of the larger pipe size.

In the case of the R-magic exhaust (@ 65mm/2.5in), they seem to take a bit of a hit in the low end, but make up for it overall in the high end. So overall, they show a net gain in HP. They also are lighter than stock too.

Its not a clear x size will give y result type of trade-off. I'm simply going for a general guideline, based more on what is available to be bought for the RX-8.

sosonic

I'm an NA Auto, so I want to preserve the low end. If I was MT, than I would not care so much, as I could drop it at something crazy like 6,000 to 7,000 RPM.

In daily situations, you spend a lot of time below 4,000 RPM. So that is an argument for keeping as much low end as possible when going through traffic, even for an MT.

My later moves will be SC (waiting on Cobb or SC ECU flash and see what happens to 1st buyers) and then revisit the auto torque converter issue. When I go SC, then I will go 2.5" to 3" exhaust. As NA, I keep it 2.3" to 2.5".

TeamRX8

iTrader: (25)

like I said, beleieve whatever you want

all the verbage in the world isn't going to accomplish anything other than taking up forum space ...

the weight loss means zip on the dyno. but just to summarize, I have a custom filter on a stick that supposedly loses power, a custom log exhaust manifold with oversize short primaries and large bore exhaust that supposedly loses lowend power, etc. Yet I make more Tq/Hp than OE from 2500 - redline

I could care less if you respect me, just wake the f-ck up ... sheesh

TeamRX8

iTrader: (25)

no, sorry Charles, I'm addressing Sosonic, though I'm not sure why I'm wasting my time ...

sosonic

I think the SC will work better with an AT. The boost is instant and more power is given in lower rpms. Because of the torque converter issue, this helps the AT in 0-60, 1/4 mile, and in general as a daily driver. The biggest issue with the AT is getting it going from a stand still... Once the AT is moving... Its much closer in performance to its MT cousin.... especially if you have an ATF cooler, flash the ECU, etc....

As for weight... It does nothing on the dyno, but it helps on the street or race course. You get the "feeling" of more power and better track/drag times if you drop enough of it and maintain or increase HP. So the weight factor can be a plus/minus type of thing. You might be doing some weird stuff, but if you keep dropping weight you could be going faster. What would the RX-8 be like if it was 2,700 pounds or less?

TeamRX8... don't you think you are being a bit hostile? Do your thing... Win your races... no need to get a stroke over anything... Hell, I'll even root for you...

However, I'm going to do and believe as I like. I'm not about launching insults over the Internet, causes as you know, that is even more of a waste of time than almost any thread. Everybody, that I've seen, just wants to go a little faster and get some thrills from it...

TeamRX8

iTrader: (25)

I just love it when someone with little/no practical RX-8 experience suddenly thinks they're Mr Expert because they read it on teh intraweb ...

TeamRX8

iTrader: (25)

LOL, don't make me show you how put nitrous on an RX-8

obvious j/k ...