13b-rotary-vw

Alright,

I would like to know if anyone out there has ever dyno tested primary exhaust piping that have been as long as 5, 6, 7 feet long???

I also like to know if anyone has attempted any exhaust systems that have used primaries that are in the range above 4 feet??? And if so please post the results...

I have actually made some primaries that are about 7' 10" long, I intend on placing a "Y" pipe after this and running another 7'10" out to the back (This is going on a VW beetle) and this is all just to see the results...

I will post the results as soon as I can obtain them...

gracer7-rx7

iTrader: (16)

I personally don't know but I doubt that long. There is probably a point of diminishing returns. Try searching on posts by rotarygod. He might have had some comments about something that nutty.

13b-rotary-vw

As a matter of fact,

I read a few of his posts on tunning of intake runners and such, and in the thread there is reference of having long primaries of 94.5 inches to obtain a specific tuning (more over all power for the power band) Here's one:

I thought I'd have some more fun by doing some more math. I'll skip the long stuff and get straight to the answers.

For a stockport exhaust on a 2nd gen n/a, the perfect length headers for short primary and long primary systems are as follows:

To tune at 6500 rpm (stock intake tuning frequency)
This is the best number to tune for based on stock gearing.
short primary: 23.1"
long primary: 95.4"

@7000 rpm
short primary: 21.37
long primary: 88.5"

@7500 rpm
short primary: 19.89"
long primary: 82.54"

If you used the streetport template on the exhaust
@6500 rpm
short primary: 21.8"
long primary: 90.2"

@7000 rpm
short primary: 20.17"
long primary: 83.69"

@7500 rpm
short primary: 19.89"
long primary: 82.55"

Notice how the streetport is tuned lower for the same length pipe than the stock port is. This only gets worse the larger you go. The intake numbers work the same way.

Knowing the above numbers, somebody go measure the pipe length on a set of Racing Beat headers and tell me why the hell they are tuned so well? Are they? No! This only gets worse if you are porting the engine. They tune too low in the rpm range for a short primary and are not nearly long enough for a long primary. Also knowing the above numbers does anyone want to speculate on how well the Pace Setter headers tune your engine? I'll give you a hint: The front exhaust runner is over 8" longer than the rear!!!

The reason that a long primary works so good over the entire power band is that we are now utilizing the 1st, 2nd, 3rd, and 4th order waves twice each! This gives us a much broader power range.


So, in doing my own research I figured that having long tubing will extract lower resonances and increases the range of other harmonics. I don't have any equipment to verify anything and I'm just experimenting and having fun with a theory of my own, and I think that it's cool that someone else has had the same ideas that I have. The funny thing is that I haven't seen anyone try it. I do agree that it seems impractical but who knows what I may find on this journey...

13b-rotary-vw

I had a finding today,

I noticed that my engine has a much harder time starting without my exhaust on. I took them of today and attempted to fire it up and realized that I had to crank it for a while. This happened a few days ago as well...

When their on it fires up like nothing. Just a quick jolt of the ignition and it's on. Also there's a pretty good lobe on it as if there was some kind of porting done, but that could be bad tuning. I got to find out if there is a port job on it, that would be cool.

Is there some overlap on stock engines??? I've heard that there is a small amount even in stock engines. Maybe my over kill pipes are drawing at idle???

Well just wanted to put this down on paper...

13b-rotary-vw

OK,

So I had to skip the 7' primaries system for a more practical system (simply wraps around the engine). It's still fairly long. I don't have perfect numbers but, I think the primaries are around 3' long now into a "Y" that then is about 2.5' long into a cherry bomb type muffler that is 1.5' long as well. Over all it sounds great, the idle is quiet, mid range is strong, and top end is pretty loud. There's a hint of ping in the sound which is from not having any presilencers...

I will try to do the ridiculous primary system later once I have a better way to monitor their diffrence in power. For now I needed a system to get this project on the road...I will post pic's ASAP...

Evil Aviator

My 1Gen RX-7 had 10' primaries, and it worked great. A long primary system isn't ridiculous unless you do something ridiculous like wrap it around the engine.

See the Racing Beat Rotary Tech Tips to save a lot of time and trouble:
http://www.racingbeat.com/mazda/perf...gurations.html