RX7Tuner.

I have a 91 N/A with RB True Duals. I love the way it sounds with the pedal to the floor but there strange noise upon deceleration, almost like a fluttering of air. The engine is a fresh rebuild with a moderate streetport (not exhaust). This was my first porting job and I am curious if this is the source of the noise. The car is currently running off Megasquirt but it made the noise before I made the ECU switch.

I would love to hear what you guys think is causing this stange "fluttering" upon deceration.

The link below will take you to the video of the car accelerating and decelerating. Note: When I move the camera in the video the sounds gets worse. The most accurate sound of the phenomenon is in the first 20 seconds.

http://www.youtube.com/watch?v=XxUmud-JLic

Thanks for all your help,

Brian

Aaron Cake

iTrader: (1)

That's weird...

Two thoughts:

1. Do you still have your airpump? If so, have you modified the ACV or the air lines? The pump can make some weird noises when it dumps air, ranging from a mooing to a quacking.

2. Perhaps it is something lightly rubbing a heat shield.

RX7Tuner.

The airpump was recently removed. It made the noise with the airpump and without it. I don't think it's a rubbing because the fluttering noise coencides with air rushing out the pipes. If I was to put my hand over the pipe you would see it bounce along with the waves of the noise.
Interesting note, before I installed megasquirt only the drivers side would flutter. After the switch, they both do it at the same time for the same interval.

Thanks Aaron. Keep up the suggestions!

Brian

RX7Tuner.

ACV is also removed.

Anyone else?

ABCDEfb

I would check the exhaust hangers. I know the sound corresponds with the pulses of air coming out of the mufflers, but vibrations would be occuring at the same time as well.

RX7Tuner.

Allf the hangers are mounted using the rubber o-rings. It isn't rubbing anywhere on the body expect the rear bumper.

I kind of thought it was porting. Does anyone think porting could cause this? I'm clueless.

turtle m3th

iTrader: (2)

Not sure if this would be the culprit or not but do you have any pics of the header? Because I would think that if you still had a 2 into 1 that y-pipe could be causing uneven distribution of the exhaust pulses and could cause the sputtering. Also if that were the case the gasses would meet abruptly and disperse never having a chance at a smooth merge. I could be completely wrong just thought I'd share my thoughts.

turtle m3th

iTrader: (2)

One more thing. What size piping are you running on those exhausts because I don't know what an N/A rotary with the lack of back pressure sounds like but it's just another point i'd look at.

RX7Tuner.

Good thought. I am running a true dual header so each housing gets it's own pipe the whole way back. I think the piping is 2 1/4" but I'm not sure. It's whatever RB's piping is for the N/A.

Curious to see what Rotary Resurrection thinks maybe even RETed if he's in a good mood...

turtle m3th

iTrader: (2)

Is 2 1/4 what they use for their dual muffler exhaust with the y-pipe because if you did use the same size per side i would surely think that there would be a lack of back pressure. I'm thinking that the RB exhaust is 2 1/4 after the header and just splits into a 2 1/4 y-pipe which means the whole exhaust system is 2 1/4 for both ports. That straight run before the y-pipe will cause the engine pulses to merge together and create a consistent flow and back pressure and the video is kind of showing symptoms of a car running without an exhaust. It revs up smoothly but when it is revving downward it sputters. I'm not certain but what I think that means is that you have effectively doubled the size of that RB exhaust because there is no choke or merge point in your system, kind of comparable to running a single muffler 4 1/2 exhaust on a normal 2 to 1 header N/A, which might even sputter less because of the 2 to 1 header. yada yada yada, I talk to damn much.

rotary-guy

iTrader: (24)

i had the road race system on on my car and it never made that sound, thats a funny sound though.

turtle m3th

iTrader: (2)

Scratch what I said earlier, i just checked out the RB road race exhaust and it also is the same style setup as RX7tuner's but just by looking at the pictures the tubing seems smaller than 2 1/4 per runner. Racing beat states that the piping is 2'' o.d so the i.d. is around the ball park of 1 3/4 per runner which should be fine so as far as the problem goes i'm stumped. Good luck with figuring that out though.

MidnightOwl

It sounds like it does it during accel also, it's rather faint.

RETed

Sorry, I don't do non-turbos...

It sounds like resonance to me.
It's the sorta sound some Hondas with really bad exhaust systems sound around here.

Try this test...
Plug up one side - use your foot (with shoes!) - so that all the exhaust goes through only one side / muffler.
Does the sound change?


-Ted

mikaz

You'd be completely blocking off one rotor am I right? If it was a collected system with a y-pipe you could try this.

RETed

Doh!
This is why I only do turbos. :P


-Ted

mxd

iTrader: (1)

imo its whistling due to almost no aggressive curving in the exhaust system and that is one super smooth flowing exhaust, May we get an image of the underside? I kinda like that sound.

or the engine is hooked up backward

BLUE TII

iTrader: (9)

Okay an idea here.

When I crank over a short block it sounds like that warbly wistling would if it wasn't at the end of a long tube. One note as the apex seal crosses the exhaust port opening and another when the apex seal crosses the exhaust port closing.

It doesn't sound like this when you crank the engine over with the throttle plates closed.

So, I say you should remove your dashpot so the throttle shuts quickly to keep the engine from pumping air on decel.

Now you might get some backfires and rotary fireballs, but those are cool right?

rotarygod

iTrader: (1)

It sounds like a true dual to me. You can really hear each individual pulse from each muffler and if you hit frequencies or velocities that the muffler can completely dampen out, all you hear is the air turbulence. Different mufflers will give different effects. I remember when I had mine I thought it was neat how you could listen right in between them and you'd get stereo sound with pulses alternating back and forth. I personally hated the true dual sound and dumped it but that was just me. The powerband was nice though.

staticguitar313

iTrader: (2)

this why i love my Borla duals

Juiceh

iTrader: (4)

The true duals in this video don't sound like that though.
http://www.youtube.com/watch?v=e-Mon1iiv4Q

Your borlas are not true duals....

Sideways7

iTrader: (7)

I think that was the point....

Also, that really doesn't sound like an engine noise to me. Is it definately 100% coming from the exhaust? It really sounds like something rotating in the engine bay thats hitting something else. Weird.
Do you happen you have pics of the ports? I doubt its the cause, but maybe some of the experts could shed some light on it.

RX7Tuner.

The noise is definitly comes out of the pipes. If I put my hand over the muffler, the pitch of the fluttering will change. I should take another video for you guys. I don't have a pic of the primaries but I found this on an old thread of mine...

https://www.rx7club.com/attachment.p...4&d=1163116868

Brian

javrosario

iTrader: (5)

One question I asked, did you use NA or Turbo rotor housings? Maybe it has to do with the turbo rotor housings not having the diffuser in the exhaust sleeve?

RX7Tuner.

Interesting you bring that up. I used N/A housings but I ground out the center section of the diffusers, making them very similar to turbo diffusers. Do you think this is the source of the noise?