Noise reduction

bcarter

bcarter

New member
We have been set a task by our local track. We need to come down from current 98 db to 92 db, at 3500 rpm, from 50 feet. Mine ran 102db in the last race, but I had a cracked pipe.

In our class we currently run 12A street ports, Racing Beat headers, back to two 2 1/8" pipes to the rear of the car, then an unspecified muffler.

Many of us run a straight thru 3" dynamax, which is really a pipe straight thru, with some stuff around it.

How are we going to reduce the noise without restricting flow, and reducing HP??? We have been working at going faster, and we don't want to slow down, nor do we want to spend a lot of money in the class.

I know Racing Beat has these Power Pulse pre-silencers, but they are bucks-a-rama$$$, and do they restrict flow?

Any ideas?

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G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta
 
Well, as you know, the key to making power in a rotary is air flow on the intake and exhaust side. In IT racing, we can't do any porting so these are critical to us.

If you add some backpressure, you may see a slight increase in torque but a drop in power.

If I were you, I would build a modular exhaust. First 3rd is what you have, 2nd third for another muffler and last third for what you have. Then when you go to another track, you can swap a straight pipe in at the second third spot.

Bottom line is that sound requirment is going to be REAL TOUGH for you rotary guys to meet.

AB

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Andy Bettencourt
06 ITS RX-7
FlatOut Motorsports
 
The most significant item you can do is increase your muffler volume. Keep the straight through design, but look for places to put mufflers. Also, go with the biggest muffler(s) you can find to maximize the volume. For example, a Dynomax bullet muffler (4" round x 23" long) is only 128 cubic inches of tuning volume. If a Dynomax round muffler (6" round x 21" long) can be packaged, the tuning volume will jump to 445 in3. This will quiet the car down. Finally, as you have noted, make sure what you have is in good shape. Not so noticeable, the packing inside some mufflers will disintegrate over time, making them louder.
 
I am using the 3" round inlet/outlet dynomax now, and it has always been loud. I think we need to put in something between the header flange and the muffler, but what? These long thin presilencers seem to fit but, which ones??

[This message has been edited by bcarter (edited October 03, 2002).]
 
BCarter,

(Paging a racer from SF region with an Rx7)

I have been to a few races up at Laguna in an Rx7 but NEVER found an exhaust sytem that was quiet enough AND didn't kill the HP.
So frustrating that I've since made arrangements to race something other than a rotary every time there.
The sound level at Laguna Seca is 92db at 50ft at all rpms. Their meter is on the driver's right in the middle of an uphill straight. I have found that the Supertrap disc series is very good at quieting them down and will allow you to tune as close to the dB limit as you wish, but my seat-of-the-pants dyno and stopwatch suggests that they kill the HP. There are less restrictive mufflers of the straight-through design but without stainless steel packing they will not live long in a rotary environment.

If your meter is always at the same place perhaps your placement and direction of the exhaust exit will be more critical.


[This message has been edited by Quickshoe (edited October 03, 2002).]
 
I'm from SFR, but I can't really tell you, since I've never run at Laguna, and I have no idea how much power I've got..

Sorry

PaulC
 
How can RCMP reduce the Road Course to 92db? Are they going to restrict the nitro funny's or rails? (I doubt it!!)

What's RCMP really trying to do?

Sorry folks, I don't mean to hijack the thread, but I'm wondering if bcarter has more info on the noise reduction.



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Allen Brown
#36 IT1
[email protected]
 
When an event brings in enough revenue to the area they can waive the noise requirement.

The Laguna noise limit is not enforced by the track but SCAMP, some Carmel/Monterey Coast protection group or something like that. I know that the CART weekend and superbike weekends, (maybe the Historics too) are not having to keep the noise down.
 
Hey Allen,

Beats me what's up, Art (Track Owner...)told me just about a month ago that he was lowering the limit startting 2004. I saw him again a couple of days ago, and he brought it up again, sort of saying because my company sponsors the CC series, I was the one to lead the way for the noisy Mazdas. He also said I had the noisiest car at the track...

From the considered opinion of this esteemed group, we are going to have a hard time getting down there without losing HP.

There must be some presilencers that we can put in the pipes that will drop 6 db, without hurting the HP???


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G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta
 
I would call Racing Beat. They've got the presilencers you mentioned, plus a ton of other parts, and they might have an idea of each one's effectiveness. It's my recollection that they don't actually affect the HP, as they are straight thru devices. The expense comes from the need to withstand the incredible heat.

John Herman has a good point regarding tuning volume, and those will add to what you've got. Plus the weight is in a good spot!

If you're running a fuel cell, is there room to turn the outlet 90 degrees and add another muffler transversely?

And finally the outlet should, as I'm sure you know, point away from wherever the measuring device is! It's a strange way of taking a reading! 3500 rpm at 50 feet???? How do they know you're at 3500 rpm?

Our local track, Lime Rock Park, in CT, has, I believe, an agreement with the local community, that restricts the number of unmuffled events a year, as well as prohibiting any racing or track activity on Sundays.There is a little church right across the street, visible from start/finish!

A racer named Ken Payson, who wins most of the Spec Miata races, always goes across the track at lunch and fishes in the Salmon Kill Stream, or so he says....I think he's going another 100 feet and praying!
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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[This message has been edited by lateapex911 (edited October 05, 2002).]

[This message has been edited by lateapex911 (edited October 05, 2002).]

[This message has been edited by lateapex911 (edited October 05, 2002).]
 
Re: racing beat pre-silencers

I know that these do a real good job of quieting the cars down. Not sure about the HP though. There are 2 guys that share a 1st gen that is Supercharged. Neither one of them are incredibly fast. Not sure how much is car/hp related and how much is driver. One driver is about 4 seconds a lap faster than the other but neither one are able to keep up with a good ITS car on the long straights. Maybe the pre silencers aren't properly sized to flow the volume of air that the supercharger adds? It is quiet though. When the car is on track, the distinctive whine from the supercharger is louder than the exhaust when heard from the paddock. Maybe frequency related vs. distance? Who knows but it's not any louder than a stock Viper at full song.
 
Carter,
Try www.mazdatrix.com for the presilencer information. Look under race exhaust .I'm curious to know if there is any power loss . I believe they are 2.25" straight thru.They are heavy ,but placed low and in a good spot for ballast. You may have to collect your header outlet 2 into 1.
BB
 
Originally posted by BrianB:
Carter,
Try www.mazdatrix.com for the presilencer information. Look under race exhaust .I'm curious to know if there is any power loss . I believe they are 2.25" straight thru.They are heavy ,but placed low and in a good spot for ballast. You may have to collect your header outlet 2 into 1.
BB
Click to expand...

I THINK that these units from Mazdatrx are actually sourced from Racing Beat. The prices are the same, (130, american) and the specs identical. The RB units appear to be stainless (and the catalog description lists the components as being stainless) while the M-trix peices are photographed as black.

Power loss is described as "virtually none" by RB, and they describe the unit internally as being a straight stainless pipe with perforations, with a stainless steel wool packing, so I can't imagine much loss.

Take TWO, and call us in the morning!


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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]
 
Unfortunately all of us racing with bcarter run 2-into-2 headers and dual pipes all the way to the muffler so would that not preclude presilencers?

This new db change really pisses me off. I switched to the Dynamax straight-through from a Walker last race and picked up 1.5 sec a lap. I used to be the quietest car of the RX-7's.
 
Originally posted by Skid:
Unfortunately all of us racing with bcarter run 2-into-2 headers and dual pipes all the way to the muffler so would that not preclude presilencers?

This new db change really pisses me off. I switched to the Dynamax straight-through from a Walker last race and picked up 1.5 sec a lap. I used to be the quietest car of the RX-7's.
Click to expand...

Skid & BCarter-

I wouldn't think the dual nature of your exhaust (same as mine) would be a problem. I was considering adding these to my system, as I occasionally run close to the dB limit, (but I am more often safe 101 dB + or -, so I haven't pulled the trigger), and I was considering running them in a staggered configuration, (one after the other). I have taken a quick fitment look and it appeared fine, but I haven't mocked it up with a virtual cardboard test peice.

Racing Beat lists their Road Race header as 2" OD with a .125 wall thickness for an ID of 1.75", while their presilencer is 2" ID. I'm not sure if that will work with your post header pipe thickness or not.



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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]
 
I checked in with Racing Beat, and they don't figure the presilencers are going to cut the actual noise by much, just lower the tone. Also called Mazda Comp, and they have no real answers either.

Hmmm.... Good thing we have some time to figure this one out.

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G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta
 
Carter,
Mazdatrix has a download video/audio of a 2gen with their "dual system" which consists of a race header with dual outlets through two presilencers to an uncollected HKS or RacingBeat catback exhaust. They measured 89dcb's at 50 feet. You could fabricate a similar dual system using seperate presilencers to a dual exhaust system in the stock location and perhaps subsituting 6"round by 16" long Dynomax stainless race mufflers. It would be louder than Mazdarix's but less restrictive. If your sound test is at 3500rpm( half loud for a rotary) you may be OK!
BB
 
I have a street GSL-SE with a 2 into 1 header, a modular section, and the stock muffler. If I put a straigt pipe in the modular section, it is too loud for conversation on the freeway. If I put in the stock CAT(for smog testing), the noise is reasonable. I also have a RB presilencer I can put in the modular section. The RB presilencer is noticeably louder than the cat, and only slightly quieter than the straight pipe.
My ITA rx-7 runs 98dB with a 7" round X 3" pipe straight thru muffler. IT gets to run 101 dB at Laguna, and I don't try to run in the 92dB quiet group. The Spec-7's that run Laguna at 92dB put a 90° bend and run a second muffler parallel to the bumper. The tow loop on the left side makes a good muffler hanger. Sucks if you get bumped though....

Tak
SFR #29 ITA
 
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