Miata List Archive

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Mail From: AbeFM (Abraham Mara)

Bonnie,
A little off topic, but what's an expansion chamber? Since I don't have to
meet any rules, do I need one? :-)

I'm slowly getting more and more into the idea of just making my own
exhaust, trying to pick up whatever tips I can to do it right.

Also - are high-flow cats good for anything? They are a lot of money - and
while I want a CAT not to pollute, if they are 1/10th as good as a stock
one, why bother putting it in?

I'm sure there's a tradeoff, and I can imagine durring smog checks any cat
(high flow or not) will be problematic. Maybe I just need a standard
exhaust dump on a lever like a freind of mine once had. When I really want
the power, I can just pull a lever.
-Abe.
P.S. It's nice to see you can find a place to play racer like that. I'm
jealous - wish I had more time, money, and of course motivation.


----- Original Message -----
From: "Bonni Weatherwax" <(email redacted)>
To: <(email redacted)>; <(email redacted)>
Sent: Friday, March 04, 2005 7:12 AM
Subject: [Miata] WAS: Still more NC pics NOW: Exhaust system requirementsper
SCCA GCRs


>> Still illegal? Maybe, maybe not.
>
> ***Actually, you got me to check out the current SCCA GCRs and I can't
> find
> anything like what I stated earlier about the exhaust! I know I heard or
> saw this somewhere . . . possibly just in my region. Nonetheless, even if
> that rule was in effect, it seems that it's been lifted.
>
> FYI, a few statements from the GCRs:
>
> 12.1 (3.) "Vehicle sound emission is not a constant factor which can be
> trimmed to barely legal (in the manner of engine displacement or vehicle
> weight); sound emissions may vary significantly from morning to afternoon,
> and day-to-day; therefore, the competitor is advised to target his sound
> level at least 2 or 3 decibels under the limit to allow for meteorlogical
> variations."
>
> ***Please note that the above rule is generally applicable at most
> racetracks, even on open track events. I had a Honda driver get busted
> twice for going over the 103 db limit at Thunderhill at one of the TEAM
> events. WOW. Most tracks have the 103 db limit -- out here, Laguna Seca
> has, IIRC, a 94 db limit (or close to that, can't remember, and have never
> failed sound). I have heard of guys that simply include a catalytic
> converter on to reduce sound. I personally use a two-foot resonator with
> a
> straight pipe, and it works. Only thing I worry about in my setup is that
> in SM, expansion chambers are not allowed. Someone might protest my
> resonator thinking it's serving as an expansion chamber.
>
> Anyway, the SM rule says (17.1.9), "The factory exhaust system beyond the
> OEM front down pipe may be replaced prodiding:
>
> a. Said replacement system retains the original configuration, e.g.
> single
> tube design, and the tubing diameter is a maximum of 2.25".
>
> b. The pipe may end anywhere after the rear subframe. Forward of the
> subframe, the pipe must follow the original path configuration of the OEM
> exhaust system."
>
> c. No expansion chambers. A single muffler may be added.
>
> d. The system meets all event specific sound requirements (see GCR Secion
> 12)."
>
> ***Part b above is probably where I got the idea that one can't use a
> relocatable exhaust tip. Looking at it again, it seems that you can put
> it
> just about anywhere.
>
> All the GCRs mention about the ITC (Improved Touring Class, which is
> broken
> down further into classes such as ITA, a class that I run my Miata in)
> exhaust requirements, as far as I can find is:
>
> 17.1.4 (reciprocating engines section D, part g): "Any exhaust header and
> exhaust system may be used. Exhaust shall exit behind the driver, and
> shall
> be directed away from the car body. Original exhaust system heat shields
> may be removed. A suitable muffler may be necessary to meet sound control
> requirements (see GCR Section 12)."
>
> ***So to end this book of an email . . . there's what the SCCA says about
> its racers. You can probably now understand why I am seriously
> considering
> leaving the SM class for the ITA class. I love running with dozens of
> Miatas, but ITA is much less restrictive. Decisions, decisions!
>
> BW
>
>
> _______________________________________________
> Miata mailing list
> (email redacted)
> ftl.realbig.com/mailman/listinfo/miata
>

This read-only message was archived from a public mail list.
Mail From: 1138 (Joe Perez)

It is a hollow cavity which is tuned such as to provide extras
horsepower. It does so as follows:

1- An exhaust pulse is emitted from the cylinder
2- The exhaust pulse travels into the expansion chamber where it rapidly
expands.
3- The trailing edge of the expanding gas pulse creates a slight vacuum
which helps to draw additional mixture out of the cylinder
4- The leading edge of the exhaust pulse slams into the end of the
chamber, creating a reverse shock wave
5- This shock wave pushes some of the mixture that was drawn out back
into the cylinder, with the end result that the VE (Volumetric
Efficiency) is slightly higher.

So far as I'm aware, these devices work only on two-stroke engines. It's
the huge funnel-shaped pipe you often see aside dirtbikes and the like.



Abraham Mara wrote:

> A little off topic, but what's an expansion chamber? Since I don't
> have to meet any rules, do I need one? :-)