Miata Mailing List: May 1997, Message #158
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From: 	"M. Roadster Performance" <percy@mroadster.com>

Subject: Re: exhaust diameters-long one

Date: 	Thu, 1 May 1997 23:33:31 -0500



>Could someone run down the definate list of cat-back diameters?  As I'll be

>turbo charging in the future, I want to find the largest diameter pipe from

>the cat back.  The exhaust's I'm interested in:

>

>Jackson, Stromung, FM, Turbo Tuff, Borla or any others you care to mention.

> I've seen 2 1/4 and 2 1/2.  Any bigger than that?  Problem is I've seen

>conflicting posts regarding the diameters and want to get them straight

>before I plunk down the money.

>

>Frank Mowry



[on a side note]



CAR CRAFT DID and exhaust anaylsis some months back. It was interesting in

that they gave the horsepower ratings for specific sizes of tubing.



It went something like:



2"-200hp



2.25"-225hp



2.5"-250



The writer mentioned that for all out performance on the high end to err on

the higher side (1/4" up). This applied to turbos and s/c's too.



Normally aspirated engines and street/daily driving we were to err to the

exact size or down 1/4".



Too big of a tubing makes for not enough velocity to expel exhaust gases.

Too little size obviosuly makes for restriction and loss of power. BUT you

also get some low-end power wth smaller tubing and high end power with

larger tubing.



I'm also learning is that muffler design affects the flow of gases much

more than tubing sizes or inlet/outlet sizing.



I had a local Borla Distributor and Banzai Motorsports guy give me the

break down on muffler design of Borla last week. He stated that mufflers

tend to fall into two camps. Modified engines types and the chambered

versions.



The modified types usually are straight through...Borla utilizes this

somewhat on the turbo mufflers by bending the exhuast path into a loose "s"

curve BUT putting baffles in the path. At low RPM the exhaust uses the "s"

path for I assume low-end response (which is usually lost with straight

trhough designs) and gases tumble over the baffles. At high-rpm  the

baffles (resemble ailerons on airplane wings) kick in to guide the exhuast

path straight (?!)



Pretty nifty.



BUT, from what I gather chambered mufflers not engineered for modified

engines don't help and may hurt a modified set-up. Many standard cat-back

systems are deisgned for normally aspirated engines. The deisgn in the

muffler is chambered to take full advantage of exhaust pulses in a normal

engine. By "timing" the exhuast outlets, it makes for more efficient flow

of exhaust and thus more power.



GReddy makes two exhaust systems that I know of that do this. One cat-back

for normally aspirated engines and another for turbos. The tubing on the

turbos are 2.25". On the standard, 2".



Now granted this opinion is influenced by my friends at Banzai and Borla

and GReddy. I could be wholly wrong but....the Banzai guys drag with a

Mitsubishi Eclipse running Borla turbos with 2.25" pipes at 230 hp. If they

didn't believe what they told me, they wouldn't be running it either.



I'm running the stock Miata pipes at 1.825" with a Borla turbo muffler.

Great low-end punch. Street driving I'm getting Car and Driver's Sebring

times of mid-7's. (Of course, HKS intake, RB headers and TSi's help I'm

sure)

Me and Mr. Mustang keep pace up until 80mph. After that, I'm outta breath.

Turbo time? Yeah, probably soon.



But the top end is needful and I'll probably upgrade to 2.25" pipes with a

Borla Turbo Carbon fiber unit. Then the (R&D) MRoadster Turbo with take up

residence to replace Mr. Eaton. Not a personal thing but the sound of 10psi

has a lot of appeal to me. Mr. Eaton will be dropping into the '98 I'm

planning to buy. (But if my sources tell me right, heck I'll just order the

1998 with the factory turbo)



My 2=A2.





p.s. by the way....last day at job #2 is the 16th. I'll be full-time owner

guy as of the 19th of May. Thanks everyone for their support in this BIG

decision.























Percy & LTLVPR

Owner Guy



M. Roadster Performance

San Ramon, CA



http://www.mroadster.com



"Less clients, more personal attention"  -Jerry Maguire



1991 A-pkg, Super "M" Spec-123,000 miles