Exhaust

Cory · Post by **Cory** » Wed Jul 17, 2002 7:56 am

I have access to a 2.5" Mandrel bent exhaust for my mx5 (off a tubo MX5), My
cuurent exhaust is the standard one with the cat converter removed and a
turbo muffler to reposition spare wheel.

I was wondering whether changing to the 2.5" exhaust is going to make a
noticable difference or if it is worth while doing at all

Cory

Gary Morrison · Post by **Gary Morrison** » Wed Jul 17, 2002 11:10 am

Theoretically I think 2.5" is too large for a normally aspirated car. 2" is
usual. Theory would have it that at 2.5" you lose some exhaust gas velocity,
and hence power. That right Fletch?

shaznandy lockyer · Post by **shaznandy lockyer** » Wed Jul 17, 2002 4:27 pm

Cory <cory@lakelandsteel.co.nz> wrote: Two and 1 half inches is too large a diameter for a N/A MX-5 engine
I have access to a 2.5" Mandrel bent exhaust for my mx5 (off a tubo MX5), My
cuurent exhaust is the standard one with the cat converter removed and a
turbo muffler to reposition spare wheel.

I was wondering whether changing to the 2.5" exhaust is going to make a
noticable difference or if it is worth while doing at all

Cory

shaznandy lockyer · Post by **shaznandy lockyer** » Wed Jul 17, 2002 4:32 pm

yes , it's to do with 'scavenging' exhaust fumes.Maintaining desireable back pressure prevents exhaust fumes re-entering the head, via exhaust valves(open) during de-celleration and gear changes when exhaust pressure/velocity drops...
Gary Morrison <gm@paradise.net.nz> wrote: Theoretically I think 2.5" is too large for a normally aspirated car. 2" is
usual. Theory would have it that at 2.5" you lose some exhaust gas velocity,
and hence power. That right Fletch?

Scott · Post by **Scott** » Wed Jul 17, 2002 4:47 pm

I've got a 2 1/4 inch exhaust and no cat. It definitely made a
difference but from what I've been told, 2.5 might be a bit to large and
you might get a bit of "cackle" noise (although some people like this).

Only problem for me is now, I've changed the timing as well and advanced
it a couple of degrees and I think it's running a little rich as someone
else mentioned a while a go and is also "painting" my bumper. What's the
best/easiest way to adjust the mix?

Gary Morrison · Post by **Gary Morrison** » Wed Jul 17, 2002 5:56 pm

Actually I think its the velocity creating a vacuum effect which scavenges the
exhaust fumes. A larger pipe has lower gas velocities and the momentum in the
flow is not as high... Also in a well designed extractor the resonances in the
tubes assist with the same thing, again, as I understand it.

jeff · Post by **jeff** » Wed Jul 17, 2002 6:33 pm

from my extensive net surfing experience, I gather that diameter/length is more critical in the exhaust manifold runners down to the collector, where the pipes, uh, collect. You don't want oversized pipes here for the reasons Gary mentions.

The stock exhaust pipe (ie after exh.header) however is < 2 inch diameter, with some yukky bends around the rear axle. A mandrel-bent system will definitely be better than stock. Big pipes here shouldn't hurt your torque curve much if at all. If you search miataforum there's some discussion about this.

and yes, "yukky" is a technical term

Fletch · Post by **Fletch** » Thu Jul 18, 2002 7:29 am

This is what I understand too...

My recollection is that the "optimum" size for a 1.6 is 2 1/4 " and for
1.8, 2 1/2" ...

So, if you've got a 1.8, its fine... It would still be worth trying...
might be a better compromise than the stock one?

Its only held on by two bolts and some rubber hangers, so it wont take long
to try it...

Fletch.

Cory · Post by **Cory** » Thu Jul 18, 2002 7:54 am

Thanks a lot guys for the advice. I Think i will keep the exhaust as it is
for now

Does any one know where i can get my hands on some headers (dont need to be
new)

Cory

lou Girardin · Post by **lou Girardin** » Thu Jul 18, 2002 8:40 am

It's not ideal, but it's better than the stock system and if it's cheap, why
not?

Michael Hay · Post by **Michael Hay** » Thu Jul 18, 2002 8:59 am

Speaking of exhausts... what's the effect of removing the catalytic
converter
i'm assuming mine has one as it was imported at the end of last year.
wasn't the weather fantastic over the weekend? i drove up to Leigh for a
couple of days and had the top down the whole time... a bit chilly
but worth every minute... roll on summer!

mike_y

lou Girardin · Post by **lou Girardin** » Fri Jul 19, 2002 8:18 am

I found the effect on power to be negligible, unless your cat is blocked. The
worst effect was the raspy exhaust resonance, I fitted a resonator to reduce
it.
Lou

Mark Roberts · Post by **Mark Roberts** » Fri Jul 19, 2002 9:40 am

Speaking of exhausts... what's the effect of removing the catalytic
converter

When your missus goes to work at 07:00 you can hear her all the way
up the street.

Mark Roberts · Post by **Mark Roberts** » Fri Jul 19, 2002 3:33 pm

I found the effect on power to be negligible, unless your cat is
blocked. The worst effect was the raspy exhaust resonance, I fitted a
resonator to reduce it.

Lou, could you elaborate on that pls?

Ta

Dotti P · Post by **Dotti P** » Tue Jul 23, 2002 11:18 pm

Well~it definitly made a much louder noise~
And perhap u'll find ur car go a bit faster!(psychologically)

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