Maybe a dumb exhaust question

I'm not sure how to think about this.

I have a '97 miata with a Jackson Racing supercharger. (7000 miles
and I love it). I'm thinking about getting a JR muffler and maybe a
low restriction cat. Though I like the idea of a more interesting
exhaust sound, I'm most interested in the power aspect of the upgrade
and here's what I can't figure out...

On one hand, the supercharged engine is producing more exhaust
gas volume that needs to get out the exhaust pipe. Therefore, it
seems that less restriction in the exhaust system would benefit a
supercharged engine more than a normally aspirated one because there
would be more back pressure to be relieved.

On the other hand, since the cylinders are being packed by the
supercharger, it seems that exhaust back pressure might matter less
because of the positive pressure of the intake system.

If I had to guess, I'd say that less back pressure would make the most
of the positive intake pressure produced by the supercharger by making
it easier for the incoming fuel/air mix to fill the cylinders and
therefore a better exhaust system would benefit a supercharged engine
more. Does anyone have the real answer?

Does anyone have this combo to comment on?

Thanks very much

In article >,
Dave Smith > wrote:

> If I had to guess, I'd say that less back pressure would make the most
> of the positive intake pressure produced by the supercharger by making
> it easier for the incoming fuel/air mix to fill the cylinders and
> therefore a better exhaust system would benefit a supercharged engine
> more. Does anyone have the real answer?

Correct, a supercharged engine benefits from a less-restrictive exhaust.
However, unlike a turbo motor, a supercharged engine's exhaust is louder
than stock, so a compromise may be necessary between flow and noise.

Dave -

As usual, Lanny speaks truth.

When I talked to the folks at FM about getting the dual exhaust system
I currently have on my '97, the first question they asked was, "Do you
plan forced induction, and if so, how?"

When I answered, "S/C", they told me to buy the normally aspirated
version of the exhaust, otherwise the exhaust note would be
over-bearing.

I've had "old time" MG and Triumph roadster owners tell me how much my
N/A duals sound like their '60s & '70s cars; authoritive without being
obnoxious.

- L



On Wed, 27 Dec 2006 03:50:59 GMT, Lanny Chambers
> wrote:

>In article >,
> Dave Smith > wrote:
>
>> If I had to guess, I'd say that less back pressure would make the most
>> of the positive intake pressure produced by the supercharger by making
>> it easier for the incoming fuel/air mix to fill the cylinders and
>> therefore a better exhaust system would benefit a supercharged engine
>> more. Does anyone have the real answer?
>
>Correct, a supercharged engine benefits from a less-restrictive exhaust.
>However, unlike a turbo motor, a supercharged engine's exhaust is louder
>than stock, so a compromise may be necessary between flow and noise.

'97 STO, "Chouki"

// Change TEJAS to TX to reply via eMail //

Thanks very much guys. I'll have to think about this a little more. I
checked the forums at miata.net and there's some info, I'll post a
specific question over there.

Have fun!


On Tue, 26 Dec 2006 19:23:49 -0800, Dave Smith >
wrote:

>I'm not sure how to think about this.
>
>I have a '97 miata with a Jackson Racing supercharger. (7000 miles
>and I love it). I'm thinking about getting a JR muffler and maybe a
>low restriction cat. Though I like the idea of a more interesting
>exhaust sound, I'm most interested in the power aspect of the upgrade
>and here's what I can't figure out...
>

On Tue, 26 Dec 2006 19:23:49 -0800, Dave Smith >
wrote:

>I'm not sure how to think about this.
>
>I have a '97 miata with a Jackson Racing supercharger. (7000 miles
>and I love it). I'm thinking about getting a JR muffler and maybe a
>low restriction cat. Though I like the idea of a more interesting
>exhaust sound, I'm most interested in the power aspect of the upgrade
>and here's what I can't figure out...
>
>On one hand, the supercharged engine is producing more exhaust
>gas volume that needs to get out the exhaust pipe. Therefore, it
>seems that less restriction in the exhaust system would benefit a
>supercharged engine more than a normally aspirated one because there
>would be more back pressure to be relieved.
>
>On the other hand, since the cylinders are being packed by the
>supercharger, it seems that exhaust back pressure might matter less
>because of the positive pressure of the intake system.
>
>If I had to guess, I'd say that less back pressure would make the most
>of the positive intake pressure produced by the supercharger by making
>it easier for the incoming fuel/air mix to fill the cylinders and
>therefore a better exhaust system would benefit a supercharged engine
>more. Does anyone have the real answer?

Since the effect of the supercharger is to make the engine act like a
bigger engine by forcing more air and fuel through the engine than at
atmospheric pressure, a pressurized intake engine has a greater
exhaust mass that has to be pumped (forced) into the exhaust system.
The exhaust should be upgraded to accommodate the greater volume of
gasses. An exhaust sized for a normally aspirated engine will
produce a lesser maximum power output due to greater pumping losses as
the back pressure acts as a brake on the rotational speed of the
engine.

The supercharger is taking power from the engine to compress the air,
so you are starting with an engine that has a greater "accessory" load
than stock, this load is much greater than any push you may get from a
pressurized charge. The intake manifold pressure has a negligible
effect at higher rpms because the piston may be moving so fast that
the pressure at the piston face may be much lower than at the valve
port when fully open. Remember that the intake valve area is less
than half the piston face area, and the chamber is designed to make
the air charge swirl to aid fuel dispersion.
>
>Does anyone have this combo to comment on?
>
>Thanks very much