Another question - mufflers

[Sean]

Had to replace the muffler that had holes in it, so asked for something with a little more of a note - I was looking to drown out the road noise to begin with...
Muffler guy started by telling me he reckoned it would be a waste of time hearing a MB engine anyway, as they sound crap. Which was an inauspicious start. Then I was told that all Suzuki engines rely heavily on backpressure to run correctly, and that by adding a more open muffler I would be losing performance and efficiency.
So what's the deal? I asked him to fit something anyway, and he fitted some kind of hotdog looking thing that definitely makes quite a bit more noise. Except it does seem to run worse now. Sounds good once it comes on the cam, but below there it's a bit wobbly, and there's plenty of burbling on over-run.
Anybody got ideas or advice? Suppose I could just admit I was wrong to start with!

Sean

[Brayden]

Short answer: Yes backpressure levels are important with such a small engine, but I can't see how removing a muffler and fitting a resonator/hotdog would decrease the performance. Sure, going for a full 2" system on an F5A would kill it with excess backpressure, but altering one component couldn't make that much difference.

Back when I was an extremely poor uni student I cut off my back muffler and put a straight bit of pipe on - it was loud and burbled on overrun, but I liked it.

My advice - give the motor a tune and see if that helps. You won't stop the loudness or burbling without fitting a rear box, but the hesitation may be to a small difference in backpressure that can be tuned out.

[Josh]

Sure, going for a full 2" system on an F5A would kill it with excess backpressure...

Excess backpressure or the lack thereof

[Brayden]

Larger piping causes excess backpressure - it is basic thermodynamics. Within reason, there is no such thing as having too little backpressure, which is what the majority of people think with regards to going bigger with exhaust piping. (Bigger pipe = less backpressure... not true)

Having larger exhaust piping causes an increase in backpressure because the volume of expelled gas remains the same as before, but due to the increase in pipe diameter the gas moves slower, and what happens when hot gas is slowed down? It cools before it can escape the system, which forms a bottleneck for the hot gas being forced out of the motor. If you want to go bigger in piping, you need to match this with a set of extractors and free-flowing components in order to move the exhaust gases at an optimum rate where it can escape before cooling occurs.

So with an exhaust it is a balance between diameter, overall length, bends, and components used. The standard exhaust system has its inefficiencies in the manifold and components, the size of the piping itself isn't the worst aspect, although being half an inch or so bigger would be a definate improvement.

[pullbackandgo]

So that would mean fitting a smaller diameter muffler would cause less backpressure, benefiting performance....? I'm pretty sure it's around the other way, if you push gas into a pipe, it will maintain that pressure and velocity until it encounters a change in the piping, eg larger piping. When it reaches the larger pipe, yes it does slow down, but the pressure will decrease aswell. You can demonstrate this by puckering up ya lips and blowing out, and then release your lips to a larger hole, pressure in your mouth drops.

Correct me if I'm wrong, but I'm pretty sure thats what I learnt at uni...

[cuz]

Get an old front muffler. Drive a 12" screw driver thru the baffles.

[Brayden]

So that would mean fitting a smaller diameter muffler would cause less backpressure, benefiting performance....? I'm pretty sure it's around the other way, if you push gas into a pipe, it will maintain that pressure and velocity until it encounters a change in the piping, eg larger piping. When it reaches the larger pipe, yes it does slow down, but the pressure will decrease aswell. You can demonstrate this by puckering up ya lips and blowing out, and then release your lips to a larger hole, pressure in your mouth drops.

Correct me if I'm wrong, but I'm pretty sure thats what I learnt at uni...

Fitting smaller diameter piping / mufflers than standard would indeed cause a decrease in performance as well because of the physical restriction, like blowing air through a straw as opposed to a postpack tube. You're also correct in saying that with larger piping the pressure would drop in relation to the velocity of the gas.
The problem with your theory is the "thermal" part of the thermal dynamics that I was talking about.
Say for example you put a 2" exhaust system on an F5A engine. The valves open and force out hot gas at a fast velocity. This gas starts to slow down as it cools and the pressure decreases due to the large piping - but the engine is still trying to force out more hot gas faster than this slow moving gas. What has happened? The slow gas at the rear has formed a kind of plug, which increases the pressure in the system. Get it?

So the idea is to work out the volume of what exhaust gasses your engine expells, and use that in relation to the length of your system to work out the optimum exhaust pipe diameter to ensure the gas remains at the right pressure to be expelled before it cools or slows down too greatly.

It's all a science, and bigger is definately not always better.

[pullbackandgo]

Yeah, my bad. I realised after my post that you were actually refering to heat dynamics. It's fun to disagree though!!!

[Brayden]

It suuuuure is.

[pullbackandgo]

[Brayden]

But to answer the original question, 1.75" is the largest I would go on an F5A or stock F8B.

[Josh]

I would have thought that there would be a marked difference between the total gas outputs of the two.

[Anarki!]

my muffler is almost fallen off!

i am disgraced driving this car at the moment, there is a guard with rust, the paint is faded, there are some mechanical problems which need addressing urgently, the interiour is filthy and i am appalled with it's condition!

As a method of resolution, i have cashed in it's rego and am now gonna begin the long, painfull but worth it process of restoring this vehicle to it's former glory.

This includes a new exhaust system as it's rooted!

cheers,

Anarki!

[Brayden]

I would have thought that there would be a marked difference between the total gas outputs of the two.

Yes but you are an idiot.

Its not like we're talking the difference between a 2L and a 5L V8 here (which would only need about 2.25" and ~3" systems respectively.)
253cc's would not make a noticeable difference in exhaust volume output.

[Josh]

Fair enough. I was just asking the question.

I would still have thought that regardless of how small a displacement 253cm³ is in the scheme of things, that adding an extra 253cm³ to a system designed around a displacement of 543cm³ would throw the maths out a little.