So im trying to work out the final buts and pieces for when i fit up my engine in a week or so and im trying to suss out what sort of exhaust systems ppl are running in their NA A series cars these days

Im going to be running a fairly modded engine and it does have a full set of extractors 4 into 1 which is 2" out and im looking to find out what sort of set ups are being used with good results, i was looking at a mid muffler only but then started to think about the need for a system with back pressure (first NA car i have built up in close to 12 years) again

Any help that could be offered up would be appreciated

Steve

I used 2 inch for camed engine with twin su, was good. I used a straight through muffler but a offset one should flow well. Will drone on freeway
2 1/4 or 1/2 press bent will be ok as the bends typically restrict the flow by 1/4 inch in
Pipe diameter too. So 2 1/4 peas bend similar to 2 inch mandrel.
You won't need mega pressure as a series not renouned for torque

The term "back pressure" is a long used misleading term. What you're actually tring to achive is optimal velocity in the exhaust to enhance scavenging. Fast moving gasses have momentum and pull on the last bit of exhaust in the cylender as the valve is closing.

So long as there's no sudden and large changes in pipe size it probably doesn't matter that much what comes after the collector. It'll have a small effect on tuning but provided it's not so small it's restricting flow it'll just move the torque peak up or down a small amount. Unless every other part of your induction, timing and cam are highly tuned it wont really matter.

basically building up an A14 with full port and polished head, twin valve springs, over sized inlet and exhaust valves, flat top pistons running 10.8:1 comp, twin side draught 40mm dellortos and a 296/297 cam as well.

So thats yeah i guess you could say it is going to be fairly highly tuned but it is deffs going to stay NA, im still thinking about just running a mid muffler and no rear but then again if i do that then i can always put a rear on if i need it down the track

Mandrel bent 2"

lamb_daiquiri is right

all depends what sort of dollars you want to spend too. personally i think the length of the extractor pipes and type of merger collector system you use is most important. i haven't seen a set of extractors in aus with a proper merger collector, mind you its an a-series (dont hate i like the a) and i would put 2" mandrel bent system on there with a half decent muffler not a gay as cannon

what is a mid muffler? is that also called a hot dog?

you will definitely need at least a straight through muffler, and even they can be loud and horrible if your not into loud exhausts

the long standing trend is to go 2inch on an a-series, mandrel bent is probably a waste of money for it

mandrel bend is good if you want 3inch but it wont fit, so you put 2.5 mandrel bend to get as much as possible out of a smaller size

The stuff below is somewhat generalised, the idea is sound, I've tried to make it 'accessible' to the average person.

DAvid Vizard did a pretty good article on this some years ago - it might turn up on car craft dot com or similar.

If we 'could' get it ideally zero backpressure is the go for power. Naturally it'a about as neighbour friendly as living next to an insomniac rock guitarist in terms of noise level however. There's absolutely no performance or economy benefits from back pressure, it's a myth at best, or maybe it's being spread by toyota fans desperately trying to get their 5k to have a chance against your A series :)

If you had access to a flowbench you can actually test the mufflers and work out the flow required (it's about 2cfm per horsepower iirc but don't quote me on that).

Another way we can measure things is simply tapping a line (run a length of copper line, coil it whatever,to cool things) and test the actual back pressure at peak hp rpms (and you can take a reasonable stab at that rpm level based on your combo, or simply measure it from 1000rpm before it, up to redline,)

Anyway - realistically you are losing power (to a significant degree) whenever it gets above about 2-3psi. Let me be clear about this, you're going to 'have' to cop some back pressure to get the noise down (in all likelihood) BUT if you can keep the back pressure down to around 1-2psi, then the relative amount of power lost is bugger all, whilst still having the noise reduced to an (arguably) reasonable level.

There's 'yet' another thing you can do to get some noise suppression with a relatively minimal amount of power loss. And this is in addition to the choice of main pipe size and muffler selection.

What I'm talking about is a pressure wave termination box. What you do is basically this. You 'want' the outgoing exhaust pulses, as they leave the main collector, to pull the next cylinder's exhaust out (or help it at least). So we 'still' want those pulses at the collector. But beyond that we don't need them and (relatively speaking they will be responsible for a lot of the noise signature) we can even get slightly more efficient flow trhough the muffler by terminating these pulses or more accurately smoothing them out into more linear flow.

The trick is simple enough - you run your normal final (or main) collector - around 2 inches in diameter, possibly 2 1/4 for an absolutely hard core ultra rpm a15 or larger motor, but likely 2 inch for anything short of that, then after this collecter, you have a _significant_ and 'instant' increase in the cross section area of the exhaust, then you 'taper' (or funnel shape) it back to the main exhaust size (2 inches, maybe 2 1/4) and back toward the rear muffler.

In 'theory' the hot-dog or resonators put in the exhaust before the main muffler do this to some extent, but they aren't nearly as effective as a properly designed one.

The only 'problem' is finding enough underbody space to fit this pressure wave termination box. Generally the larger the better, but if you could get it to around 3-4 litres internal capacity, that'd be enough to do the trick, 4-6 would likely be better but unlikely to be easy to fit.

Apart from the above described theory, you could basically think of it a bit like this, it almost 'fools' the exhaust gases into thinking it actually has a collector that vents to the atmosphere, rather than it then going through a muffler.

You can play with final collector length (I've already mentioned where you'd be aiming for as far as diameter goes) Somewhere around the 8-12 inch mark is probably where you'll get the most benefit.

For most streeters, you'll actually find a bit of mid range (and no loss of peak) power with longer primary pipes that most off the shelf headers for the a series powered datsuns. I suspect that for the most part their length is what it is due to the issue of clearance as it passes the gearbox bellhousing region. It's not enough of a difference that it makes any off the shelf headers 'rubbish' just icing on the cake stuff (primary pipe diameter is by far the most crucial issue).

I've done a couple of diagrams with approximate volume. The one at the top is better volume, but could probably benefit from a slightly more gentle taper at the rear (I couldn't be bothered doing the math and putting a 4th example there, but just a bit less of an angle)

The middle one is what might more easily fit without having to bash the floorpan up wards for clearance, but suffers a little from less volume, so might have to be a bit longer.

That's the basic shape, and easy to fabricate If you wanted to you could also radius the 'tail' end near where it meets the exit pipe. You could also look at making an 'oval' shaped one - it would work better if it is round, but if you can't fit it then an oval shaped one is better than none at all to be sure...