datto12qld- I missed the uploaded picture - sorry I didn't respond to your post any sooner!


believe it or not I have seen that pairing (1 and 3 paired, 2 and 4 paired) on some honda pipes. I'm ignoring your cylinder numbers, and assuming it's 1 2 3 4 inline as it normally is, but the _shape_ and design you've pictured as I've said I've seen it, but I don't know what the hell they are achieving with it. Yes number 3 (since it fires after #1) will get help being sucked out by number one's tail end, but then after 3 exhaust passes along, it's 3 times the wait (vs how long #3 exhaust phase happens after #1s) so #1 would get comparatively less help (and the same goes for 2 and 4 - 2 would get 'good' help, 4 would get less).

THe only pairings that make sense (to me, and if someone can explain the 'uneven' pairings on an inline 4, please let me know, and WHATEVER you do guys, don't get it mixed up with what they do on V8s, as they have a bunch of added problems due to the fact they don't fire evenly spaced events on each cylinder bank - it's not left, right, left right etc, it's acutally left right left left right left right right (and back to left) so the 'pairings' on v8 exhausts are different and for a specific reason.

On an inline 4, if you are going tri-ys, you pair up 1 and 4 (which fire 360 apart, even stevens each way) and 2 and 3 (ditto) then they alternate evenly at the primary collectors. then the secondary pipes will get an alternating (evenly) pulse as they discharge into the collector.

someone elses extractors for sale

is 'right' in terms of primary pairings on an inline 4 that you wish to make a tri-y

One weird thing pairing 1 and 3 and then 2 and 4 will create on an inline 4 - is the exhaust pulse of 3 will 'overlap' 1 (and so will 2 overlap 4) and end up coming out and sounding like one large continuous pulse out of one secondary pipe, and likewis the other. you might actually start to make the engine sound like a boxer engine (i.e. vw or possible subie 4) which can have similar uneven firing cycles (per cylinder bank) like v8s do. This 'overlapped' exhaust pulse situation is actually what gives v8s (and boxer 4s) their distinctive 'glug glug glug' exhaust note, esp noticeable at low-mid rpms.

Ok - so let's look at some real world examples.

For a streter, I'm going to assume that the redline would be perhaps 6500rpm on a 1200 (which they'll tend to have no dramas with, imho, due to the relatively short stroke). I'd also have to factor in that being a smaller engine, at some point you have to accept a little bit of compromise. They lack capacity, so even with excellent volumetric efficiency, they'll never produce the torque a 1.5 could. Which means that once we've maxxed out torque, the only thing to do is produce it at a higher rpm level (which means more hp, since hp is a product of both torque and rpm) - and then of course re-gear the diff, so that the torque at the rear wheels ends up being higher (and more in line with what a 1.5 would achieve at the wheels with higher gearing. On which note, pet peeve of mine is that 'higher ratio, higher gear ratio' actually technically mean two different things, and often end up being argued to the ends of the earth. Which is why I choose to go with the wording 'higher geared' or 'lower geared' . Let's say stock is 4.1:1 - to go to lower gears would be a 4.88 (since that's a real world existing ratio) and going to higher gearing would be 3.7 or 3.9:1 (again real world optoins for those of us using a h165 diff). Anyhoo so this 1200 would have to rev harder for a-b and freeway driving, but the actual peformance would at least be a little closer to the 1.5

So getting back to the nitty gritty -

for a streeter - I'm going on the basis of 6500rpm redline for a 1200, and for a 1500 (even though more than a few people here have spun them way higher, I'm looking at what is likely practical enough to be able to retain stock factory engine life) I'll put it at 6000rpm. I could almost make a case for a 7000rpm redline on a 1200, and it'd likely last fine, but it would also sacrifice enough lower rpm manners that it would render it a lot less worthwhile. The a14 is somewhere in the middle, but I'd put the redline at 6000rpm since it's got enough capacity to produce good torque/power without big rpms.

that means for an exhaust to 'work' for these two engines, remembering the pipes 'work' 1500-2000rpm either side of their sweet spot, let's see (I'll use 1500rpm, so that we can make the most of it, rather than assuming they are ideal 2000rpm either side of said sweet spot.)

I'll also assume that, given their age, it's likely most people would be running a rebuilt engine, and so are likely on their first oversize. there's probably a few piston options, but for sake of simplicity, I'll assume a 0.5mm overbore (about 20thou). This would actually produce a capacity of :

1200 + 0.5 = 73.5 x 70 = 1187.52 : 296.88 or 18.12ci per cyl

1400 + 0.5 = 76.5 x 77 = 1415.08 : 353.77 or 21.59ci per cyl

1500 + 0.5 = 76.5 x 82 = 1506.97 : 376.74 or 22.99ci per cyl


So - for the street 1200 - 6500 redline, you'd want extractors with primaries which 'peak' at 5000rpm, giving you the best of the sweet spot from 3500rpm through to 6500rpm

We can't get pipes any old diameter, so looking back the most common size I have seen is in fact 1 3/8, but it's possible, esp if going custom , to look at anything from 1 1/4 to 1 1/2 - at least hypothetically

As I've mentioned, I usually allow for 1/8th inch less than the outer diameter (how they are measured, for obvious reasons) of the pipes to acount for tube thickness, and to provide the 'real' inner diameter the exhaust gases 'see'

I'll quickly calculate the various cross section areas and multiply by 88200 to simplify the equations:

for a 1 1/4 pipe, which is 1 1/8 i.d.

0.99401955054989551685732075799078 x 88200 =

87672.524

And so on (not showing all working, post is long enough as is)

1 1/4 primary = 87672.524
1 3/8 primary = 108237.684
1 1/2 primary = 155862.265

1200 + 0.5 = 18.12ci per cyl
1400 + 0.5 = 21.59ci per cyl
1500 + 0.5 = 22.99ci per cyl

R = (P x 88200)/C


So with 1 1/4 primaries:

1200 = 4838 rpm or 3338-6338 range - which isn't bad at all for a streeter!

1400 = 4061 rpm or 2561-5561 range - ok for streeter, a little small for a modestly improved streeter

1500 = 3814 rpm or 2314-5314 range - ok for 100% stock, small for improved streeter

primary length (based on rpm range they produce, which might not be where someone was aiming, but where they'd all work harmoniously given the primary diameter

1200 somewhere around 30-40" (as long as 38-40 if you need a touch more midrange
1400 aroud 35-45"

1500 37-47 inches would all work 'right' but you'd likely be trying to tweak just a little more top end out of it, and go for something in the 30-32 inch range (which would give a slight gain at higher rpm, but likely sacrifice just as much lower down. Basically you'd only do it if you couldn't get bigger primaries and had to try and make the most of it, so to speak.


with 1 3/8 primaries

1200 = 5973 rpm or 4473-7473 range - bigger than ideal for a streeter, suit competition oriented (esp rally where you don't go as much for peak hp, as you would mid range torque and wide a powerband as possible)

1400 = 5013 rpm or 3513-6513 range - about as good as it likely gets for a moderate streeter.

1500 = 4708 rpm or 3208-6208 range - excellent match - slightly better than for the same pipes on a14

lengths:
1200 28-32 (ideal) or possible 38-40 if trying to get the primaries to work down lower (i.e. you couldn't get smaller diameter primaries, and are tying to make them work at a more streetable rpm. Once again though -28-32 would match the primaries best - so they all work best at the same rpm, no compromises)

1400 38-42 (ideal) or shorter if you need more top end.

1500 40-44 (ideal) or shorter if you need a little more top end.

I've never seen 1 5/16 piping used for headers (1 11/16ths also comes to mind. there's no big tech reason, it's just that tooling to bend (esp mandrel) those particular sizes is pretty rare, so they just aren't available (or at least readily or cheaply). But if you could hypothetically get one, then it'd be about as spot on for a full circuit racer spare no expense build. You might even consider running stepped headers with 1 3/8 primaries and then a step out to 1 5/16 if you could get it somewhere ( using it for just the last 'straight' section before the collector so it didn't require bends). This is of course speculation on my part.

with 1 1/2 primaries

1200 = 8602 rpm or 7102-10102 range - full race only - and likely bigger than any a12 would want - drag race only most likely *(yes i know some shorter stroke mini engines and dattos have been known to spin in that territory, all internal parts will be regular consumables)

1400 = 7219 rpm or 5719-8719 range Very well suited to circuit racer, and even drag racer.

1500 = 6780 rpm or 5280-8280 ramge Excellent circuit race rally, competition use.

primary length for 1 1/2

1200 - likely under 24 - possibly even under 20 inches
1400 - probably 27-29 inches
1500 probably 28-31 inches.

So those are the ballparks - based on the theory. In practice, exhaust port idiosynchrasies, and cam timing and rod:stroke ratio (and bore/stroke ratio too) can affect how hard the initial 'push' or tail end of the pull of the exhaust pulse is. Very very very generally, if the stroke is longer, and or the rods are shorter, you'll get away with a slightly larger than ideal primary pipe.

On top of all that, NOTHING replaces testing. So if you happen to know people who have been hotting up datsun a series motors for decades (and doing so successfully, and preferably with access to a dyno, or at least some decent on track testing) - they'll know very intimately all the little shenanigans that these motors demonstrate, and would know which side of these guesstimates to look at.

For the record, you'll likely find the primary pipe diameters _very_ accurate, whereas the primary _length_ would be close, but not 100% perfect. Thankfully (and I've mentioned this before) primary pipe length has far less impact apon output and power curves - so you can get away with a little bit of latitude there.

Next - secondaries. If we aren't going to 4>1 pipes, then we're likely going 4>2>1 - and the 2 'pairings' are 1 & 4, and 2 & 3. What probably isn't apparent, is that even with a very wild cam (and I should have thought about this before I posted the earlier stuff, as i was still thinking too much in terms of inline 6s and some v8s) the exhaust pulses _in the secondaries_ don't overlap each other. What this means is that you don't actually need (or in many cases want) to run secondary pipes that are any bigger than the primaries. So if a 1 3/8 primary 'works' it will as a secondary. HOWEVER, we've mentioned stepped primaries. Well instead of running stepped primaries, the other option is to go to a slightly bigger secondary diameter, which will essentially produce the same result.

So with that in mind, if you establish that the ideal primary length is 1 3/8, you could either run 1 3/8 secondaries, or possibly 1 1/2 (unless 1 5/16 is available?) secondaries.

The final collector is generally sized according to flow/hp characteristics. As a rule of thumb, on everything short of a full race 1200, you'll do very well with a 2 inch system. A full race 1200 might put you into 2 1/4 territory. Mild a15s will be ok with 2 inches, though you'd do ok with 2 1/4 for anythin above mild.

In the diagram of the pressure wave terminator box, I should have drawn the headers in there. Basically if the space permits, the total 'length' from the start of the collector to where its 'tail' ends inside that termination box, would be from 12-18 inches, but don't be afraid to try anything from 10-26 . Ideally you could rig up a couple of flanges and different thickness/length pipes and be able to adjust the position of the pressure wave terminator, forward or backward, to 'tune' the length of the collector - which is about the only time (save for an open unmuffled exhaust) you'll be able to actually find any differences in collector length and output/power curve. I can do a quick ms paint drawing of how to make the collector length adjustable without costing the earth.

Last little tid-bit - I haven't got experience with 1000 of them or anything, but experience (such as it is) with jet-hot coating shows it to work. It looks 'good' too, and helps keep the heat in (keeping the heat in is good, it keeps the heat energy and exhaust scavenging working optimally, it also protects the engine bay from heat, and on the same basis prevents carbs/fuel from heating up and pushing you closer to detonation) Basically the jet hot coating will allow you to run a touch more compression safely. Now as nice as it looks, I'd suggest ganging up on it - run the jet hot coating (obviously only get it coated after all work is done to the exhaust as required) - and ADD the heat wrap insulation tape around the headers. This will mean even less temp is radiated out of the exhaust through the pipes and into the engine bay - win win.

For nearly every application I can think of, for practicality, you can consider secondary + primary length = the total length of the two, as the primary length when setting the overall lengths. For example if a specific length primary was required for 4>1s - lets say 32 inches, then if you had 4>2>1 pipes and the primaries were 14 inches, and the secondaries were 16 inches (allow for 2-3 inches for the primary collectors of course) they'd perform pretty much the same.

What else - for supercharged applications, there's differing opionions but generally the consensus is you need a slightly bigger exhaust. If a 1 3/8 primary and 2 1/4 collector worked for the rpm/output of a 1.5 litre normally aspirated, then the same engine supercharged, with the same general rpm range, would tend to benefit from 1 1/2 primaries and a 2 1/2 collector. you won't 'lose it all' with the smaller system, and some argue that that is a better bet since it'll run a little better at more common conditions - like 2/3-3/4 throttle for spirited but not hammer and tongs driving. The same goes for nitrous - you'd go that one 'size' bigger. however in that instance, I'd actually make the case for hte smaller or NA size. This is because supercharging, you can use it all the time, nitrous only sparingly, it's just too expensive for street stuff (and illegal of course in Aus on the street) - so you'd go for the exhaust system that works best for NA - since that is what you'd have 99% of the time

I think that's about everything :)

I'm only a 1\4 of the way through that long post but already I can see where I have infact harmed my A14 through use of a restrictive exhaust. When I drop it back in it's getting the exhuast that came with the engine.

That's some awsome info there Jmac!

V-Tec.... "All of the Lag... None of the Boost" ha ha.. go the Honda...lol,
I've got that book too, it's awesome, 4 stroke performance tuning,
it's up to the 3rd update..

Variable length intake runners, it's where it's at.

So jmac, since this is a datsun 1200 site, how about some rough dimensions for primary, secondary pipes. Or recommendations, for a street A12/15 in a 1200 on exhaust dia through out.

Obviously the cam, the intake system, valves, ports etc etc have a large take on it so that was a pretty useless question but say for a standard a15 since many people are running them as a base upgrade on the a12. Or even the usual a15, quite mild street cam (very broad term i know), and 40mm sidedrafts as most people jump straight to them.

-chris

funny you should mention hondas
i know a guy (here comes that yarn) with a b18 VTEC and everything he does to quieten it down kills the power
so as a result it freakin screams its butt off and goes like you wouldnt believe. BUT it attracts unwanted attention
backpressure? maybe what these guys had discovered was gas speed? get the exhaust going faster and it will suck out the last bit?
had a mini at one point too fun car

have a look at pro stock pipes those guys test and test and test. nothing goes on the car unless it works. they enclose their intakes in a box so noone can see them and hang covers over the sides of the motors while in the pits.
ive heard they even shorten the intake runners during a run as the rpms go up

so buy a flow bench and dyno, make 100 different sets of pipes, test em all and let us know how you went

Legend, thanks jmac, that was awesome reading. I saved it as a word document so I don't lose it! I had a couple of questions about the wave termination box. I realise it stop pulses travelling back up the exhaust.
with it being that far down the exhaust system does it help with power or noise or both. Also how critical is its position along the exhaust system, does it have to be quite close to the collector or does it just have to be before any resonators/mufflers?

After reading this on was poking round on the american hytech exhaust website and noticed they have a patent on the wave termination canister design except they run smaller versions in each primary.

This post ended up being pretty long, I did it in a few stages as time permitted. If I've missed the end of a sentence or something isn't clear, please let me know and I'll edit the post and sort it out.


First things first - I keep repeating this, but it's important - I didn't come up with this theory/technology. Plenty of extremely intelligent people who make me look like an idiot worked it all out, tested, refined it, and were also generous to share it (sometimes for a very very reasonable price of a book, other times for free!). That list of people would include (and I'm missing a few no doubt) people that would be well known and others who wouldn't, depending on the specific car marque/circles one occupies. They also might be as infamous for getting some stuff a little screwy and getting other things on the bullseye (which is probably a necessity in car modifications, if you never went down the wrong path occasionally, you've never been anywhere etc). That short list would include the likes of : Smokey Yunick, David Reher, Clive Trickey, Carroll Smith, David Vizard, Larry Widmer, John Lingenfelter, Bill Jenkins, Dave Emmaneul, Jim McFarland, Doug Roe and a bunch of others. I'd also add in Mark Ellis (rip) , who I knew extensively through posts/emails on moparmarket, but sadly never got to meet in person.

The stuff I'm posting below represents my understanding of things, and that understanding was effected by the writings of the above people (and many others). I'm not doing any of this for profit, nor am I directly duplicating anything they've written, so hopefully the post is reasonable and not any attempt to rip them off in any way. I understand if the mods have to edit/delete it.



ok lets talk about why extactors 'work' -

#1 is of course that they reduce backpressure massively (well most do, though some factory exhausts over the years are actually pretty good in this respect - two that come to mind are the cast iron dual outlet torana xu-1 manifolds (for some Aussie content) and for overseas guys, the 'rams horn' chevy small block exhausts. Not perfect but far less restrictive than other factory cast iron exhausts. Less back pressure means the piston moving up the bore doesn't have to push as hard to get the exhaust out. Simple enough, a reduction in pumping losses. On 'most' setups from the 70s backpressure is relatively high, as it's 'quiet' it's also a killer on power output.



There 'was' a myth about requiring backpressure to function. This is about as logical as saying Paris Hilton is a virgin. There's _ZERO_ benefit (for power levels) for having exhaust back pressure, plain and simple it's an old wives tale. For the record, I had an exhaust done by a place local to me around 10-12 years back and I specified 2 inch system (*it was a 1275 mini, in a fairly mild combo, probably putting out around 95-100bhp). I 'picked it up' and they informed me that they had 'done me a good turn' and put a 1 3/4" system on there as otherwise 'there would be no backpressure' - arrggh. I needed the car that day, as aside from being a fun car, I blew the auto in the other car, so needed the transportation, but I never went back there. Point being, that attitude still prevailed at some places in the late 1990s, it might still today at some places so buyer beware. If anything we've probably gone a little the other way, where people are fitting 3-4 inch exhausts on small 4 cylinder cars. Above and beyond the exhaust size/design that gets rid of back pressure (it'll never be truly zero all the time, but let's say 'gets rid of back pressure for all intents and purposes') - any bigger than that will just cost more money, for no benefit, probably drone a lot at part throttle, and present clearance issues both from the ground, and between the body and the axles etc. It also likely gets a bit more attention from the boys in blue, which I'm pretty sure nobody in their right mind wants!

So basically the first big thing of note is the reduction in backpressure. This applies to the exhaust header/manifold/extractor, and of course the rest of the exhaust from there back.


In the most simple of terms if you can't get an exhaust place to make an exhaust system to the dimensions you require, and they don't have a lot of either competition experience in general, or don't want to tell you why, just a condescending pat on the back and a 'we know best' only to give you a restrictive exhaust because 'you need backpressure' - just find somewhere else. The better places out there aren't necessarily the dearest either, ironically enough.

----

Before I go on to #2 I have to add in some more theory. An engines output is of course a product of the whole combination of parts - head/port work, intake manifolding and carburettion or efi configuration, the bore and stroke (and hence capacity) and the exhaust system, and of course the cam profile. And that is a big generalisation

The longer an intake valve is open, the more time the engine can breath in. Believe it or not, they actually close the intake valve _well_ after the piston has reached the bottom and has started to rise. At high enough rpm (and it's all relative, if the cam is short enough, this can happen at lower rpm, for a little while at least) the air coming in has momentum, and even though the piston has started to move back up, it will continue packing in the intake charge. The higher the rpm, the later you can close that intake valve and still have beneficial cylinder filling. But with later and later intake valve closure, at lower rpm, the air isn't moving fast enough to have enough inertia, so the piston can actually push it back up the intake tract - losing some of the cylinder filling, a phenomenon known as flow reversion (*in this case intake flow reversion)

Similarly, at the end of the exhaust stroke, the exhaust valve might not close till after top dead centre, for similar reasons, the exhaust might get enough momentum up to keep plucking the last remnants out of the exhaust. Of course at the end of the exhaust phase, we begin a new intake phase, and the intake valve itself will be starting to open just as the exhaust is beginning to close. This is known as valve overlap.

Imagine we have a very restrictive exhaust system, at lower rpms, when there is valve overlap, it might actually be less difficult to push that last bit of exhaust up and out the intake valve!. This is dreadful, as it slightly heats up the intake charge, as well as meaning that it gets pulled back in the cylinder for the next cycle, taking up valuable space that could be occupied by air and unburnt fuel for more power.

So on a stock engine, with stock exhaust, it's not uncommon for the intake/exhaust overlap to be relatively small.

Now let's imagine you have a free flowing exhaust, you can safely run more overlap and not have intake port contamination with exhaust gas reversion. That's fine when the throttle is open. No big deal. BUT when the throttle is closed, like at idle, it's hard to suck the intake air/fuel in there, but there's this open exhaust, with no major resistance to gases being sucked bakc in the cylinder (which is different to them being pushed into the intake port at wide open throttle). At idle, it can suck some exhaust back in there, and hence have less air/fuel in there at idle, and produce (relatively) less intake vacuum at idle, and a rougher idle. In the past, to alleviate this problem (which as mentioned is only occuring with a closed throttle/idle scenario) they used a cam (i.e. when talking about performance/non stock cams with more duration) with wider lobe separation angle, less valve overlap, and a smoother idle was to be had. The 'problem' is that this generally resulted in a cam with too wide a lobe separation angle for optimal mid-range output (which is more important than peak power for just about anything except land speed salt cars and tractor pulling). Adding an anti-reversion step at the exhaust port exit, at the extractor flange helps resist this reverse flow, so allows a cam with tighter (or in this case more optimal) lobe separation to be used without as much of a detriment to the idle quality.

Which moves us on to

#2 - exhaust scavenging.

Imagine a near perfect 'primary' header pipe. With the right length and diameter even if it just went to open air, didn't meet with any other pipe' - if it was combined with a cam with the right overlap (and duration etc in general) to work over the same rpm range, as the tail end of the exhaust rushed out the exhaust valve, it'd create a negative pressure wave that would create a momentary low pressure in the cylinder, and not only 'pull' out hte remnants of the exhaust, but it'd pull in some intake through the intake valve and with the right timing, that would be left in the cylinder, so it 'kickstarts' the intake flow without the piston moving down much to initiate it, which means you can get the cylinder filled beyond 100% with the right combo.

David Vizard has written that this outgoing exhaust pulse can have a 300% stronger 'pull' (and I know that's the wrong terminology, but it'll do for the sake of explanation) than the piston moving down. That's absolutely the truth. It's also a 'fact' claimed by the people trying to sell tuned length extractors as 'negative supercharging' as if it's a new technology or something, and they WAY exaggerate the potential increases. What those chaps DON'T seem to realise is that SURE it's 300% the pulling power, but it's at a time when the intake and exhaust valves are only open about 50-70 thou or less (and closing in the case of the exhaust) - and we all know that ports probably only flow 10% or so (I pulled that figure out of the air, whatever the actual one is in any given circumstance, the main things is it is WAY smaller than peak or mid range flow) of their potential - so it's NO WAY KNOWN going to provide a doubling of torque or some other such nonsense claim. So if you ever see an article by Vizard on this issue, keep that in mind. He isn't in error in any way, it's just some people have chosen to mis-interpret it. All I am saying is the gains are real, they are worthwhile, but they are icing on the cake, not 'triple the power' or anything.

Ok, so we see that the outgoing exhaust pulse not only can remove all the last traces of exhaust in the cylinder, it can also pull some new intake charge in there. A _very_ handy thing. So then if you take it a step further, it's not hard to see why having the exhaust header primary pipes meet up at a certain point further back, at a collector, then each outgoing exhaust pulse from one primary, could help pull out the exhaust pulse of another cylinder about to hit it's exhaust stage, which makes that cylinder's exhaust cycle more efficient, which will then help the next cylinder's exhaust cycle and round and round we go.

So basically 4 into 1, various collector designs, that are all designed to do that. The merge collector basically does it better than the less optimally angled collectors, but they are all better than nothing. Think of merge collectors as being collectors where the pipes are all angled slightly toward the centre, instead of having a funnel at the end to redirect them, they are aimed dead on, and produce the best negative wave. Due to this improved aim/direct path, they can actually run a slightly smaller main collector (a choke point is a term I've heard used for it as well) that then tapers back out to the main size. Done right it increases the negative wave/exhaust scavenging even further, of course too small will still be too small.

At this point, without a lot more experience on it, I'd be reluctant to go tighter than 2inch OD for a 1.5 litre full race motor, tapering back out to probably 2.25 or 2.5, and I'd always wonder whether 2.25 as the narrowest point might not be better.


------

Stepped headers are a good and relatively modern refinement. IF you have an engine where you want to have your cake and eat it too, these are it. They let you get most of the torque boost of the smaller headers at lower rpm, and the larger headers at higher rpm. Ultimately the precise length of each is a matter of dyno testing, but you'd find it hard to go wrong with primary pipes going 10-12 inches, then going up to the next size for the rest of the primary. The added benefit of a stepped header is that at the 'join' it adds yet another anti-reversion step. All win win. You'd still be running a 4 into 1 setup, just that the primary pipe diameter goes up one pipe size at around 12 inches of length. Aside from having the best of both worlds kind of thing, the exhaust is still expanding a little as it leaves the exhaust system, so this slightly larger pipe a little further along has merit. Of course you don't want to go too far and lose momentum

--------

Obviously all of this stuff only works optimally when there isn't any significant exhaust back pressure. If there was, the exhaust velocities would not be reached, and even if they were, they'd be pushed against a restriction. Bottom line, they won't work nearly as well.

The next problem is of course noise. Once apon a time race cars could be loud, real loud, and a racer witout tinnitus (ringing in their ears even when there is no sound) would be a rare thing indeed. So now we have to adhere (of course it's always been the case on the street, but recently, in Australia at any rate, even old cars have now been sort of retro-legislated against, and the 'acceptable' noise level is pretty bloody low. Aside from reflecting sound waves against one another (which cancels out that sound wave) - which some (but not all) mufflers attempt to do, some of the reduction in noise will inevitibly be at the expense of raised backpressure, and therefore reduced power.

Being realistic, we'll have to accept some backpressure. I haven't seen too many hard figures on it, but if you weld a few bungs into exhaust systems, and find anything above 3-4psi, you are losing power, and if it's like 10psi, you are losing a lot of power. Down around 1-2psi, and yes you'll lose a little, but nowhere near as much. So the goal has to be, realistically, to reduce noise as much as we can with as little increase in backpressure as we can, but inevitibly we must accept a little back pressure.

So how do we do it? Well unfortunately not all mufflers were created equal, so you might have to experiment. If you can find a modern variant (in design, not just in name, as there has to be a hundred cheap knockoffs that use the name but are terrible internally) of the 'turbo' muffler, you'll have to test it. You might also find that a 2.25 muffler works well, but you only have a 2 inch system (for argument's sake) so you weld on a flange to pipe exhaust in from your system to the slightly larger muffler, and either keep the 2.25 out, or flare/funnel it back to 2 inches on the outlet.

To find out what muffler to go for, you might need to look at some sources on google where they do comparative tests back to back and show various brands and which is the quietest for each flow rate. In terms of flow, David Vizard has discussed at length testing he has done, and came up with a rule of thumb that you need 2.2cfm muffler flow (tested at 1.5 in Hg which is about 20in H2O) per flywheel bhp in order to fall into that ideal range where you get sufficient noise reduction, but sacrifice as little power as possible. So on a full race A series - I'm guessing as much as 160bhp, or more, for 1.4 ish litres, you'd need a muffler that flowed 350cfm or more.


No big deal there.

But in the middle - after the last collector and before the first (and only, if it can be done) muffler you need to add something. You need something that will allow the exhaust pulses going out the collector to still be pulses, which means the extractors etc work, but that dilutes/mixes them so that it's more like a slow continuous flow into the muffler. This will 'fool' the extractors into working at one end, and still allow the exhaust to exit at the other and the process of stopping the 'pulsed' sound will in and of itself help quieten the exhaust too.

The 'thing' is called a pressure wave termination box. To make it work, after the exhaust goes through the collector, it has to be presented with a significant change in cross section area/and a large container volume, so the whole lot can still exit said pipe. Then you basically funnel/direct it back to the first muffler. Just how big that 'container' has to be is somewhat dependant on engine size (and whether all cylinders are dumping into it in the case of a v8 vs a 4 or inline 6). The precise size would be down to testing, but a good enough rule of thumb would be to aim for perhaps 6-8 litres. The bigger the better, but once you get past around the 6-8 litre mark (for an a series up to 1500cc) the increases will be stuff all. Basically it is 'enough' volume to allow the exhaust coming out of the collector to act as it if were venting to the atmosphere.


The design doesn't have to be outrageous, and it doesn't have to be perfectly round top to bottom and side to side (ground clearance issues might force your hand on that one). The collector 'exit' should be a straigt cut, and should be inside the 'box' a good 2-3 inches before it's cut off, so that there is even 'atmosphere' behind it a little (gases do funny things). The 'funnel' shape back to the exhaust need not be fancy, just generally make sure it's 'flatter' than a 45 degree angle (probably 30 degrees would be even better) to be the final piece to make sure it doesn't try and reflect a positive wave back up the exhaust (which is done on 2 stroke exhausts on purpose!) Make it as wide and tall as you practically can - to give the biggest change in cross section area, and beyond that extend its length or alter the tapered section to get the volume up. Is this magical? Nope. Is it a closely guarded secret? Nope. Just decent application of principles found through a combo of theory, science and relentless testing.

The pressure wave terminator/cancellation 'box' or cannister in function actually does what a resonator (hot dog shaped thing) is supposed to do, but rarely does to any significant degree. It's just a better conceived/designed tool and does the job far better.





Second to last, though it's not quite as big a deal, if you get all your ducks in a row with regard to the above stuff, about the only thing else is to try and position that muffer as far back as possible - i.e. behind the axles not in front.

Very last thing. This came up in a post by Larry Widmer on his 'oldone' forums. Unfortunately the forums are down. Larry is not without his critics, but he did work for aircraft designers for the US military among other things, so does have a decent handle on fluid dynamics and so forth. I don't have the post saved or anything, but am pretty sure 1/2" was the correct measurement)


Anyway, he did a rough calculation, and if you got a piece of thick material (he used pvc pipe, which would survive the relatively more moderate temps right at the exhaust tip, but i've never located a source in Australia that is the right combination of overall diameter and thickness, so you might have to look at making one from alloy (which would experience galvanic corrosion eventually, it press fit into the exhaust tail pipe!). Anyway, it has to be around 6mm wall thickness (and thicker wouldn't hurt), and you drill holes (alternating pattern) of 1/2" diameter all around and along it's length, then slip than into a tight fitting outer pipe (i.e. the exhaust, so on a 2 inch exhaust, you'd find 2inch pvc with around 6-8mm wall thickness, and slip into a 2.5 pipe that was welded to the outlet (or similar, so the actuall inner diameter of the pvc or alloy matches the main exhaust pipe diameter). That massive saturation of holes doesn't present a massive flow restriction, but it does cancel out (via reflection) a particularly noisy range of frequencies that he observed on the 4 cylinder engines he worked with (in this case 1.6-2.2 litre honda 4s, but it should still apply). This won't quieten it across the board, but can hit some of the worst offenders. It'd cost around $5-10 to try it, can't hurt to experiment.

CAPSLOCK!

Jmac, please carry on.

HI ALL I HAVE A SET OF A15 HURRICANE 4 INTO 1s I GOT FROM PERFORMANCE EXHAUSTS IN RINGWOOD THEY SEEM TO BE MADE WELL THEY LINE UP WELL WITH PORTS AND GASGET THE 4 PIPES ARE 38mm INTO 50mm AND THE WELDS ARE THE RIGHT WAY AROUND . I HAVE PICTURES ON MY COMPUTER BUT CAN NOT FIGURE OUT HOW TO POST THEM HERE . PS I THINK I WILL HAVE A PROBLEM FITTING THEM IN MY KB10. THERE'S NO ENGINE IN THE CAR AT THE MOMENT SO CAN'T TELL YET

Bring on the data Jmac. Im loving this stuff. Perhaps you could do some sort of document for the Wiki that is focused on the a series extractors.

So much to learn so little time...