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 saleis '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 :)
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