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heys,


has anyone had any experience or heard from anything about 5zigen exhausts and headers for the na6?

i ran into there website and they seem to have lots of different exhaust systems for miatas.

cheers lads or even what people find is best for a na6 exhaust system (naturally aspirated power)


cheers

This is a poorly worded question.

Whats done to your car, what will the future mods be.

if you are never going to replace the stock cams then any headers will be fine and work as well as each other within 1-2 hp of each other. Exhausts are easy, run a 2 inch mandrel bent system on a stock or mildly modified system and run the biggest noisiest straight through mufflers for the best power. Mufflers kill power, simple as that, so do resonators and catalytic converters, so replace that with a filthy big high flow one as well.

Dann

Well atm I have a air intake from mx5factory and a 6speed from a nb.
I want some NA power and want to know the best route for it as I plan and hope to get it installed next week, if parts are available for a decent price.

If you want some REAL N/A power dont buy an exhaust first. Buy it after you have fitted your cams.

If you want a proper long winded explanation just say so.

Dann

NitroDann wrote:If you want some REAL N/A power dont buy an exhaust first. Buy it after you have fitted your cams.

If you want a proper long winded explanation just say so.

Dann

Please explain.

Warning, long technical post


Ok.

A few key points to remember.

Overlap,
This is what its called when your intake and exhaust valves are open at the same time in the same cylinder.

Charge momentum,
This refers to the fact that air and exhaust both weight something, if only a little. Momentum is mass multiplied by velocity. So the faster the air is going, the more momentum it has.

Duration,
This is the amount of time the cams open the valves for, the longer they are open, the more fuel and air can get in (or out in the exhaust cams case).

Some super basics,
4 strokes have 4 strokes....
First is intake, this is the piston going down, the crankshaft turns through 180* between top dead centre and bottom dead centre. During this time the intake valves are open to allow air and fuel in, and the exhaust valves are closed so that only intake air is let in not exhaust.

After this first 180* (the first stroke) the piston is going back up, this is the compression stroke, both intake and exhaust are closed so that the air and fuel are able to be compressed ready for combustion.

As the piston gets to the top of the cylinder the spark plug fires the mixture and sends the piston down. This is called the power stroke, or combustion stroke.

The piston comes back up once more with the exhaust valves open and the rising piston pumps out the exhaust gases.

Ok, so now we have covered that, you need to know that the valves dont open and close at the very top or bottom as the piston changes direction. Infact not even close.

EG: the intake stroke lasts 180*. If the intake valve was open for 180* it would have to be pushed open by an intake cam with 180* of duration. But some people have 256* Tomei cams, and I have 275* cams.

The reason is that the intake charge (the air and fuel mixture) and the exhaust gases have momentum.

Lets start at the exhaust stroke,
Imagine the piston is going up, and the exhaust valve is open.
The exhaust is heading out of the valve at thousands of KPH. Thats a lot of momentum even with a lightweight material like gas. And remember that at 6000rpm, the intake valve was open letting intake charge in just 1/100th of a second ago, so the intake charges momentum is pushing up against the back of the valve just like if 100 people run through a door single file and the door suddenly closes.
So what we can do is open the intake valve while the piston is still going up, the momentum of the exhaust gas shooting out the extractors sucks the inlet charge in, as does the pressure wave behind the intake valve.

So now the intake stroke is happening, the exhaust valve has finally closed, and the piston is traveling downward sucking intake charge in. The intake charge is coming in at huge speed. because of this momentum the intake valve can actually close after the piston has started rising for the compression stroke. At low rpm, and therefore low intake charge speed the piston would be pushing air back out the intake, but at high rpm the momentum is still pushing that air into the engine with the piston going back up the cylinder!
As I said, at low rpm, this does push air backwards out the intake valve, this is what causes cars with big cams to lope (lumpy idle).
But its now easy to see why big duration cams work but only at high rpm right?

Big exhaust cams have a similar story,
After the compression stroke is ignition right? Wrong.
Ignition actually occurs while the piston is still traveling upwards during the compression stroke. remember each stroke is 180* right? Well, 18* of advance refers to the ignition event happening 18* in advance of the piston reaching top dead centre.
This is because it takes a fraction of a second for the explosion to really start going, and as its starting up, the piston makes its way part top dead centre and starts coming back down for the power stroke.
If you look at a timing map you will see that the ignition timing increases in advance as the revs go up, it then flattens out at maximum torque and then rises again after peak torque. This is because peak torque rpm is the rpm that the cylinder is filled with the most fuel and air mix and because its a bigger bang (thus making more torque) it explodes faster, therefore needing less advance.
Now while the power stroke is happening the cylinder might get 3/4 of the way down and the exhaust valve is already opening. Thats because 90% of the explosions energy has already been used up turning the crank, and we need to get the exhaust out as fast as possible to allow intake air in. So the exhaust valve opens as the piston is going down, and stays open until after the pistons has reached the top and is coming back down again during the intake stroke, as was outlined at the start.






OK.....
Anyone still awake?

Why is it important to know all of this?
Well, in a stock engine, this overlap doesnt happen. With big cams it does.
So remember that while cylinder 1 is on the exhaust stroke, cylinder 4 is on the intake stroke right? Same with cylinder 2 and 3 , because firing order is 1-3-4-2.

In a stock engine while the exhaust is rocketing out the extractors at 5000kph on cylinder 1 and the intake stroke of cylinder 4 is open, is exhaust valve has already closed.
But on a car with big cams, and therefore overlap cylinder 4's exhaust vlave is still open a bit. its about to close, but for the moment its open right, and so is the intake valve right? So the momentum of cylinder 1's exhaust can actually pull the exhaust out of cylinder 4 and therefore suck some intake charge in!

This is called scavenging, and is worth something like 12% (15kw anyone? free 15kw? any takers?).


BUT....
This scavenging can only happen effectively if the first big powerful hit of exhaust gas from cylinder 1 shoots into and through the collector of cylinder 4 at the exact moment that its intake valve is open AND the exhaust is at the exact same time (during the overlap period).
This may be for only one 200th of a second at the rev limiter, fortunately the exhaust gas is traveling at thousands of KPH and so can easily make it to the collector in time.
The trick is having the collector EXACTLY the right distance from the exhaust valve.
This is able to be calculated fairly easily. BUT the calculations are affected by the cams a HUGE degree, and as such extractors for one cam will on no way be in tune for another setup.
Noob tips:The reason it needs to hit the collector at the same time as the paired cylinder's valves are open is that this is where the extractor pipes join each cylinder together.

So if there little to no overlap on stock cams, why do headers give us better power?
Well because the stock ones are of very poor build quality. The pipes are smaller than the exhaust port, so the air has to go past a filthy big ridge from a big hole over a sharp 90* edge into a smaller hole. and then the pipes are not mandrel bent, and then they squash down to 2/3 of the pipe diameter as they reach the collector.
Aftermarket extractors give us power ONLY due to better flow.


TL;DR?

If you are never going to do serious engine work, like cams, or valves or porting, or compression, any old headers that have smooth bends etc will be fine and one set wont be more than 1-2% better or worse than the next.
But by now you can see that if you are going to get some cams you need to get them first, then carefully choose extractors and then exhaust pipe diameter to suit them or you could be robbing yourself of 12% right across the powerband, which may well be 20rwkw on a well sorted aspirated setup.


Dann

Well... took a couple of reads, but that was a great explanation Dann.. I have some confusement though (you'd think??)

NitroDann wrote:In a stock engine while the exhaust is rocketing out the extractors at 5000kph on cylinder 1 and the intake stroke of cylinder 4 is open, is exhaust valve has already closed.
But on a car with big cams, and therefore overlap cylinder 4's exhaust vlave is still open a bit. its about to close, but for the moment its open right, and so is the intake valve right? So the momentum of cylinder 1's exhaust can actually pull the exhaust out of cylinder 4 and therefore suck some intake charge in!
This is called scavenging, and is worth something like 12% (15kw anyone? free 15kw? any takers?).

I'm fine with the four-stroke process itself and on its own, and understand now what the cams are doing and the overlap, but when they are combined into a bank of cylinders with separate intakes and exhausts, I'm a little confused as to how the extra air is being drawn in..

Cylinder 1 on exhaust - If cylinder 4 ready for intake and exhaust valve is still closing due to big cams, shouldn't all the exhaust have exited cylinder 4 already (pretty much)? What exhaust is there to 'pull out'? Also, if the exhaust side of the cylinders is plugged into headers, and the cylinders are all isolated from each other, how are they interacting?

Closest I can visualise is that with the overlap of both the valves open, the cylinder 4 is drawing air from both intake and exhaust during the intake stoke. What am i missing?

Time for me to google?

NitroDann wrote:If you are never going to do serious engine work, like cams, or valves or porting, or compression, any old headers that have smooth bends etc will be fine and one set wont be more than 1-2% better or worse than the next.
But by now you can see that if you are going to get some cams you need to get them first, then carefully choose extractors and then exhaust pipe diameter to suit them or you could be robbing yourself of 12% right across the powerband, which may well be 20rwkw on a well sorted aspirated setup.

This works great for me, as without this thread, I would assume the correct order to chase more efficiency in the engine is to do the headers/exhaust routine. I am interested in cams in the future; so I might just look at doing it before i change exhaust... does the engine need tuning again? or a new computer? or does the stock ecu just sort itself out?

SOmeone should make this sticky... but in a good way..

Please also explain and work out my top speed for a 1k run what stats do you require!

Just to clarify Dan's ideas of real NA power is probably MUCH higher than you are thinking (directed to original poster). There are off-the-shelf cams that work fine with the stock computer and the mass produced headers. Are you willing to spend lots of money and having something that can hold pace with a particular kind of vehicle for instance? Are you willing to sacrifice low-rpm drive-ability? Is there a real dollar figure limit? Those are all big factors in the advice you get.

Right now the details about tuned length exhaust manifolds are only relevant if your building an all-out race motor...even then I've only seen tuned length headers on a few race cars which operate in the 9k-10k+ RPM range and they're only eligable in low restriction race classes with lots of money thrown around. So while interesting probably not really relevant to the original poster

Deen it's a venturi effect, the velocity of the air exiting the 4th tube into the collector of all the pipes and exiting out the combined exhaust creates a vacuum as it goes past the #1 cylinder tube which assists in pulling air out. Momentum keeps the air from #4 going out the exhaust else the whole manifold would be pressurized. The time difference for the exhaust charge to reach the collector need to be in sync with the next cylinder starting the exhaust valve opening for scavenging to work. This will only happen at a certain RPM and improve/slowly decay as you leave that point. I don't think a basic formula for doing it would be that accurate though. It'll be affected by air temperature, intake manifold, head characteristics, intake design (whether it's pressurized) etc. The big tube manifolds I've seen on N/A cars are far and few between at the track. Tune and CR should alter it again I'd think as increased cylinder pressure should increase exhaust velocity... AFAIK when people go to that degree of tuning they have access to an engine dyno

A lot of what hellman says I agree with 100%.
Having said that, anyone with some time an a calculator can choose a set of aftermarket headers that will suit their engine pretty close.

Long story short, its better to buy cams and do headwork, and then do some dyno runs (part of tuning anyway), then you will know plenty enough based on the torque curve, timing map, and with some basic math can work out if the Racing Beat 4-1's will work better, or if the FM tuned length will be best.

And then after that with 100 bucks worth of exhaust slip joints and half an hour of dyno time you can easily change the length of the primary pipes a few inches either way, and find the perfect setup.

Dann

Hamxster:

You say you are fine with 4 strokes but not how cylinder 1 can pull extra air into cylinder 4 because they arent connected.

They are, where each of the 4 header tubes meet at the collector. When the exhaust pulse rushes through the collector it forms a vacuum in the other 3 cylinders, including its paired cylinder.

Now re read the section.

Dann

ah ok... didn't look as far as the collector (where the headers come together) to see 'where they would interact'.. makes sense now.

Not sure that for non-technical people, we would be able to marry the exact header lengths to the cam duration though. We're just likely to get headers off the shelf, and we're stuck with however they are.

Is is then possible to match up those headers with cams that would best suit it, if we aren't able to modify the headers further? What do we have to look for and how to we calculate that?

Buy Graham Bells 'Four stroke performance tuning'.
It has a whole chapter on it. And for 30 bucks being able to make even 5hp more its great value. There are many other books also but this one I would recommend.

Its very easy to get perfect headers, Just do the math and buy the closest ones. Then just before the collector cut them. Now get an exhaust shop to make you some slip joints. These are the joiners where they flare the end of pipe the so it just fits over the other pipe.
Get a few lengths made, so you can try up to 2 inches shorter and longer and when on the dyno just swap them to change header length. Its a 5 minute swap and a few dyno pulls away.
When you get it right, half an hour later, take a note, and get that slip joint welded on.

Woopee! Custom headers and plenty of power for cheap.

Im considering buying some tighe 805 cams, which are quite common and developing the perfect header for them, so they can be offered as a proper proper performance package. But still bolt on ready for street cars.

Dann

Nice set of posts dann, how is that key pad on your nokia feeling. ;-)

Sent from my GT-I9100 using Tapatalk

Its fine but my thumb tips hurt lol.

Dann