What size cat?

[alexpav]

Hey guys
Getting a hiflow cat...
I have a 2-1/4 or 2-1/2 (not sure) Apexi exhaust/Headers....What size cat should i get to get max flow (inlet, outlet, center barrel)

Cheers
Alex

[sailaholic]

Match your exhaust, so should be 2.25. Outside size depends on how much space you have to play with

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[NitroDann]

As big as you can afford and will fit. Period.

Dann

[sailaholic]

No point having a 3 inch cat and a 2.25 inch exhaust.

Check you're state legislation on what is legal, qld a 100 cell it's legal.

best price I'm getting for a reasonable unit is about 140 from the states for a spun magnaflow one. But I'm a bit limited in my od, so I'm looking for something small

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[NitroDann]

Yes there is.

the inlet and outlet may be 3 inch but the core doesnt flow anything like a 3 inch piece of straight pipe. Hence wanting bigger.

Ive yet to EVER see a road car with a cat that flows like the pipe it replaces.

Dann

[sailaholic]

No there is not.

Dyno results -> http://www.importtuner.com/features/0610impp_catalytic_converter_removal/viewall.html

for people who don't want to follow links. It's a 1999 Civic with a CAI, header and a cat back exhaust.
Ceramic Magnaflow = 3 hp and 2 ft-lb over stock
Metal Magnaflow = 4 Hp and 2 ft-lb over stock
Delete / Test pipe = 5 HP and 3ft-lb over stock.

Seeing as 2.25" is good to 200 flywheel HP, and is already bigger than stock, going a bigger will have no noticeable benefit.

Alex, have a look at this website, http://stores.ebay.com.au/Greyhound-Performance/MagnaFlow-Converters-/_i.html?_nkw=2.25&submit=Search&LH_TitleDesc=1&_sasi=1&_fsub=13617825&_sid=83841015

May still be better out there but will give you an idea on price. Depends also if you want a bolt in (flanged) replacement. I'm not so worried about that as i need a new exhaust anyway.

FYI, from a muffler shop a 2.25inch cat was 220 odd using a quick quote over the phone.

[NitroDann]

Yeah but why would you run an exhaust substantially bigger than needed, and stuff your scavenging, if you were worried about the exact right cat?


Your example makes another horsepower with the test pipe. So even though the 2.25 inch exhaust was FAR bigger than optimal, or needed, for the car the 2.25 cat wasnt.

Anyways, this site has zero engine development threads, and users often post questions as if they are developing an engine, and the truth is 90% of the options given as responces will give the same seat of the pants feel to OP, so what difference does it make to argue about 1 hp?

If anyone wants a serious exhaust development thread. start one ill gladly join in

Dann

[sailaholic]

Yeah but why would you run an exhaust substantially bigger than needed, and stuff your scavenging, if you were worried about the exact right cat?

1. Alex's extractors are i'm assuming 2.25 inch, i'm also assuming that the rest of the exhuast has been made to suit. If it's still standard from cat back, a bigger cat will make even less of an difference. Yes the extractors are probably a bit big for a standard motor, but that's what the "shops" make them to. SO, working from there.....no point spending more money on a bigger cat.

If you were talking about me, than no I'm not running substantially bigger than needed, and yes the rest of the exhaust will be sized to suit (2.25 mandrel bent)

Your example makes another horsepower with the test pipe. So even though the 2.25 inch exhaust was FAR bigger than optimal, or needed, for the car the 2.25 cat wasnt.

Wrong Assumption, sizes were not stated in the thread i linked, they only said it was aftermarket. In fact, the original cat in the test was OEM, so size hadn't been upgraded at all. Yes, the SAME SIZE AS STOCK, and all you lose is 1hp and 1 ft/lb over a test pipe.

Anyways, this site has zero engine development threads

It's been picking up lately, and continuing it when questions like this are only going to help that. I think we would both agree Wozzah has been a big help in this area

users often post questions as if they are developing an engine, and the truth is 90% of the options given as responses will give the same seat of the pants feel to OP, so what difference does it make to argue about 1 hp?

Maybe they aren't developing an engine, maybe they are just researching for the future?
And they won't always give the same seat of the pants feel, the cheaper option will make the wallet in the back pocket stick into their ass more. ;) Besides your the one arguing for more expenditure, but I'm glad you've realized that it wont matter to buy bigger.

If anyone wants a serious exhaust development thread. start one ill gladly join in

You've Joined in already Dan, serious thread right here, keep this rolling and maybe we can get this on the sticky list ;)

Nick

[NitroDann]

OK to paraphrase myself and a few others...

heys,


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

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

hamxster asked for a detailed explanation...

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.

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

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?

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?

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

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

This is a good start to understanding of proper exhaust tuning.

As you can see, choose an exhaust size to suit the peak power you will produce, not the power you WANT to produce, and then the rest is in the cams.

Dann

[sailaholic]

Edit, assume that was for the "sticky". Good collection of information.

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[NitroDann]

Most of that info will be in one of the stickys soon if I get my way.

Dann