First, an engine is an air pump. It needs to be able to breath. Open up the air intake and you will see a little benefit, not leaps and bounds. After flowing through the engine the exhaust gases must be released, this is the purpose of the exhaust. The factory manifolds are okay, for stock heads. A header will outflow a stock manifold anyday, but do you need it? If you open the intake then a header will help a little more. Headers work based on a scavenging principle. The exhaust gases leave the cylinder head easier through the larger tubes and once they start flowing rearward it creates a suction which helps to pull the rest out through the system. I noticed a large improvement over stock when I installed my first header. Others may argue differently, but ask them why don't racers use the stock manifold if they are so efficient.
Second, Consider the way your exhaust is flowing. From engine to rear of car, so why put a large exhaust system on the car and still have a bottle neck (manifold) at the source of the exhaust. This is backwards, really you should put a header on then a larger exhaust, otherwise you won't notice much difference besides sound from a large pipe and muffler.
Exhaust is one of the best mods you can do to an engine. The better it flows the more efficient the engine will be, therefore more power and better gas mileage. Headers offer smooth insides and gentle bends, unlike exhaust manifolds. If you are going to put the money into exhaust you should also put some aside for a header. It also improves the sound of the exhaust.
6HP. Reality. 6HP. This is the peak difference between the stock exhaust manifold and headers on a stock L6 motor.
Some of the cheaper headers actually lost HP. For you FlowMaster fans the FlowMaster Muffler lost HP too. Nissan Motorsports headers were the best. Now for this 6HP you get longer warmup times, louder exhaust (even if you think it sounds cool, ask your neighbors) gasket leaks, eventual rust out and other troubles. Now here is why the race teams use them. 6HP. They need that 6HP. Are you a racer? Do you want to change out your headers every two to three years? Do you have time in the morning for your car to pop and sputter while it goes through an extended warm up? Do you want to change exhaust gaskets every year? My stock manifold into 2.5" exhaust with Thermal resonator and muffler tops out just short of 150mph. Do you need to go faster? It always amazes me when I read on a forum like this about a guy who has an obvious vacuum leak and three people pipe up and tell him to "put headers on it man!" Perhaps common sense isn't so common?
From what I understand with no other modifications at all, you should get more HP out of headers, because as Matt said the gentle bends are a lot easier for the exhaust to leave than the sharp jagged exhaust manifold. After all the faster the exhaust can get out, the faster the gas can get back in. The Z's seem to have a lot of backpressure, so I would think a header would help even a stock Z, but I have 3 other questions relating to them, Does anyone know if there is a SMOG legal header setup for CA? Do they HAVE to be louder? or can you wrap them with something to make them as quiet as a manifold (I am probably different from most sport car people , I like a QUIET stealhy performace car), and 3 with a Turbo can you get a set of headers that will work with the Turbo? Or would that not be neccessaray with the Turbo?
Common sense would tell you that headers will give you better HP. Dyno tests of six different exhausts tell me and you if you please, that even the best headers are only worth 6HP, and the cheaper brands sometimes negative HP. You can buy headers with an EGR fitting, that is all you need to be legal in CA unless you have a newer L6 then you need the O2 sensor fitted as well. Wraping will not make them quieter but it will improve another problem I didn't mention in my previous post. Underhood heat. Headers give off incredible heat. The wrap will cut down on this but the wrap will cause them to rust out sooner as well. You can get headers made for a turbo set up but that would be way too expensive and unneccasary. You need all the heat you can get to the turbo.
I will tend to side with Rob, since his points about rust, 6 hp gain and heat hit home with me, especially living beside the Atlantic Ocean.
i appreciate everyone's input. Made my choice a lot easier!
Plus I have no money
Yes, headers do increase hp. BUT, only in engines that can benefit from the increased exhaust scavenging and smooth bends. Installing a header doesn't improve a stock engine. Why? Because a stock engine just isn't producing enough flow to warrant a header. Also, headers need low backpressure to properly scavenge the exhaust, so a larger diameter exhaust is necessary. As far as sound, headers always increase the exhaust sound of a car. Why? Because they are expelling exhaust at a higher velocity, and the movement of air is what creates sound, which is why higher velocity air = louder exhaust sound.
Headers do work, if installed on the appropriate application. So, unless you have a ported head and proper induction, I wouldn't waste my time.
Personally, I run the Nismo 1 5/8" header. I'm very pleased with the quality and performance of this header. I would never use or recommend anything else. Of course my engine is a 3.0L with 10:1 compression, 2.5" exhaust and triple 45mm Webers. My engine redlines @7500rpm, so I actually need the header to expel all that exhaust, especially at higher rpm. The stock one was just too restrictive in my application.
When is a header appropriate?
It's a combination of several things, here are some guidelines. These are based on experience and from talking to others, so don't quote me on this, but it seems to hold true to me.
1. Compression Ratio above 9.5:1
2. Longer duration/high lift cam. Something like a 270 duration and .450" lift.
3. Increased induction. Either with more carbs, ported heads, bigger valves, etc...
There's no sense in installing a header if there isn't any more air than stock to expel!
Thanks for all the information, The heat factor did not occur to me. I think the stock Turbo Manifold will suit me just fine, as I do NOT like excessive noise, nor do I like replacing gaskets every year (YUCK!). So I guess headers will not be part of my car. Is there any advantage to having the stock exhaust manifold smoothed out inside, either by a pro, or by dremel?
When dealing with cast iron (stock manifold) especially on turbo apps, heat plays a critical role in the wall thickness when the manifold is being designed. Yes people do port them and open them up, but we are talking tiny gains if any and heat cracks usually result. It is fairly easy to take a dremel and clean up the stock piece including gasket matching. But do not go overboard or you may find your self looking for a replacement.
One more thing about headers, like I said you will see little improvement on a stock engine. And if installed correctly using the proper torques and tightening sequence, with quality studs and locking nuts a header will seal just as good as the stock manifold. Flange thickness makes a huge difference as does gasket quality. I have run the same header and gasket for three years without a single leak and it is nowhere near rusting out. Maybe if you wash your engine with salt-water and let it sit wet for extended periods the headers rust quick. Bottom line you get what you pay for. My motor drastically loses power with the stock manifold and it doesn't like reving over 6000 rpm. With the header it bounces off the 8500 rpm rev limiter faster than a stock motor hits 4000 rpm. Headers can definetly be beneficial.
This is one of the most intelligent discussions of headers I have seen in awhile. Now, those of you considering headers need to do some soul searching. My engine falls into the catagory of engines that could benefit alot from headers and yet I choose not to use them. It isn't a money issue with me I just realize that I would not be happy with the loss of driveability. Now from the posts I read here I can extrapolate that most of your engines are near stock and wish as you might that is where they will stay. For that reason I again point out, you don't need headers.
I do have a header and from reading the previous posts, i qualify well for one. The heat issue could be a problem, unless your header is ceramic coated like mine. This holds the heat into the header nicely, my engine compartment dosent get as hot anymore. there are no signs of any rust either. It is actually warrentied against any rust!!
The mounting flange was deifinitely superior to the stock one too.
The answer to getting a stock manifold ported is Extrude Hone corporation. They force an abrasive paste through the manifold to smooth and contour any rough stuff that is inside. The answer to ceramic coating is Jet Hot coatings. BTW, If a part is jet hot coated, old or new, it will be covered by a lifetime warranty by JetHot. I have couple of buddies running SCCA formula cars and their headers are coated. They look new after years of running at 12000 RPM, with no leakage, warpage, or any other ill effects.
I am running a pacesetter armor coat header. I bought it like 2 years ago and it still looks new! It is not of equal length primary design, unfortunately I didnt understand this stuff until recently. The collector is coated too. Looks nice and the flange is so flat you could use it as a straight edge.