There are many opinions and even some test data regarding various exhaust systems for our 928s. There isn't any one answer to the question of, "What is the best system for my car?". Cost is always a consideration as is sound quality, and of course power increase.
Choices for aftermarket systems usually come to:
1. Single pipe or dual pipe.
2. Headers or stock manifold.
3. Crossover style
4. Catalytic converters.
I'll present some information I've found from my own experimentation and research to help you decide on a system that will fit your needs. I have done most of my testing on the 32v engines and most of that has been on my '90GT.
1. Single pipe or dual pipe. A single pipe system will usually weigh less, cost less, and be simpler to install than a dual pipe exhaust system. The argument in favor of the dual system is that the dual pipe exhaust is easier to mount closer to the underside of the car and is less likely to hang down and look unsightly. The smaller diameter dual pipe mufflers are easier to conceal and usually do a better job than the larger single pipe muffler. I like the sound quality of dual pipes vs. single pipe systems, but that is a personal subjective opinion only. Dyno testing I've done show that both single pipes and dual pipes with equivalent flow capacities will make equal top end power. However, the dual pipe system makes more torque below 5500 rpm.
To see dyno charts of single vs. dual pipe exhausts, click here.
Pipe size is important and the 32v engines respond well to the larger size pipes. If you use a dual pipe system, 2.5" OD pipe will give you more torque throughout the rpm range than either the stock pipes, or 2.25" OD pipe. For a single pipe system, use 2.5" pipes from the manifold to a 2 into 1 merge collector, then a 3.5" pipe to the rear. 3.5" single pipe works better than a 3" pipe.
To see dyno charts of pipe size comparison, click here.
2. Headers or stock manifold. The tubular stock exhaust manifolds on the 32 valve engines are well designed, don't leak, don't break, are light weight, and all the 32v engines already have them mounted on the engine. The stock manifold has less surface area than any header design and will radiate less heat to the engine bay and surrounding components such as motor mounts. Heat radiated by headers can be minimized by ceramic heat barrier coating, and/or covering the pipes with insulating material. All of the currently available headers have to be installed with the engine in the engine bay and are difficult at best to install. Custom headers can be designed to enhance a certain area of the power band and can show a performance increase over the stock manifold. Because of the variation in header designs, it's nearly impossible to make a blanket statement regarding a comparison between overall power increase for headers vs. the stock manifold.
Qualified blanket statements: Headers should make more power than the stock manifold, and generally that's true. I've seen several dyno comparisons on cars that had improved exhausts using the stock manifolds where the stock manifolds were replaced by headers. In all cases, the headers produced more low and mid range torque than the stock manifold. Above about 4500 rpm, the increase from the headers is more spotty with some showing more power, some about the same, and some less. Since no header system will bolt directly to the stock exhaust system, you have to be careful when evaluating performance claims from adding headers. What I mean is that adding headers will automatically get you a different, and hopefully improved, exhaust behind the headers. You may not know how much of the gain is attributable to the headers, and what part is attributable to the rest of the exhaust. My '90GT routinely made ~325 rwhp (SAE corrected) on a Dynojet before the 2/6 rod bearing disaster and I've seen up to 342 rwhp actual as measured on the Mustang Ranch Dynojet at San Jose, CA. That's pretty good for using the stock manifolds. If you have a stroker, or supercharged motor, and also want to maximize the mid & top end power, strongly consider using headers. I believe the stock manifolds will be a source of flow restriction. Especially on the stroker you need to increase breathing above 4500 rpm and headers (Devek level 2) will help with that.
In choosing which headers sytem to use, we don't have a lot of choices. I think Devek stands out as the one supplier of headers who has been at it a long time and (through testing) has improved their design over the years. It is safe to say that their current design will produce a power gain, over the stock manifolds, at all RPM ranges. Unfortunately, Devek is no longer in business. Sometimes their headers come up on the used market. Headers manufactured by MSD are available too, but I have little knowledge of their effectiveness nor have I seen dyno charts showing results of their use as compared to either stock manifolds or any other headers.
3. Crossover style. There is a lot of discussion and questions regarding the "crossover". Common questions are: What is it? Do I need one? Why? What if I don't have a crossover? I'll try to answer these questions.
First. What is it? A crossover is the term usually given to the exhaust pipe or passage that connects the exhaust pipe on one side of an engine with the pipe on other side. Obviously this does not apply to the inline cylinder configuration such as a straight 4 or 6. It does apply to engine configurations of the "V", and opposed (Boxer) type.
Second. Do I need one? You do if you have a V8 engine with a 90 degree (dual plane) crankshaft such as the Porsche 928 engine. There are very few 180 degree (single plane) crank V8 engines. I believe all production V8 engines have the 90 degree crankshaft. This style is chosen because it provides a smoother running engine with reduced 2nd harmonic balance problems. V6, or opposed 6 cylinder engines do not need an exhaust crossover.
Why do you need an exhaust crossover? When you look at the firing order of a 90 degree crank V8 engine you'll see that two cylinders on each side fire in succession. One right after the other with no cylinder on the other bank firing in between. Let's check. The Porsche 928 firing order is 1,3,7,2,6,5,4,8. Cylinders 1 - 4 are on one side and 5 - 8 are on the other side. You can see that cylinders 1 and 3 fire in succession on one side and cylinders 6 and 5 on the other side fire in succession. The cylinders fire every 90 degrees of crank rotation and the exhaust valves are open for longer than 90 degrees. This means that the exhaust pulses overlap in the exhaust system for those successive firing cylinders. This causes inefficient scavengine for those cylinders as well as "overloading" the exhaust pipes on that side when the two succesive firing cylinders exhaust almost together. Headers can help since the primary pipes help to isolate one cylinder from another. However, the overlapping exhaust pulses still end up in the collector together. Tom Cloutier's "crossover" style headers connects one of the cylinder pairs to the collector on the other side to keep the pulses even in the collectors and pipes. With this style of exhaust header a separate crossover is not needed.
What is the effect of not having a crossover? Of course you'll get a decrease in power due to the poorly scavenged cylinder pairs and overloaded exhaust pipes. The other drawback is a very unpleasant sounding exhaust. The sound is a little hard for me to describe, but some terms come to mind; Rough, Like a Harley, An old Chevy pickup, Annoying.
The uneven exhaust pulses is what gives our V8s the pleasant rumble, or beat, that we usually like. The trick is to leave some of it, but have it in the background accompanied by a nice smooth melodic (howling) sound when the power is turned on. Not the flat drone of the typical ricer with a 3 lb Folger coffee can muffler. There are two types of crossover. The "H" style and the "X" style. The H crossover is simply a pipe that connects the exhaust pipe from one side to the other. It is simple, cheap, and marginally effective. The usual front engine, front transmission, V8 engined car configuration leaves little room for an efficient Ott (V2) crossover up front near the exhaust manifolds so they typically will have the H crossover. Some Ott (V2) crossover styles are simple 90 degree intersections of the two pipes. This is more effective than the H style, but not as good as is possible. The stock 928 dual cats have a modified H crossover in front of the cats. It is about as good as you could get with the H configuration.
I have researched the crossover quite a bit and I've come up with a design that I believe is optimum for our 928 cars. We have the advantage of having sufficient length in the space occupied by the stock dual cats to permit a nice X configuration crossover with excellent scavenging properties. It also allows for catalytic converters to be fitted to stay street legal. The basic X configuration scavenging crossover design I use is suggested in A. Graham Bell's book, "Four-Stroke Performance Tuning". I refined the design for best performance compromise on the 32 valve 928 motors with results verified by several hundred dynojet trials. Ott (V2) pipe design.
As you would expect, removing the cats, or replacing them with smaller cats, will increase the sound level. The cars with the large center mufflers ('86.5 through '88 and GTS) can use theX pipe along with a RMB and they will sound very nice. Not too loud when cruising on the freeway, but it does command attention at full power and above 3500 rpm. The '85 and '89 and later S4 & GT with the small resonators will be satisfactory for most people if the rear muffler is left installed. Using a RMB on these cars will usually be too loud. The rear muffler doesn't make much difference in power whether it is on or replaced by a RMB. Click here for a picture of my dual cat replacement Ott crossover pipe.
The stock dual rear exhaust can be replaced by dual 2.5" pipes for an additional 12 to 15 hp. Click here for a picture of a complete exhaust system with Ott (V2) pipe, high flow cats, and 2.5" rear exhaust.
4. Catalytic Converters. Like it or not, catalytic converters are here to stay. Even if regular emmissions checks are not required in your area, it is a requirement to have the cats. Fortunately, cat manufacturers have improved the choices so cats do not have to be the power robbing devices they once were. Most catalytic converters have a ceramic substrate upon which the catalyst is deposited. The ceramic is in the form of a grid with holes spaced at 400 per square inch. This ceramic "biscuit" is about 2 inches thick. The typical cat has two or three of these biscuits. Different catalyst is deposited on each biscuit. The front catalyst is known as a reducing type and it's job is to disassociate the oxygen from the nitrogen in the NO component of the exhaust. The next biscuit and catalyst is an oxidizing type which takes that free oxygen, and additional oxygen from the air pump, and combine it with the CO to make CO2. The ceramic biscuits with the holes don't flow real well since the ceramic can't be too thin or it will be so fragile it would break easily. The only way to get better flow is to use a cat with a large area. That means one with fewer biscuits, such as two rather than 3, and the larger 4"x7" oval style rather than the small 4" diameter ones. These can be difficult to fit in a limited space. Random Technology has developed a type of cat that uses thin stainless steel as the substrate rather than ceramic. The advantage is that they can get a higher ratio of open space to substrate than when using ceramic. They also deposit the different catalyst types on one cat section rather than two or three to further reduce the overall length. The disadvantage is that it is more expensive. The Random Technology part number 702500 is a 2.5" in/out cat with an overall diameter of 4" and approximately 10" length. It is reported tohave excellent flow characteristics and be able to clean exhaust sufficiently to not need additional air from an air pump.