Exhaust Manifold & System Information

We would say that a good exhaust system couldn't make your engine produce more power as a straight piece of pipe can make your engine work properly, but a system that is unsuitable will cripple it. A well-designed system is made to reduce noise without losing power. The exhaust manifold is the most important part of the exhaust system, but it will only work properly if the system it is connected to is of a good design, e.g. unrestricted and of the same diameter as the exhaust manifold chosen. If it is not, it will reduce the efficiency of the manifold. In some cases we may recommend a system of a larger diameter. 
The exhaust manifold is one of the most important parts that control the output of an engine. You can spend endless amounts of money on tuning your engine internally, but if the exhaust manifold's design does not complement it, your engine may not produce the power you expect. It may move the torque band into a different place, causing all sorts of problems. For example if the manifold you have chosen to fit on your engine is designed to give an all round mild improvement over the standard item, it would be useless to fit it to a full race engine, which would be strangled and as the engine begins to come on cam at quite high rpm, the exhaust gases would be totally restricted & probably generate nothing like the power expected. If your engine has been designed to produce large amounts of torque at low rpm for a particular type of motor sport, e.g. National Hotrods or grasstrack racing, if you were to fit a full race exhaust manifold it would not help the engine at lower revs to produce its full potential, since the manifold is more suited to higher rpm.
We have done extensive testing with both Vauxhall and Duratec engines, spending hundreds of hours testing manifold designs on the dynamometer & just by changing the diameter of manifold pipes as little as a 1/16", or its length by only ½", you can get quite dramatic changes in the characteristics of an engine. If the manifold you have chosen to use is badly made, or of the wrong design, the problems you will experience will increase as you tune your engine further as time goes by. This is because when the engine is only mildly tuned, although the exhaust manifold is still important with mild cams fitted, the manifold is called upon less to help with the charge of fuel rich air being drawn in to the engine on the overlap. Overlap is when the exhaust cam is about to close and the inlet cam is just opening, but as the engine is tuned further the amount of overlap is usually increased. As this happens the manifold then becomes more important because it is used more and more to help with the incoming air fuel charge which the engine needs to generate power. If its design does not match the characteristics for which the engine has been designed, the manifold could be at its most efficient at the wrong rpm and have past its optimum when the cams begin to start working. Or maybe the complete opposite, the exhaust manifold may not be at its best until the cams have past the working rev range.
In the case of a fuel injection engine, if the manifold you choose to run is not to the same spec that it was originally programmed on and is totally different in its design, not only could it stop your engine from producing the power, it is likely to change the fuel requirements of your engine. So it could even it by under or over fuelling unless it is reprogrammed to suit
Frequently asked questions about exhaust manifolds: 
1. Do I really need a manifold that long?
Answer: If that is what the engine spec has been designed & tested on then yes.
2. I have an exhaust manifold a mate of mine says is the best he (or she) has ever seen and it works on his engine, so it will work on mine won't it
Answer: It may be the best manifold ever but until it is tested on the dynamometer in a back to back test we cannot know and even if it is better your engine will almost certainly require re-jetting or re-mapping to suit. 
3. The man who is making the exhaust manifold for me says he cannot make it fit unless he makes one of more of the pipes shorter than the others, will it matter? 
Answer: About the worst thing you can do on any exhaust manifold is to make the pipes uneven. What your exhaust manufacturer usually means is if you want the pipes that accurate then the job has been under-priced.  
But if you decide to ignore any of the above don't come running to us saying that your engine doesn't produce the power we promised, since we only know what power your engine should produce when it has been built carefully with the correctly matched and accurately made exhaust manifold and good quality exhaust system. 
There are two basic types of exhaust manifolds the 4>1 and 4>2>1. The 4>1 manifold is traditionally used on an out & out racing engine. This is because they work well at higher RPM but they are not very good at low of mid range RPM. The 4>2>1 manifold normally works better in the lower & mid range RPM areas but is not as good at higher RPM. We have developed a range of exhaust manifolds combining the best characteristics of both these types of manifolds, which involves using unusual length primary & secondary pipes. We would recommend a custom made exhaust box be made for your car, since the box specifications will depend on your requirements as regard to use & noise limitations. 
BTB Exhausts have made our exhausts & manifold systems for many years; the design, construction & accuracy has always been to a high standard & we have been able to develop increased performance with their help. Unfortunately, in recent times many companies have appeared on the market claiming to produce systems of equal or better quality, this has caused us quite a few problems when people have bought inferior products & then found their engines don't produce the performance that we have quoted, we have then received phone calls & emails asking us why the engines do not perform. We have found it difficult in recent times to be competitive selling the high quality BTB manifolds, so we have now asked BTB to deal direct with any customers of ours & supply the systems at the cost we would pay for a single item. We are still selling a limited number of exhaust manifolds (see below).
We are continuing to develop new designs of manifold to work in line with the ever increasing performance of our engine specifications. When an engine kit is purchased from us, we will supply a drawing so that the manifold can be made to suit that specification. The drawings can be purchased separately, so you can have the manifold made by a company of your choice however if it is not made exactly to our specifications, you will not get the performance.
Remember the exhaust system & manifold is as important as any other part of your engine & if it doesn't work you will have wasted a large amount of money tuning the engine. Also don't forget that labour is getting expensive & a good quality exhaust manifold & system do not come cheap.
*Important Note: 
If you have purchased one of our engine kits and want to achieve the full performance, you will need to have an exhaust manifold and system that matches our recommended design exactly. We have spent a considerable amount of time on the dyno, rolling road and track testing to get the designs we have developed to produce the ultimate performance. Even if you are not running our engine kits, we believe our manifold design will allow you to improve the performance of your engine.
If you can find a manufacturer who has produced an off-the-shelf manifold that exactly matches our design, we recommend that the exhaust manifold is fitted when you initially install the engine to ensure it fits your installation correctly as fitting at a later date may mean that the off-the-shelf one may not fit. Some modifications to the shell, engine mounts, cross members and sump pan (if running wet sump) may be needed, as everyone fits engines in different positions (especially rear wheel drive) and we cannot guarantee that it will just bolt straight on. It would be impossible to us to have produced a manifold that suited every car & installation. We have made all our manifolds using a donor car that we hope is characteristic of the majority of installations.
Even if you are considering a custom made exhaust manifold and system for your car, we would recommend that you ensure you speak to the company that are manufacturing the exhaust for you before completing the installation so that there is sufficient space for the exhaust manifold and system to be made and fitted correctly.
We have been working with BTB Exhausts Ltd to produce a new exhaust box which incorporates a catalytic convertor. BTB have been using our SBD Duratec Westfield to produce a suitable hybrid cat silencer, which was used by Matt Hilliam in the 2009 Westfield Speed Series. Please contact BTB direct for more details. 
BTB Combicat Box