You are likely familiar with the Burns Stainless double slip merge collectors used in many top-notched racing turbo-exhausts. But did you know that the double slip concept is also an excellent choice for many naturally aspirated applications, particularly street cars and other fuel injected applications. The purpose of the double-slip collector is to minimize exhaust leakage, while allowing for a slip-on collector. A slip-on collector is often times preferred over a weld-on collector for ease of disassembly and to help alleviate thermal stress which can lead to premature failures.
In a race car, some exhaust leakage may be tolerable, but in a street application, exhaust leaks are verboten for several reasons. First, in catalyst equipped cars, leaks of exhaust gas from ahead of the catalyst are untreated and add to air-pollution. Second, exhaust leaks can be dangerous to the vehicle occupants and the noise a nuisance. Finally, leaks can also allow air into the exhaust system causing erroneous lambda sensor readings resulting in less than optimum performance. This last reason also affects many race cars that rely on lambda sensor readings for proper fuel metering and tuning.
Burns takes special care when sizing collector slips, and in a perfect world, they should not leak. However, the tolerance in tubing size and the possibility of misshaped tubes can cause a poor slip fit between primaries and the collector, leading to leakage. When we build headers, we take special care to assure that the primary tubes are properly sized and shaped to assure a perfect fit. We use a hydraulic tubing expander for this purpose. Since many fabricators do not have easy access to this type of equipment, a possible solution is to use a double slip collector.
A double slip collector is comprised of a standard slip merge collector and an extra slip-sleeve that is welded to the exhaust primary forming a double-slip arrangement. Exhaust sealing is achieved due to differing rates of thermal expansion. As the exhaust heats up, the inner primary tube heats up, and expands. Since the collector slip is not in direct contact with the exhaust gas, it heats up less, and therefore expands less providing a tension seal between the two sections of tubing. Finally, the outer double-slip sleeve heats up even less, expanding even less causing an even tighter tension seal.
During a recent discussion with race engineers from an upper-echelon race team, I was told that they were able to reduce the fuel consumption of their endurance race motor by over half a mile per gallon simply by tuning their motors with headers equipped with double-slip collectors. This may not sound like a lot, but at a 24 hour race like Daytona or particularly Le Mans, it can make a huge difference.
It should be noted that another good application for double-slip collectors is in tight spaces where a standard 2" long slip cannot be accommodated. We do not recommend using single-slips shorter than 1-1/2". For applications where shorter slips are needed, the double-slip is a great option.
Any of our merge collectors can be ordered as a double-slip collector. For a 4-1 collector, the additional cost is approximately $60. Double slips cannot normally be retrofitted since a wider spacing between tubes is needed to accommodate the sleeves. Also, double-slip connections can be purchased individually for use in other sections of your exhaust.