The exhaust system is an area often overlooked, for street or racing, especially in relationship to valve size changes. The proper matching of the exhaust system size with valves, carbs, cam, and head work can provide better all around power, mileage and cooling for street or racing. Let's face it, you can't get it in if you can't get it out. Any restriction of the exhaust limits the intake or will not adequately remove hot burned or unburned gasses from the chamber, thus not allowing a new cold charge to start the filling cycle. What is critical here is to provide the proper balance of the system. That's where our well over 30 years of testing and developing on nothing but the VW air cooled engine really shows up for our customers and the reason for our total product line of exhaust systems.
As, I have stated for years, and in my GB 801-HEADS technical article, using "any" valve larger than stock "requires" the removal of the restrictive heater boxes and the use of at least GB 932S 1-1/2" complete larger pipe exhaust system. Even larger intake valves alone will not provide the performance or cooling that the stock size will, unless the corresponding exhaust valves and extractor are installed to complete the matching system. In fact, such a bastard combination will normally reduce the air speed badly in the intake port, making the engine hard to tune in the lower RPM's. These engines often run hot and suffer from flat spots off idle. When bigger jets are installed to fix the flat spot, then the engine gets too rich in the mid and higher RPM as the air speed starts to return to normal.
The engine will not have much more top end (if any) because the small exhaust valve becomes the restriction in the chain. If bigger exhaust valves are installed, then the stock heater boxes and exhaust are the restriction. That is probably why all car manufacturers (before smog laws) used a formula to achieve a starting point to establish the proper balance from the intake valve, to the exhaust valve, to the exhaust pipe size.
With extractor exhaust systems the size of the primary pipe, collector shape and size, the pipe out of the collector to the muffler or the stinger (megaphone) size and taper all become critical in the system. Even too large of a collector and/or the size of the pipe from the collector to the muffler can diminish the tuning and driveability of the system just like any other size relationship. We should all be able to relate to water. So visualize that water is running down a shallow, wide stream. Now, it comes to a narrower location and the water speeds up, or if the stream gets wider it slows down.
This is exactly what happens in the exhaust. When the air goes through the collector it speeds up. It continues at about this speed until it opens into a larger chamber at which point it creates a vacuum behind it. The length of this smaller size is what makes the proper amount of vacuuming occur at the proper point all the way back into the chamber. That is why our systems are 'Extractor Exhausts' and many other companies are simply headers.
As today's VW engines get larger, produce more power and/or higher RPM is required, the need increases for better and larger tuned exhaust systems. I, with the aid of Dean Lowry, developed the first "competition" regular collector (GBE931, 1-1/2") extractor exhausts for VW's in 1965. I have continued improving designs with the development of the merged 1-1/2" (GB 932) system in 1969, the 1-5/8" (GB 933) in 1970, 1-3/4" (GB 934) in 1972, 1-7/8" (GB 935) in 1977, 2" (GB 936) in 1987 and 2-1/8" (GB 937) & 2-1/4" (GB 938) in 1992. Continuing to improve our designs, I have moved the #2 & #4 pipes for better pulley bolt clearance (for ease of valve adjusting) when our GB 442E Equalizer pulley is used.
The collector has been moved down for better apron clearance and straighter exiting into the collector and stinger. All merged systems have slip joints at the #2 & #4 pipes to allow for different engine widths and ease of installation. With the introduction of the 2", I have added swivels on all 4 cylinders (GB 935 and larger) that allow rotation for alignment with the slip joint attached to the merged collector. GB 933 and larger systems have special double thick flanges to prevent distortion.
In many applications, the rear apron may require clearancing due to the positioning and size of the larger system's collector. We provide some systems for both street (mufflers must fit) and race cars. Due to the size and required location, the 1-7/8" and larger will require (furnished on request) a 1" spacer block to be welded to the engine support (or often called a traction bar) to clear the #3 and possibly the #1 pipes. On some wheelie bar applications, a dropped engine bar (print available with system upon request) may be needed as well as a spacer.
All Berg merged systems include a stinger (megaphone) and our exhaust flange & collector gasket(s). Appropriate nuts for mounting the exhaust system to the engine are included. Mufflers (dual) are only available for GB 932S & GB 933(S).
Naturally, we still offer our special (GB 932S & GB 933S) merged systems that have the #1 & #3 pipes moved to clear the thermostat and bracket, so proper warm up can be achieved to prevent premature wear. Muffler with mounting brackets available separately.