Torque & Horsepower

Will a larger exhaust system reduce torque?  

To vastly oversimplify a complex subject, it could shift the torque curve, resulting in a loss of torque at certain r.p.m. ranges.  Moreover, it could be possible to replace a system that helps scavenge the combustion chambers at specific tuned r.p.m. with one that reduces exhaust velocity and scavenging (or worse, results in increased reversion).

Generally speaking, as long as the air-fuel ratios are correct, there's no need for any backpressure in an exhaust system. (The old wives tale about engines needing some backpressure is a result of engine problems resulting from improper AFR after a backpressure reduction) Thus, an exhaust system only needs to be big enough to eliminate backpressure at the targeted r.p.m. ranges, and no larger. Moreover, the larger an exhaust system is, the more it tends to weigh.

However, primary tube size, primary tube length, collector length and collector size are also variables that a Car Crafter can manipulate to increase volumetric efficency through scavenging.  Thus, components of an exhaust system can be too big for proper scavenging. They can also be too short, too long and too small.

To optimize an exhaust system for street/strip cars, the best thing to do is to use a tuned header with the best compromise of primary tube length and size together with (1) optimized collectors for the horsepower and r.p.m. levels anticipated, (2) a larger cross-section "wave termination" section after the collector, and (3) mufflers/tailpipes that are adequately sized to prevent backpressure.  See, David Vizard, "How to Build Horsepower."  Unfortunately, almost none of the commercially available exhaust systems contain a "wave termination" section. Thus, most street exhaust systems are far from optimized.  
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460_BBF_Turbo-in-CC (formerly Dr511scj) "This guy has no life other than posting endlessly on carcraft.com." -- Car Craft, July 2005
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October 1, 2003: " I'm thinking a couple of...turbos, blowing through an old Powerstroke intercooler...on a Super Cobra Jet-head 460 would be mad cheap and make sick power."
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"I have no problem with your...talking to several versions of yourself...or pointing out our failure to do a turbo story ...." --Douglas "CC/Rambler" Glad

Another way of approaching the problem is to develop a variable area exhaust system. In other words, an exhaust system that routes the exhaust through smaller flow path at low r.p.m. and opens additional flow area at higher r.p.m.  

This is most often done through the use of "active" mufflers (silencers with a pressure-sensitive butterfly valve inside), most commonly to reduce "boominess" and "drone" at lower constant r.p.m. cruise conditions.  These systems tend to add backpressure when the butterfly valve is closed and, thus, are hardly optimized for low-flow torque. But they could make an engine effectively "see" a smaller exhaust at low r.p.m.  

A more sophisticated, yet seldom seen way to employ a variable area exhaust system would be through a primary/secondary piping system with a gate valve. The gate valve controlling exhaust gas access to the secondary piping would only open up when the flow was sufficient to cause backpressure in the primary system.   The costs, extra weight, and difficulty in emplementing automatic control of such a system would probably outweigh any benefits of increased low-speed torque.  Moreover, the scavenging and reversion effects would be more affected by the header/manifolds used, which would be "upstream" from the primary/secondary piping system.  
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460_BBF_Turbo-in-CC (formerly Dr511scj) "This guy has no life other than posting endlessly on carcraft.com." -- Car Craft, July 2005
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October 1, 2003: " I'm thinking a couple of...turbos, blowing through an old Powerstroke intercooler...on a Super Cobra Jet-head 460 would be mad cheap and make sick power."
-------
"I have no problem with your...talking to several versions of yourself...or pointing out our failure to do a turbo story ...." --Douglas "CC/Rambler" Glad