imo for exhaust on the i6 just stick with 2.25 pipe from the header back and put on whatever cat/muffler combo you like the sound of. if you put too big of an exhaust pipe on it you'll loose power. every naturally aspirated motor needs back pressure to run properly.
goes with that quote from 5-90.
louder may sound cooler to alot of people but it doesn't really mean more power.
I'll agree with the bit on the 2.25" for a stock engine (2.5" for a stroker - no real need to go larger than that for any NA 6-242 variant,) but I'll argue the "engine needs backpressure" point. You are correct about going with too large a pipe being bad for power output - but not due to the backpressure point. Keep reading:
Authoritative literature (Taylor, Morrison, Blair) all agree - any backpressure will harm scavenging and contribute to the pumping losses already present. This is why there is an optimax pipe size (primaries and secondaries) to exhaust - too small, and there's not enough flow area to evacuate the gasses properly. Too large, you get internal turbulence and flow restriction ("backpressure," in simpler terms) due to uneven cooling of the exhaust gas pulse.
In fact, you want to get as close to zero backpressure as you can - better still, you'd like to see slight
negative backpressure at the exhaust collector(s) - because the slight reduction in pressure between the exhaust pulses helps to pull out the next pulse, and creates a mild "pulling" effect that improves scavenge efficiency with less pumping effort.
You can get away with some backpressure on supercharged engines due to the "positive feeding" of the intake stream (particularly with a Roots or a Whipple - both of these are positive-displacement pumps, rather than simply relying on the increase in air molecule velocity created by the centrifugal,) and backpressure is unavoidable on turbosupercharged engines (due to the location of the turbine in the exhaust stream, usually at the manifold collector, and as close to the exhaust valves as possible. This is why exhaust manifolds that mount turbos are "log-style" parts, with short primaries that can be measured in a handful of inches from the valve proper to the exhaust plenum.) Also, the "optimax pipe size"
after the turbine goes out the window - because you don't have "pressure pulses" anymore. They've all been broken up by the turbine wheel, and you just have residual pressure to move the gas (turosupercharged engines aren't quite as efficient at scavenging as supercharged engines for this reason - which has much to do with why turbo units are often selected to run higher boost pressures. This is workable, however, because a turbosupercharger is driven by the thermal and kinetic energy of the exhaust gas flow - which is otherwise wasted, because it doesn't do anything to drive the vehicle. Figure that the energy released in combustion is divided roughly into thirds - one-third goes out the exhaust pipe as gas flow and heat, one-third goes out the radiator as heat, and the remaining third [or so - typically 30-35%, with 40% at this point being good] being converted to kinetic energy to drive the vehicle. A conventional supercharger, however, shows a lower net drive energy output because it uses the drive output energy of the engine to also drive the compressor. The turbosupercharger isn't "free power," but it comes close to it by making use of energy that is otherwise wasted.)
NA (Naturally Aspirated) race cars would
love to run open ports on the heads - but do not, because that's a very good way to have cold air hit the back of the exhaust valve head and warp the thing, if not crack it outright. This is why you will see primary tubes on racing cars - even if that's all they have (yes, I know the Top Fuel vehicles are heavily supercharged and run right on the ragged edge of hydrolocking - but you'll also see the same exhaust arrangement on dedicated racing vehicles with NA engines as well. Anything else introduced backpressure into the system, which causes an increase in pumping losses, which reduces engine output.)
Further reading on the topic can be had, as mentioned, in definitive literature on the topic:
The Scientific Design of Intake and Exhaust Systems, by Philip H. Smith and John C. Morrison (I always think of this one as "Morrison" - "Smith" brings my kid sister to mind first...)
The Internal-Combustion Engine in Theory and Practice, by Charles Fayette Taylor ("Taylor")
Design and Simulation of Four-Stroke Engines, by Gordon P. Blair ("Blair")
Other literature that can be considered definitive groundwork for pretty much anything WRT automotive engines:
The High-Speed Internal Combustion Engine, by Sir Harry Ricardo (I often cite this as simply "Ricardo")
Internal Combustion Engine Fundamentals, by John Heywood (I'll usually cite him as "Heywood.")
The first three may still be purchased readily new, but the latter two have been out-of-print for quite some time (this makes them no less authoritative - they've typically used as baseline references for most more modern works, because the basic principles haven't changed a whit.) However, Heywood and Ricardo may be found digitally, which I leave as an exercise for the apt pupil.
For someone wanting further study of forced induction on the automotive engine, I recommend
Supercharged! and
Maximum Boost, both by Corky Bell (and, being modern works, both are still readily available through Bentley Press.)
(Yes, I have all of these - and more besides. I got into mechanics quite some time ago, performance mechanics shortly afterward, and I've always wanted to know
how and
why things work - which has lead into some heavily abstruse study on topics most people don't want to touch - including fields like petrochemistry, organic chemistry, thermodynamics, nuclear physics, quantum mechanics - "the dreams that stuff is made of" - and a number of other subjects.
All as self-study.
(There are times I've been asked questions, given answers I later found were accurate, and still spend a half-hour sitting and wondering "Where the Hell did I learn that?" My wife loves it - she sez that she doesn't need to buy an encyclopaedia, she has me. In fourteen years, she hasn't asked me anything I haven't been able to answer off the top of my head, or find the answer in ten minutes. I wonder if she's trying...)
