Honda fits only short branch ( 4 into 1 pipe ), cheap & heavy cast iron
exhaust*manifold for its engines <2.2 litre, even a F22A's *'s twin pipes
are short.  Result is lower though adequate torque @ low rpm ( as during
buyers' test drives ), but very low ( <½ as much ) torque & mpg @ high
( >3000 ) rpm, this difference rises with rpm.
http://circletrack.com/techarticles/73598
My local-made 4-2-1 * [i] uses aluminised mild steel  [ii] is 13 kg <
original *  [iii] has longer twin pipes than a F22A's *  [iv] twin pipes'
interiors' total cross section area is 60% > original * ( single pipe )'s.
In 6-02 I saw a new Civic vtec 1.6 litre engine with very short * : a waste
of vtec ( no way is torque / mpg esp @ high rpm maximised ).
www.turborick.com/gsxr1127/gasoline.html para 10.2(1) indicates how
bad a civic's * is.  Many car makers ( incl Nissan in Sunny 130Y, Hyundai
in Sonata 2.4, Proton in Waja 1.8 ) save on *, because 99.99% buyers
don't test drive on highways, or know / experienced a difference between
short & long branch *.

My F20A had crude carbon-core cables made by Sumitomo, efficiency
was low ( even by 1990 std ) : longest cable has 12.7k ohm
www.magnecor.com/magnecor1/truth.htm  I've changed my F20A's plug
cables 4x, present set ( German cables ) has just 0.2 ohm/ ft : when
warm & w/o load, my F20A can idle @ just 600 rpm, with load then
550 rpm ( without shaking / stalling ).  Plug gaps are 2mm.  All these are
impossible with crude carbon-core cables.  How low can your original
spec engine ( with crude carbon-core cables ) likewise idle @ ?
Carbon can absorb RFI, but cannot conduct as well as metal : high
efficiency cables makers coil metal wires around carbon cores, then use
these coiled wires to conduct : spark size Ø expands from ½ to 3 mm,
10x brighter, colour turns fr deep blue to bright yellow, 5x as loud.( in
open air ).
@ high rpm, any engine with 1 small coil & crude carbon-core cables
will have very small sparks, low torque & loud exhaust noise.  This
inadequacy is esp bad in a 2 litre SM4 accord ; combined gear ratio
3.1323 is > those of 2 litre Toyota / Nissan models, e.g. 9.78% >
Nissan Cefiro' s 2.8530 : 75 mph requires 3341.7rpm, but a Cefiro
requires just 3043.7 rpm, on the same 205/65R15 tyres.