yes indeed bro, diameter tuning unlocks probly 40% of ur exhaust potential, its the length of ur header and downpipe to first bullet that makes the big difference

try this, if u r on 4-2-1, assuming extractor length is 15" like mine, then use 37" downpipe length and put ur bullet there. if header length is different, just makes sure header+downpipe length = 52". +- 1 inch is ok.

use 1.6" piping for downpipe, this will flow good up to 4500rpm on myvi engine. with myvi exhaust open BBDC at 30 degree, 52" of header+downpipe would give u boost at myvi peak torque 3200 rpm. for max boost effect, ur bullet internal pipe volume has to be minimum 2X engine cc i.e. 2.6L of bullet volume

if 4-1 then 52" starting from extractor flange to end of downpipe then put bullet there.

i tested this theory on kelisa, 34" downpipe to boost at 3500rpm, i felt boost at 3400rpm which made me happy coz it was really close to my approximation.

try this first and let me know the result, then i can propose something more to lengthen the boost duration

oh yeah, avoid openpodding ur intake, it does not do good for touge tuning, keep ur airbox, modify it to let airflow in easily up to ur rpm habit and call it a day. once air can flow easily thru ur airbox, there is no need for aftermarket drop in. use oem filter that was highly R&D-ed, designed and tested purposely for myvi

yes, this technique applies to all conventional 4 stroke engine, the difference is the length of ur exhaust pipe depends on ur engine camshaft profile particularly the exhaust valve open BBDC. refer to ur manual for this data and i can help u approximate urs

it works even if u got engine with variable cam timing (vvti etc) coz i think only the intake cam is variable but exhaust cam has same profile across rpm

in ur manual at engine mechanical section. usually there is a table showing engine cc, bore stroke, compression etc, there u will find intake/exhaust timing

bro, i need the exhaust open valve degree at BBDC to estimate, and would be good to let me know:
what setup u have on ur car now?
ur driving habit as in the rpm u usually drive in?
what kind of power u would expect?
do u prefer more straight line acceleration or faster speed recovery (torque-y)?

oh how so?..every car manual usually states the cam timing degree, its ok, whats ur engine? i try look for the exhaust open BBDC for u. without it i cant do anything much really

300km with 26.31 liter RON 95  ?  11.40 Km/L ... ... is this Auto car ?

u will be surprised with how different the car goes with 0.1" pipe change. increasing pressure in exhaust increases gas velocity in squares i.e. 2 pressure increase = 4 velocity increase, 3 pressure = 9 velocity. so u can imagine how 0.1" change can make lots of difference

if backpressure happens, the velocity will be reduced in same fashion too

i only know straight flow setup bro. have to ask others who have experience with sflow in their setup

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Added on October 17, 2011, 6:54 pm

may i ask what is the muffler spec? and any mod on stock piping?

what is the diameter of that muffler?

ok this is tough..mind sharing ur driving experience before and after u mod?

ok i can probably say that the muffler actually raised ur powerband from 2.8k to somewhere near 3.2k, reason can be the straight muffler opened up a bigger path so the gas velocity now takes higher rpm to match ur usual driving habit.

but just want to see other factors, what do u have in between the new muffler and the 4-2-1?

did u find more power at higher rpm than before?

apart from colder air at night giving more power, pressure wave tuning will change at night too because of exhaust wave speed is slower at lower temperature so meaning if during day boost is felt at 3500 rpm, at night time the boost might be felt at 3,000 rpm

in general, i believe an engine performs differently between day and night especially when we have 2 extremes of shiny hot days to breezing rainy night coz engine performance overall is largely affected by temperature..exhaust speed, intake air temperature, thermal efficiency etc...

who knows in rainy days where air is dense with moisture, we got free water injection upgrade for a while haha

do u like the power u have now in higher rpm? or u like the setup but wish the starting boost stays about the same rpm as before? or anything else that u want to improve?

general rule for collector length:

18" - 24" for higher rev range (6k-7k peak), longer 30" - 40" for lower rev range (3k-4k peak) and if u wish to tune it further down slight higher than stock exhaust, use as long as 50"

u will feel higher boost with bigger pipe diameter midbox but since noise is an issue, if got space why not install another midbox right after the first one? the bigger the pipe diameter the better but if scared too noisy then keep to same diameter on what u have now but the potential would be limited. it muffles better than bullet and if the 2 midboxes are close enough to each other it would form a bigger termination box than what u have now and thats good for power overall, but some exhaust shop would refuse to do this, i think it would work.