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Old 5th April 2006, 18:22   #1
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Default VTEC with cut exhaust- good or bad?

ok heres a question thats had me by the b*lls the whole of today.. my friend has a old honda city vtec whose exhaust pipe got neatly chopped of just before the muffler.. by neatly i mean there are no obstructions to the flow, so its become a kind of illegitimate free flow system.. here is my question... is this gonna aid or hamper the performance figures?

the way i see it there are 2 possibilities (duh)
1) the performance will improve because it is now a free flow exhaust system so the engine doesn't need to contend with back flow blah blah...
2) the performance will fall because teh engine management system has been configured to work best with the old specs of the car, ie with a fully functioning muffler...
but here is the catch.. then main aim of an ECU is to adjust the various parameters of the engine in accordance with changes the engine is forced to contend with(which in this case is the exhaust) so as to keep the engine as efficient as possible... so the ecu shud theoretically adjust the engine parameters and hence make the best possible power/torque etc..
see wot i mean by by the b*lls??

Last edited by superunknown : 5th April 2006 at 18:23.
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Old 5th April 2006, 18:37   #2
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I think you should ask for your b*lls back.

Just cutting off the muffler doesn't make it a "free flow". Do some searching on the site and you'll get enough info on what a free flow really is.

And yes, a car without a muffler is a very bad thing.
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Old 5th April 2006, 20:33   #3
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Also u can visit

www.automechengineers.com for more info.
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Old 5th April 2006, 21:15   #4
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If u need an increase in power, this ain't gonna help. U'll only be getting the sound..

For performance, u'll need to work from the headers all the way to the tail pipe...
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Old 5th April 2006, 22:35   #5
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It will be just noise and not a tuned sound, and total loss of low end torque. The -ve terminal of the battery needs to be disconnected for a few minutes or the ECU to be reset which will allow the ECU to start recalibrating again.. but again as I had mentioned earlier ..it will be only be a loud racket nothing close to a tuned sports exhaust.. u need to mod headers also... etc etc
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Old 5th April 2006, 22:44   #6
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Your "free-flow" still has to contend with the catalytic converter, which is a major obstruction to the flow. That alone is worth 5 bhp on the old Vtecs.

But once the cat is removed and you have a straight pipe from the valve to the cut-out, it should be interesting. With a well-designed set of headers, you would have minimum back-pressure leading to a centre/side exhaust. Which is basically what some of the drag cars are running.
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Old 6th April 2006, 00:00   #7
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ok i see some posts and i have answers..not exaclty wot i was looking for but neway....
boom shiva - "a kind of illegitimate free flow" system is NOT a "free flow" system, but thanks for the advice on reading up tbhp for further info-the thought would have never crossed my mind
secondly this happened by mistake to a car ive never seen/driven so no im not trying to be smart and get more performance from a vtec by cutting the exhaust
v1p3r, i do understand that the cat will still remain as a source of power loss, but what i wanted to know is if the ecu of the car will be able to adjust for the free-er flowing exhaust gases(NOT FREE FLOW EXHAUST lol!!) as the muffling action( what ive coem to understand is nothing but bouncing back and forth of the gases in the muffler to get harmonic balance with exhaust valve opening) will be eliminated..
chetanand-wont the ecu be able to adjust on-the-fly, cant removing the muffler be compared to say decreasing the outflow of gases(reducing back pressure) in conditions such as idling?.. or do i haev another tbhp bashing coming my way
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Old 6th April 2006, 09:26   #8
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Hey bro, didn't mean to sound like I was bashing...

Btw, if the cops catch your friend without a muffler, they'll have a field day extracting bribes, so please get the car fixed ASAP.
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Old 6th April 2006, 13:52   #9
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For the ECU to adjust anything, it would have to be valve-timings. This would affect the whole combustion cycle. And it would also mean that the ECU has to have some sort of a pressure sensure, which I'm not sure exists on the OHC Vtecs.
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Old 7th April 2006, 00:05   #10
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Quote:
Originally Posted by v1p3r
For the ECU to adjust anything, it would have to be valve-timings. This would affect the whole combustion cycle. And it would also mean that the ECU has to have some sort of a pressure sensure, which I'm not sure exists on the OHC Vtecs.
I also dont think there is a pressure sensor.
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Old 7th April 2006, 00:38   #11
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Quote:
Originally Posted by superunknown
chetanand-wont the ecu be able to adjust on-the-fly, cant removing the muffler be compared to say decreasing the outflow of gases(reducing back pressure) in conditions such as idling?.. or do i haev another tbhp bashing coming my way
No ecu will NOT treat the idle condition low back pressure as back pressure at other RPMs .. becoz :
1] ECU knows when the engine is idling, the TPS tells it that your engine is idling. The IAC motor/valve comes into picture and the job of ecu is to keep the engine just breathing and not worry about other params.
2] But when you are speeding at WOT , the ECU again knows what is happening and starts communicating with a different set of sensors and controlling different valves etc and not even bother about AFR
3] similarly at mid range the ECU is doing something else ..worrying about the AFR .. and stuff like that..
So if u have a low backpressure exhaust, there is greater flow of intake charge during exhaust stroke (due to valve overlap)...
that means ECU will have to step up and make corrections to mantain AFR , EGR, timing etc etc tect etc ..
FYI .. this is just a basic introductory description of what is the difference in backpressure to a ECU at idling and backpressure at different load conditions..
(AFR is just one parameter ..there is a combination of multiple other parameters also in the picture)
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Old 7th April 2006, 03:38   #12
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Quote:
Originally Posted by v1p3r
Your "free-flow" still has to contend with the catalytic converter, which is a major obstruction to the flow. That alone is worth 5 bhp on the old Vtecs..
5bhp!!! whhattt???
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Old 7th April 2006, 07:03   #13
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Quote:
Originally Posted by rareone
Also u can visit

www.automechengineers.com for more info.
The correct URL is http://www.automechexhausts.com/
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