sipstrassi Posted May 21, 2007 Share Posted May 21, 2007 Hi just aquick question. I Have a N/A just take 2nd decat out and put in new RSR exhaust. (have the baffle in - very noisy without)) The car seems to have lost some horses and does not pull as well as it did. How can i get the power back - put in second decat, take out baffle (risk getting stopped by police) Thanks in advance Pete Quote Link to comment Share on other sites More sharing options...
Ric Posted May 21, 2007 Share Posted May 21, 2007 Its been mentioned that the NA relies on backpressure for max power, so removing you're cats will be getting rid of the back pressure you require thus loosing you power Quote Link to comment Share on other sites More sharing options...
sipstrassi Posted May 21, 2007 Author Share Posted May 21, 2007 Right thank for that...back in it goes. Will taking out the baffle with the 2nd cat goes back in make a difference? Pete Quote Link to comment Share on other sites More sharing options...
Al Massey Posted May 21, 2007 Share Posted May 21, 2007 i had both my decatts out but i had to put one back in as i lost torque,it was well noticeable when it was out so once back in the power and torque were back Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 21, 2007 Share Posted May 21, 2007 Its been mentioned that the NA relies on backpressure for max power, so removing you're cats will be getting rid of the back pressure you require thus loosing you power Well whoever mentioned it is talking rubbish. Quote Link to comment Share on other sites More sharing options...
Animal Posted May 21, 2007 Share Posted May 21, 2007 I have an RS*R GT2 on my N/A with no cats & the baffle plug in and never felt any drop in horses Quote Link to comment Share on other sites More sharing options...
mikeyb10supra Posted May 21, 2007 Share Posted May 21, 2007 Well whoever mentioned it is talking rubbish. I took this stance the other day Chris and got shot down for it Quote Link to comment Share on other sites More sharing options...
Thorin Posted May 21, 2007 Share Posted May 21, 2007 Well whoever mentioned it is talking rubbish. Pretty much a myth, unless you had an older car without fuel injection that couldn't adjust the air/fuel ratio to compensate. Quote Link to comment Share on other sites More sharing options...
hoff Posted May 21, 2007 Share Posted May 21, 2007 I was thinking of getting the Whifbitz 2nd decat for my 94 NA. Will this then have an adverse effect on my power? I was also thinking of getting one of nics manifolds in the future. Quote Link to comment Share on other sites More sharing options...
letmeshowyou Posted May 21, 2007 Share Posted May 21, 2007 Chris, what is your take on the cause? I've heard of a few N/A supra's that have suffered a drop in torque from decatting and a very free flowing exhaust (not trying to start a row, just curious ) that have been better with a cat put back in. Quote Link to comment Share on other sites More sharing options...
hoff Posted May 21, 2007 Share Posted May 21, 2007 I thought its fine as long as you stick to a 2.5" pipe. Quote Link to comment Share on other sites More sharing options...
Digsy Posted May 21, 2007 Share Posted May 21, 2007 Yeah, I have experience with NA cars that have lost midrange torque after fitting a bigger exhaust too. Not so sure about "relying on backpressure for max power" though. Quote Link to comment Share on other sites More sharing options...
PAG Posted May 21, 2007 Share Posted May 21, 2007 i have both cats out on mine with a silent hi power system and i think that the cars more responsive and im pretty sure its up on power aswell but have no proof as havent had it on the rollers Quote Link to comment Share on other sites More sharing options...
JustGav Posted May 21, 2007 Share Posted May 21, 2007 Please correct if I am wrong, but by putting in a free flowing exhaust you don't 'loose' the horses as such, however the engine dynamics have changed and by being more breathable the powerband moves up the rev range a tad. Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 21, 2007 Share Posted May 21, 2007 N/A's, particularly modern N/A engines, rely on a tuned resonant length of both intake and exhaust to pull torque up at desired points. Going to a massively big bore exhaust is like radically shortening it. Fiddling with stock air box and fitting an HKS "filter" et cetera alters the tuned length of the induction system. Back pressure, or lack of it, is NOT the reason torque peak areas move about! Quote Link to comment Share on other sites More sharing options...
Digsy Posted May 21, 2007 Share Posted May 21, 2007 I thought it was mroe to do with the exhaust backpressure giving the intake charge something to work agaist even thought the exhaust valve was open (i.e. it wasn't all just blowing through). Its possibly got more to do with exhaust header and secondary pipe tuning that total exhaust backpressure. Quote Link to comment Share on other sites More sharing options...
JustGav Posted May 21, 2007 Share Posted May 21, 2007 I thought it was mroe to do with the exhaust backpressure giving the intake charge something to work agaist even thought the exhaust valve was open (i.e. it wasn't all just blowing through). Its possibly got more to do with exhaust header and secondary pipe tuning that total exhaust backpressure. Yeah, okay, I see my error of explanation... By changing the exhaust to a bigger bore, it would change the resonance frequency (causing the engine to breathe via the exhaust at a different time/frequency), this is required in order to time the waves at the same time the exhaust ports open, so not only does the engine push the combusted material out, but the resonance frequency would set up a wave, if timed correctly the removal wave will help pull the air out of the exhaust ports.. I assume then the intake is the opposite, utilizing the push wave at the correct time to further push in more air into the chamber. On the opposite side, the wave would also help keep the fresh air in the chamber while the exhaust valve closes. My thinking/explanation more correct now? Quote Link to comment Share on other sites More sharing options...
toyotasuprauk Posted May 21, 2007 Share Posted May 21, 2007 Please correct if I am wrong, but by putting in a free flowing exhaust you don't 'loose' the horses as such, however the engine dynamics have changed and by being more breathable the powerband moves up the rev range a tad. When I fitted the HKS Superdragger to my NA with both cats still in, it felt like I lost power at the lower end of the rev range and gained a little at the higher end so would agree with this. Getting fed up of the noise though so thinking about going back to stock. Quote Link to comment Share on other sites More sharing options...
JustGav Posted May 21, 2007 Share Posted May 21, 2007 Exhaust scavenging, that was the phrase I was trying to think about... Quote Link to comment Share on other sites More sharing options...
Chris Wilson Posted May 21, 2007 Share Posted May 21, 2007 Yeah, okay, I see my error of explanation... By changing the exhaust to a bigger bore, it would change the resonance frequency (causing the engine to breathe via the exhaust at a different time/frequency), this is required in order to time the waves at the same time the exhaust ports open, so not only does the engine push the combusted material out, but the resonance frequency would set up a wave, if timed correctly the removal wave will help pull the air out of the exhaust ports.. I assume then the intake is the opposite, utilizing the push wave at the correct time to further push in more air into the chamber. On the opposite side, the wave would also help keep the fresh air in the chamber while the exhaust valve closes. My thinking/explanation more correct now? Eloquent and accurate Quote Link to comment Share on other sites More sharing options...
Tricky-Ricky Posted May 21, 2007 Share Posted May 21, 2007 You also have to consider that fitting a larger bore exhaust will also cause the exhaust charge to cool quicker, which in effect will slow the gas escape further, which will sap power,which is why you see manifolds etc lagged to keep the gas temps high, its all a question of balance and dynamics Quote Link to comment Share on other sites More sharing options...
hoff Posted May 21, 2007 Share Posted May 21, 2007 So whats the advise, will I be ok putting in Whifbitz 2nd decat? Quote Link to comment Share on other sites More sharing options...
Thorin Posted May 21, 2007 Share Posted May 21, 2007 Yeah, okay, I see my error of explanation... By changing the exhaust to a bigger bore, it would change the resonance frequency (causing the engine to breathe via the exhaust at a different time/frequency), this is required in order to time the waves at the same time the exhaust ports open, so not only does the engine push the combusted material out, but the resonance frequency would set up a wave, if timed correctly the removal wave will help pull the air out of the exhaust ports.. I assume then the intake is the opposite, utilizing the push wave at the correct time to further push in more air into the chamber. On the opposite side, the wave would also help keep the fresh air in the chamber while the exhaust valve closes. My thinking/explanation more correct now? Yup, and with more air getting sucked into the chamber means that older carb engines could run leaner, resulting in burnt valves I understand. Quote Link to comment Share on other sites More sharing options...
Thorin Posted May 21, 2007 Share Posted May 21, 2007 So whats the advise, will I be ok putting in Whifbitz 2nd decat? yes Quote Link to comment Share on other sites More sharing options...
hoff Posted May 21, 2007 Share Posted May 21, 2007 Sweet. That'll do me nicely:) Quote Link to comment Share on other sites More sharing options...
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