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Exhaust Power gain = none!

Seriously, how many expect massive gains from a back box???

Think about it - the engine produces power (AT THE ENGINE) and dispatches it to the wheels with exhaust gases fired out the back (TO GET RID OF THEM) So tell me, changing the back box, which has nothing to do with an engines power - how does this give a power increase at the flywheel/wheels

IT DOESNT - is everyone delusional - just think about the principle for at least one second! An exhaust gets rid of exhaust gases which WERE used to produce power - do u think if u take out ur complete exhaust system from the manifold the power will be more? NO but ull get a lovely noise.........;)



its allowing the engine to breathe better

it may give you a few bhp

remember more air in more air out

if you can speed up the flow of gases the engine can chuck out more.

Cupsize - imagine trying to running a 10000 meters, but you can only breathe through a burger king drinks straw ! Your lungs are still the same size and power, just less efficient !

Wait a second cupsize is right, many exhaust systems will give you less power. Even the ones who dont will probably give you more power top end and less torque. Its important to choose a system carefully.

As noise goes, a bigger exhaust doesnt mean louder. On aston martin db7s vantages there are three sports exhaust systems, the loudest one doesnt have the biggest bore as the noise is being forced through a smaller area.

LOL exhaust systems do not give you less power, only poorly designed ones ie too big or little diameter. They will however change where the power is.. You would be looking at 3-4ish bhp or so from a back box...... not a lot but you do notice it on motorways as mid range is where it improves the power.

hmm, well surely that would mean some backboxes fit in that criteria, and so dont give gains? Fitting a sport backbox is likely to decrease torque as much as it increases power. Im not saying this is the case for all of them, but enough. Same with cats. Most modern cats dont restrict much power, by fitting a decat you will get better throttle response, fooling people into thinking it has more power.

fitting a back box will do nowt........nadda...nothing....and as ROb says, it can leas to a loss through poor design, even if its the SAME bore.

i have tested most of teh system s on the 172 and only real gains froma full system are a max of about 2-3BHP....something you will NEVER feel.

its more about looks and noise,and it doesnt matter, as long as you dont loose power, its ok i guess.

and exhausts are VERY important to how much and where you make power. you could loose more than you thought by changing to a straight through system or indeed taking it all off. Tunned lengths, back pressure and gas velocity are only a tiny amount of things you need to consider when making an exhaust. Now the manufacturer spends alot of time and money amking one that is quiet and not restrictive, then performance manufacturers do no testing or development but to go out and copy the system bend for bend and take out the silcencers.....although admittdedly, some do go to some lengths to develop a system. But i can ssure you magnex is not one of them....LOL

well, they all suggest that!

Jetex, read their site, they said their cup system gave the racers a gain from 187bhp to 200!!! LOL....its gone now..wonder why, i complained after testing and said it gave 2bhp at peak...asked them to more replies....but its cheap...and not too bad......

Well benr if there was no gain to be made from having a straight through back box then why do racing cars have them and not baffles?? I have a K-tec system on mine I think I can notice more mid range/top end with it..... even if its not given me more bhp its made my car more powerful where I normally use it!

Hope I havent been mis sold then? Have just ordered the titanium system from David at K-Tec and he said I would see a noticable improvement as long as the application was correct.

Not expecting a great improvement by any means but I take it "noticable" is over 5Bhp?

Steve- As i was saying probably throttle response is better, not power. Though with a filter and de-cat combo you will get a power change which is noticeable.

Sangy- noticeable is when you notice it!

Thanks for that I think?! Guess it cant do any harm and it is a good system. Am taking on the Midland RR day so thatll be the proof of the matter.

Well, i use the K tec system over here for the HK market, as they are the best you get for the 172, well obviously apart form the 900 quid orbisound system but thats another story.

K-tecs you wont be disappointed is NICE. And David is a very nice and helpful guy.....his products are great.

anyway, steve.....why do race cars use race systems....because, they are after a different thing than a rd car. They deisng a system that will provide the least restriction, they run lots of overlap so back pressure is a non starter. They need to pass alot more gas, which is hotter and expands more, leading to a larger volume of gas from the start. noise isnt so much of a problem, and if you look, race system are rarely a single staight will see bends which are calculated and tested via trial and error to make the best use of the sound waves to help move gasses as fast as poss and scavenge the cylinder when the exhaust port is open.
  Clio v6

Quote: Originally posted by RobFenn on 07 January 2003

I didnt think it was that bad! Oh well!

One of the best exhaust makers in the UK is Hayward and Scott by the way.
And a snippet from their website

"Just released! The new EVO 7 exhaust giving approx gain of 17 BHP with cat pipe, centre pipe and a straight through back box ( 97 decibels ) with a tail pipe size of your choice. A complete system with a big bore down (75 mm) will give approx +25BHP.
The system is as supplied to a leading mitsubishi supplier."

Maybe I am a little bias?

From what ive read they do take their game very seriously, there best form of advertising is word of mouth so they cant be too bad? 97 decibels, nice! just legal!
  Clio v6

Quote: Originally posted by BenR on 07 January 2003

define legal?
The noise limits that apply are as follows: Bedford

101dB(A) at 2/3 maximum revs ½ metre from the exhaust at an angle of 45 degrees.

Drive by:

87.5dB(A) at a minimum of 60mph measured 20 meters from the centre line of the vehicle accelerating maximally.

well, the bedford is the autodrome right, they impose thier own noise limits.

There is NO actual legal noise limit as noise (in db) varies with temp, altitude, humidity etc etc....and you cant imply one noie regulation for all atmospheric conditions, its impossible and unfair. BUT, the police CAN pull you over and give you a (forgot the name) ticket which means you have to go get noise tested. Now, not many garages can test for noise, so go to one, and ge them to write that they cant test and produce that to the police.....end of story.

This is what a mate told me.....and the only reason he knows is that hes the is cheif trainor for the transport dept (soz dont know the official name LOL).

Tell me this then:

How exactly do u notice 3-5BHP in a 172, maybe in a 1200 but I mean come on! U notice noise and that has a placebo effect to make one think "oh yes, this is much better....." ad lib

U may gain 3-5 BHP along the range somewhere but ull loose it somewhere else.

My advice - stick a GrpN replacement Green element in which has 2 layers only (New Demon Tweeks motorsport mag with Motorsport news this week) and do nothing else! At least ull know ure not loosing power - how many people have got a shock when they rr their car and oh dear its got 130BHJp at the wheels after serious mods have been done

  BMW 320d Sport

Oh dear...of course an exhaust makes a difference to the engines power output, its not just a case of venting the exhaust gases straight out from the head!Exhaust tuning on a normally-aspirated engine is something Im not even going to get into but yes you can get power out of an engine with exhaust tuning. Not a huge amount but it can be done. Probably easier to lose power with exhaust mods than gain it though unless youre very careful and know what youre doing!

With turbo cars, replacement exhausts claim big dyno results because thats how a turbo works, the flow of the exhaust system is directly related to the force that drives the turbine, so more boost pressure is available or at least available earlier on the throttle, by improving the exhaust flow. Of course without the turbo being capable of the additional boost pressure and the fuelling/management being able to keep up with the extra boost, the exhaust mods will just over stress the turbo or lean out the engine, losing power. But if everythings in place you can see big gains on a turbo with a performance exhaust.
  Lionel Richie

Quote: Originally posted by Jonto on 07 January 2003

Cupsize - imagine trying to running a 10000 meters, but you can only breathe through a burger king drinks straw ! Your lungs are still the same size and power, just less efficient !
Or try running the 10000m with a straw in your mouth and a cork up yer ahem!!!!

I heard somewhere that if you exchange different designs of well designed exhausts (like the original for a good aftermarket one) what you can do is shift the area under the torque curve about.

The area does not change, so for a gain in one place you will lose out somewhere else, but you can do things like iron out flat spots, shift the peak torque further up the rev range (for more power), or put max torque where you spend most of your time.

yes, but with limited results.........using cam profile is a far easier way of moving the power band about.

the back box on a car is designed to give pressure and create a vacuum to suck out the gas! sounds strange but the velocity of the gas leaving the system box creates a vacuum behind it.....thats why an exsaust thats too big will lose back pressure and thus performance!" the whole 1000m thin is true for the rest of the system!


ClioSport Admin
  Audi A4 v6 & Clio

BenR, I can tell you Magnex DO go to lengths to produce their exhausts, because they wanted my 172 for a few days (in Oct 2000) to design the exhaust around and do testing on.

Seems to me that reno put alot of work in designing the original exhaust system to give the standard engine the maximum amount of power in the right place, all these big bore exhausts do is sap the power in the lower rev range(where you need it the most) and give a little more when nearly redlining!

yep...i think they spend ages workin out the correct size and pressure required at each stage of the system... or you could just fit a scaf bar and fit that!