Car Mechanics (UK)

THE IM­POR­TANCE OF BACK PRES­SURE

- Iceland · Belgium · Belarus

Klar­ius’s Doug Bent­ley ex­plains why back pres­sure is so im­por­tant to con­sider, es­pe­cially as am­a­teur tuners get it so wrong: “After­mar­ket sports ex­hausts are, usu­ally, the first place we look for an ex­am­ple of how to get things right and wrong. The qual­ity varies so much, from be­ing works of art to low-grade, generic big bore muf­flers. Sports ex­hausts are usu­ally tuned for loud­ness, which tends to be achieved by using fewer sound-dead­en­ing ma­te­ri­als and baf­fles within the si­lencer boxes. Should the pipe be a wrong/ in­ap­pro­pri­ate di­am­e­ter though the sys­tem, a freer-breath­ing lowend ex­haust sys­tem can ac­tu­ally re­duce en­gine out­put. This is be­cause any en­gine is de­signed for a spe­cific flow of gas through its pis­ton cham­bers, valves and ex­haust sys­tem. The pis­tons rely on a de­gree of back pres­sure to push against, in or­der to bal­ance the gas pres­sure within the en­gine to max­imise power and torque.

“As an anal­ogy, if you blow through a small di­am­e­ter drinks straw, the pres­sure in your cheeks rises and can be­come un­com­fort­able. When you choose a wide straw, and the pres­sure re­duces, there is only at­mo­spheric pres­sure re­main­ing and you soon run out of puff. With an en­gine, should you re­move the ex­haust from an en­gine en­tirely, it will run poorly, mis­fire and pro­duce less power. Con­versely, if you im­pose too many re­stric­tions into an ex­haust sys­tem, the en­gine will labour un­nec­es­sar­ily. Get­ting the bal­ance right is crit­i­cal.

“Should an ex­haust sys­tem start to blow, the back pres­sure drops, en­gine power will re­duce and both fuel con­sump­tion and emis­sions will rise. In these cases the ECU might put the car into ‘limp-home’ mode or, in ex­treme cases, shut the en­gine down en­tirely. We have also wit­nessed some cheap after­mar­ket ex­hausts that have dis­as­trous ef­fects on en­gine per­for­mance, ef­fi­ciency, emis­sions and wear, due to their in­com­pat­i­ble back pres­sures. This is why Klar­ius places such an em­pha­sis on only sup­ply­ing Type Ap­proved sys­tems.”

A fur­ther con­sid­er­a­tion is noise. It is il­le­gal to mod­ify a car’s ex­haust so that it is louder than it was when new. When measured sub­jec­tively by an MOT tester’s ear, it is a valid fail­ure point. The po­lice are also en­forc­ing the law and the gov­ern­ment has been re­ported as tri­alling ‘noise cam­eras’ that are de­signed to crack down on, what the for­mer Trans­port Min­is­ter de­scribed to the press as, "nuisance driv­ers".

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