As Jim says - some (most) turbos require back pressure to maintain the integrity of the oil seals. In modern cars that's provided by the catalyst; those who remove it without investing in uprated seals will shortly wish they had. For efficiency, assuming the bearings can take the pressure differential, the bigger and fatter the pipe on a turbo system the better; this means the turbo is blowing into as low a pressure as possible which keeps the spin and compression up.
On a normally aspirated car, it's far more critical to pay attention to the timing of the exhaust pressure pulse. The exhaust is a tuned system and the impedance of it changes wherever there are changes in the structure; branches (4-2-1) or baffle boxes or cats. When the impedance changes, there's a reflected pressure wave moving the other way - and at low pressure. The idea is to arrange that at certain frequencies - engine revs - that wave arrives back at the manifold as the exhaust valve opens. The longer the pipes, the lower in the rev range this arrives - potentially massively improving the scavenging in the cylinder (though of course this is also dependent on the exhaust cam profile and lift). So 2*2 into 1 will usually give low end torque, while 4 into 1 will have torque higher in the range. (Though that can be complicated because sometimes a 4 into 1 can arrange so that the pulse arrives at the cylinder no opposite the one that started it - with a 4-2-1 system, you arrange that matched pairs 4-1 and 2-3 form the branches). With a turbo, the exhaust pulses are minced up by the impeller blades, so the exhaust *length* is irrelevant. Er, the second half of this is of no interest to you... aye well! Neil On Jul 11, 8:26 pm, Jim Hearne <[email protected]> wrote: > My 2.0 zvh turbo has a 2.5" exhaust with only one little straight > through silencer and must have pretty much zero back pressure. > It doesn't seem to cause any problems. > The only problem i've heard of like this is some turbo designs need a > certain amount of back pressure for the oil seal on the exhaust turbine > to work properly. > > Jim > > Arn1905 wrote: > > I was just wondering if there was any way or formula for working out > > back pressure in the exhaust system? > > > Im looking to replace the exhaust system on the frst engine with a > > custom made one, im hoping to use two harley davidson screaming eagles > > for the exits, but am unsure on what the back pressure would be and > > the effect on the power/turbo? > > > IF not ive got a top gear near me i can take a trip to and ask a few > > questions. > > > Thanks > > > Arnie --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Quantum Owners Group" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/quantumowners?hl=en IMPORTANT NOTE: All information presented herewith is provided on an "As Is" basis, without warranty or the implication thereof. Neither the Quantum Owners Club nor the individuals associated with the Quantum Owners Club or in the preparation of the above information shall have any liability to any person or entity with respect to liability, loss, or damage caused or alleged to be caused directly or indirectly by the instructions contained within this or related message(s). -~----------~----~----~----~------~----~------~--~---
