Hey guys, Whats the most efficient tube size for a 12psi boosted engine? 2.5, 2.75 or 3inch? looking for best torque response, not best hp figure. discussion
For a turbo charged engine the best exhaust is none, for a road car this isn't practical. So its all compromise, you want a turbo charged engine to make full boost quickly to make full torque. So for torque response as you have called for and the options 3" as is the most free flowing. Side info to help make a decision, just aim for a horsepower figure and let torque be a by product. Exhaust pipe HP flow 2.5" single max 232hp as twin 463hp 2.75"" single max 283hp as twin 566hp 3" single max 339hp as twin 679hp reference to calculations: http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/
I know 2.5 flows more than that Was that taking into account the engine size feeding the exhaust pipe size?
It's not 'absolute maximum flow' but more of a good limit to keep the power efficient, if you're pushing over those guidelines then you'd probably see good gains stepping it up. Although a bunch more noise going 3" so either more mufflers or less pleasant daily duties.
Are you going to be modding it more than what you have listed? 3" will give you the same as 2.75 plus extra noise. Maybe wven the same power output as 2.5!
Couple of graphs showing changes from TWIN 2.5" to TWIN 3" guys at moderate power levels. Before/after 2.5", 3" stock turbo car Upgrade turbos, exhaust change only, same day dyno I'd go 3" if possible.
Not really, 12psi on 98 will see about 400hp atw max (upgraded turbos and other flow mods) so from the numbers MORBOOST posted even twin 2.5" is almost enough, given those numbers are at the engine stepping up to 2.75 would be resonable, 3" is massive overkill. If you're not fussed about noise and have diplomatic immunity then no worries going 3", if occupant comfort, neighborly love and flying under the radar are important it's a PITA.
If it helps, i have 2.5" dual system, straight pipes from primary cats and make 620HP. My exhaust is about 4' long though from heads to exit.
http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/ An example of the type of information available in one of the links. You can see there are a lot of variables to consider, as you mentioned you were interested in the torque figure rather than maximum horsepower, the RPM figure to use would be the RPM at maximum torque. So do the calculations & let us know what you work out. "Okay, class. Excellent question, Jaded. Pay attention now, because this will be covered on the test. Several assumptions: 1) This is a gas engine we are talking about, 2) it is a four-cycle gas engine, 3) Combustion will be stochiometric and complete, 4) the compression ratio is about 10:1, 5) the engine is throttled (no variable valve timing), 6) normal aspirated engine (no turbocharger), and 7) volumetric efficiency (the amount of air that makes it into the cylinder during the induction stroke) is 1.00 (actually it depends upon the RPM and intake manifold pressure, but work with me here.) First, it should be intuitively obvious to the most casual observer that the amount of air that passes through the engine in will be equal to the engine displacement times the RPM divided by 2. For an engine of 3 liter displacement going at 3000 RPM, the amount of air pumped for minute will be 4500 liters. That will approximately be the intake volume flow for an engine with the throttle wide open. If we assume that the throttle is only open 33%, the intake volume flow will still be 4500 liters, but the pressure will be one-third of an atmosphere. The equivalent mass of air will be the same as 1500 liters at one atm of pressure. Neglecting the addition of the fuel mass, the mass of the exhaust gas will be the same as the mass of the intake gas. From the ideal gas law we know that the increase in volume of the exhaust gas will be proportional to the increase in absolute temperature. If we assume an intake temperature of 80 deg F, and an exhaust temperature of 1800 deg F (reasonable assumption, depends upon compression ratio), the absolute temperature will be 540 and 2260 deg Rankine, respectively. The volume increase will therefore be 2260/540, or 4.185. For the hypothetical 3 liter engine running at 3000 RPM and full throttle, the exhaust gas volume will be about 4500*4.185, or 18,833 liters/min. At one third throttle the corresponding flow is 6277 liters/min. Since one cubic foot is equal to 28.3 liters, the respective CFM flows will be 665.4 and 221.8, respectively. How about the contribution from combustion products? Assuming stoichometric combustion, there will be one pound of fuel burned for each 14.55 lbs of air. Air is 21% oxygen, so there is 3.05 lbs of oxygen available to burn each pound of gas. A reasonable chemical approximation for gasoline is octane, which has a chemical of C8H18. The molecular weight is (12*8+18*1)= 114. The combustion formula is C8H18 + 12.5 O2 ==> 8 CO2 + 9 H20. For each 114 grams of C8H18, there will be 12.5 moles of oxygen consumed, producing 8 moles of CO2 and 9 moles of H2O. For gas volume purposes, since equal moles of gas produce equal volume, the volume of exhaust gas replacing oxygen will be equal to 17/12.5 = 1.36. The volume percentage of oxygen in air is about 21% (not exact, but work with me here). This volume will be removed, and replaced by exhaust gas with a "volume" of (21*1.36) = 28.56%. The resulting post combustion volume is (79% + 28.56% = 107.56%) of the pre- combustion volume -- assuming no temperature increase. So what do we have? Combining the increase in volume from combustion reactions and thermal expansion, an engine with a 3 liter displacement running at 3000 rpm with the throttle wide open will have an exhaust volume (at 1800 deg F) of 665.4*1.0756 ~~ 715 cubic feet per minute. For the throttle one-third open, the exhaust flow will be 238.6 cfm. I'm tired, so I may have made some calculation errors, but this should get you in the ball park for exhaust flow."
Haha, I was going to say he'd grow out of it but if I remember correctly you're not in your early twenties so maybe not everyone does.
:agree: True only now I have $$$$ to go buy lots of toys,my Dad is 90 still fit so I can only hope it runs in the blood line :rolleyes2::zlove::br:
Re: 3" System Since fitting the 3" exhaust system, what would you say would be the most noticeable improvement that you have identified?
