sn95_331_GT_ yellow

anyone who buys a car without gears already in it should slap the previous owner lol

Moonshine

What the hell is goin on in here?

amoosenamedhank

I'd rather have a turbo (or positive displacement supercharger) with stock gears.

tbirdscwd

Probly need to adjust the tune a bit too. Going from 4 cats to none and opening up the exhaust can definitely influence the car's need for fuel and timing.

Rockwell28

Who is being being labled as a troll mr. Moderator. Tbird that sounds more logical than some things said in th past on th subject. And its old purp

945LSTANG

^you ya big ole' bitch!

945LSTANG

BTW, that was a joke rockwell. Don't take this one seriously like you usually do.

PurpleIcedGT

Pick on Lizzy and you got another thing coming Rock.


Secondly, whats old? I cant understand wtf you say so I politely ask wait....what?!?!

scottybaccus

It looks like you read one technical article about a particular muffler and set all your understanding on something fancy they said there.

I never said cars needed back pressure. I said that the placement of the torque curve is influenced by the rpm at which the engine builds exhaust (and intake) velocity that promotes scavenging. This is fluid dynamics 101. Gaseaous materials (air and exhaust) behave as a fluid. Restriction is a factor of orifice diameter AND length. There is a sweet spot for any fluid flow scenario where flow is maximized when volume matches flow capacity, creating a velocity that encourages suction (scavenging). In a fluid model, you would recognize this as a syphoning effect. You can't get a syphon flowing unless the volume fills the pipe and creates suction. Lesser volume results in the cavitation (turbulent spaces in the gas or fluid) and loss of suction that you see when you didn't pull enough fluid to fill the pipe. (low rpm, low volume, big pipes, lost scavenging, low torque). Suck hard enough on the hose to fill it with fluid and you get a natural suction effect from the resulting velocity of the fluid. This velocity is the momentum of the fluid that resists cavitation and promotes scavenging. (moderate rpm, moderate flow volume fills larger pipe, scavenging resumes, torque is increased). Now go further, have the engine begin to exceed that sweet spot, fluid (or exhaust gas) is being forced into the pipe, restriction limit is encountered where the fluid does not compress and volume can not exceed the capacity of the pipes. This increases as pipe diameter is decreased, OR if pipe length is increased. (Carb jets are tuned this way, same size orifice flows less when it is longer, look it up in any jetting catalog) The result is a sudden drop in the torque curve where the engine exceeds the flow capacity of the exhaust. (I just agreed with your back pressure point). (high rpm, flow capacity of pipes exceeded, torque falls off)
In summary, increase exhaust capacity and you still have the same range of scenarios, they just occur at a higher rpm. This is plain physics and beyond opinion. You aren't debating me, your debating scientific facts that apply in ang hydrodynamic scenario. It doesn't matter if it is fuel injected or carbureated. Naturally aspirated engines (not forced induction) obey the same physics.
Your examples are a miss. Top Fuel dragsters are using forced induction. This pushes the upper limit of the engines torque curve by raising the operating pressure of the exhaust system beyond what the engine could do naturally aspirated. They run really short pipes because they want nothing in the way of the gasses exiting the cylinders under this forced pressure and the rules do not require a full exhaust system. Some teams even tailor the angle of the zoomies to promote down force on the chassis during a run because the pressure out of the pipes is so high. Your diesel engine analaogy is incorrect also because a naturally aspirated diesel engine is running at a much higher compression ratio as a requirement of the fuel. This means much higher cylinder pressures when the exhaust valves open so the exhaust pressures in the same size pipe are higher than a gasoline engine. That only means that the diesel engine appreciates reduced restriction sooner. The higher torque already achieved in a diesel engine is a result of the higher cylinder pressures that it operates at.

Now what was this thread really about?

scottybaccus

OH yea, I remember. Increased exhaust flow can benefit any engine, however too much of an increase will result in a loss of LOW END torque as the torque curve is moved up in the rpm range. Go google any old hotrod magazine, exhaust catalog, tuning advice, How to Build a Small Block whatever, and you will get the same answer.

For a STREET car, too much is sometime really too much. You don't go to school or work at 6500 rpm. You need to build torque at 1500 rpm to leave a red light, unless you like tickets. Our OP here simply overshot his "Happy Place", and yes, opinions vary so your happy place might be somewhere else.