Can someone explain to me why an automatic seems to lose low end power with an intake/exhaust upgrade?

I have experienced this first hand but I don't know in technical terms, why this happens.

 

I think its to do with backpressure.. when I had my 2.5" system just fitted I noticed lack of torque.. they added a 2.25" resonator which lessened the flow and gave back low end torque

 

5MT vs 4AT

 

You would have to be very specific in describing exactly what kind of upgrades you are talking about.

It is not really backpressure so much as constant system pressure in the exhaust and intake which increases intake and exhaust gas velocity (which is what makes it an upgrade).

Here's an example: for arguement's sake lets say your exhaust leaves the engine at 20 psi during low-end acceleration. In the stock 2", or 2.25" configuration with cat, pre-cats, and backflow muffler you get restrictive points that might up the pressure in certain places to 30/40psi creating considerable backpressure and is now classified by many to be a restrictive configuration. Now lets say you take all the restrictions out, get short headers, straight pipe the cat and make all the tubing 2.5", or 3" with a straight-thru soupcan muffler. Now the exhaust still exits the engine at 20 psi, but now that there are almost no restrictions the 20 psi continues to expand as it travels down its path and by the time it exits from the tailpipe is only a measly 5-10 psi. This has effectively slowed down your exhaust flow and does not count as an "upgrade". Although at high speed the input pressure will go up and the air rushing past the tailpipe might helpfully suction the gasses out so at top end it might be a considerable upgrade. The ideal system will have some low-impedence restrictions to maintain some pressure throughout the bandwidth, such as a cat, a resonator, and a dual tipped muffler. Then your 20 psi might exit at 25 psi during 10mph driving while still accelerating your high speed pressures, and now you have effectively made an upgrade to your exhaust. Same idea goes for the intake, but in reverse direction.

An automatic is more susceptible to these types of bogging issues since the PCM does not push each gear to redline, and therefore keeps the pressures in the low-mid range most of the time for smooth engagement and fuel economy, however the physics of gas pressures does not change.

 

It's not just automatics that lose power with those mods. Manuals experience the same losses. Automatics just make you notice it more because the gearing is taller, they don't give you clutch control, and they don't let you hold gears as easily.



As for why those mods lose low-end torque:

It's not backpressure. Backpressure is ALWAYS bad. It's all about preserving the velocity of the air stream. At high RPMs, the engine moves the air fast enough that you get the best benefit from keeping the pathways big. At low RPMs, though, forcing the air to pass through a smaller passage makes it move faster. That means it has momentum, which means it will actually suck more in after it. It also helps for the plumbing to be shaped so it irons out any resonances that might hurt the steadiness of the stream.

The stock intake and exhaust are well optimized for low-end torque. The intake flows just fine, and is shaped and chambered to get rid of resonances; the exhaust doesn't flow that well but the Y-pipe is equal-length and that goes a LONG way. Aftermarket parts may flow better, but they are larger and don't have that shaping to them, so they lose on the low end.


EDIT: Beat to the explanation...

 

Thanks d00df00d and L0R1DA!

 

edited.......

 

Torque loss is one of the reasons I swithced back to my stock tail pipe & intake. Which is the VQ30's strong point. This is the other strict sound ordinance