Cold Air Intake, Headers, and Exhaust

[Marty]

It seems like these performance mods all claim improved fuel economy. I have a 1999 Toyota 4runner, 4cyl 2.7L engine and am wondering - for someone who drives economically - whether I'd see much improvement. It seems that the cold air intake, and exhaust system, would help in the higher RPM range, but might cause lessening in the low end. The headers seem like they could help the low end. Maybe they all work (ideally) in concert with each other? If so, that is about $1000 bucks worth of modding. Any opinions???

The truck is pretty well stripped down (running boards, passenger mirror, roof rack, back seats, bug deflector, rear spoiler - all gone); running Amsoil 0W-30, Amsoil air filter, and oil filter; just installed ScanGauge II. That is all so far, but I feel there is more that could be done and I like to keep the project going.

This is a very inspiring site, and I'd much appreciate any thoughts or feedback.

Cheers,

Marty

[shovel]

I have yet to ever see one single dyno sheet demonstrating a reduction in torque resulting from a reduction in exhaust restriction. I have seen countless dynos demonstrating an increase across the board, with a naturally greater increase at higher rpm's - but still a net increase at low rpm's as well.

When can we put that old story to rest?

[chuckm]

CAI is good when you've got the pedal smashed and the throttle plate is wide open. In those situations, most cars tend to run rich simply because it the engine is starving for air. Since cold air is more dense, it reduces this problem slightly. At part throttle, a CAI is largely neutral and perhaps a slight negative. The engine isn't starving for air and cold air is more viscous, translating into greater pumping losses.
I've never really looked into the whole exhaust side of the equation.

[Backtobasics]

The engine is an air pump. These items make the pump more efficient. While they are geared more towards HP (speed sells), they ultimately decrease the restriction in the pump. Yes, it is possible to go overboard, but a basic air intake that smoothes intake flow, lowers restriction, plus exhaust that is similiar in diameter to stock, but lower restriction in muffler and piping design will let it breath.
On my 1997 Ram, I have the intake, headers, high flow converter, exhaust, quality ignition components, and electric fan.
The difference is noticeable.

[JohnnyGrey]

When can we put that old story to rest?

This is definitely NOT a myth. An increase in exhaust diameter results in a reduction of exhaust velocity and therefore scavenging. Yes, a larger exhaust will open up your top end because there is a limit to exhaust gas velocity, but when you lose that velocity on the low end, you lose power. The same principle applies to the intake runners.

[Marty]

Thanks guys, all comments taken on board. Looks like the only chance I have for an exhaust, is a catback system make by Borla (OEM). I wonder if that might be a good place to start. Then add some OEM performance headers, and leave the cold air intake out of the equation (unless it would be needed). I don't usually - except for the freeway and being late - take it much over 3,000 rpms.

[Backtobasics]

Originally Posted by JohnnyGrey

An increase in exhaust diameter results in a reduction of exhaust velocity and therefore scavenging. Yes, a larger exhaust will open up your top end because there is a limit to exhaust gas velocity, but when you lose that velocity on the low end, you lose power. The same principle applies to the intake runners.

Consideration needs to be taken into account on the configuration, size and type of exhaust system under the vehicle stock. Compression bending, large multi pass restrictive muffler, unnecessary bends in piping all conspire to slow the velocity. In many cases, a higher flow, straight through design muffler, in stock location could provide benefits, if the stock piping is designed well.

[Backtobasics]

Originally Posted by Marty

Thanks guys, all comments taken on board. Looks like the only chance I have for an exhaust, is a catback system make by Borla (OEM). I wonder if that might be a good place to start. Then add some OEM performance headers, and leave the cold air intake out of the equation (unless it would be needed). I don't usually - except for the freeway and being late - take it much over 3,000 rpms.

Borla, JBA, and Doug Thorley all offer cat back exhaust systems for your 4 Runner.
You have the option of a high flow muffler in stock system as well.

Bear in mind that most of the "cold air" are not per se cold air. Many draw air from the same location as stock. Most have an air dam of some sort to keep hot underhood out (built for performance crowd). That same intake (heat shield removed) could offer the lower restriction, and breath hot under hood air for quicker warmup, and better atomization and better MPG that result.

[shovel]

Originally Posted by JohnnyGrey

This is definitely NOT a myth. An increase in exhaust diameter results in a reduction of exhaust velocity and therefore scavenging. Yes, a larger exhaust will open up your top end because there is a limit to exhaust gas velocity, but when you lose that velocity on the low end, you lose power. The same principle applies to the intake runners.

Yeah i see those words written on automotive message boards all the time.

What i have never seen is a dyno sheet or any other actual evidence proving them. Ever. Just words and theories.

Just dozens of dyno sheets showing net increases in torque and horsepower across the board from idle all the way up, from increases in exhaust diameter.

[TOOSTUBBORN2FAIL]

how about this, i know of 3 dyno graphs, granted this is on a race car, however it shows 3 things

header, catback, muffler, lowest hp and torque

header, catback, no muffler, showed gains at all rpms, in both hp and torque

header, no catback, no muffler, showed gains at all rpms above the dyno with the catback, in both hp and torque, almost 20 more hp vs the first dyno with muffler and catback

car was a dodge neon, with a properly contructed long tube header, to the stock cat, with 2.5" piping i believe (2.25" stock), stock muffler.

i'll try to find those dynos again and post them, if i can get persmission