The point of Long and Short Intake Runners

[Farquart]

I'm new to the car world, I understand almost all engine concepts, except for what would be the advantage to having longer runners at low rpm's, and switching to shorter thicker runners at high speeds? I understand thicker means more air, but why shorter?

 

[SHOtimer]

It doesn't switch to the shorter ones, they add them inaddition, they are the "secondaries". Think of it as a 4 barrel carb. When the SHO motor passes 4k rpms it opens the secondaries (the bottom shorter intake runners), which is similar to flooring a 4 barrel and the 2 secondary barrels open up. Then length doesn't really matter, it is just that the secondaries are position underneathe the primary intake runners on the intake. Hope this makes sense.
Doug

 

[projectSHO89]

Actually, the length of the intake runners is VERY important. It has to do with the resonance of the air mass in the intake.

"In general, long runners tend to improve low-speed torque and fuel economy, while shorter runners improve high-speed torque and power at a slight expense of fuel economy."

Suggested reading is pages 32 and 33 of Charles O Probst's "Ford Fuel Injection & Electronic Engine Control" book.

Happy reading.

Steve

 

[SHOtimer]

Well, that proves my ignorance. Well that is my new thing to learn today.
Doug

 

[NotSoSlowSHO]

projectSHO89:

"In general, long runners tend to improve low-speed torque and fuel economy, while shorter runners improve high-speed torque and power at a slight expense of fuel economy."

hehe... 'slight'??

Im sure all of us are well aware of how lousy our gas mileage gets if you have the engine buried in the 4k+ range often... especialy with the latest gasoline prices

 

[SHOZ123]

Intake runner math

Intake runners explained

 

[sdpatt]

I have found the www.howstuffworks.com site to have some excellent information. It sounds as if they are describing the SHO's staged intake in the last paragraph.

The downside to using smaller diameter intake runners is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflows at higher speeds it is better to have large diameter pipes. Some carmakers attempt to get the best of both worlds by using dual intake runners for each cylinder -- one with a small diameter and one with a large diameter. They use a butterfly valve to close off the large diameter runner at lower engine speeds where the narrow runner can help performance. Then the valve opens up at higher engine speeds to reduce the intake restriction, increasing the top end power output.

 

[Farquart]

Yeah! I love that website, it explains so much stuff, most of which I will never use in the real world, but it's good to know.