Short runner valves

[Prestonguillory14]

Why does it feel like my short running valves in my intake are not opening when I mash the pedal. No matter how much I try to get them to open I cannot get them to open at full throttle. If I ease into it at about 3500-4000 rpm they will open but when I mash it they won’t open. Any ideas?

 

[Ken226]

Why does it feel like my short running valves in my intake are not opening when I mash the pedal. No matter how much I try to get them to open I cannot get them to open at full throttle. If I ease into it at about 3500-4000 rpm they will open but when I mash it they won’t open. Any ideas?

The short runner valve isn't activated by throttle position alone. The valve opens, shortening the runners for more airflow @ higher rpm and high throttle input.

At wide open throttle and lower rpm, the manifold pressure will be pretty high and you'll get better performance with long intake runners, so the valve stays closed.

When the engine speed (rpm) gets high enough that the long runner pulse scavenging effect becomes more restriction than help, Somewhere around 4800rpm according to AI, then the PCM will open the valve:




A good example. The old early 90s L98 engine prduced alot of torque, but peak HP was about 4500rpm. It's intake had a fixed, long runner system. A very, very long runner intake. Torque was awesome but it fell flat on its face above 4500 rpm:


The next generation engine, the LT1 was the opposite. It only had a fixed, short runner intake. Much better high RPM horsepower, but had poor low RPM torque:







Our Hemi's have the best of both. Below about 4500rpm, you have long runners and good torque, from a similiar but opposite effect of that which makes headers effective.
Above 4800'ish, the valve switches you to short runners and you the high RPM airflow that helps make peak horsepower.

 

[hunterdan]

If you take off from a dead stop, you should feel the difference from when they open. For me, my truck was a **** off the line until about 3500 rpm, then it pulled hard. My intake was gummed up and probably still is to a certain extent, from an over full catch can just dumping oil and other debris from the pcv system into the intake manifold where it eventually pooled up in spots. Running a bore scope through the intake I could see the inside of it was coated in a nasty oily substance. After several intake cleanings (it's been too cold to pull the manifold off), it's gotten much better. If the valve doesn't open, it should trigger a CEL. If it's something you're concerned about, pull the pcv line from the bottom of the throttle body, and use crc intake cleaner into the vacuum port as directed and follow the directions. For $15 can of cleaner, it won't hurt.
But as I said if the valve doesn't open, or is slow to open, it should trigger a CEL. So, if you're not seeing or feeling much of a difference, it's possible you need a cleaning or tuneup.

 

[Prestonguillory14]

If you take off from a dead stop, you should feel the difference from when they open. For me, my truck was a **** off the line until about 3500 rpm, then it pulled hard. My intake was gummed up and probably still is to a certain extent, from an over full catch can just dumping oil and other debris from the pcv system into the intake manifold where it eventually pooled up in spots. Running a bore scope through the intake I could see the inside of it was coated in a nasty oily substance. After several intake cleanings (it's been too cold to pull the manifold off), it's gotten much better. If the valve doesn't open, it should trigger a CEL. If it's something you're concerned about, pull the pcv line from the bottom of the throttle body, and use crc intake cleaner into the vacuum port as directed and follow the directions. For $15 can of cleaner, it won't hurt.
But as I said if the valve doesn't open, or is slow to open, it should trigger a CEL. So, if you're not seeing or feeling much of a difference, it's possible you need a cleaning or tuneup.

Thank you!