For those that don't know, the dashpot is an emissions control device to help burn unburnt fuel each time the throttle is closed while driving. It is a simple diaphram that uses a valve to let air in quickly, but release it slowly, easing the throttle plate closed when released over a very short period of about .8 seconds. (See diagram at the bottom)
I know that a lot of Supra owners like to vent their blow-off valves to atmosphere instead of properly recirculating like stock. But this causes large spikes in unburnt fuel flooding the engine when letting off the throttle after boosting, and often causes the engine to stumble or stall. This is due to the air being metered by the air flow meter(AFM) and when vented to atmosphere, the ECU assumes that air is going into the engine and adds the amount of fuel it thinks is needed for that air.
My aftermarket blow-off valve is designed to vent to atmosphere and I got tired of the stumbling and stalling, and I thought that the dashpot, releasing the throttle much more slowly after blow-off, could solve this problem. Obviously a standalone or Maft-pro would eliminate the AFM and the problem, but I found a temporary fix:
I cut open my dashpot VTV, and found it had a check valve inside that allowed air in, but then shut to force outgoing air through a tiny pinhole in a small metal plate. My goal was to simply slow this outgoing air to hold the throttle plate partially open for a much longer time after letting off the throttle. So I used silicone to glue that pinhole shut, and glued the VTV back together, so it now functions ONLY as a check valve.
I tee'd in a K-Sport manual boost controller (spring/ball-valve type) to act as the pinhole for the outgoing air. I found that by loosely tightening the screw on the boost controller to put very light pressure on the spring ball valve, it allowed the dashpot diaphram air to creep by very slowly. I then adjusted the dashpot set screw to set the RPM at which the dashpot holds the throttle open (1500-2000 RPM)
The end result keeps the throttle open at 1500-2000 rpm for a few seconds before returning to idle, and effectively eliminates all the stumbling and stalling problems I was having before.
Notes: I first tried using a regular check valve, but found that it did not close when the throttle was let out slowly. Only the OEM dashpot VTV internal check valve was sensitive enough to work 100% of the time.
Also: the success of this mod depends on air creeping very slowly by the internal ball valve of the K-sport boost controller. If the valve was to seal perfectly, it would hold the throttle open permanently at the selected RPM. I had this unit kicking around and it worked, but I don't know if other units or new ones will work the same way. Experimenters, please report back! :biglaugh:
P.S. Before this mod, I was once approaching a corner with my music cranked loud, and my engine stalled because of the above problem and I didn't hear or notice. I had to unexpectedly pull REALLY hard on my steering to avoid the curb because the power steering stopped working when the engine stalled!
I know that a lot of Supra owners like to vent their blow-off valves to atmosphere instead of properly recirculating like stock. But this causes large spikes in unburnt fuel flooding the engine when letting off the throttle after boosting, and often causes the engine to stumble or stall. This is due to the air being metered by the air flow meter(AFM) and when vented to atmosphere, the ECU assumes that air is going into the engine and adds the amount of fuel it thinks is needed for that air.
My aftermarket blow-off valve is designed to vent to atmosphere and I got tired of the stumbling and stalling, and I thought that the dashpot, releasing the throttle much more slowly after blow-off, could solve this problem. Obviously a standalone or Maft-pro would eliminate the AFM and the problem, but I found a temporary fix:
I cut open my dashpot VTV, and found it had a check valve inside that allowed air in, but then shut to force outgoing air through a tiny pinhole in a small metal plate. My goal was to simply slow this outgoing air to hold the throttle plate partially open for a much longer time after letting off the throttle. So I used silicone to glue that pinhole shut, and glued the VTV back together, so it now functions ONLY as a check valve.
I tee'd in a K-Sport manual boost controller (spring/ball-valve type) to act as the pinhole for the outgoing air. I found that by loosely tightening the screw on the boost controller to put very light pressure on the spring ball valve, it allowed the dashpot diaphram air to creep by very slowly. I then adjusted the dashpot set screw to set the RPM at which the dashpot holds the throttle open (1500-2000 RPM)
The end result keeps the throttle open at 1500-2000 rpm for a few seconds before returning to idle, and effectively eliminates all the stumbling and stalling problems I was having before.
Notes: I first tried using a regular check valve, but found that it did not close when the throttle was let out slowly. Only the OEM dashpot VTV internal check valve was sensitive enough to work 100% of the time.
Also: the success of this mod depends on air creeping very slowly by the internal ball valve of the K-sport boost controller. If the valve was to seal perfectly, it would hold the throttle open permanently at the selected RPM. I had this unit kicking around and it worked, but I don't know if other units or new ones will work the same way. Experimenters, please report back! :biglaugh:
P.S. Before this mod, I was once approaching a corner with my music cranked loud, and my engine stalled because of the above problem and I didn't hear or notice. I had to unexpectedly pull REALLY hard on my steering to avoid the curb because the power steering stopped working when the engine stalled!
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