pboglio
20+ Year Contributor
- 1,801
- 90
- May 8, 2004
-
Palos Heights,
Illinois
There has been some discussion lately about holding boost on T28's. Well, I've been having trouble holding boost from 6000-7000 rpm. Its keeping me limited to about 35 lb/min airflow on my T28. I can hold 20 psi then it drops to 17 psi near redline.
I have the hard parts to pull the 38 lb/min compressor limit, but that can't be done on low boost. Boost is power no matter what parts are on the car. My car can handle 21 psi on pump gas and a 4 psi boost drop near redline is worth a ton of airflow. Now, I have the FP 17 psi actuator which should hold the boost, but it doesn't. Well I started to take some measurements to see what the hell is up.
Here we go: FP 17 psi actuator cracks at about 16 psi with air pressure hooked up to the nipple. FP actuator diaphragm is 1.94" in diameter. This would require an internal spring which starts to deflect at about 47 lb of push force. Stock T25 actuator cracks at about 11 psi input pressure & same wastegate diaphragm as the FP actuator. This means an internal spring which starts to deflect at about 33 lb of push force.
Now, somebody might say, why not add a helper spring like a carb spring to the FP unit.
That would definitely increase the spring rate no doubt, but it would also raise your minimum boost accordingly, and maybe beyond what the motor can handle anyway. Not the answer to the problem.
O.K., so I pulled my old HRC wastegate actuator upgrade out of storage. Hmm, interesting whats this? The diaphragm measures a whopping 2.44" in diameter. So big in fact it was sitting INSIDE my passenger fan assembly. Well thats why I replaced it with the FP actuator. HRC actuator cracks open at about 16 psi at the nipple, same as FP's unit, but the internal spring is MUCH MUCH MUCH stiffer, my calculations have it deflecting right around 75 lb of push force.
So lets recap, small wastegate diaphragms can run a stiffer spring up to a point, then you have almost no boost "control" or adjustability. Actuators with big diaphragms can run ultra mega stiff springs and still run boost or high boost, just like a good external wastegate does.
Now, I'm bringing all 3 actuators to work and measure pull force with a force gauge to get exact values. But the T25 actuator I could EASILY pull open by hand by pulling on the rod. The FP 17 psi actuator I could also pull it open by hand, although with a little more effort. The HRC actuator on the other hand I needed a vise grip and I had to put the actuator between both feet and pull 2 handed like a heavy dead lift to even budge it, that is how STRONG the internal spring is on the HRC unit.
When I get my car back together I'll do a few logs to see if the HRC actuator actually maintains boost to redline or not. Cheers.
I have the hard parts to pull the 38 lb/min compressor limit, but that can't be done on low boost. Boost is power no matter what parts are on the car. My car can handle 21 psi on pump gas and a 4 psi boost drop near redline is worth a ton of airflow. Now, I have the FP 17 psi actuator which should hold the boost, but it doesn't. Well I started to take some measurements to see what the hell is up.
Here we go: FP 17 psi actuator cracks at about 16 psi with air pressure hooked up to the nipple. FP actuator diaphragm is 1.94" in diameter. This would require an internal spring which starts to deflect at about 47 lb of push force. Stock T25 actuator cracks at about 11 psi input pressure & same wastegate diaphragm as the FP actuator. This means an internal spring which starts to deflect at about 33 lb of push force.
Now, somebody might say, why not add a helper spring like a carb spring to the FP unit.
That would definitely increase the spring rate no doubt, but it would also raise your minimum boost accordingly, and maybe beyond what the motor can handle anyway. Not the answer to the problem.
O.K., so I pulled my old HRC wastegate actuator upgrade out of storage. Hmm, interesting whats this? The diaphragm measures a whopping 2.44" in diameter. So big in fact it was sitting INSIDE my passenger fan assembly. Well thats why I replaced it with the FP actuator. HRC actuator cracks open at about 16 psi at the nipple, same as FP's unit, but the internal spring is MUCH MUCH MUCH stiffer, my calculations have it deflecting right around 75 lb of push force.
So lets recap, small wastegate diaphragms can run a stiffer spring up to a point, then you have almost no boost "control" or adjustability. Actuators with big diaphragms can run ultra mega stiff springs and still run boost or high boost, just like a good external wastegate does.
Now, I'm bringing all 3 actuators to work and measure pull force with a force gauge to get exact values. But the T25 actuator I could EASILY pull open by hand by pulling on the rod. The FP 17 psi actuator I could also pull it open by hand, although with a little more effort. The HRC actuator on the other hand I needed a vise grip and I had to put the actuator between both feet and pull 2 handed like a heavy dead lift to even budge it, that is how STRONG the internal spring is on the HRC unit.
When I get my car back together I'll do a few logs to see if the HRC actuator actually maintains boost to redline or not. Cheers.
