One thing I wanted to make sure I got right before dropping in the motor was that I had the correct wastegate springs fitted and properly preloaded. I plan to run these turbos up to 32psi and if I assume that the ΔP will be 2:1, then exhaust manifold pressure will be up to 64psi. The area of the wastegate orifice is .75in², so this means that the wastegate will need to hold up to 48lbs of force.
In order to measure the wastegate holding force I used a bluetooth load cell designed for rock climbing training and clamped it to the center of the wastegate flapper arm.
Playing around with different springs and levels of preload I found that the minimum strength spring that could hold 48lbs of force resulted in a cracking pressure of 22psi! This means that in order to guarantee I didn’t have any wastegate opening before desired, my minimum boost pressure would be almost 20psi. I’ve driven cars with manual boost controllers and super heavy wastegate springs and they suck to drive. On top of that, tuning them can be a major pain in the ass. Luckily I figured out a pretty slick solution to the problem.
I converted the Mamba wastegates to a twin port style and will use a 4 port wastegate actuator. I had thought of doing this a long time ago, but I couldn’t think of a simple way to seal the lower half of the wastegate that wouldn’t require some pretty precise and tedious fabrication. Luckily, rummaging through my collection of unused seals, I found that a valve stem seal from a 6.8L Ford Triton motor has an ID of 5.5mm, which is just barely smaller than the wastegate actuator arm. The OD of the valve stem seal is also an almost perfect fit into the lower half of the wastegate canister. I tapped the existing unthreaded hole on the bottom half of the actuator canister to 1/8NPT and used a 1/4″ hose barb. I milled a recess for a washer that would hold the valve stem seal in place. The other side of the valve stem seal is held in place by a washer, being pressed on by the wastegate spring.
With the dual port setup I am able to use a wastegate spring that results in only 9psi of cracking force, but can hold more than 50lbs of force at 10psi of boost pressure. With this new setup my minimum boost should be less than 9psi, assuming that the wastegate orifice is large enough. I should also be able to make whatever boost pressure I want, without any worry that the wastegate is blowing open.
To make sure that the new setup was reliable I connected the new 4 port turbo control valve to a switch and 12V. I connected a regulated 35psi of air pressure to the boost side of the TCV and actuated the wastegates 100 times. The bottom of the right side actuator has a small leak above 15psi, but it’s not enough to be concerning. Here’s a video of the test:
https://www.youtube.com/shorts/5G90hfl1Oyk