Hobbs Switch Boost Response Enhancement
Operation and Theory of Design -
The 1.6L 4XE1-WT engine as installed in both the Impulse RS and Elan M100 is equipped from the factory with a form of electronic boost control. The main purpose of this electronic boost control is not to regulate maximum boost, but to enhance turbo spooling. It does this be controlling a solenoid valve (VSV) that opens or closes depending on system conditions, affecting wastegate response.
Wastegate actuators by nature of their design are progressive valving devices. As boost increases, the wastegate will open progressively to vent off more of the excess exhaust gases. This has the effect of reducing the response of the turbo as pressure rises closer to the maximum set pressure - slowing the responsiveness of the turbo.
In the open and powered position, the VSV enhances spooling by bleeding off nearly all of the pressure transmitted to the wastegate sense line, effectively forcing the wastegate into remaining completely closed until boost has reached a preset threshold, then the VSV closes into the unpowered position, allowing the mechanical wastegate to assume full control over regulating maximum pressure. The OEM setting for when the VSV opens and closes is not precisely known, but the threshold is below the maximum allowed stock boost setting.
If like me, you have performed the wastegate bleeder mod, but are too cheap to fork over several hundred dollars for a decent aftermarket electronic boost controller, then there may be something to be gained by altering the response characteristics of the boost control VSV. The most effective means of doing this involves a relatively simple installation of a Hobbs-type pressure switch, however, in conjunction with the installation of a properly calibrated FCE circuit, the benefits of this modification are questionable since the FCE circuit automatically raises the threshold for VSV operation anyway. IOW, the jury is still out on this one . . .
The Hobbs pressure switch (~$45) is adjustable to switch on or off at any pressure level from 0.5 to 24 psi and can allow for tuning the system to hold the wastegate completely closed until just below the set maximum boost pressure. So if you have your bleed valve set to provide a maximum boost pressure of 17 psi as I do, setting the Hobbs switch to 15 psi should theorhetically provide for faster turbo response up to the 15 psi threshold at which point the mechanical wastegate can resume normal boost regulation duties. This translates to faster turbo response as well as improved acceleration and shift response. Unfortunately the difference is relatively slight and I do not have data to back up this claim, but it makes too much sense to ignore that there is a benefit. So if you're looking for every ounce of go-fast that you can muster on a budget, this may be a worthwhile modification to perform.
Parts Required -
Installation -
The actual installation is very quick and easy with the two objectives to this installation being to override the OEM VSV control circuitry by grounding out the ECM control wire, and to splice the normally closed Hobbs switch in series with the VSV control circuit. More simply, at or near the VSV which is located underneath the front outside portion of the intercooler support bracket, cut the white and purple wire. Splice the Hobbs switch in series with the wire and run a ground wire to the ECM side of the switch. That's it!!!
Setup and Calibration -
Tha Hobbs switch with the above part number is shipped preset to 15 psi. If you regularly have your maximum boost setting higher than 15 psi, that's probably a good setting to leave it at, but I recommend a 2-3 psi difference for best performance. If you run closer to 15 psi or have not installed a fuel cut eliminator circuit, then you'll definately want to adjust the switch down a few psi. If it is set too high, this will be rather obvious as either the fuel cut will engage, or boost pressure will fluctuate by several psi as the VSV cycles on and off. This is one reason not to use the VSV as a means to regulate boost pressure, as such wastegate valve flutter may be destructive to the wastegate or possibly the turbo over time.
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DISCLAIMER: I MAKE NO PROMISES AS TO THE USEFULNESS, SAFETY OF THIS MODIFICATION, OR IT'S EFFECT ON DRIVETRAIN RELIABILITY. IF YOU BLOW SOMETHING UP, YOU DID IT, NOT ME. AND IF YOU DON'T KNOW WHAT YOU'RE DOING, ASK SOMEBODY THAT DOES!
Copyright, 1998