regulator-based MBC for 5kcstq, 15psi [long]

[Ken]

I changed my '88 5kcstq over from WGFV operation of the WG to what's been called the "Timmerman Mod".  It's basically a pneumatic manual boost controller.

As some or most of you know, this puts regulated boost pressure on the upper chamber of the wastegate.  This allows for the wastegate to remain closed until the boost pressure on the bottom WG chamber exceeds the regulated pressure at the top.

My attempt at understanding how this works, is that below the pressure the regulator is set at, pressure is equalized on both sides of the WG diaphragm, and the wastegate is held closed by spring pressure.

Then, as the lower wastegate pressure rises above the pressure in the upper chamber, the WG will remain closed until the pressure differential exceeds that which the WG spring will hold.  So, let's say my WG spring is supposed to be a 1.8 bar spring.  So, with my car set at 15psi, that means that the WG spring is providing about 12psi of boost, and the regulator mod is providing the extra 3psi, by providing a regulated 3psi to the top of the wastegate chamber.

The regulator is one of the cheap ones you can get at Sears or Home Depot.  I'm running the Home Depot version, which is a Campbell Hausfield(sp?), and cost under $20.  That, some hose and clamps, and a "T" fitting was all it took.  The regulator is located behind the firewall in the cowl area.

My car is fairly quick at 15psi.  While dialing it in at the high side, I think I may have heard some knock, which caused me to get my foot off the pedal quickly of course.  What I basically heard wasn't what I thought/expected  knock to sound like (diesel rattling type sound), but did sound like some kind of "new" metal-parts-doing-something noise that I've never heard before.  I have it dialed back to where that noise doesn't happen.  I would like to have a wideband 02 sensor setup, fer sure.

Of course, this is a very simplified boost control setup, and my TAP ECU is not in control of boost pressure anymore, if it even was before.  Not that it was much with the 1.8 bar spring installed.

I'm considering swapping back in the stock WG spring, since I can control boost with the regulator, and it may help WG responsiveness at the set boost level.  I don't know if this would be better, but it seems like it would.

I was originally collecting parts to convert a WG cap over to adjustable style, so I could get some of the boost I had when I blew the lower WG hose.  But then I read that it wouldn't increase max boost, just the WG cracking pressure.  That makes sense.  I may still do the mod, just to be able to tweak things.

Btw, if you take out the WG allen bolt from the underside, you can just push out the plug, you don't need to drill a hole and pull on a screw or anything.

Where can I get the valve spring shim washers that are recommended as WG spring seats, so that the spring doesn't bind when adjusting the preload?

A word of warning to others who may be wanting to cryogenically treat parts, I think the process made the WG diaphragm brittle and possible cracked it.  It was all stiff and torn, and I would have remembered if it was that way when I bought the donor car.  I actually couldn't get the WG to fully seal, which I think was the diaphragm being stiff.  Luckily I have a couple replacements now.  A cheaper way would be to take out the diaphragm before treatment.

Anyway, for those of you who fully understand, can, and are willing to, switch to manual boost control, I recommend the mod.  It's a cheap mod that could be used to bring the boost up to just below the fuel pump cutoff at 1.5 bar, for a little extra boost.  Unlike a lower WG chamber bleeder type of a setup, it's a closed system so metered and boosted air isn't let out, which seems better.

Thanks to Ben Swann for the "Boostin'" webpage compilation, to Paul Timmerman for the idea, and to the lower WG hose that blew to show me the extra power potential.

Now, I'm interested in single-pass intercooler options, and need to install a bypass valve before something becomes its own blowoff valve.

Let me know if there are any questions.

Later,

Ken