Turbo Questions... (Please move to Technical)

[Y-TRY]

As I mentioned in another thread, I had a chance to really put my car to the test this weekend- hitting the burger stands and local spots and cruising around. There's a couple strange anomolies I want to see if you guys can help with. It's my first time dealing with turbos, but I know the function of an engine pretty well and have applied that into trying to diagnose what's going on:

Problem: The car seems to stay revved quite a while when letting off the gas. For example, if I'm on the freeway and let off the gas, put the car in neutral (5spd) and coast it down the exit ramp, it stays revved up to 2k or so and very slowly settles back to idle, and quite often even loads-up and dies at the light.

What I think is happening: At first I quipped "vaccum leak". What it really seems like is happening is that the turbos are still spinning right along, well after I let-off the gas. The wastegates would control max boost, and the BOV controls spikes. But there isn't anything to keep the turbos from continuing to spin after letting off, other than their natural decelleration. Point is- it sure seems like the turbos are still forcing air past the throttle blades when my foot is off the gas(because they are spinning disproportionately with the throttle). I don't know if the air is actually overpowering the throttle springs and opening the blades, or if it's just pushing through the partially closed gap (from idle setting). So it's acting like the throttle is sticking, when it's just the turbos keeping air pressure against the topside of the blades. As long as the air continues to flow past the boosters, they are continuing to add fuel. Eventually, the turbos slow, less air is forced through, and there's a moment where the car floods, killing it.

I plan on running through this situation, or mimicing it, without the hat on to test this hypothesis. It's really the only thing that makes sense, though. Can any of you guys confirm or deny this sort of problem, or how it may be resolved? Are my theories all wet?

Another problem: My wastegates aren't opening. (HKS Standard 40mm, external) All the lines are secure and they are getting good signal to both ports.

Suggestions: If I want the wastegates to open at 10psi, do I need 10psi springs in each one or 5psi springs in each one? I know that's a stretch but is it a possibility? Do vaccum lines work like hydraulic? ie- will the entire line show the same pressure regardless of how many times it has been T'd? Or does each port require a dedicated vacuum source?

I'm just throwing this stuff out there as possibilities but I'm still scratching my head. There aren't alot of blow-through carb/twin turbo old Camaros out there to steal info from. I figured that this is the best place to ask. What do you think?

[LIL QIK]

It's always been my understanding that if you want them to open at 10# then you need 10# springs in both wastegates. Same as if your pressure doesnt' change because of two turbo's........just the volume. If that's makes any since.

[Blown353]

Wastegate springs: if you want 10 psi of boost, you need to run 10 psi springs in each wastegate. As far as them not working, where do you have their reference pressure line coming from? Typically you hook the wastegates ports via hose to fittings directly on the compressor housing outlets. You can also reference off the intake piping or the hat, anywhere before the carb is OK. Do you have any kind of boost controller, manual or electronic? That may be your problem, either not functioning, not set correctly, or plumbed incorrectly.

As for your carb sticking problem, what it sounds like is happening is exactly like you describe, residual boost in the hat/intake piping after letting off. What this does is pressurize the bowls of the carb and will blow extra fuel out the idle circuit. With the throttle closed this makes things very rich and can cause the engine to die. This isn't a problem when you're in the throttle because manifold pressure under the carb should be the same as pressure above the carb, but this will definately cause problems once you let out, such as going dead rich and killing the engine when you back out of boost. It won't shove any extra fuel out the boosters as the boosters are above the throttle plates and will see the same pressure the bowls do

What you can do to check for this is run 2 pressure gauges-- one below the carb (say from the manifold vac or PCV port on the carb) and one pressure gauge hooked to the hat. Go for a ride, step on it, and then back off and watch for a boost differential, i.e. the pressure in the hat being greater than manifold pressure. As soon as you back out of the throttle you'll want as close to 0 psi in the intake piping as you can, otherwise the differential pressures will cause a rich condition as the pressure in the hat (and therefore the bowls) will dump extra fuel into the engine.

I am not a blowthrough turbo guru, but I see a potential way around this is to run a centrifugal supercharger style surge valve that keeps the intake piping open to atmosphere under cruise and closes when you step on the throttle and vacuum drops; this way when you back out of the throttle, the valve goes full open, dumping the intake tract and bowls to atmosphere, relieving all pressure in the bowls. This should alleviate the rich condition when coming down from boost. What I don't understand is your BOV should be doing the same job, but not staying open at cruise as a surge valve would.

The HKS BOV's are adjustable, you may have too much spring tension in it; i.e. it only dumps down to say 5 psi and then closes, which may leave just enough boost in the bowls to flood the car when you back out of it. With the blowthrough I'd say you want it to start opening at say 0.5-1 psi of pressure to minimize the amount of pressure the carb will see when you back off the throttle.

Hopefully someone with blowthrough turbo experience will chime in as I'd like to know the real solution to the problem, my workthrough is just an "in my head" visualization of the problem. I've had centrifugal supercharger experience (both blowthrough and EFI) and turbo experience (EFI), but never blowthrough turbo experience.

Troy

[Y-TRY]

Thanks. These are great ideas.

As I understand the Wastegates, they will only open when the manifold sees boost pressures higher than the spring rate. They are just along for the ride until they see 10psi (or whatever) then they open up. They dont open at all until then, and close as soon as the manifold sees less pressure. That way they let the boost build only to the max setting. Kinda on/off like a carb powervalve does.

I thought the BOV only senses surges. The SSQV actually senses sudden spikes in vacuum, signaling that the throttle has abruptly closed. Then it opens only briefly. (sounds like a passenger screaming "weeeooooo", pretty funny) I have not played with the adjustment screw. I'll do this next. What should I do there? Is there a way to loosen it up where it doesnt need such abrupt changes in vacuum to open? It'd be neat if I could make it operate when I ease off the throttle.

I don't think there is any sort of mid-throttle bypass valve like a centrifugal uses. The wastegates and BOV seem to be just hanging out until I'm operating in boost or getting out of boost suddenly.

I have the wastegate and the BOV ports all connected to actual manifold pressure, not the hat or turbo housings. We built a little "vacuum log" with several ports in it to feed all the parts needing vacuum signal, because there is only one port (tapped into the carb baseplate) for 8 or so components.

If I've got any of this wrong, let me know.

[camcojb]

I believe the wastegates have to be plumbed BEFORE the carb, not in the intake manifold under the carb. I think that's your problem. Try routing them to the turbo housings, should be an 1/8" npt threaded tap there.

Jody

[hotwheels]

yea, mine take the sensing from the turbo itself.............the bov is only to help relieve pressure after you intend on getting out of the boost.......helps insure that your turbo doesn't spin backwards........




hotwheels of turborides

[Y-TRY]

Okay, I've got all this and I've adjusted the BOV. It seems to have helped, as now I can hear the "whoosh" when I casually let off the throttle for switching gears and decelerating. The screw was all the way in before and I really had to be on it and let-off abruptly for it to open.

FYI-- The research I've done on where to source the wastegate and BOV lines has confirmed what I thought: Most stock-style hook-ups are done at the turbo for simplicity, but for the best signal it it should be sourced directly from the manifold. The signal from the turbos themselves can be altered by restrictions in the tubing and intercooler. So the pressure at the turbo is likely different than what the engine is actually seeing. If the post-turbo tubing and intercooler are in any way restrictive (which all are) it will read a higher boost than the engine. Either way works, but sourcing the vacuum at the manifold gives the absolute more precise reading than at the turbo.

I also think I have the wastegates working better, too. With an open stretch of road I was able to got WOT while in overdrive so I could watch the boost gauge. It stopped right at the 9.7# where the springs are set. So that's good. I'll start tuning for that now with the controller and carb.

It doesn't seem, however, that this has fixed the problem of the turbos producing residual boost in the bowls and flooding the engine, as previously diagnosed. My next question is this:

Why have I not seen any turbo applications with the centrifugal-style BYPASS valve in lieu of the BOV and/or Wastgates? Is it because there is something different with how the supercharger produces boost compared to the turbo, that somehow negates the use of a bypass? Or is it simply because I've never seen a turbo/blow-through at all, thus never seen the symptom or it's solution? (Maybe this isn't a problem with EFI'd turbo cars so it's never been needed) It seems like the Bypass valve would work great in a turbo application and could possibly fix the problems I'm having. There must be a reason why I've never seen it used. Have any of you guys had experience with this type of valve in a turbo application like mine? What are the pitfalls? What kind of info can you give me?

[hotwheels]

your wastegate is just what it's name says, it takes the extra exhaust that is not needed to maintain your level of boost and dumps it to the atmosphere or to your exhaust pipe, depending on you you have that exit routed.......the wastegate is mounted inline of your turbo's on the exhaust piping.......the bov is lined up on your air inlet to the engine, this helps ensure that your turbo's don't stall or spin backwards when you let off of the throttle........so your wastegates are releasing exhaust and your bov is releasing air.........if your bov is connected right, it will get it's signal from the intake manifold and the wastegate will get it's signal from the turbo...........that is how i set mine up and it seems to work very well..........without actually seeing your set up, it is kind of hard to help to solve your idle issue........anyways, i hope that helps in the understanding of a bov verses a wastegate..........if i can help with anything, feel free to ask, later, hotwheels of turborides

[BigAl...]

ytry, do you have a WB in your car? I'm wondering if your afr is really rich on decel? it seems like your bov is still not sensetive enough and when you close the throttle it's still drawing fuel from the boosters. with that said I'm wondering where you have the boost/vac signal to your afpr at??? this might be a problem also.

The purposes of the bov are to keep the comp. wheel from slowing down to much between shifts (on manual trans only [obviously]) and avoid a surge in the charge tubes that might cause the shaft to shutter and ruin the bushings. it's virtualy impossible to spind a turbo in reverse. When you don't have a bov or it is set to tight you here that che che che che sound, imagine the shuttering that wheel and shaft (that btw are spinning at about 100,000rpm) are encountering .

ytry you might consider a supercharger baypass valve like mentioned earlier by blown353.

just my .o2 worth, don't spend it all in one place

[Y-TRY]

Thanks for all the help and info, guys. There are certainly a few options and possible problems.

I have a good grasp on what/how the wastegates and BOV work, and what their function is. My prior knowledge is only affirmed by your responses. I think the problem may remain to be what BigAl stated: the BOV still isn't letting off enough pressure, like it's staying open for too short a time. It's as loose as it goes right now, though.

I have the car apart right now to get all the tubing and whatnot coated, so I haven't been able to address this problem since. I did get a good tune on it right before pulling it down and the problem is pretty much gone, but it still dies maybe 1 of 10 times if I don't blip the throttle as I'm hitting the brakes. My feet get to moving like I'm Lars Ulrich.

I'll let ya'll know what I come up with when it's back together. Right now I'm also trying to fix a mean oil leak at the rear pan seal and come up with the best way to fix a problem with the headers....