dgeesaman

iTrader: (7)

I recently bought the Service Highlights book and started reading more deeply about the stock turbo control system setup. For anyone interested in learning this, I recommend this book - it explains a lot of the 'why's behind the car's design.

Questions:
1) Why is there a separate turbo control and precontrol flapper door? In theory, you could simply duty control the turbo control door to create prespool as well as parallel turbo operation.

(I suspect the TCA/TC flapper are too clumsy to spool the secondary well, or thinking in the opposite way, using the precontrol door alone for secondary operation would choke the flow too much - am I on the right track?)

2) Why does the wastegate and precontrol actuator sump pressurized air back into the primary charge? Couldn't it just sump back to atmosphere?

(My guess here is that using the pressurized charge as sump makes for a lower net pressure over the restrictors, which means the orifice size in the restrictors isn't so sensitive, plus it adds a dampening effect to smooth the duty cycle. Or is it some type of fail-safe to prevent overboost or excessive spooling if the charge pushes a major leak?)

3) Why have duty control on both the exhaust and charge sides? In theory, couldn't you just use the exhaust side to vent, and not use a charge relief valve or air control valve at all?

(My suspicion is that having purely mechanical blow-off valves in place is a backup in case the duty control side fails, and system lag means you need both to prevent spikes)

4) Since we have both exhaust (input) and charge (blow-off) flow controls, which one acts first? (Do the ACV and CRV vent in normal operation?)

Dave

soru81

Very interesting questions, tough I didn't quite understand all of them...

1) I think the turbo control valve port is too big to offer fine control over the amount of bleeding air to the secondary. The amount of air for prespool is substantially smaller than the air needed for running it, so the port for the precontrol valve is much smaller, so it is a better tool for controlling bleed off quantity at small amounts. Also, the duty cycle and open loop (for turbo control, and turbo precontrol) doesn't control the actuator movement very accurately. If the TCA would be replaced with electronic motor for moving the flapper door, then it would be probably possible.

2) I don't understand this one quite. I think you're assuming the air is sumped to the pressured air in the intake. As far I see it from TCA and TPA solenoids goes to the intake before the turbo - meaning there's vacuum inside instead of boost, so it can, hmm, maybe decreases response time for bleeding the actuators?

3) I don't get this one as well. Where do we have duty control on the charge side? Charge relief? it's only for the second turbine while it isn't charging yet. Air bypass (the real BOV) isn't duty controlled, it is just taking a signal from the intake manifold (vacuum or boost).

4) The control for turbos is actually exhaust control = wastegate. BOV is just for safety: not to overpressure the intake before TB and not to forcefully brake the turbo down.

dgeesaman

iTrader: (7)

Thanks, that's what I guessed.

Well both the wastgate and precontrol actuators see two lines: one coming from the pressurized size of the intake (between the primary and y-pipe), and one coming from the duty control solenoid. So it seems it's using the solenoid as the air inlet and the intake as the outlet. Which is why I was wondering why it's set up that way. The more I think about it, I suspect this: when the charge is at/above 10psi, the charge air balances against the pressurized air coming from the solenoid, causing the actuator to open the door and drop the charge pressure. When the solenoid is off, then if the charge builds up and the restrictor line acts as the inlet, opening the wastegate or precontrol and controlling itself. I think damping is not an issue.

Question doesn't make sense - the wastegate and precontrol are duty controlled, the ACV and CRV are not duty controlled - they are only pressure-actuated blowoffs. I understand now.

Ok, so another way of saying it is that duty control is done from the exhaust side, and the ACV/CRV serve only to blow-off spikes of boost that occur during transitions and as backup in case of component failure.

Is it true that the reason why using only blowoff is bad, is because without controlling the exhaust, you'd constantly overdrive your turbos on partial / no load?

Dave

soru81

Ahh, I tought you have meant the line from the solenoid to the intake before the turbo - I tought where the solenoid bleeds pressure out of the actuator, which is just before the intake air entering the primary turbo (vacuum).

I think it's the other way around: The restrictor line is always inlet and line to the solenoid is outlet. When accelerating, the pressure comes from the primary housing through the restrictor line. If there were no solenoids (wg in precontrol), the pressure in the actuator would raise to the level where actuator starts to move the rod (7psi) and opening the wg. This would make the boost stable at some level let's say 7psi. When the throttle is let off, the exhaust air from the engine is so small, that the turbos aren't charging - the wg is closed tough, but they aren't charging. The solenoids are here just to help raise the boost level so, that they virtually decrease the pressure that actuator sees - thus not opening the wastegate yet. I don't see any way in operation, that the solenoid air would be as inlet to the actuator and primary housing the outlet.


Well CRV still has its own CR solenoid, which is controlled trough duty cycles by the ecu. Oh, but actually if I think again, the ecu doesn't really have a duty cycle for CR solenoid, it's just on and off switch, and it doesn't have duty cycles defined - as far as I understand. So can it's ecu controlled, but not exactly duty controller?

I don't know is my understanding of duty control is accurate, but I understand at something being controlled with some rate in a time unit (like injectors and precontrol, wg control solenoids duty cycles).


Well this is the main job of BOV: when boosting, and the throttle is left off -> throttle butterflies close imediatelly, but the turbo is still spinning -> potentially creating huge amounts of pressure of the intake turbo side with IC all the way to the throttlebody, and forcufully braking the turbo (this can damage it) -> therefore the BOV get vacuum signal from the intake manifold (which is after TB) and bleeds off pressure saving the pre-TB-intake-system from overpressuring and turbos from agresive braking.


Well yes, if there would be no wastegate, the turbos would constantly spin at maximum revs for the give exhaust from the engine. Blow off would be reducing the boost entering the engine, but the turbos would still spin like they would for highest boost. Similarily, you can overdrive your turbos if you have a boost leak (if the WG actuator is taking boost signal from the intake manifold, which doesn't see actual boost because of the leak and therefore the actuator is holding the WG shut) - but not with the stock rx7 boost control setup, but this does not hold true for some other turbocharged cars.

dgeesaman

iTrader: (7)

That's correct. I just looked at the vac. diagram - and mistakenly thought the wastegate/precontrol were connected to the pressure chamber lines, which is why I figured they were the source. Actually, they connect to the engine intake by the injectors, which makes sense as an outlet. So as you said then, the wastegate and precontrol always inlet from the primary charge, through the restrictor, and into the actuator. The ECU then duty-controls that actuator to control the amount of boost.
The reason why the CRV has a solenoid is because the CRV opens before the ACV. (the secondary charge tops out around 8 psi during the prespool phase). So the CR solenoid just shuts off the CRV when the secondary comes online.
Right, its just like fuel injectors: it cycles on/off to essentially be open a percentage of the time. If done at the right frequency, it will control flow.
OK, that makes sense, and it does happen every time the throttle is let off after boosting. So it's for venting the extra boost created while the turbo de-spools. The CRV is for venting the secondary when the let-off occurs during the secondary pre-spool phase. Now that I think about it, I should have thought of that considering that aftermarket BOVs hiss on every upshift or let-off.
OK, your writing was a bit confusing there but I think I get it. In the RX-7, the exhaust is controlled to extend turbo life. Otherwise (in the case of a BOV-only setup) the turbo is constantly spooled according to the exhaust flow. Do singe-turbo setups still use any exhaust-side controls, or do they just stay spooled and use BOV? (I'm not a single turbo guy, obviously).

Thanks for all your help. Hopefully we're both learning here.
Dave

broken93

Exhaust-side controls are always necessary... venting boost is not an effective way to control boost, nor is it a stable one. Without a WG, there is no control of turbine speed, possibly causing catastrophic failure of the turbine and compressor wheels as well as the bearing assembly.

KevinK2

most of your answers are here:

http://www.autosportracetech.com/RX-7/rx7stuff.htm

(from Newbie sticky, under "turbo" ... link needs to be fixed)

alberto_mg

where did you get the book?

dgeesaman

iTrader: (7)

Thanks, I've read that all through 100x. The key word in my posts is 'why' things are as they are. I'm trying to build an understanding, not just a picture. I know a couple of questions are more general, but the bookseller who I ordered the Corky Bell book from finked out on me.

I'm tempted to re-write the 'how the tc works' thing in a different method: instead of discussing each component, and throwing it at the reader shotgun style, I'll build it up. Start by showing the primary turbo system alone (a single turbo setup). Then add to the diagram a secondary turbo and show the valves/switches to make it come online at 4500rpm. Finally, add the prespool controls to the secondary and show the whole picture.

The 93 Mazda Service Highlights manual is available from the dealer. I got mine from Ray at Malloy for about $40. The turbo section is quite short, I may scan it and post it if you want.

OK, that makes sense, but I don't frequent single-turbo discussions enough to know that. I'll have to do some digging to see what those manifolds and controls look like. (All I notice before was a big turbo which appeared to be connected right to the engine block). (Noob question, that one. Sorry)

Thanks for everyone's help.

Dave

dgeesaman

iTrader: (7)

Well I must be charged up today or something.

Here is the turbo section:

http://web.newsguy.com/geesaman/Mazd...ual_Turbos.PDF

Dave