Zac's 1998 RX7

[kyteler]

Intense man, sucks about selling it also. Still, at least you enjoyed it for a while and got some learning done with it.

[ProZac]

Chur. Yeah, was good times. Hopefully it wont be too tricky to sell. It basically a completely stock, unusually tidy S7 Type RS. It'll have a couple of under the bonnet mods though, better intercooler, upgraded twin turbos, so should go a little better than stock, but still look really standard.

[yoeddynz]

Big congrats on making a mini me!  I see a very awesomely built go kart in the future....

[ProZac]

Cheers dude . Yes, go karts, quad bikes, all that fun stuff for sure .

[yoeddynz]

Go karts with track evaluated geometry and custom built ecus etc etc

[ProZac]

I've been doing some work on the 7 over the holidays. I had till the 8th off, so got a couple of days in at work before we went back to get some interesting jobs on this thing done. Not before i fulfilled a promise to wifey though, and got stage 1 (of about 10, eugh) of the front fence built:

Brick columns are original (but were shorter), were shorter with brick panels in between. I cut apart the brick panels, cleaned off the bricks (the ones I didnt break, anyway...) and had the columns extended using these bricks to match the rest of the perimeter fence. New privacy / acoustic panels are 200x50 T&G retaining wall. Stout :-). We've had some really strong winds since, and nothing fell over, so I feel good about it ;-). But enough of home type stuff, more car type stuff.

FD RX7's and their twin turbo systems.... For some reason I really like them. They're cool, weird, and kind of unreliable. I'm trying to cure as much of the unreliability issue on this car as I can, while giving it a little performance tickle up. First step was to actually understand how the system really works:

As a quick run through of the operation, I'll look at the system as it would operate if you were cruising at around 1800rpm, and floored it, keeping it floored till red line in second gear. Remember, that's how you get sweet speeding tickets.

Above is a picture of the stock exhaust manifold, with it's outlet ports that connect to the turbos facing up. The primary turbo bolts on the front (the bottom, in this picture) and the secondary turbo bolts on the rear (the top, in this picture). For the moment we'll ignore the little outlet port in the middle, and come back to its function a little later on. The passages inside the manifold are linked, so gas entering the rear manifold inlet port can exit the front outlet port, and vice versa. The manifold is a super nuggety piece of cast steel, with an interesting flapper valve, which we'll call [FV1] from here on out.

Initially, at 1800rpm this flapper valve is as shown above, closed against the entry to the secondary turbo, preventing it from receiving any exhaust gas and operating. All the engine's exhaust gas is therefore exiting the front manifold outlet port, operating solely the the primary turbo. Sweeeeeet, fast boostage for max skids.

 

As you can see, [FV1] is connected to an external linkage on the manifold, which is in-turn connected to a grunty double sided diaphragm actuator (not shown). When you hit around  3500rpm (not 100% on the exact figure, or if its solely RPM based) the system reaches the switch over point. At this time, the actuator connected to the external linkage is commanded to move... But the pressure in the exhaust manifold will be pushing against [FV1], keeping it closed. This is why [FV1] has a slightly odd construction, its a 2 stage unit. The first part of the actuator movement opens small orifice in the middle of the valve face, releasing enough pressure from the exhaust manifold to then allow the actuator to them open [FV1] the rest of the way...

[ProZac]

Which gets the system to this point. Everything is open, your turbos are both supplied with the rotary engines finest supply of exhaust gas. Max skiddage ensues till the redline, you're all happy.

 

But what about controlling the boost level? Iiiiinteresting stuff there.

The boost control system is split into a couple of phases, pre switch-over point, and post switch-over point. We'll start with the boost control before the switch over point. Mazda did something pretty clever here:

That's the exhaust manifold with the secondary turbo turbine housing attached. I've removed the cast cover on the turbine housing to show another flapper valve, henceforth referred to as [FV2]. Pre switch-over, only the primary turbo is operation. We're about to have a surplus of exhaust gas in our manifold, which will push out primary turbo to extreme speeds and overboost our poor little motor. Best we relieve that eh? Currently the secondary turbo is sitting there idle, not receiving any exhaust gas. Late to the party it's probably having a cry and a quick masty. We need to get some exhaust gas to the secondary turbo ASAP! [FV2] and that middle outlet port on the exhaust manifold that I said to ignore earlier come into play here. The small passage feeds [FV2] which is now opened by (yet) another actuator. This has two effects, it relieves exhaust manifold pressure to control the overall boost level, and supplies the secondary turbo with a small amount of exhaust gas to begin spooling it up.

 

[ProZac]

So what about boost control post switch-over, once both turbos are receiving a complete supply of exhaust gas and operating? Its a completely conventional internal wastegate system, with the wastegate flapper valve [FV3] located in the primary turbo housing.

That's the primary turbine housing, showing inlet exhaust gas from the manifold, and where it can exit to the dump collector via either the turbine wheel, or [FV3]. Control of the boost both turbos create is achieved by just this single valve because at this point of operation, the exhaust manifold / primary turbine housing / secondary turbine housing system is one big open area essentially. Doesn't matter where in that area [FV3] is located, when it opens it'll bleed off exhaust manifold pressure, slowing down both turbos, therefore reducing output boost. There are bound to be a bunch of complicated airflow dynamics at play here also, but I choose to ignore those for simplicity, as I'm a simple kind of guy.

 

Have you spotted the big missing link so far? To discuss that, we need to move to the compressor side of the turbos.

The primary turbo has a completely conventional compressor side connection. The outlet of the compressor housing heads to the intercooler, and then to the throttlebody. Sweet, simple, my favorite.

However, the secondary turbo needs some form of control on its output. If the secondary turbo compressor outlet was also directly connected to the intercooler, it is therefor also connected to the primary turbo compressor outlet. If the primary turbo is operating, and the secondary turbo isn't, the output boost from the primary will head back through the outlet of the secondary turbo, spinning it backwards and reversing the rotation of the earth. Now, as superman taught us, this is how you travel through time. I'm not a fan of being my own grandfather, so we need to avoid this at all costs.

This is the pipe connected to the outlet of the secondary turbo compressor housing. Its in need of a clean up sorry, but I haven't gotten around to that yet. Its got another valve in it, which although you cant see, you can see the linkage and actuator. The valve looks EXACTLY like a throttle valve. There is yet another valve in this system which is missing from this picture alas. Its connected to the open end of the rubber pipe, and its a plastic valve that looks EXACTLY like a factory blow-off valve.

So how to these work?

Pre-switch over, the pipe is configured as above. As the primary turbo begins to build boost pressure, the throttlebody valve above is closed, preventing this pressure from entering the secondary turbo outlet and spinning it backwards, then being wasted back out to atmosphere.

As [FV2] begins to open, bleeding off exhaust manifold pressure and begining to spool up the secondary turbo, the missing valve not pictured above (but connected to the open end of the rubber hose) is open, venting to the air box. This allows the secondary turbo to spool up, but without actually applying its output to the intake system.

At switch-over point, the position of these valves is reversed. The secondary turbo is pre-spooled by the bled off pressure which is controlling the boost level currently, the throttlebody valve opens, connecting the output of the secondary turbo to the inlet system. The non-pictured plastic valve closes, preventing a massive intake system leak to atmosphere. Two turbo's, lots of exhaust flow, awesome! The pre-spooling of the secondary turbo is how Mazda manages to negate much of the 'Valley of Death' that sequential twin turbo Subarus suffer from.

I've read some internet conjecture about the timing of the operation of these two valves. According to internet legend the non-pictured plastic valve closes before the throttlebody valve opens. This means the secondary turbo is spooling up, and its outlet air has nowhere to go. This causes it to surge, overspeed, and be spinning super fast in preparation for it being introduced to the intake system as the throttlebody valve is opened. Haven't read any literature from Mazda to that effect, but its a cool thought, and I read it on the internet, so its as good as being carved in stone I rekon.

So yeah, sequential twin turbo system. Sick. Works bloody well when it works. There is of course the vacuum / boost / one way valve / accumulator / electrical system all laid on top of these mechanicals which has to be in perfect order for the system to function as intended... Sweet as!

[ProZac]

So part of the work I did while at work over the break was to clean up all the turbo housings, they all came up pretty nicely. The secondary turbine housings are well known for cracking really badly, but these replacement ones I've got are in really good nick, which is great

That's the exhaust manifold, primary and secondary turbine housings, and the dump merge collector. It makes for a bloody chunky unit, and I'm sure contributes to a large amount of the heat issues under the bonnet of an FD. It heat soaks pretty quickly, and has such a migh mass it'll have heaps of heat to emit once the engine is off. Your engine bay stays warm for hours.

With my plan to rebuild / upgrade the twin turbos I've got a few key milestones to hit:

*) Increase size of factory wastegate port. Once you free up the exhaust a bit, its common to have boost control issues. A larger internal wastegate port should help with this.

*) Measure and document geometry of replacement turbine and compressor wheels, so the factory Mazda turbine and compressor housings can be machined to match.

*) Assemble garret T25 based CHRA's and have them balanced.

*) Figure out what needs to be made / machined to fit the garret T25 CHRA's to the Mazda housings. The placement of the CHRA in relation to the housings will determine the location the measure geometry needs to be machined into, to make it all work.

 

First to be knocked off was increasing the size of the wastegate port. I chose to do this on the mill as work, as its got a sweet probe that let me find the center of the existing port. Plus, it takes all the actual manual work out of it, because as well as being pretty simple, I'm also lazy.

Yeah, I'm not a machinist, but this setup turned out to be rigid enough. I needed that much stickout on the endmill to clear the wastegate flapper.

Took it out to 31mm. The flapper is 33.5mm, so leaves around 1.75mm of sealing surface each side. Not that these ever actually 'seal' per-say, but will still let it do its job.

Was much easier than grinding them out by hand, which is how I've done it in the past.

[yoeddynz]

You need to write more threads because I enjoy reading them. Its like a really interesting class with a good teacher.

As you were.. get back to it.

[ProZac]

Chur buddy. One day I'd love to be that crazy math or science teacher at a secondary school ;-).

[ProZac]

Next milestone I've decided to tackle is the measurement of the compressor and exhaust wheel geometry. I reckon this is probably the hardest part of the whole turbo endeavor as the data and machining has to be pretty damn close for the turbos to be efficient. I had a couple of ideas about this, but decided to start relatively simple and use the lathe.

I spun up and arbor that the compressor wheel is a really good fit on, no wiggle, but it'll rotate freely. I started at the front edge, and moved the tool in till you could *just* feel the edge of it as you spun the compressor wheel by hand, and noted down this x-axis value. Moved back in the z-axis in 0.5mm increments, repeating the process till I got to the end of the blades. Not an ideal method by any stretch, but I got some data out of it to work with as a starting point.

Popping the points into solidworks helped to visualize things, and got me here:

I didnt expect this to be an ideal fit, as I hadent corrected my measured radius for the 0.4mm radius of the tool in the lathe, but as a starting point I rekon its a good one :-).

[ProZac]

We've been using FeatureCam for CAM stuff at work, and its not to shabby, but I wanted to have a play with Fusion360. So far its far more intuitive, and there is a heap more support for it out there available on the interwebs. I cammed up a toolpath, modified the post processor a little to work better with our lathe controller and spun up the measure profile into a test piece.

Its not bang on perfect yet, but its most of the way there. A few more measurements and tweaks, and that'll be the compressor wheel geometry sorted :-).

[NickJ]

16 hours ago, yoeddynz said:

You need to write more threads because I enjoy reading them. Its like a really interesting class with a good teacher.

As you were.. get back to it.

Exactly that, these posts are making for good afternoon reading between stints in the shed

[yoeddynz]

Highly jealous at the tools you have to play with.

We just bought a mill.. but not CNC though. I might add some DROs and then pretend its CNC..

[SHGWAG]

Looks like progress is coming along well!

[Willdat?]

On 1/15/2017 at 11:44, ProZac said:

Chur buddy. One day I'd love to be that crazy math or science teacher at a secondary school ;-).

Nah - teach Engineering. That's where the magic happens.

This year I had a student scratch build this.

 

[ProZac]

Holy shit that's epic. What age group are you teaching?

[Willdat?]

He was Year 13 - designed in Inventor and 3D printed over a few weeks. I teach from Year 9-13, it's a scam of a job as motivation is pretty high .@yoeddynz hung out in our dept a few years ago so should be able to testify to how sweet it is.

Another student designed and built this in class -  would be a lot easier now we have a laser cutter...

http://www.instructables.com/id/Acrylic-Skeleton-Clock/

 

[yoeddynz]

It is pretty sweet. I enjoyed my time working there. Pretty much all the students were OK at that age ie not annoying dicks.

You'd make a good teacher Zac. I can totally picture you in socks and sandles (just like Will)