Picked the car up last Wednesday. Yoshi had the heater's radiator repaired at a radiator shop. They ordered a new core and reassembled the unit. Cost from shop was $200. Yoshi also found the shifter boots torn and replaced them. Parts cost for both heater (O-rings replaced) and boots (inner and outer) was around $160. Total for the whole job was $778. Car is "drivable but not recommended" by Yoshi due to large exhaust smoke/steam at cold startup. Yoshi thinks coolant seals are leaking into combustion chambers. I know that the turbo oil seals are also leaking oil into the intake air tract. The smoke is probably a combination of those things. Once fired up and running, the smoke goes away. Carole drove it home and it's been a garage queen since then. I still haven't run a "champagne" test.

Our car is now #2 in line for an engine overhaul at Neptune Speed. Yoshi will call when he is ready.

 

I think that's very reasonable for the heater core repair. The thought of getting upside down under the dash these days for me isn't pleasant.
I hope you don't have to wait too long to get it in for the coolant seal issue. The seal grooves can pit/rust over time. Still, a lot of miles of smiles out of your car and you would have spent money on anything else...maybe more. I think it's the right call.

 

Thanks, Sgtblue.

Yesterday I ran what I can only describe as a "catastrophic champagne test." I used a Lisle funnel and filled at the upper (above thermostat) coolant fill hole, to just into the narrow-diameter neck of the funnel. Car was overnight-cold. I started the engine and let it idle for quite a while, waiting for the thermostat to open.

While the thermostat was closed, I saw a very few bubbles. The level in the funnel rose very slowly until it was about 1/3 full. Then the thermostat opened in a rush! Boiling coolant shot out of the funnel and all over the engine. This did not stop, even after I shut the engine off, although it gradually settled down, but not before a lot of coolant was on the floor.

So either the thermostat was stuck closed and popped open all at once, or this is normal for this engine. I don't know which, but the test was not what I expected. I thought that as the thermostat opened (gradually) I would see more bubbles, or not if the coolant seals were intact.

Any comments? The temperature indicator during normal operation of the car goes to about 9:00 or a little lower, never above that. I know it's non-linear and has a "dead" area around that point, but it never has gone higher. And of course with no pressure in the cooling system, boiling will occur at a lower temperature, but should it "explode" like that, all at once?

In any event, we will be having the engine overhauled soon, and are not driving the car much at all.

 

Speculation....you have a coolant seal failure. That failure was in a position on the iron that caused it to pressurize the cooling system behind the t-stat. When the t-stat finally began to open, all that pressure was released...out of Lisle funnel.

 

Also, an air lock (air behind the thermostat) could have caused the thermostat to think the coolant was still cool and when it finally opened, the engine temperature was above boiling. Once the restriction was released, the coolant boiled and spurted out.

This scenario also could indicate a coolant seal failure because the air bubble should not have formed.

 

Thanks for your comments, and we are waiting for Yoshi at Neptune Speed to call & tell us to bring the car in for an overhaul. He is currently overhauling another FD.

 

Still waiting for Yoshi, who e-mailed that he is nearly finished with the current job on another FD. I ordered a new set of BNR Stage 3 turbos from the RX-7 Store, which should arrive here around the first of April. Yoshi will replace the OEM turbos when he does the overhaul. Hopefully that will help with the large amount of oil smoke out the exhaust when starting from cold after a long period of sitting in our garage.

Any advice on whether I should change engine oil type from my classic Castrol GTX SAE 10W-30 to something more appropriate? Are synthetics still not recommended for this car? Even if recently overhauled? What is the current thinking?

 

When I purchased my FD last year I looked around for current recommendations about oil also. I didn't find anythiing that conclusively said that "synthetics are good and stop worrying about it".

Seems like the safest thing to do is still to stick with non-synthetic. The only thing you really gain from synth in modern cards is that it lasts longer, but with the fact that these cars burn the oil, it's not like you're going to have extended oil change intervals.

I ended up going with Castrol GTX 20w50 based on my research.

BNR Stage 3 turbos are a nice upgrade--do you have a boost controller and gauge to take advantage of this?

 

In terms of CFM, is the CHRA's from the BNR's equivalent to the stockers? If not then:

Do you have an ECU to compensate for the change in CFM due to the BNR's? Or are you guys installing a widewband to monitor AFR's?

 

Surprised you weren't aware...... BNRs are twin T28s, and as such they flow much more than the stock Hitachi units.

wstrohm, my brother's FD that set the overall BNR twin record runs Idemitsu 20w50 at my suggestion, has for many years now

 

Thanks for your replies. Regarding the BNRs, we are sticking with the stock waste gate and expect no more than the 10 psi max boost of the OEM turbo system. In fact I would expect the standard 10 - 8 - 10 pattern. No change is planned to the fuel system.

Regarding oil, I will consider switching to 20W-50, but this is just a street driven car, not raced. What record is the "overall BNR twin record?" Boost level?

 

As I pop the lid off this can of worms, I'll say that I use synthetic in my car mostly for its performance in high heat environments...like you tend find in turbocharged rotaries.

 

@Sgt.Blue,

I remember reading somewhere that "synthetic oil has a very searching action" and as a result can "creep through" gaskets, more so than dino oil. Although I do not know if that is Mazda's reason for not recommending synthetic, and >23 years have passed since the manual was written, have you ever regretted using synthetic? (I use Mobil 1 SAE 5W-20 in our 2008 Ford Fusion, because that's what Ford recommends. Engine is a 3.0 L "Duratec" V6.)

 

I thought the BNR's required fuel system upgrades and tuning even at stock boost levels?

Did you consider changing to 99-spec turbos? These can be used with OEM injectors and even stock ECM AFAIK safely at stock boost levels.

 

Nope, no regrets. And actually no leaks since a rebuild nearly 9 years and ~35k miles ago. I think the old ester oils didn't play well with some types of gaskets, but understand that's been corrected. The "creep" factor may be true. But the usual culprit on our cars is the pan gasket. I sealed it with Hondabond and a Garfinkle brace back in 2007 and it's still dry as a bone.

 

I did look at the 99-spec turbos, which are about $700 more expensive than the BNRs. But the BNRs are "advertised" to be more robust, having 6.35 mm diameter shafts vs. the 5 mm shafts of the other units, plus somewhat heavier construction throughout.

As for the fuel upgrade and tuning requirements, I have a mental block against understanding that. If the FD's MAP sensor reports 10 psi gauge pressure in the intake, I just cannot see how it matters what turbo has produced that. Max pressure is set by the stock waste gate, not by the turbo itself. If the BNR is capable of delivering more air mass at 10 psi than the OEM turbo into the same manifold volume, how is that possible? (Unless the air is much cooler due to compressor efficiency, I just don't see it.) The turbos are just not that much different.

I'm sure that at high rpm the OEMs are giving up boost due to having less capacity than the BNRs, so the BNRs can hold the 10 psi up at higher airflows, but that's a different issue, and a definite advantage to the BNRs, and they no doubt do need more fuel, but our car is just not driven that way.

 

I think you'll be fine. Even if there's a density difference, the stock ECU runs pig rich.

 

The bnr's are twin gt28s shoehorned in to the stock turbos. They definitely do flow more air than the hitachis. Every turbo does not flow the same amount of air at any given psi, that is why there are different turbos in the first place. Does that mean you need a fuel upgrade to run the bnr's? That I do not know. You'll have to wait and see.

 

Especially from cold! I had an AFR meter on my Miata, but not on Carole's FD. But it does run fairly rich. Probably around 11:1 on the OEM setup.

 

A few years ago when I did my mechanical resto, Brian at BNR mentioned that he used 2 types of seals. One worked with synthetic. The other required non-syn or they would leak. Suggest you confirm if this is still prevalent and which seals you got on your turbos.

The variability of the seals and better spool of the 99 twins is what led me to buy them over the BNRs.

 

Excellent advice, thanks! My turbos have yet to be made, and I queried Jason at RX-7 Store to ask about this.

 

The motor's compression is pressurizing your coolant system. It's why your heater core o rings were blown out. And it will occur again if you keep running the motor. I assume you don't want to pay twice for those repairs.
It's why your mechanic is advising against you running the motor.

 

It certainly looks like there was pressure in the coolant system. Remember, though, that the pressure cap on the Air Separation Tank is rated to hold pressure in the system only up to "16.4 - 20.6 psi" (page E-7 of the 1994 workshop manual). Above that the cap valve should release, and I have checked it and it does work.

It could be pressure at that level from combustion plus the 23-year-old heater core that caused the leak, but if the cap meets its spec, the system (including the heater core) should normally be able to handle at least that much pressure. Also, the O-rings were not leaking, it was a pinhole in the heater core itself. Yoshi did replace the O-rings as well as the heater core, though.

 

Yeah I thought I remembered that they were larger, but I wasn't sure by how much nor remembered the details. Thanks for the info


CFM is measure of flow while PSI is pressure and yes you can have identical pressure readings while having different flow rates. That is one reason why some turbos provide more power at the same psi, because they provide more flow.

Technically, 10 psi is just a "reference point" that we use as we know that the volumetric flow rate that the stock turbos produce is 'safe' (due to being a factory configuration). But that's all it is, a reference point. But that 10 psi reference point is actually useless when we swap out turbos since the CFM completely changes. Hence, a wideband is not unwarranted because it will tell you if your AFR's are in check.


He may be fine indeed. However, when it comes to engine safety I would much rather be safe than sorry. Because man.. I would feel like a total ******* if my wife's engine blew (and possibly turbos with it) because I was too cheap to spring a couple of hundred bucks for a wideband check.

But in any case wstrohm keep us updated on whatever route you take. It will be a good contribution to the the forum as I am sure many others would love to swap out their stockers for BNR's and still use their OEM ECU.

 

Easiest solution would be to take it to a shop and have it run up on a dyno. They don't have to tune it, they just need to do a few pulls to measure AFR and verify that the stock ECU is handling the extra output. I'm pretty confident the BNRs aren't going to be a challenge at stock boost levels, but better to be safe than sorry.