jimlab

All production 3rd generation RX-7s were turbocharged as delivered from the factory. While a prototype of the FD built on an FC chassis may have been powered by a naturally aspirated engine, NO naturally aspirated version of the FD was ever put into production by Mazda.

If there are any that are naturally aspirated now, they were converted after they left the factory. Not to mention that a 13B-REW converted to naturally aspirated operation without swapping in higher compression rotors would never be capable of 190 horsepower... "smart guy".

jimlab

Obviously you can limp the car somewhere if you absolutely have to, but high rpm use while "naturally aspirated" would be a definite no-no. You'd probably also want to slap a filter on the open intake piping while you were at it, just to keep things out of the engine...

Interestingly enough, this is one thing that the RX-7 can do that the Supra can't, unless it has an HKS VPC installed to eliminate the MAF sensor. If you pop an intake hose loose, you get clouds of black smoke as the computer injects enough fuel to compensate for all the air being pulled through the MAF sensor by the turbos... but which isn't reaching the engine. You can barely keep the car running, let alone limp it anywhere, and you'd better have a 10mm socket or a screw driver on hand to repair the problem, or you've just become a pedestrian.

RX7SpiritR

iTrader: (1)

lol, ok ok. Sorry, maybe I got too excited. I know I'm pullin the 190 HP from somewhere...I think it was a version of the 2nd gen. But sorry if I got a little cocky or something and pissed someone off! LOL, just tryin to sound smart and it didn't work!

spooledUP7

If only I read the third page, I wouldn't have missed such a great opportunity. Rats!

spooledUP7

As always Jim, truely interesting. I could never figure why a turbocharged car would nead anything more than a manifold pressure sensor and TPS to regulate fuel consumption rates. (othere than Temp and O2 for finite)
Why would you need a Mass Air Flow sensor?

ArchangelX

Jim..where the hell do you learn all of this? Experience? jimlab

rynberg

Just to add fuel to the fire, I thought I'd post my dyno numbers on a run when the inlet side IC coupler came slightly loose. I made 181 hp on a couple psi of boost....so I think my estimate of 150 or so is in the ballpark.

jimlab

The RX-7 has what is commonly referred to as a "speed density" system. It's very simple, and at WOT it simply injects a pre-determined amount of fuel based on a fuel "map" (look-up table, or grid). The amount of fuel is determined by engine speed, intake air temperature, and intake manifold pressure. The downside of a speed density system is that it can't compensate for modifications, and the ECU maps must therefore be reprogrammed to maintain safe air/fuel ratios.

The Supra has a "metered air" or MAF (mass air flow) sensor-based system. A heated "element" directly in the air flow is fed a given amount of electrical current, and changes in airflow cool the element proportionately. The MAF sensor reports to the ECU how much current is being required to maintain the temperature of the element, and the result is used to inject the appropriate amount of fuel for that amount of airflow. This type of system also uses engine speed, intake manifold pressure, and intake manifold temperature readings, but the main difference is the MAF sensor.

The downside of a metered air system is that there is a bulky meter directly in the intake path prior to the turbos, and on a single turbo car, this can become a bottleneck. The usual remedy is an HKS VPC (Vein Pressure Controller) which basically converts the Supra to speed-density, for all intents and purposes, and as a fringe benefit, allows primitive fuel tuning, and allows the user to run upgraded injectors, all without touching the ECU.

The upside of the metered air system on the Supra is that you can throw a downpipe, cat-back, intake, and a boost cut controller (BPU, or basic performance upgrades) and bypass the wastegate actuator with a single vacuum line, and all of a sudden you're running 17+ psi of boost with a system set up from the factory for 11 psi, and you don't even have to touch the ECU or the fuel system. That's why horsepower on the Supra is so cheap until you reach the single turbo stage.

As for knowing about the black smoke thing... I drove a MKIV Supra Turbo for 3 years as my daily driver. After getting a front mount intercooler, I took it out for a test drive and *PHOOMP*, blew off one of the new IC piping connectors. I found out the hard way that the Supra produces a smoke screen worthy of one of James Bond's cars when the connection from the turbos to the intake is broken. Luckily I had a small set of tools along with me because I never head out on test drives without some after the Trev Dagley "Exxon Valdez" incident with my RX-7. Don't ask.

rotary-tt

I'll second that a NA 2nd gen makes 146-160 hp as was said above. You don't get 190+ hp unless you have a Turbo II.

I've blown off a turbo coupler and have driven NA on my FD. I didn't notice any smoke but it was at night

ArchangelX

Jim, You told us not to ask so I won't. But I think Exxon Valdez explains it all...

Sad thing is, that was a couple of years ago, huh?

SO what made you switch from a Supra to an RX-7? And which do you prefer, just outta curiosiyt?

Shinobi-X

This has turned into quite an informative thread. Thanks Jimlab.

jimlab

The RX-7 shouldn't smoke. No more than normal, anyway.

The Supra smokes because it's running pig rich; far too much gas is being injected for the amount of air that's actually making it to the engine because the ECU is trying to compensate for how much air the MAF sensor tells it should be making it to the engine. The result is billowing black clouds of unburnt fuel.

The RX-7, on the other hand, doesn't care (as much) about how much air is coming into the engine, it just cares about the pressure in the intake manifold, more or less, and really only at WOT. Since there's no boost being made, it's happy as a clam running in closed loop mode, adjusting the air/fuel ratio based on 02 sensor feedback.

fdracer

if a turbo fd makes 255hp at 1.68 bar of absolute pressure, an n/a fd at atmospheric pressure will make about 152hp.

jimlab

Well, I'll give you the "condensed" version. After my first engine let go, I started having a ported engine with 3mm ceramic apex seals built by Hayes Rotary (which is another story, but one I've posted before) and while it was being built, wanted to keep the car running. Trev had a couple spare junkyard engines and reluctantly sold me one (he was one of those people who liked having a lot of spare parts just for the sake of having a lot of spare parts, I think) and we installed it in my car. Check the picture below... junk yard engine compared to my ~6,000 mile engine. Can you tell the difference?



We were tag-teaming to get it done quickly on a weekend, and Trev was handling some things while I was doing others. I specifically asked him to swap over the oil cooler lines from my engine to the other one, and thought he'd done so. When we took the car out for a test drive after bolting everything together (at 1 AM, when all really good adventures should start) we found out the hard way that he hadn't switched the lines.

Apparently the junkyard engine had sat on couplings at the end of the oil cooler lines at some point, and gravel or some foreign substance had distorted the coupling to the point that one of them (the line from the engine to the oil cooler) had distorted enough that it did not slide fully onto the oil cooler coupling, and the clip that is supposed to retain the house didn't catch in the groove into which it's supposed to clip. We found this out later, much later.

Bottom line, at 80 mph and 14 psi, the oil line let go. 100+ psi of oil pressure emptied the pan almost immediately and in the process greased the left rear tire. If it had been both tires, I probably would have lost it, and as it was, we fishtailed down the road until I got it under control. I shut off the engine as soon as we were pointed in a straight line. For a long time after, we called that adventure Mr. Toad's wild ride.

We coasted into a BP Station and discovered the "problem". I trotted over to a 24-hour Safeway and bought more oil while Trev tried to put the coupling back together and find the clip. Oil was everywhere under the hood. Keep in mind that this is one of the last '95s with only around 6,000 miles on it at the time. My hood liner was soaked, everything was coated in oil. I was not happy.

We dumped the oil in, started it up, and it immediately puked all of the oil onto the ground, since the clip was still not holding the coupling on. That's when I discovered that Trev had most definitely not switched the oil lines. We ended up walking a bit over 4 miles to his Dad's house, grabbed more oil, MY oil line, and a few tools, and drove back to the BP station. 10 minutes later it was up and running, but completely soaked with oil. From the engine bay to the back of the car, the entire underside was black and dripping. I drove it back to his dad's house and left it (it was about 4:00 AM at this point) because I had to get home and get a little sleep before going to work.

I ended up throwing out my hood liner and degreased everything as best I could, but I just cleaned the last of the remaining gunk off the underside of the car early this summer. I had a spare couple of hours to lay under the car and wipe it all clean with a can of Sherwin Williams grease remover. Fun. Obviously the engine bay is spotless, it's been repainted twice since then.

A couple... September 1997. Trev died in the wreck in June of 2000.

I had the RX-7 first, bought it in April of '97. I bought the Supra in January of '99 and used it for almost three years for my (reliable!) daily driver, until I sold it and bought the Z06 in October of last year.

The Supra was far bigger, obviously, and could haul a ton of stuff (You can put an FD 5-speed in the hatch, for example, or nearly 500 lbs. of bushing components) and never failed me once, even though it saw over 100 passes at the strip at BPU power levels. It also won me a trophy; 1st place, Power 6 class, 1999 NIRA Northwest Nationals. When I sold it, after driving it hard pretty much from day one, compression was still 160-161-160-159-160-161.

I didn't dislike the Supra, it was just time for something different and the 405 horsepower rating of the (then) new '02 Z06 was all it took. The Supra was also an automatic (all I could find at the time) and I wanted a manual transmission again. And a car with a warranty that had enough power that I wouldn't (hopefully) be tempted to modify it. I voided the warranty on the RX-7 by converting to full non-sequential and full mods about 3 months after I bought it...

ArchangelX

Thanks for sharing, and answering my questions, jimlab.

It helps to have a little insight on the whys and whens...

mp5

iTrader: (3)

I drove my Fd N/A when I first got it.
I gunned it for the first time on my way home and heard a wooshing sound

I limped in to the next garage ,popped the hood and saw one of the black rubber couplers from the i/c had come off,put it back on and off I went.

You really would not want a N/A FD,they suck!

C.

Red-Rx7

I had to drive my vehicle with no boost from KC to TX when I first got it. At that time, I just installed the Haltech E6K, and couldn't get the solenoids to work correctly with sequential twins.

I had it all hooked up correctly, and come to find out(stupid me), one of the relay's I was using was bad.

But, to find this out, I drove to RP. I limped along, and it did well. I couldn't go over 45% throttle or the turbos would surge. It makes a pretty nasty noise, and you realize it quickly. Just be careful, and the car runs peachy-keen.

Mike

ArchangelX

Nah..it wasn't that I wanted an FD. I just remember'd discussing the fact that my WRX is about 120 NA which is a 2.0, and I was wondering what a 1.3 liter RX-7 without the turbos would be pushing.

The fact that it's got THAT much HP even though it's designed as a turbo car that's only 1.3 liters is simply amazing.

The rotary is VERY cool...