Are the JDM cars faster, stock for stock, due to catless downpipe?
 

Let's compare 92-95 cars. The US cars came with a precat. This is likely more restrictive than the JDM cars with catless downpipe. The JDM cars have acccess to higher RON fuel in some places, like 100 RON which is more like 94 or 95 in the US.

Mazda didn't advertise power differences between the two, but I'm thinking the US engines are overrated, the JDM engines are underrated, or they're both overrated due to some sketchy stuff like Mazda did on the Rx-8. The power numbers are rated on an engine dyno which allows all sorts of mischief.

Thoughts? I'm tempted to dyno my mostly stock car with the JDM downpipe and stock catback to do some back to back comparisons, inspired by https://www.rx7club.com/3rd-generati...added-1104322/ would be a month or two before the car would be ready as it needs other things.


Here is what I'm curious about:

1) baseline with all stock USDM intake and exhaust, USDM, stock US ecu
2) stock everything except JDM downpipe
3) stock everything except racing beat catback
4) JDM downpipe, stock cat, racing beat catback, stock intake, which is what my car will have (after pillow balls and other stuff are done(.

Maybe I will do this study myself to answer my curiosity, if nobody has tested before. The assumption is that the US and JDM ecu are tuned the same, when in reality there may be differences due to the downpipe cat on the US car.

Sounds like a good experiment. If nothing else, it will show the difference exhaust actually makes.

Banzai did something similar a while back, but it was not this exact set of circumstances.

Vince

I'd enjoy seeing what, if any, difference there might be. But damn...intentionally R & R'ing downpipes with the engine in the car doesn't sound like much fun. The only guy I can think of into self-abuse like that was Roseann Barr's ex. Kind of hard on expensive gaskets too, isn't it?

Also, I've got a lot of questions about how the stock ecu really controls boost on a stock car. Especially as it relates to gear position. I need a loading dyno to figure that out. Need to be able to control rpm ramp rate.

Maybe swapping the parts around between dyno runs isn't so bad If you're already working with brand new gaskets and have a vehicle lift.

Hard to say but I don't think it would be that big of a difference. The precat has a VERY fat pipe with the cat in it, I think the idea is to keep the amount of flow that can go through it the same. JDM downpipe is 2.5" or so ID.

The big disadvantage to the precat is it's a SUPER hot oven which heat soaks the engine bay like crazy and it is very prone to internally breaking up and clogging which is a huge performance penalty.

But, I dig your experiment.

BTW, I would LOVE to have access to an engine dyno and strap an FD motor to it and try swapping parts out and testing things. That would be SO cool.

Dale

I'd be interested to see these results. I'd also like to know if the JDM downpipe results in slightly higher boost, as I think I saw when switching from a stock US downpipe to aftermarket.

You probably know this already, but a Dynapack hub dyno sucks for simulating actual loads. I've run my car with a Dynapack and then with a Mustang dyno, twin turbo spool and transition looked pretty similar to real-world on the Mustang with big heavy rollers but the Dynapack did all sorts of weird things... I think the little stock turbos were messing with the dynapack's PID control. Engine speed logged during Dynapack runs always look weird, it seems to adjust the load on the engine in search of a constant acceleration rate... cars don't do that in the real world, the acceleration depends on how much power it makes. On my car the turbos would spool, then the dyno would load the engine and it would overboost a little, then the boost control would react, then the dyno would unload the engine a little, it was no good.

Post the dynapack runs, or shoot me a PM.

Dynapacks are like having a water brake engine dyno for your hub. It's different than roller dynos, which have an exponential road load vs vehicle speed curve. Engine dynos are similar to dynapacks. Unless you have a special road simulation algorithm, they are meant for steady states and fixed rpm ramp rates regardless of the transmission gear. The dyno controls the engine speed, the engine doesn't control the engine speed by the power it generates. That's a lot to wrap your head around I admit.

There are things you can do with dynapack style dynos (or roller dynos set up in a similar way) and engine dynos that you can't do with roller inertia dynos (common dyno jets) so easily. It's a different kind of tool, that's all.

Horsepower is NEVER advertised on the OEM side based on a vehicle dyno jet pull, or based on a road load simulation used on an engine dyno. Its always engine dyno with fixed water and air supply temperature in a lab, and with differering methods of controlling intercooler temperatures. It's either done on a fixed ramp rate (transient method) or a steady state method, such as holding for example 6500rpm/WOT (advertised peak power rpm on a stock FD) for 30 seconds and averaging the data.

So the dynapack runs are actually much closer to the way advertised horsepower is tested. I'm not saying that's better or worse than a standard inertia dynajet. But it is different. Dyno jet pulls are the de facto standard for measuring gains on aftermarket parts though.

Screenshot of the dynojet runs attached. This is a pretty weird setup, mostly stock with the exception of aftermarket downpipe, Xcessive Manufacturing LIM with 4x ID1000 injectors (primary injector bungs blocked off), AEM Series1 EMS running about 10psi boost. Stock intake, stock cat, stock muffler. This was the first time I dyno'd the car, unfortunately I never thought to do baseline pulls on the stock ECU before changing setups. The dyno wideband was measuring a pipe stuck in the exhaust, AFRs measured by the sensor in the downpipe were more rich by about 0.5-1.0 AFR. I doubt the stock ECU would have let boost fluctuate as badly as my aftermarket ECU & novice tuning skills, but these settings resulted in nice smooth boost in the real world. I'll see if I can dig up ECU datalogs from those runs, I think I still have them saved somewhere.

We can take that to a PM conversation. I need time based data to make sense of that, rpm on the X axis doesn't work because you don't know the ramp rate based on the plot.

Yes sorry to derail the thread, I'll find logs and PM you sometime. Interested to see your results.

I have had the chance to drive a stock USDM LHD car and a stock RHD JDM car but this was going back 7+ years ago. Both cars were of a similar sort of mileage, both were of a similar sort of year (both were non-sunroof, rats nest cars)

Objective, seat of the pants feel was the JDM car accelerated sharper and felt to boost quicker than the USDM car did. Both felt to have the same sort of power but the JDM would likely have got from 0-60mph first. Marginally. I only ran them about a half a mile up the street outside the warehouse, around a roundabout at the top of the street and back to the warehouse. I remember wailing on both pretty mercilessly as they were going to become spare parts after that. I remember coming back and saying to my Dad that the white car (JDM) was faster than the red car (LHD USDM). I'd love to see some sort of dyno comparison of both specs to confirm this.

I later cut the USDM car in half for parts as it was not able to be driven on the road in my country and was only useful for wrecking purposes. The funny looking US downpipe with the cat in it, we laughed at it and then promptly threw that in the scrap metal bin.
Also the accelerated warmup system thing was quite comical too. We had never seen all those EGR bits and pieces before.

^ thanks for the anecdote.

I am now thinking I should do the dyno runs myself. That wouldn't happen for a couple months.

Another thing to consider does the odd shape of the jdm Downpipe(to clear the steering) hinder some of the benefits from it not having a cat?

I was thinking the same thing.

I actually have a JDM downpipe and ECU at the shop. The next completely stock car that comes in for a downpipe install I will run on our dyno and get a back to back comparison

That pipe is similar to the HKS but I seriously doubt the JDM pipe vs US pipe will effect power much if boost is kept the same.

The problem is when you increase the flow the boost jumps up and the power along with it.

Very interested to see this, thanks. It helps that you have done all these other tests on the same dyno, so the numbers are easier to compare.

i was going to say, if you want a JDM ecu, i have two.

one is a stock N3A7, which is ~93 m/t.

the other is an RE Amemyia chipped N3A7. its the bottom level chip, so its meant for an intake/exhaust but thats about it. unlike the old US chipped ones we used to run, this one is remapped everywhere. idle is different, boost control is radically different.

since i'm running a stock Fc turbo with an FD ecu, i've noticed that the boost control does actually learn a little. the first run after connecting the battery is different than the second. just by a bit, but its there.

the JDM cars were rated at 265 vs 255, but it was also PS vs HP, so difference is small.

i actually don't have the 1/2 gear switch hooked up, i have wondered what it does. i have read it was just for boost control, and since i had no trouble passing smog i think that may be the case

If the JDM cars are rated 265ps, that's 261hp. So 6 more hp than the 255 the US cars are advertised to have.

I wish I knew more about the Re Amemiya and other chipped ECUs. If I'm running stock intake with JDM downpipe and Racing Beat catback, that is a very mild modification. I don't really want to turn up the boost.

If the stock USDM and JDM tunes are in fact different, it's probably richer on the USDM car to protect the precat under boost.

If only my country had a dyno, I would gladly dyno my 95% Bone Stock JDM RX-7 for you guys, because all that came with my car is an Fujitsubo FGK Catback exhaust, on top of that, i'm also interested in how much power my car is actually making.

Re-Amemyia used to list what they did for each chip, but they took it down recently.

they had a B chip, which is for stock engine/injectors/turbos/cat. boost, is .9p/.8s. the speed limiter is removed, as are the boost and rev limit (i think). so far it sounds like a pettit or M2, but then they also retuned fuel and timing, and there was other stuff listed too.

the A is like the B, but without cat, boost is slightly higher again, usually its written on the ECU.

and then they had one for a single turbo, and or a custom tune. these are the ones where you would be just buying the ECU because it already has a PROM chip soldered into it

so these chipped ecus have the parts in them to make them programable so you could buy one and tweak it. my ecu has a cable in it, so it can plug into something.

you should add #5, try a 99+ ecu. they do run WAY better.

The problem is that a 99+ ECU does not just plug in to a USDM 93-95 AFAIK.


Matt

On chipped ECU's -

Back in the day a company called Techtom released a setup for dealers to chip ECU's. They would solder a daugherboard to the ECU with a removable EEPROM socket and the shop could change fuel and timing maps and whatnot and burn chips. It was also popular in Japan to get a "chip tune" where you would bring your car to a shop and they would dyno tune it, burn a chip, and off you go. Many vendors had chips for common setups, hence the different versions of chips.

Pettit Racing and other US vendors basically did the same thing with the Techtom setup.

This is all early 90's technology. Even if you could find a Techtom programmer you probably couldn't get it to run.

Also, at the end of the day chipped ECU's still have the same limitations of the stock ECU - 3000 RPM hesitation, you have to have the airpump or the car runs rough due to the stock ECU compensating for O2 sensor readings that are fouled by airpump air, etc.

The later 16-bit ECU's from 96+ in Japan are supposed to be way better as far as driveability and such, but that's not from first hand experience. Hopefully Mazda got rid of the 3000 RPM hesitation. Those ECU's won't plug in as stated, very different connectors.

Dale

This is all early 90's technology. Even if you could find a Techtom programmer you probably couldn't get it to run.


I bought a previously ROM tuned Knight Sports 4 Beat JDM ECU and had it re-ROM tuned by Knight Sports to my specs.

They have a worksheet where you put in your modifications and what cuts/rev limit you want.


Also, at the end of the day chipped ECU's still have the same limitations of the stock ECU - 3000 RPM hesitation, you have to have the airpump or the car runs rough due to the stock ECU compensating for O2 sensor readings that are fouled by airpump air, etc.

Only time I experienced the 3,000rpm hesitation on stock ECU was when I broke the double throttle solenoid (so DT was always open) and got into the boost too much on a cold engine- don't boost a cold engine hard!

I didn't check to see if 3,000rpm hesitation was still there with the ROM tuned ECU, but I did notice immediately the ROM tuned ECU ran noticeably smoother than stock. Probably because they don't have to meet emissions.

Also, with ROM tuned you can choose your "limits" to the stock ECU Fuel Cut boost limit and Rev Limit and retain full factory knock control.

That is something even the stand alone ECUs do not offer.