I tried to use NGK iridium IX (LKAR7BIX11S) which supposed to be the correct plug for our cars, but when I compared with the original plug, there was a big difference.
The original plug extends longer into the cylinder than the IX. I heard stories that changes like this affects the compression ratio, so I decided to order the OEM instead.
Kinda weird that NGK will make a compatible plug like this.
Anyone else come across this?
Left is original, right is LKAR7BIX11S

 

I've noticed the same and was curious if I got some fakes, so ordered another set from a different vendor, but they haven't arrived yet.
But now, I'm thinking that this is as intended.
I searched online for "spark plug reach" and looked through a number of articles.

According to enginelabs.com article:
"For street engines, the extended-nose is good for part-throttle efficiency. The short-reach is far better for high output engines where the longer ground strap could get hot enough to cause pre-ignition."

Your photo doesn't show it, but the Laser Iridium (OEM) has longer ground strap, along with the longer reach, so it would be better for part-throttle efficiency.
This matches at least two forum posts I read saying that the car performed better with Iridium IX plugs.

Also, everything I read says that shorter plugs are "safe" in the sense they cannot cause physical harm to any components, as long as it's not SO short that the threads are exposed. But these plugs have the same thread depth, so that's not a concern. Beyond that, too short of a reach will lead to reduced power, because spark position is too far away from the combustion mixture.

Based on these findings, I am going to install the Iridium IX plugs when the second set arrives and I confirm that I have the genuine parts. In the worst case, I'll notice something not quite right and go back to Laser Iridium, but I'm actually expecting them to work just fine.

Update add-on: NGK specs list the same "Reach" of 26.5mm (1.04") for both, but the Laser Iridium states, "Extended Projection," vs. Iridium IX's "Projected." So that's another indicator telling me that they know what they're doing. Note the Ruthenium HX version (92274 / LKAR7AHX-S) appears to match Iridium IX's design (Projected and not Extended Projection).

 

Thanks for the informative reply.
It would be hard to determine at this point without actually putting it on a dyno.
I saw a vid that was for a Suzuki swift, and basically they changed the plug to one that had shorter protrusion(racing type) and they lost 3-4whp on the dyno.
They initially though it was due to lost compression, but the comments say that it was more due to the change in ignition position.
The difference between the OEM and IX is not so extreme as this example, but it is interesting info.

Let's see how your replacement IX will look.
Thanks!

 

very interesting , i replaced the OEMs with the IX plugs . still have the originals , yea they have the longer reach , never noticed the difference .
the IX plugs were the fix for the part throttle issues i was having . the car being very sensitive to having work the gas pedal just right starting off
in 1st gear or i could get this annoying stumble starting out . would have to throw it in 2nd to get out of it . been driving a stick well over 40 years ,
never had this kind of problem with a clutch . the IXs largely eliminated this , noticed right away how much stronger the car felt coming off
the clutch . . not sure what they mean by " part throttle efficiency " . haven't noticed any drop in performance or mileage

 

Sorry for the delay. I was waiting for the weekend to work on it.
I now have a set of new Laser Iridium plugs and two sets of Iridium IX plugs. I'm absolutely certain that they're all legit NGK.
I just replaced the plugs in the car, but I won't try to drive it until tomorrow. No point in starting it up to go nowhere. Sorry for the cliffhanger.
I used 13 ft-lb for the plugs and 70 inch-lb for the coil packs.
I never found the right torque for the coil packs, but every service manual I have has different numbers LOL
Nissan Frontier: 62 in-lb.
Toyota FR-S: 75 in-lb.
Mazda Protege: 69.5-95.4 in-lb.
Subaru Impreza: 106.8 in-lb.
I guess it's not that important.

Same reach (thread depths) but different projection (Laser Iridium has Extended Projection).

 

I drove around a bit just now with the plugs swapped to Iridium IX and it drove just fine.
I might even say it felt better, but placebo effect and all that.
As for the placebo effects, it felt a little better at very low RPM (more torque).
That weird torque dip or stumble at ~2300RPM was still there, until engine fully warmed up. Maybe it was a little less. Maybe it was the same.

Edit: I just re-read Bill Bosco's post above. Maybe this isn't all placebo. It did seem easier to get it moving from stop. That's what I meant by "more torque" at very low RPM. And yes, my Fit is also a manual.

I found a nice photo of the plug from under the head on this forum:
https://www.fitfreak.net/forums/raci...-teardown.html

Just search for:"You can see the spark plug right in the middle of the combustion chamber."

You can see the Laser Iridium's extra length past the threads poking out.
Iridium IX would poke out much less since there's maybe 1.5mm less past the end of the threads.
Seems fine to me.

 

Thanks for the reply!
It seems that the difference is negligible to our butt dyno. I switched to new laser iridiums instead, and it feels like I have a little better response on lower end. I also noticed that dip from 2-3k is still there too. I see it pulling timing when it does that, but not there once the engine is fully warmed up. Perhaps new plugs just refreshes what the engine lost from peak performance?
The difference is so small that the ecu can adapt...