ITB Airflow Shenanigans
 

Ok, this'll be a long write-up I think. Lots to take in, bare with me.

So I'll start with the basics:
1999
Exintake cam swap, stock exhaust cam
OEM japan header
Stock Midpipe
Borla Muffler

The intake setup is what I'm going to talk about as it has changed a lot recently, but wanted to give a background first. I'm going to give some Virtual Dyno numbers too, and can provide pics later.

I'll start with when I had the stock intake manifold with a cold air intake (along with the mods above). Car made about 118whp/100wtq.

Then I put some 42mm ITBs on. However they had a Yamaha R6 flange on them, so the port area was 25% smaller than the Miata port (airflow pathway was 42mm throttle body bore > Yamaha R6 port > Miata port). Car made more torque down low but choked out up top around 5500 RPM. Made about 116whp/105wtq. After this setup I was led to believe the car just needed that R6 port to be eliminated and take that restriction out.

Then I upgraded to 45mm ITBs with no neck down (it was just 45mm bore > Miata port). Car made 118whp/97wtq.

So to gather all of this and analyze. First I am kind of surprised that the ITBs haven't really done anything (I was expecting at least 130whp). My manifold pressure has stayed the same throughout every pull on every intake setup. At 2500 flooring it I hit 98kPa, and at keeping it floored to 7000 I go down to 95kPa. That being said, could my headers/midpipe be bottlenecking me this whole time (maybe a plugged up cat too)? Every ITB setup I've seen has a nice exhaust to compliment, but I don't. I do plan on getting racing beat headers and a midpipe in a few weeks. Another possible lead into this is my back pressure. I haven't measured it but if I stand behind my tailpipe and feel the flow at idle, it feels insanely stronger (velocity) than both my friends' Miatas (NBs too).

I started to think about exhaust with the R6 ITB setup but thought the obvious smaller port size was the culprit (which did have an effect, but not as much as I thought obviously). I'm also thinking about the more specific things like port velocity, but my numbers seem so low to even start considering this, and given the setups I've seen I see no problem with my 45mm setup. Maybe a better set of cams?

More than happy to show you dyno files from VD if you want to see as well as a log. Thanks, sorry for the essay.

The intake you started with was NOT a significant restriction at the power level you are at, something else is.
Until you test other parameters continuing to make changes to the intake will provide further frustration.
What is your exhaust back pressure at the header collector?
ITBs are best suited to a serious combination of compression and large duration cam profiles.
They are commonly the "last" thing you do to a combination. Often they are employed to separate intake pulses to improve engine response in the lower end of the engines operating range.
If you calculate the airflow potential of most ITB set ups it is substantially higher (2-3 times) than the engines they sit on.
They separate big cam overlap (and backflow into intake) to each individual cylinder. ITBs allow 25-30 degrees more lobe overlap than most multi-port intakes.
Large cams require more compression to operate properly.
You have a bad to the bone induction set up on a relatively stock engine. I would have expected total power to remain the same with a small reduction in low and mid-range torque.
Your intake looks way cool. Made me look into 3d printers. Thank you!
Build a bad-assed motor and you will be set.

Thanks for the response. I'm definitely on the path to do supporting mods like exhaust. I'm going to be checking back pressure there very soon to verify as it seems like the culprit. I saw the itbs as an opportunity to begin serious mods and the amount it cost me to make the setup was totally worth it ($300-400). Exhaust will come in a few weeks and I can update you guys when it happens. Stoked for the future! And yes 3D printing is something I hadn't considered ever. Found out about a material that could withstand the heat and it's been so great.

Ecu? Compression/leakdown numbers? Type of dyno used/correction factor/elevation? Same dyno for all runs? Same conditions?

Those numbers are about right for a 100% bone stock 99, maybe even a bit light.

As the other guy side, stock '99 intake manifold is not holding you back hardly at all, it's the second best OEM manifold, giving up just a couple of HP on top end to the squaretop.

Megasquirt, compresion when I bought the engine was 180-190 across all 4. I use virtual dyno so trying to see if it's also credible. I haven't taken it to a real dyno yet but plan to in a month or so. I never correct it with temp/pressure. The last 2 out of the three setups are nearly identical conditions, while the 1st is a little different, so a possible variance there. Totally agree though, seems a bit light for a stock engine with a little bit of mods. Maybe my dyno info is off. I'm going to be putting a nice flowing RB exhaust package on the car in a few weeks so I'm thinking that'll be the light.

A good thing to note is I did mess around with the tune a little more and got into the low 120s and 103ftlbs or so.

Without comparing your VD results with real dyno results they're no good in terms of overall comparison, only good for comparing before/after on your own mods.

The fact that you gained nothing when adding ITBs was covered pretty well by technicalninja above.

Agreed. Awesome thanks for the reassurance!

Wanted to share an interesting snippet of something that struck my mind yesterday while picking up some headers. In my city, when I would floor it, my max pressure would be 97-98 kPa as mentioned in my original thread. While on the freeway yesterday (about an hour away from my city), I had no problem hitting 99-100 kPa. Then it struck me, elevation also contributes, duh lol. The elevation of my city is 774' while the city an hour outside of my town was only 92'. Can't believe I glanced over that

Happy to say the issue is solved! It was a simple goof on my end, I had thought I had a 4.3 diff but I found out I had a 4.1. Car now makes 144whp and 120wtq!

Could you post the dyno chart? I would love to see after a few more mods.

So far my "dyno" results have been based on Virtual Dyno so I don't have a real number yet. I'd like to go to a shop soon for some pulls. The virtual dyno ones are a good estimate and can send those if you want to see them