A look inside the VTCS intake.
 

I'm going to upgrade injectors and pulled the manifold off my '04 MSM motor. A few general observations;

- Wow, is it filthy in there. Between the EGR and PCV its ALL kinds of grubby in there. The filth extends up the port in the head right up to where the injector fuel spray washes it clean.

- There is a really large resonance chamber built into the lower half of the manifold. Unlike the VICS system, there are no butterflies between the chamber and the intake tract, just a passage that connects the chamber to the plenum all the time.

- The VTCS butterflies seem like an obvious impediment to airflow even when open. My understanding is that VTCS serves only to tumble the intake charge under certain circumstances and is purely a North American market-only emissions fix. My natural inclination is to remove them.

HOWEVER... Things aren't always as they seem, so before I hack it out I have a few questions.

First, is it possible that when open the butterflies may have a beneficial "airfoil" effect? (Seems unlikely, just trying to think it all through..)

If removing the VTCS is the way to go, how "removed" should one go? Just take the flaps off and leave the shaft? Pull the shaft, plug the one "outside" hole and leave the hole between ports? Plug em all?

Welding inside the bores looks like a huge PITA. Any thoughts on JB Welding them up instead?

Pictures rule. Here are a few.

https://www.miataturbo.net/attachmen...ine=1360084678
https://www.miataturbo.net/attachmen...ine=1360084678
https://www.miataturbo.net/attachmen...ine=1360084678

Loving your detailed pictures (though all this observation has been done before extensively). Still - keep it up.

From my understanding (and numerous threads that I read) the vtcs butterflies do their thing only when the car is cold. When car is warmed up they are 100% open 100% of the time. They are for emissions and warmup idle only.

I really don't know why they did this to the 01+ specifically, since none of the previous years had this and don't really have warm up idle issues. I'm guessing vvt has something to do with it, but that's just a guess.

Most people remove everything: flaps, shaft, etc. and either epoxy the holes or weld em up.

My understanding is that by closing the butterflies at idle (whether warm or cold), the resultant turbulance is supposed to create some beneficial effect insofar as idle emissions are concerned. This is mostly anecdotal.

I've never seen a before-and-after dyno comparison, however the "common wisdom" seems to be that the entire mechanism (including shaft) can be completely removed, and the resultant holes patched with JB Weld. I believe I heard this from Keith Tanner, though I can't find a direct attribution to cite.

the butterflies increase port velocity to help mix the charge when the engine is not up to temp and atmoziation is poor.

In my experience, the butterflies reduced the amount of warmup fuel I needed when the car was cold. You can re-test this with an open or closed loop engine management system by just opening and closing them during cold startup.

they are a huge impediment to airflow in a turbo application. even when cold, the increased airflow from the turbo backed up behind the plates and choked the car above about 2000 rpm.

plug all the shaft holes with JB weld or high temp epoxy.

I believe Keith Tanner/FM did before and after dyno tests and found almost no gain.

Key word bolded for free mod.

Gracias. Actually I'm looking around more and he does recommend pulling them for "...performance engines"

Just remember the stock computer will not run well at all while the engine is under 160 degrees and may throw a check engine light. Important if the car is to ever need to pass OBD2 inspection again.

Quicksteel or epoxy to close the holes? I'm about to put this intake on my daily.

Also I observe FM's dyno plots with a GIANT grain of salt. Maybe its due to the high elevation, maybe something else, but they're almost always completely different from anything anyone else ever posts.

I just plugged the outer most shaft hole. Saw no reason to fiddle with every single hole. No reason for it. See posts dated Nov 23rd 11:02 Pm and 11:14 pm. http://grassrootsmotorsports.com/for...p/55057/page1/

FM dyno plots are problematic because of the elevation.

When you increase elevation, you can add in a "correction" factor. This usually works very well for its intended purpose.

But you see, the "correction factor" is "intended" for naturally aspirated engines.

Now consider that the first significant use of turbochargers was to "fix" the problem with inadequate air density at high altitudes (WWII aircraft) thus improving high-altitude performance and increasing the operating ceiling.

So what happens when we apply standard altitude correction to the dyno plot of a turbocharged engine?

Well, first consider N/A engines. Intended for normal corrections, if the air density is only 85% of sea level density, then engine output is only 85% of sea level output. This is generally accepted as true across the entire RPM range, and the model works very well.

Now on to turbochargers...
First off, there is a distinction that MUST be made in boost control: There is a significant difference between a boost controller which operates off of "relative" control as most of our MBCs operate and "absolute" control which would normally be an EBC operating off of the MAP sensor. Such a difference will produce substantially different results on otherwise identical cars on an identical dyno at high altitudes. Without knowing that this difference exists, you would not understand why a dyno might produce results that seemed "unrepeatable" or "unreliable" between different control systems.

So what happens when you "correct" a turbochargers dyno plot using a standard correction factor?

Well, if atmospheric pressure is only 85% at your altitude, you would correct your dyno to the tune of about 17.6% to equate to sea level (because 100 is 117.6% of 85). At sea level, a N/A car sees 1 bar of pressure, and at altitude, intake pressure drops by .15 bar to .85 bar. .85 bar is 85% of 1 bar. The same relatively-controlled turbo car at sea level might see 3 bar of pressure, and pressure would drop the same .15 bar at altitude, so that car would see 2.85 bar of pressure at altitude. This is 95% of the same car at sea level. If we apply a 17.6% correction to 95%, we get a reading which shows 111.7% of actual power, or a power rating which is falsely *inflated* by 11.7%.

Now using the last car, an absolute referenced turbo car, at sea level it sees 3 bar of pressure. At altitude, because boost control is referenced absolutely, it still sees 3 bar of pressure, which is 100% of what the same car sees at sea level. If we apply the 17.6% correction to this car, it inflates our sea-level power numbers by the full 17.6% Thats 5.9 percentage points higher than the exact same car with relatively referenced boost control. Contrary to initial perception, this 5.9 percentage point difference is increased at lower boost levels, all the way to a whopping 15 percentage point difference if your boost control is set to 1 bar. (I know, who the hell would boost their vehicle to 1 bar?)

So why apply correction to turbocharged vehicles?

The most significant factor in high-altitude turbocharged performance is turbo-lag. For those sea-level drivers who say "turbo lag" doesn't exist, drive your car to 13,000 feet and you'll notice that it takes noticeably longer to spool that snail when you drop it into third at 60mph and step on the throttle. You'll also notice that boost threshold is moved to higher RPMs. What's the easiest way to show potential customers the responsiveness of a turbo at sea level when you're at 4,700 feet elevation? Apply an industry standard dyno correction. This also has the side effect of showing higher power levels at full-spool, but hey, that's not bad for business.

How to fix the problem?

Well, within reason, you can't.

One option is to build a non-standard correction curve which corrects down to zero once the turbo is at full spool while maintaining the appearance of proper spool-up, but this would be completely different for every single setup tested - and the only way to figure out what that correction curve looks like would be to dyno the car at sea level, then dyno the car at altitude, and then apply a non-standard correction curve to make the altitude dyno look like the sea-level dyno...but if you're going to get a sea-level dyno in the first place, why on earth would you do the extra work of getting an altitude dyno and building a non-standard correction curve? You cannot use this correction curve on any car and apply it to any other different car.

The second option would be to build a pressurized dyno booth with high-volume air pumps, and regulate the booth to 1-bar of pressure. You would need to connect the car exhaust to an outdoor vent, but the exhaust would need to be sealed to the vent, and the vent would also need to be regulated at 1-bar so there's not a relative vacuum on the exhaust producing higher than expected results. This would work perfectly, but remember, the key phrase here is "within reason" - it would be extremely cost prohibitive to build such a booth simply for better looking graphs.

Fooger is correct, from my experience. I live at 7k+ feet, and recently had my car dyno'd at a reputable shot in Albuquerque (5200 ft elevation or so?). The corrected results were 240whp, the uncorrected results.. 204whp. Virtual dyno results? 220-230whp. lol.