Good day

A new member .

After having pent some time looking into fuel vapor systems and water foggers ,  I decided to venture into trying out HHO .

I am running my maiden trips on my spiral unit - low LPM just to test on standard ECU without any modes.Subaru Legacy 2.5 NA (non-turbo)

I have a few questions which I am hoping some more experienced members can offer some degree of clarity .

1) Entry point to use for HHO into engine.

a- Air intake

b- T-into PCV input to allow Mani vacuum to pull HHO gas into engine inlet branch .

Note in my case the MAP sensor is between the throttle body and Pcv input .

2) With pcv inlet pipes from either side of the engine drawing air into crank case from the Air box ,  - would this not pull HHO into the crank , through the engine back up to PCV input .

3) With higher LPM outputs what happens to long downhills where the foot is not on the pedal - would this cause any HHO build-up .

4) Can a stand-alone HHO system improve the milage without and ECU tampering ?

I am assuming if one looks at LTFT's one can gauge in percentage the fuel saving ?

Thanks in advance.


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Many of the answers to your question are located here.

Hey Bradley, So a good Omen(rule of thumb) is This is Rocket Science once you have this down then you understand this presuming your willing to learn the in's-out /Do`s& Don`ts as said by Gabet!

What specs of said Subaru Legacy 2.5 Engine as is??  

1. Safest place to put HHO gas unit is on the PCV outlet that connects back to the Intake manifold w/T-connector!   SAFETY IS OUR #1 PRIORITY!

2.!WRONG! The Pcv is a one way valve that dumps vapor/Oil droplets  back into the intake manifold NO HHO is going into the valve cover !

3. Baseline test 1/2 Lpm HHO idle & 1Lpm @full throttle.

Once your engine is idling(ignition ON) HHO will be burning, however NOT a huge buildup off HHO within the HHO system.

4.(a) YES however needs further research cause NOT everyone get that same perfect result but Dont go tampering with stock manufacturer ECU if you knows nothing about it!

b.What does LTFT means?  

also a new member - 

with regards to ECU hacking, you have to at least modify the signal of the MAP so the engine can account for timing, otherwise you'll get rough running and misfires.  Further, you'll also have to account for the plethora of oxygen in the exhaust system the computer is programmed to combat with increased fuel delivery - EFIE will let you fool the machine.

to the best of my understanding, you want to have the HHO going straight into the manifold to displace intake air, so the vacuum source on the manifold is the input, but that displaces the hose to the PCV itself; the outlet of the PCV valve could be routed into the intake airstream upstream of the throttle plate (but downstream of the MAF) to remain emissions compliant (legally).  What I've done on my v6 is to T the breather line from one valve cover together with the PCV hose from the opposite valve cover, and bring that back to the intake tube, just ahead of the throttle body, at the breather's source. (I've also increased the diameter of those hoses so that there's a greater chance of the manifold vacuum pulling whatever is in there that needs getting rid of back into the engine - the moisture in that blow-by probably helps power/economy/temp management)

for question #3, with the throttle plate "closed" on a long downhill, the engine (theoretically) shuts off fuel delivery; with the engine turning and no intake air getting in, and an HHO cell attached to the manifold, I'd bet that the engine was running on HHO alone, or very close to it.

Thank you for the replies so far.

The LTFT I referred to was Long Term Fuel Trim , to elaborate on the original quest .My understanding is If the Long Term Fuel Trim is at zero and the Short Term Fuel Trims are around ZERO then the ECU is supposed to be delivering thge Stoiciometric

ratio of 14.7:1 .This is only my understanding of the closed loop system , if  one sees a positive LTFT then one can assume more fuel is being delivered , inversely if the LTFT goes negative then one can assume less fuel is being delivered by a constant load or rpm if stationary .

I was wanting to get a precise flow meter in order to test such a case by adding the oxyhydro slowly and recording the changes in the long term fuel trim.

Again I am no expert in this field but am interested in the technical understanding and hopefully I see a small difference by tomorrow afternoon .I do not want to count my eggs before they hatch but my fuel guage is looking good this week after 4 days of 100 km per day traveling to and from work .

One issue with my vehicle is it s model was a year or two before the OBD-2 standard was adopted 100 % so this Subaru I am unable to use a standard ELM327 or similar .I can read O2 sensors and #1 pulse injector time but no long term reading- bit frustrating .



An OBD-1 computer is good news - while you don’t get metrics that are as detailed, you also don’t have to fight with it to achieve mileage/efficiency gains. A simpler system that’s easier to work with rather than around will make your work much more of a joy.

Fuel trims - based on oxygen sensor feedback - Are a comparison between the base/factory fuel delivery chart the computer uses for injector pulse duration (for given running conditions) and what’s actually happening as the engine is running. Coolant temperature determines open/closed loop parts of the chart, and until you hit optimum coolant temperature, the oxygen sensor feedback isn’t taken into account (closed loop). The LT doesn’t move until the ST has reached the extreme of its range either way, +/-, and then it bumps the LT by an amount (until the ST comes back into range), which it then writes to the fuel map. And the cycle continues as long as the engine is running and as long as the sensors are functioning correctly. Check engine lights come on when sensor inputs to the computer stop making sense or are outside the range it’s programmed to expect. (That’s where EFIEs and MAP/MAF “enhancement” circuitry come into play). You can force the computer back to defaults by disconnecting it from the battery; often, to drain residual power in the system from circuit capacitance, you should jumper the +ve and -ve battery leads together rather than simply removing the +ve lead from its battery terminal. (You’ll have to reset the radio/clock, but the computer is doing the heavy work). Some manufacturers go so far as to specify a drive cycle to get the computer to “relearn” when you disconnect the battery (so look it up; I’ve found tuner/enthusiast forums are a big help there), but they all include what I call a “drive it like you’ve stolen it” part where you have to be very heavy on the accelerator/throttle/gas pedal to show the computer the high extreme for it to compare regular operation to (but don’t do it until the engine is fully warm and in closed loop fuel delivery).

Computers only do what they’re told, so if they’re “not working correctly,” they are either getting incorrect information from a broken sensor, or the information from a sensor needs adjusting on the way to the computer.

OK Glen, basicly

How much Lpm off HHO are your hhocell  putting out? presently (Litersperminute) 

What is your current/Amp draw + Wattage used per day?? What Arated  fuse are you using!

Feedback- my consumption was really looking at a posative outcome and filled tamnk today and was a little disappointed:

My last tamk wwas 8,33 l/100 km

Todat after HHO it dropped ever so slightly top 8,1L /100km.

In my assumption this is negligible and could be a result of driver habbits and other influences.

One thing I did not do was to reset ECU after starting with the HHO.

I was a little disappointed at the unit is displacing about 500mL per minute - approx 5-6 Amps .

My exhaust is definitely burning cleaner now with no smell of fuel whatsoever.

After the calculations I was very disappointed and thought of going back to the Fuel vapor system , since then I have decided not to abandon the HHO yet .

Greg my Subaru is the very first  /early OBD-2 just before the standard was strictly agreed upon so with my CRP123 I can read most of the settings but not able to sacn the vehicles diagnostics for proper interrogation of whats happening .  other ELM327's etc I have had no luck .Lauch confirmed this from their email reply .

OK where to next :

1) My little understanding of the MAP sensor and O2 sensor hardware adjustments - what would be the way forward ?

2) What would happen if I remove the Map and O2 sensors altogether and run on defaults with the HHO?



MAP sensor adjustment depends upon whether it’s analog or digital. If analog, a simple potentiometer will suffice to make an adjustment, but if it’s digital, things get a bit more electronically intricate. For O2 sensors, EFIEs are placed on each upstream (prior to the catalytic converter) sensor, and in some cases on the downstream one as well.
I’m away from my desktop computer at the moment, but when I get back to it, I’ll post some links for your consideration and further research - each car is different, and Subaru engineers have methods/perspectives that differ from the larger carmakers, so you’ll have to dig in there to try to determine how they implement OBD2 into their design.

ok - MAP enhancer:  I'd suggest signing up at so you have access to the resources there.  Once you have, search that site for "MAP enhancer" and it gives a good run-down on both analog and digital methods.  If analog (which I suspect, given what you know about the ECU on your car already), consider yourself lucky.  and DO take the advice about the 10-turn potentiometer...much easier to dial in than the cheaper 270 degree versions.  I even put a nice rooster head knob on mine.

If you're not an electronics type (like I was), this link was a big help:

And then I found a part-specific article for my vehicle which permitted me to pull it all together:

being from the same manufacturer, and drawing from the same engineering team, while the wire colours weren't the same, I was able to easily make the mental leap and gained a bunch of faith...and a fuel consumption improvement.  

I had performed a series of other modifications to my car prior to reaching this point, and I encourage you to explore your options beyond EFIE/MAP/HHO.  direct message me for specifics, but one thing I will encourage you to take a long hard look at is your Subie's grounding and Ignition.  we're all about BURNING less fuel, and those are 2 important components in that quest.

now, for the EFIE - eagle research has a version that seems to be quite well regarded, and it's fairly inexpensive  the manual is invaluable, and his youtube channel has a great video of adjusting it once it's installed.  He uses 2 voltmeters to demonstrate the voltage offset, and the sensor operation to the computer..  extend that to the MAP circuit, and then you're tweaking both ends of the equation, in and out.  he has testimonials of as much as 50% reduction in gasoline usage as best I can recall WITHOUT HHO.  (and then there's his electrolyzer with PWM...but you already have one of those)

rather than splice into factory wiring harnesses, I've bought extenders for my MAP and o2 sensor to splice into...if you're similarly inclined, there are vendors out there...some connectors can be tricky to find/make if you don't have the correct tools.

Hope this helps!!

Hi Greg- Thank you for the graet information - How can I get in touch via email - thanks Bradley

Just click message below his name.


Yesterday I decided to build a vaporisor and used the principle of the PCV as the source of vacuum and made aan H type bypass so as not to restrict the PCV vacuum pressure.Looking at the Launch data which is a little limited on my ECU I decided to scrap that and return the PCV to standard.

After a while I returned late last night and decided to run a tyre pump as the source with additional by-pass T- and valve to adjust pumped air into bubbler , from there  have the bubbler going into airbox via flash back .I straight away noticed my AF reading going negative up to -25 % on idle  -good so far .My interpretation was the sensors were noticing additional fuel and pulling back on the injector delivery  supply .Obviously I had to remove the spiral HHO unit to use space for pump .

I gave a colleague a lift to work today and did not want to let on my experiment so did not run the pump to work but on the way home 50 km I did so .Tomorrow I shall fill up and start my test per tank of fuel and see how it goes.

I am still very keen to re-address the HHO even if it serves the purpose of just is cleaning  the exhaust emissions , a combo pack would be good.

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