Leaning the carbs for altitude - on the cheap


5 posts in this topic

Posted

Someone mentioned the HACman system for leaning the carbs for altitude.  It seemed like a cool idea.  Digging deeper I learn that there once was an automatic leaning setup, but it is no longer offered.

Digging even deeper, the explanations seem to be a little obscure, perhaps so we'll want to buy something we could easily cobble together on our own.  I hate to ruin the Hacman's day, but c'mon!  My understanding is that one takes the low pressure from the (initially plugged) tap into the Bing carb's throat, mixes it with another pressure source for some reason taken behind the air cleaner screen, and uses this "mixed" pressure to influence the flow through the jets of the carb via the float bowl.  Basically, you're messing with the float-bowl vent pressure.  Lower float-bowl pressure means less fuel flow through the jets.   Elegant and brilliant, no?  Well it seems to me that the float-bowl by default vents to the local pressure inside the cowling, whatever that is, so any pressure drop thru the air filter element is irrelevant.  Plus, if we only want a leaner mix, couldn't we just admit a teensy amount of air into the carburetor throat?  Take out the throat plugs, install hose barbed fittings, connect the two tygon tubes into a Y-fitting, then route the one tube to your panel, and install a brass needle valve from the hardware store, the other side exposed to the cockpit pressure.  Closed would be full rich, and then at altitude open the valve to admit some air directly into the carb throats to lean the mix to get the correct EGT.  Here, you're not even messing with float-bowl pressure.

Now I've done no calculations, so it's possible that you wouldn't be able to lean the mix enough with this simple scheme.  In that case, we just use two needle valves, one of which connects to the carb throats as before,  and a second, which connects to engine compartment pressure.  We connect the two pressure sources together with another Y-fitting, the third leg of which then goes back to connect to the float-bowl vents (another Y required).  Now we're doing the HACman thing on the cheap.  We set the needle valve connected to engine compartment pressure to some nominal opening.  We will immobilize it after initial tweaking, maybe mounting it behind the panel where we don't normally even think about it.  We leave the valve connected to the carb throats mounted on the panel.  The panel-mounted needle valve is closed for full rich, and again is opened a bit if we want leaning, making the pressure applied to the float bowls something between engine compartment pressure and carb-throat pressure, depending on by how much we open the needle valve.  We would only have to play with the cabin-pressure needle valve in the beginning, in order to set an appropriate sensitivity for the panel-mounted valve.  Clearly it's not set right if full leaning at 10kft requires only 1/8 of a turn, or 5 turns.  In this case we reach behind the panel and adjust the other valve.  Again, EGT is our guide.

Comments?

Share this post


Link to post
Share on other sites

Posted (edited)

Ski doo snowmobiles used their HAC and DPM system on sleds for about 10 years.

I had a bunch of them and even added hac to a sled that didn't have it (just looked up the hac parts on the microfiche).

Float bowl pressure regulation...  worked really well but I did have some failures over the years.  Default fail is rich.. which is good, but the sled would only pump about 40% power when it failed.  Reasonable failure mode in a sled but not so much in a plane!

Man, I dont know... an attempt to re-create a alt compensation unit in a 2 stroke would require tons of testing, fiddling, reworking, and possibly engine rebuilding if you mess it up and overtemp in the R&D process.

Think I'd rather pick up a 600 etec and try to build a psru adapter plate for it! (Direct injection, "FADEC", and 120 HP)

Edited by Yamma-Fox

Share this post


Link to post
Share on other sites

Posted

Fails?  This would be a fully manual system.  Clearly one would be monitoring egt, rpm, and coolant temp, as in normal operation.  Shutting the needle valve brings you back to the basic configuration.  At least with leaning we have some control over egt.  Perhaps I am missing your point here?

Share this post


Link to post
Share on other sites

Posted

The Artic Sparrow mixture control works great. One turn of the control knob is equal to moving the clip one notch. 

Share this post


Link to post
Share on other sites

Posted (edited)

I have a HACMAN on my 582, and it works well. It has enough authority to lean at least 1.5 GPH at altitude (from almost 6 GPM to about 4 or 4.5), and would raise the EGT from 800 to about 950 or 1000 degrees. Using the HACMAN always seems to prevent any rich plugs. I flew across the US from Stockton CA to New Haven CT, across Wyoming, so I got to see all types of altitudes from SL to about 11K', and I could always lean to keep the plugs clean.

The HACMAN principle is simple - it has a screw valve in the cockpit that drops the carb bowl pressure by mixing some of the very low pressure area at the carb throat (the air at the throat has low pressure because the air in the throat is flowing so fast.)  The low pressure from the carb throat reduces the bowl pressure so less gas is sucked up into the throat, the HACMAN lets you  lower the pressure differential between the bowl and the throat.   It basically reduces the suction so the needle valve lets less gas into the carb, thus leaning it out. (This really just restating what Turbo wrote above, his sense of the physics is spot on).

Frankly, the HACMAN is just an air needle valve and some cheap hoses, my guess is we could make one fairly easily, but since I was flying over the Rockies and didn't want to walk home, I paid the money.

The altitude compensating HAC that Rotax sold at one time appears just like the HACMAN except it used a simple diaphragm to adjust the mixing of the two air sources. 

Edited by nlappos
Fixed some confusing language

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now