HAC System Effectiveness (1 Viewer)

[Gretsch]

Hello all. Recently back from a 1800 mile trip in the cruiser to the mountains. She did pretty good overall, but was a dog at higher elevations as one would expect. Seems timing advance is cure for high altitude situations. I did not have timing lights and such with me to adjust all that so rolled as it was. My truck has the original 2F and is de-smogged. They got rid of the HAC valve in the process of desmogging it. Considering replacing the HAC system to allow for use at higher altitude. Before I get all into that, trying to get a bead on how effective the factory HAC system was at handing high altitude situations. Did it work well enough to warrant me replacing it to handle times when I take the truck above 4000 feet elevation? Or did it provide a marginal improvement at best at higher altitudes. I would prefer something more automatic for counteracting the effects of operating these beasts at elevation to manual options. HAC was my first choice but want to make sure it really works before I dive into that setup. Thanks in advance.

 

[OSS]

Without the HAC system, the 2F "barely runs" above 4000ft. Ok that's a bit of an exaggeration, but it definitely runs MUCH better with a functioning HAC system. Especially at slow speeds. I once disconnected it at around 4500 ft and man the engine was not happy with that and ran like a tired dog (but ran). It definitely felt like something was wrong with the engine when the HAC was disconnected at altitude. At least that's my recollection of it.

 

[Gretsch]

OK thanks here @OSS. That was pretty much my experience. It 'barely ran'. Idle was way down, no power, going up a hill was ridiculous (had a nice car train behind me on several of the mountain passes. Lots of folks giving me the cruiser salute as they angrily got around me). I was up around 9200 and it was really rough. I had to really get the engine up to around 2500 RPM to get it to do much of anything.

So I am then assuming the factory HAC system handles the altitude well enough as to make it worth gathering the parts to put it back on this engine? Sound right?

Edit: The factory HAC is NLA from Toyota as I understand it. Sound correct?

 

[BlackCat]

Since you’re already desmogged, I’d just carry a 12mm wrench and adjust timing by the seat of the pants feel at altitude, vs plumbing all that back in.

 

[Gretsch]

Yeah its a sound option. Just for whatever reason it would be worth it to me to try it with the HAC. I may change my tune when I get into it a bit more. Might realize its gonna be a big PITA to get back. Seems like only 4 ports though so we shall see.

 

[GeologistFelix]

Ok, so, a little train of thought I just went through:

First, some facts:

1. HAC both advances timing and leans out the air/fuel mixture by letting extra air past the throttle through the port on the carb.
2. Richer fuel mix reduces combustion temperature, which in turn reduces pre-detonation.
3. If you have a Trollhole/CityRacer/non-smog Aisin carb, you don't have the port to let the extra air in.
4. If you have a stock distributor, you have primary and secondary vac advance diaphragms, and if you did the "$0 recurve option" of plugging the secondary and retarding the timing until it doesn't ping with the primary hooked up, you have an unused, built-in mechanism to add advance.

Which leads to me thinking the following:

1. Your mix is just going to be a little rich at altitude no matter what, unless you swap jets and retune every time you drive into the mountains. On the bright side, that means you can advance the timing a little bit more.
2. You can absolutely advance the timing by hand, but you could also just wire a switch in the cab to a vac switch (ideally, one you tore out for your desmog but haven't thrown away/sold yet), and put the vac switch in between the intake and the secondary diaphragm on the dissy. Go up in altitude? Flip the switch on. Go back down/it starts to give you pre-detonation? Flip it back off. Probably plenty to bump your idle back to normal and get a little of your lost power back.

Now, some conjecture:
1. The extra air that the stock HAC lets past the throttle is probably at least somewhat significant at idle, but I doubt it's more than like 1% extra air at WOT, since it's just a single ~3mm vac port versus the entire intake throat of the carb.
2. If anyone with a desmog is worried that running rich might ruin your cat...I think that horse has sailed, so to speak. Don't worry about it.

Lastly, some real-world experiences driving with a desmog, trollhole, and no HAC:
1. Idle will drop to like 400 from 700, but a hand throttle solves that.
2. Power will drop off, but not to the point that a 240k 2F/H42/stock 3.70 gears won't still push 33s, four adults, way too much cargo, and an extremely unaerodynamic heap of gear on the roof rack at 75-80 mph (per GPS) on flat sections of highway at 7500 feet. Hit even a small slope and that'll fall back to 65-70.
3. Offroad, it's all about 4-lo *anyway* with the 33s and 3.70s, and I think the altitude is kind of a red herring because the steep and gnarly stuff around me is also up in the mountains and >4000 feet. There's definitely some power loss, but I think the torque at the low end of the RPMs is still pretty similar to what you get at sea level.

 

[Gretsch]

The desmog was done by the PO, so I don't actually have any of the parts for this in my possession. Would have to source the HAC itself to put it back (among other things). The carb is stock aisin. The emissions sticker shows information on the HAC, so I assume this truck had one at one time and also assume the carb has the ports required to put things back. Again need to look into that some more.

I couldn't get anywhere near 80MPH at any elevation in my truck as it sits (stock gears/trans/33's). She will do maybe 75 now but that's about it without getting into the 3200 RPM range. She did not do too bad at elevation without any changes but it definitely was noticeably lacking most everywhere I went. I did not wheel it anywhere, but it didn't seem like with the power she had she could have gotten up any of the hills near where I was. It was enough to have me drop out of any wheeling opportunities. I am somewhat a purist (although you couldn't tell from OME lift and the 33's on her now), and like to try and use OEM gear whenever I can. A factory HAC would be perfect, but only if it actually helps improve performance at altitude.

I am actually more trying to figure out if the factory HAC system was any good at all at actually compensating for the effects of high altitude on these 2F motors before I go through the trouble of putting it back. So, for those of you with the HAC intact. Does it actually work well enough for it to be worth putting back? Or is does it only work marginally and doing something else would work better. Thanks for the replies.

 

[GeologistFelix]

3200 RPMs with 33s and stock transmission/t-case/axle gears should be ~85 mph actual speed, and ~75 on the speedo. 33s are about 15% larger in diameter than the stock 235s, so your speedo is consistently showing a speed 15% slower than you're actually going. (Apologies if you've already corrected for that, though!)

As for if the stock HAC setup is effective, I'm willing to say it definitely is. If you can find the parts, I think it would be the best way to solve the problem given your (reasonable) priorities re: keeping it as close to stock as you can.

 

[4Cruisers]

My stock '84 FJ60 with HAC does relatively well at altitude when properly tuned even with 33X9.50s, although the change from stock size tires dogged it down a bit. It's spent a lot of time high in the New Mexico and Colorado Rockies. Still a really slow slog on long grades like that north of Durango on US 550. It's spent most of its life at 7,240 feet.

 

[mattressking]

Desmogged, 35s, 3.73s and h42. Just took it to 13k ft and hovered around 7-9k for a week. Little slower than usual and raised mixture a little, chugged along very well. Timing set at 10*BTDC at sea level. Give the truck a proper tuneup and verify everything works, having a working HAC only helps.

 

[Gretsch]

Still a really slow slog on long grades like that north of Durango on US 550. It's spent most of its life at 7,240 feet.

That's one of the grades I encountered on this trip. Again, did OK, but seems like HAC would give it some additional help. I'll prolly take a look at what's needed to add HAC back in here and see how far I can go with it. Thanks for the replies everyone.

 

[mattressking]

That's one of the grades I encountered on this trip. Again, did OK, but seems like HAC would give it some additional help. I'll prolly take a look at what's needed to add HAC back in here and see how far I can go with it. Thanks for the replies everyone.

Functioning vac advance, HAC unit and USA carb.

 

[Cincodemustache]

That's one of the grades I encountered on this trip. Again, did OK, but seems like HAC would give it some additional help. I'll prolly take a look at what's needed to add HAC back in here and see how far I can go with it. Thanks for the replies everyone.

We still need to meet up local, but if you do move forward with the HAC my truck has everything still in place for smog and might be helpful to look and compare.

 

[Gretsch]

We still need to meet up local, but if you do move forward with the HAC my truck has everything still in place for smog and might be helpful to look and compare.

Yeah that would help a lot. Thanks.