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Discussion Starter · #1 ·
Ok I am likely overthinking this as usual. Feel free to say so. But Our new to us Onyx Xt i rated at 260Hp and I am sure that is at considerable boost pressure and related charge temps. I tow a 1600LB Scamp trailer and can maintain any speed (65) i feel safe. But In the event I travel to the higher mountains with long lengths at 6+ % grades I would typically reduce my speed to ease the load and heat generated. Is this counter productive on the OB or even the Ascent. The only cooling air passing thru the intercooler comes from forward motion and no fan to assist it. Plenty of cool air at 70 but what about at 45. Or extremes at 30 mph. Some of the grades are many miles long. Where does the slow speed become worse than higher speeds and more power needed. Is there a fan kit designed to assist the intercooler. Water spray nozzles seems to be excessive but Is IT?? I did a radiator water spray system on my 1987 26 foot motorhome to better handle the Colorado I-70 interstate in high summer temps. and the 117 deg in Utah and Nevada It ran from the 40 gal of Fresh water in the motorhome It did make a difference in extreme conditions. One could even power it from the boost pressure with a tiny1/8th 3/16th line plumbed into a small reservoir of distilled water??
 

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The STI uses a water spray on their intercooler to increase air density and power but it doesn't overall cool the engine. A water spray on the radiator like you did with your RV would be more effective at overall engine cooling.

But there is a new thing I just learned about the 2021+ Outback XT where it says to use 91 octane when towing to decrease the chance of engine overheating. That raises my eyebrows. It adds credibility to the idea that maybe the engine cooling system is marginal for towing.

In that case, if you know you'll be towing, I would use 0w-30 full synthetic oil and premium fuel. 5w-30 is fine if temperatures aren't freezing.

I think reduced speeds are fine and will probably keep the engine cooler than pushing high speed just to get air through the intercooler.
 

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Discussion Starter · #3 ·
Definitely only thinking of the intercooler air and cooling charge temps if anything.
 

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The boost pressure in the 2020 Outback XT is 14.5 psi, on top of a 10.6:1 compression ratio.


View attachment 519721
Very interesting, thanks for posting! I can see why they call for 87 in this thing, seeing this. I'm sure there's math that'll tell us what compression ends up being with the added 14.5 psi but I'm guessing it ends up around the same 12.0 as the 2.5l, or maybe a hair more. Back in my day of hot rodding engines by the time we got to 12.5:1 or so we were looking for at least 93 octane. This engine with the boost, full load, with added timing, extra heat, I can see why the manual calls for 91 octane. A little insurance against detonation.
 

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The secret to high compression high boost is the charge cooling effect of direct injection. My port injected WRX had something like 8.2:1 compression ratio and 14.2 pounds of boost.


At 10.6:1 with 14.5 pounds of boost it's the equivalent of 21.0:1 compression ratio. :eek: Now you know why I only run premium. This isn't surprising since air pressure is about 14.7 pounds and adding 14.5 pounds of boost means 29.2 pound air (not accounting for charge heat).

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Based on old school thinking it has to be a combination of:
  • The cooling effect of direct injection
  • Rich air/fuel mixture
  • Retard timing
  • Special charge stratification or swirl or squish or whatever.
edit: Also things like variable valve timing and EGR so that even though it says 10.5:1 compression ratio it varies depending on valve timing but that's more complicated than I can really wrap my head around.
 

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Discussion Starter · #10 ·
With all the controls and research to make such a great system it seems that the only real upgrade would be some additional cooling of the intercooler, Possibly Water spray or forced air movement at in high load condition. the area above the cooler is 20 inches side to side counting the flanges and 6 inches front to rear. is there a fan system to enhance air movement during extremes without inhibiting air movement normally. 2 or 3 6x6 inch muffin fans or serious cooler fans on a simple bracket from flange to flange. that still fits inside the rubber hood boot. I am trying to plan ahead for next years trip out west pulling a 16 foot casita trailer at around 2200lb.2400lb with accessories I already have a Scan Gauge 2 to start looking at these pressure and temps.
 

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Shouldn't you be more concerned with keeping the CVT cool? There are a lot of posts about transmission coolers in the towing forum.
 

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Discussion Starter · #12 ·
Yes definately. The OE CVT cooler will be replaced with a much larger similar design low restriction unit for safety and I am even considering one with a supplemental fan that can only help get air thru to the radiator and engine compartment. A little like the unit on the ascent unit??? along with higher octane fuel when towing.
 

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Before you implement your fan setup on your intercooler maybe you should try to set up a method to measure air flow into the intercooler at 45 mph. There's a chance that your muffin fans or whatever you put in there will impede rather than increase air flow unless they have serious CFM.
 

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I'm thinking that a single intercooler fan in the middle (if it's very high flow) can provide low speed benefits without blocking as much air at higher speeds - wish they tested a single center fan.
 

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Discussion Starter · #15 ·
Absolutely the fan structure could be more restriction than I gain under normal operation.. Maybe the fine mist water nozzle idea holds more promise. zero restriction just cooling effect thru Evaporation. although limited run time. easier lighter and less invasive. Distilled water should leave no residue.
 

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edit: forgot that it was already mentioned in the thread but here's the owner's manual:

By the way since my last post in this thread it has been discovered that the owner's manual since 2021 says that the turbo model should use 91 octane or higher while towing to prevent overheating.

Font Screenshot Newspaper Publication Rectangle
 

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Discussion Starter · #17 ·
I did also read the 91 octane statement in the manual. A much larger trans cooler is on its way to help get rid of heat. I also will monitor the CVT temps with my Scan gauge. I am still planning to add a water mist to the incoming air above the intercooler for long-haul climbs like I-70 and others. By using Distilled water I should avoid any build up. using two fine mist nozzles to start. can easily ad more or less nozzles.
 

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I did also read the 91 octane statement in the manual. A much larger trans cooler is on its way to help get rid of heat. I also will monitor the CVT temps with my Scan gauge. I am still planning to add a water mist to the incoming air above the intercooler for long-haul climbs like I-70 and others. By using Distilled water I should avoid any build up. using two fine mist nozzles to start. can easily ad more or less nozzles.
What does fuel octane rating have to do with CVT temps? Or am I misunderstanding your plan?
 

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Discussion Starter · #19 ·
Two very separate things . 91 octane to reduce timing reduction and water mist for air charge cooling so possibly less heat created. Then the larger cooler for CVT temps just relating it all to less heat in general. And the CVT temps were brought up earlier in the post as an additional towing concern.. Sorry if I confused the issue or thread!
Thanks
 
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