Banks experiment - altitude and hp

RoyJ wrote:

valhalla360 wrote:
- I don't think any production vehicles are running superchargers...so not really relevant but even there, you are assume they are on 100% of the time. Assuming there is a control mechanism, there can be a difference.

As mentioned, many vehicles run superchargers.

No, I didn't assume superchargers run wide open. They're throttled just like NA and turbo gas engines. But regardless of how much you open the throttle, they have a fixed pressure ratio (boost ratio).

If the ratio is 2:1, at sea level, 100% throttle, then you manifold pressure is 14.7 psi. At an elevation of 0.8 atmosphere, you'll be at 11.76 psi.

At 50% throttle, you'll be at 7.35 psi, and high elevation 5.88 psi (grossly simplified, but you get my point).

Theoretically both NA should see similar percent reductions and both turbo should see similar percent reductions but actual field tests can bring to light issues not readily apparent (like small turbos only partially negating the thin air).

With NA, I bet test results = math.

To guess a turbo's "room" left to boost at altitude, you could look at how much top end hp a vehicle gains with only a tune.

On small turbo engines like my lil Mini Cooper, a tune can gain 30% of torque in the mid range, and only 10% above 5000 rpm. We can make an educated guess at say beyond 3000 feet (0.9 atm), it'll probably start to lose power. Because at that altitude, it requires the turbo to generate 10% more boost ratio.

I stand corrected that there are supercharged production vehicles...of course, none listed look like typical tow vehicles. Most aren't even typical passenger vehicles but specialized performance vehicles sold in tiny numbers. So while technically true, you aren't likely to see many of these towing a 30' TT.

Out of curiosity are they permanently on and at max boost? If not, you might see a similar effect to turbos.
- Cruising at highway speeds on level ground at sea level, my assumption is they turn off the supercharger as it's simply not needed...similar to a turbo charger not boosting under light load. Only it's worse as superchargers eat up some of the HP to power them.
- Under heavy load climbing at altitude, a supercharged engine intended for towing would be putting out max boost.

It would probably be a little more choppy in terms of effect (on vs off rather than a more smooth matching of boost to needs) but the general effect would still be there. Or as someone suggested, they may have some sort of gearing (or equivalent function) to adjust boost to need...but again, none of the examples are intended for serious towing.

To the second part of your comment: it's nice to see the math but nothing beats actual testing as unexpected issues can come into play.