AmarilloMike

As I said before the acceleration of my 718 coupe seems much better than I would expect from just 300 HP. No doubt the PDK is a big part of that as is the 3200lb curb weight.



Amarillo is at 3,600' elevation and so on a standard day the air in Amarillo is about 90% as dense as air at sea level.. I noticed my other now-gone sports cars (all normally aspirated) had a lot better acceleration in the lowlands than they did in Amarillo. It occurs to me that the electronics could compensate for the lower density air charge in Amarillo by having the turbo increase its output.

Does anybody know if Porsche programmed the 718s' electronics to compensate for higher altitude?

Still absolutely thrilled with my new-to-me 2018 coupe. Got the metal license plate on it, the oil changed and the one year service done. I'm going to run it in autocross Saturday.

Thanks!

iliveoncaffiene

Almost all modern ECUs (even on NA cars, even on non performance cars) have AFR (air-fuel ratio) sensors and are constantly adjusting for that optimum ratio (all depending on throttle / RPM too).
So yes, I think it compensates for altitude and depending on year, your NA cars did too. The difference is that a turbo compensates so much better because it's manipulating the air pressure and can just turn up the boost. It may lag a bit more, but it will still get to the same pressure as at sea level. An N/A car does not have that ability

AmarilloMike

If it does adjust turbo boost for altitude that would make a HUGE difference in Denver!

Thanks!

iliveoncaffiene

Almost all car makers (and some aftermarket tuners supporting said car makers) nowadays test at altitude as part of the normal development process for an engine / car.

AmarilloMike

On a standard day the air in Denver has about 75% of the density of the air at sea level.

So a normally aspirated car in Denver won't have nearly the same horsepower as it would at sea level.

But with turbocharging it is possible to have that car make the same horsepower in Denver as in Galveston, Texas, again on a standard day.

But Porsche would have to program the electronics in my 718 for it to make the standard seal level 300 horsepower in Denver, or in Amarillo, as it does in Galveston.

GrantG

I believe most all of Porsche's current turbocharged cars make nearly the same power in Denver as at sea level. The lag is worse (turbos have to spin faster to develop the same absolute pressure), but they can eventually make all (or nearly all) the power. As you go further up in altitude, most of the turbochargers are not properly sized to continue to make full power (but they still make much higher percentage of full power than NA models).

Porsche tunes their turbocharged models to open the wastegate at a specific absolute pressure, not a set boost amount over the ambient pressure. So, it's possible to offset much of the loss due to altitude.

AmarilloMike

If the loss in horsepower of a normally aspirated 300 hp car is proportional to air density then the NA car would have, on a standard day, about 225 hp.

If my 718 puts out 300 hp in Denver I would have 75 more hp than the normally aspirated 300 sea-level-horsepower car.

I get your point about it taking the turbos longer to get wound up in Denver than in Galveston.

Very cool never-the-less!

Thanks for all the replies.

GrantG

Yep, turbocharging is really great in some ways at altitude. But they are really lazy off-boost, because their lower compression ratio mixed with the thin air makes them very sluggish. But when on boost, all is good. Of course, most modern turbocharged Porsches still have pretty high compression and do ok even off boost. But older ones have very low compression ratios and really suffer up here (930 Turbo, for example)...

AmarilloMike

That's very interesting to me.

Even at 3600’ there is a little bit of sluggishness before the turbos spin up. But after that - WOW!

spdracerut

Turbo cars since the 90s have adjusted boost for elevation to a limit. For example, a stock 90s supra could hang with a viper of the same era in Colorado, but not at sea level. The limits to how much boost are knock threshold and turbo speed. All the OEMs have elevation margin built into their turbo sizing. Some leave more margin than others... But as an example, the turbos on the 991.2 have a speed limit in the ballpark of 200k rpm. At sea level, they maybe spin only like 170k rpm. So the Ecu can up the boost/turbo speed by 30k at higher elevations to offset the loss of air density. *note: numbers are for example only, I haven't calculated what the actual turbo speed limits are for the two turbo sizes for the 718 nor speed in operation.

AmarilloMike

.
That answered the question I was asking exactly.

Before this thread I did not know it was common practice for manufacturers to "normalize" their turbo cars for altitude.

Thanks again for all the replies.

Best,

Mike

spdracerut

Check out this post which shows the command authority of the ecu. In this example, the ecu is adjusting for higher Intake Air Temps by increasing the boost level to maintain power. Of course, there's a limit. But you can see how Porsche tries to maintain consistent power regardless of ambient conditions of variable temperature and air density by adjusting boost level.
https://rennlist.com/forums/991/1150...l#post15968368

A common specification is a turbo must have enough elevation margin to not have to derate power at 2000m. That number does depend on the OEM and the application. In other cases, maybe 1000m, etc.

AmarilloMike

That is very interesting.

So, if the 6,000 meter rule-of-thumb holds true for the stock 718 it would have the same horsepower in Denver as in Galveston. Quite an advantage for the highlanders I think.

Thanks!

iliveoncaffiene

No advantage here - I think you're misunderstanding....
If you drove the same car from Denver to the coast (sea level), your car would have the same power the whole time. The only difference you'd notice is that in Denver the turbo would feel laggier.

AmarilloMike

Yeah - I wrote that poorly.

At sea-level, the 718 and a 300 hp normally aspirated car have, by definition, equal horsepower.

But if I dyno those same two cars at 5,000' altitude, the 718 still has 300 horsepower while the normally aspirated car is significantly below 300 hp.

So everything else being equal, the 718 has more power than the normally aspirated 300 sea-level-horsepower car when the cars are in Denver.

Assumptions in my statement:
The 718 turbos are capable, on a standard day, of outputting enough volume and pressure at 5,000' to maintain 300 hp.
All dyno tests are done on a standard day.