DA Affect on NA vs FI Engines - SETTLED - Jeep Garage - Jeep Forum

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Old 11-04-2014, 09:47 AM
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DA Affect on NA vs FI Engines - SETTLED

It's time to settle this debate once and for all. I'm not an engineer. Empirical evidence suggests that hot/muggy weather affects us a lot more than our forced induction counterparts. Thus why a Cayenne TT beats me in the summer but I beat it in the winter.

I've been doing research, found a thread that discusses it. Normally this would all be another opinion thread but there are references to the NHRA addressing this issue. Apparently they adjust DA effects differently for NA, SC, and T cars with NA cars getting the most aggressive adjustment.

Previously on this forum in some of the competition threads people have gone up in arms on each side of the argument. One poster, I don't remember, was acting like the authority on the issue explaining why turbo cars benefit more from colder weather. From the data I see, he is completely wrong. Here is why. I've attached the thread link and a few quotes.

Discuss.

Density Altitude effect question.... - CamaroZ28.Com Message Board

"The rule of thumb I've always seen used is a 3% power loss for every 1,000 feet of elevation for naturally aspirated [COLOR=blue !important][COLOR=blue !important]engines[/COLOR][/COLOR].

Supercharged engines suffer as well, and almost as much as naturally aspirated engines. Since a supercharger's speed is linear in relation to engine rpm, boost is going to drop over the entire rpm range as the air gets thinner. In other words, you can't maintain sea level boost levels at higher altitude without changing pulleys to compensate. However, if you do, you should be able to maintain the same boost response you had at sea level, relative to engine rpm.

Turbochargers, on the other hand, can often automatically adjust to changes in altitude because their speed is independent of engine rpm and limited by the amount of exhaust energy bled off by the wastegate. In other words, they can just spin faster to make the same amount of boost.

For example, if your wastegate is set to open at 10 psi at sea level, you'll probably still be able to make 10 psi without changing a thing, even at 6,000 feet, because the wastegate simply won't open until you hit 10 psi or redline, whichever comes first.

Boost response will certainly suffer somewhat, because it will take more engine rpm (exhaust energy) to produce the same boost levels, but you should be able to maintain your maximum boost level no matter what the elevation, assuming your turbocharger is sized properly and isn't being run out of its efficiency zone at the higher speed it will be turning, and assuming you don't run out of engine rpm."

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Old 11-04-2014, 09:55 AM
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Re: DA Affect on NA vs FI Engines - SETTLED

Blown or turbo cars essentially make their own environment as where n/a strictly has to rely on the weather. Like you said that's why correction factors affect n/a cars more
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