[locowill]

I have heard alot of stories and want to hear what u guys have to say about this issue. I know for a fact that the n/a correction factor is something like 25-30 % loss, from sea level to 5k feet. Then we have a turbo charged car which is a atmosphere compensated i have heard that the loss is like 6% on a good sized turbo. I can tell u this, driving back from new york the engin felt real responcive and would wrap out like crazy, as soon as i started to climb in the 3-5k ft zone i dould definitly feel the differance but when i got in to boost it felt the same as it was @ sealevel.?

and thoughts?

 

[buffhomer]

It's not the same, even at the same boost level...the boost pressure is the same but the air is still less dense. I've seen cars correct to the same factor that N/A cars do and I've seen others correct to the half N/A factor. Seems to me though that more cars lean toward the half factor, which is what the NHRA uses as well.

The faster the car, the less the difference too.

Use the bottom half of this list:

NHRA altitude correction factors

 

[Joyrider564]

From what I understand:

Let's assume atmospheric pressure at sea level is 14.7 psi, and atmospheric pressure in Albuquerque is 12.08 psi (did a quick online calculation). On a naturally aspirated engine, the difference in pressure would be 2.62 psi, or about a 20% difference in air pressure. However, if for instance you are running 15 psi of boost, that means the engine is seeing 29.7 psi of air pressure at sea level, and 27.08 psi of air pressure in Albuquerque. It is still a 2.62 psi difference, but that makes the difference only 4%, which shows while in boost, it seems why you have lost less power.

If that makes any sense, I haven't done a whole lot of calculations like that in a while so feel free to correct me if I am wrong!

 

[locowill]

so then its safe to assume 10-15 % loss if that.

 

[locowill]

Originally Posted by 1bad4dr View Post
umm, that is not entirely correct. A Turbo car will have less of a correction factor than a blower or Nitrous car, but there is still a correction factor.

that is what i said.

here is the SAE correction factor

Turbo correction factor
Skipping all the math, (if someone really wants to see it let me know), for a turbocharged engine with a constant boost pressure, (after the turbos are fully spooled), it can be shown that the indicated power and torque would scale with pressure as

αp = (Pref + Pb)/[Pdry(1 + Pb/Patm)]

where Pref is the reference dry-air absolute pressure (29.235 in-Hg for SAE), Pb is the boost pressure also in in-Hg (= 2.036*PSI), Pdry is the absolute dry-air partial pressure at the time of the measurement, and Patm is the total absolute air pressure at the time of the measurement. To get the brake power scaling, clearly the mechanical efficiency needs to be factored in here as well. (See that other thread).

Below is a table showing how the two correction factors might differ for a turbocharged engine measured at a mile high with everything else at SAE J1349 conditions; Pref = 29.235 in-Hg, RH=0%, etc. There are other assumptions that I'll not get into here. Note how the “typical” correction factor overestimates the results and gets increasingly worse at higher boost pressures. It should also be pointed out that the difference between the two will change as the other conditions change. For example, as the absolute ambient pressure approaches the reference pressure, the difference is reduced.

For the above conditions, the SAE J1349 correction factor is: CFsae = 1.254

First column = boost pressure in psi
Second column = boost pressin in in-Hg
Third column = dyno CF appropriate for a turbocharged engine (CFturbo)
The 4th column shows how much the SAE J1349 CF overcorrects the results (relatively speaking), i.e., 100%(CFsae/CFturbo - 1)

Code:
Pb Pb
(psi) (in-Hg) (Actual CF) sae/trbo-1(Dyno inflated CF)
5.0 10.18 1.178% 6.40%
7.5 15.27 1.155% 8.53%
10.0 20.36 1.137% 10.22%
12.5 25.45 1.123% 11.61%
15.0 30.54 1.112% 12.76%
17.5 35.63 1.102% 13.74%
20.0 40.72 1.094% 14.57%
22.5 45.81 1.087% 15.30%
25.0 50.9 1.081% 15.93%
27.5 55.99 1.076% 16.49%
30.0 61.08 1.072% 16.98%
__________________
what about all this mumbo jumbo

 

[Joyrider564]

In general I would assume so... there are also other things that come into play with power such as humidity, temperature, and barometric pressure (which can be calculated for the actual atmospheric pressure). Calculating using numbers from another turbo car in Albuquerque, it was about a 10% difference from actual to corrected horsepower numbers.

EDIT: From reading the mumbo jumbo, I know that my original calculations were very oversimplified. A lot of other factors would come into play, and that would be an ideal "HOLY SHIT FREE BOOST" in a perfect atmospheric world. There are also a lot of other things that come into play, but I just wanted to throw out a general idea of it...

 

[locowill]

if i can trap the same speed here, what the car trapped in new york @ the same boost level then would be someting.

 

[Guest]

if i can trap the same speed here, what the car trapped in new york @ the same boost level then would be someting.

well run the car here and than drive down to Roswell and see if the car picks up any MPH

 

[1bad4dr]

Jesus, talking about digging up an old post of mine. hahahahahahahahaha

Us F/I guys still have to run N/A while out of boost.

I agree with you Will, my car runs like a bat out of hell at Sea Level.

 

[ICBM]

I always felt a ton more off idle, part throttle and out of boost performance. In boost, well it was faster too because I saw an increase of 3psi on the boost gauge without changing the pulley (Kenne Bell 1.5L on a 5.0 Ford). This was from here to near the coast of California. So with the small supercharger, 1psi = 10hp there about. That would be a 30hp loss or gain, right?

The rough rule that I use to go by was 3% loss for boosted apps and 15% for NA apps. Of course DA will play some part of the actual differences.

3% for every 1000ft. for NA

At least on dyno runs I have seen this to be pretty close. Case in point 01smokes dyno sheets. Granted, SAE is assuming ideal conditions at 0' altitude. Look at the difference and you'll get the point. Consider we are at 5200'+ in many parts of Albuquerque and/or Rio Rancho.

 

[ICBM]

So Ron. Did you notice how much boost difference you saw while in Cali? Just curious.

 

[locowill]

what would arts graph look like if the busa was boosted?

what i mean is would there be less of a loss compared to n/a

 

[ICBM]

what would arts graph look like if the busa was boosted?

what i mean is would there be less of a loss compared to n/a

Unfortunately dynos do not know the difference if it is boosted or not.

Yes there would be less of a loss in boosted apps. As I stated earlier, I gained or loss 3psi. But I still had boost at higher altitudes which would still make more power than a NA app, right?

Can't put an exact # on it and would require some experimentation.

3% maybe a little on the lightside actually. 5% would probably be closer for boost apps.

 

[1bad4dr]

So Ron. Did you notice how much boost difference you saw while in Cali? Just curious.

With the Centri (Paxton) I was at 12psi at Sea Level. Once up here, I would see 9.5 - 10psi with an occassional spike to 11 psi.

As for the Twin Screw, I am told it is at 10 psi @ Sea Level, but I have not verified that (was scared to let her rip lol).

Up here I am seeing 9 psi.

Now, the difference between the two blowers, besides the obvious, is my motor. With the Paxton I did not have a stroker, just over bore and 9.5:1 CR.

With the Twin, I now have a 426 stroker and 9:8.1 CR.

Could this be why I am not losing so much boost? I have no idea.

 

[ICBM]

With the Centri (Paxton) I was at 12psi at Sea Level. Once up here, I would see 9.5 - 10psi with an occassional spike to 11 psi.

As for the Twin Screw, I am told it is at 10 psi @ Sea Level, but I have not verified that (was scared to let her rip lol).

Up here I am seeing 9 psi.

Now, the difference between the two blowers, besides the obvious, is my motor. With the Paxton I did not have a stroker, just over bore and 9.5:1 CR.

With the Twin, I now have a 426 stroker and 9:8.1 CR.

Could this be why I am not losing so much boost? I have no idea.

Well maybe. Remember, boost is just a measurement of restriction in an engine's ability to injest. The old 5.0s as a whole is restriction by itself. The new modern stuff as well as the larger capacity engines will certainly not see the same boost #s as if I were to put that mega monster of yours on a mostly stock 5.0. Everyone gets hung up on boost #s when really we should talk about CFM capabilities.

Yea, my old KB had a 11psi pulley and made 8psi with a spike of 9psi on occassion up here.

 

[1bad4dr]

Well maybe. Remember, boost is just a measurement of restriction in an engine's ability to injest. The old 5.0s as a whole is restriction by itself. The new modern stuff as well as the larger capacity engines will certainly not see the same boost #s as if I were to put that mega monster of yours on a mostly stock 5.0. Everyone gets hung up on boost #s when really we should talk about CFM capabilities.

Yea, my old KB had a 11psi pulley and made 8psi with a spike of 9psi on occassion up here.

Great statement.

haha I have learned A LOT over the past couple of years concerning F/I and N/A at high and low elevations

 

[locowill]

ok, so if one was to make X amount of hp to the wheels the actual hp to the wheels would be 3-5% less on a turbo application, compared to the 25-30 % loss of a n/a engine at this altitude?

 

[CHZBRGR]

ok, so if one was to make X amount of hp to the wheels the actual hp to the wheels would be 3-5% less on a turbo application, compared to the 25-30 % loss of a n/a engine at this altitude?

Seems kosher to me :headbang:

 

[CHZBRGR]

Actually, this might be a dumb question.

But does anyone know what the percentage of power loss at our elevation is for nitrous setups?

For example:
Lets say we have a regular LS1 with say, a 125 shot controlled by a window switch. It sprays at WOT from 3k to redline.

What would the difference in power be if it were at sea level and here? Saying that we only measure the power it makes ON nitrous.

Is it similar to boosted applications??

 

[Joyrider564]

That seems about right... I am curious to see actual dyno numbers from sea level to here, not the dyno corrected shnanigans.

My question is, has a turbo car had a loss of boost from sea level to mile high??

My theory is... since the turbo can control it's rpm (as a result the boost too) through the wastegate, it can always bring itself to the set boost level, as opposed to a supercharger where it is at a fixed rpm. If that makes sense...