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Old 09-30-2001, 12:02 AM   #1
subachad
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Default Max intake temp??

Hi All,

I just got my new thermometer today and have been taking some measurements.

What I'm wondering is, when you exceed the maximum compressor efficiency of a turbo it goes into like a "subsonic" mode where it just heats the air up and you start losing hp. A 300zx hits this around 12 PSI of boost.

What I want to know, what is the maximum an intake temperature can be before I start LOSING horsepower?

My max temp at the throttle body hit 67 degrees C at 5psi with no intercooler. I have other numbers as well, but I don't want to make this into a novel.

Chad
93 1.8T

Here's the results if anyone's interested

http://www.uniquemotorsports.com/sub...mperatures.htm
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Last edited by subachad; 09-30-2001 at 01:34 AM.
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Old 09-30-2001, 02:12 AM   #2
the Dabbler
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Well, how's this for a first order approximation.

Assume that 5psi is gaining you +34% more power, just by virtue of the fact you're breathing 34% more air molecules:

14.7 + 5 = 19.7
19.7 / 14.7 = 1.34
1.34 - 1 = 34%


But the incoming air is now less dense, since you're heating it up with your turbocharger. Assume change in density is:

(Original Absolute Temp / Final Absolute Temp) - 1

Temp has to be in Kelvin or Rankin.

So, assuming 25C ambient (298K), and using your measured 65C intake temp (338K), we get:

298/338 - 1 = -12%

Assume the overall power gain is:

(pressure change) x (density change) - 1
1.34 x 0.88 - 1 = +18%


So this setup should produce (approximately, of course), 18% more power. This is probably a bit low, because the original (i.e., normally-aspirated) intake temp was most likely above ambient to begin with.

Working the equations back toward the intake temp that would result in 0% gain, we get:

126.3C

Beyond this intake temp, the engine would be making less power, even though it's under boost.

I'm sure there are a number of variables in there not taken into account, but like I said, first order approximation, right?

Last edited by the Dabbler; 09-30-2001 at 02:41 AM.
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Old 09-30-2001, 03:33 PM   #3
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I never did learn to think in metric...

Subachad: 152.6 degrees fahrenheit?? Damn.. that's hot.

Dabbler: 260 degrees fahrenheit?? That's even worse!! I'm wondering how those temperatures would effect exhaust gas temperatures....

Fitz
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Old 10-01-2001, 12:47 PM   #4
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Quote:
Originally posted by Fitz
I never did learn to think in metric...

Subachad: 152.6 degrees fahrenheit?? Damn.. that's hot.

Dabbler: 260 degrees fahrenheit?? That's even worse!! I'm wondering how those temperatures would effect exhaust gas temperatures....

Fitz
Fitz:
To clarify, I didn't say that 126.3C existed anywhere in the intake path. The question was, what temperature would the intake air have to be in order to actually turn the turbocharger into a liability instead of a benefit, at 5psi. And the answer, if we can believe my math, is 126.3C.
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Old 10-01-2001, 01:01 PM   #5
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Thanks for the info Dabbler, that was great. I'm going to add your quote to the tech section of my site if it's ok with you.

On a side note, I installed a top mount intercooler yesterday. Realize, I have an L so no hood scoop for it. Stabbing the pins with my thermal probe, it was about 50C sitting in traffic on the way up to the "testing hill." A 5psi 3rd gear pull up the hill heated the intercooler to a max 69.9 degrees C. Hot Damn!!! that's fargin hot, I need a hoodscoop.

We did another test in my friends much, much faster Porsche 944 turbo with straight exhaust and 15psi. A third gear pull, (much shorter time span since the car is sooo damn fast) With a factory front mount intercooler, the temps reached 73 C at max.

Chad
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Old 10-01-2001, 04:24 PM   #6
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Quote:
Originally posted by subachad
Thanks for the info Dabbler, that was great. I'm going to add your quote to the tech section of my site if it's ok with you.
Fine with me, but I was hoping for some corroboration!

Any thermodynamicists out there care to comment?
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Old 10-01-2001, 11:46 PM   #7
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Dabbler,

I posted the info on my site. Here's the link...

http://www.uniquemotorsports.com/sub...mperatures.htm

Let me know if you would like to see any changes made.

Chad
93 1.8T
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Old 10-02-2001, 02:54 AM   #8
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subachad:

Let me know if you get any contrary opinions on your website. My reasoning makes sense to me, but like I said, I'm no thermodynamicist.

Thanks.
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Old 10-02-2001, 11:21 PM   #9
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There's another varriable you've got to take into consideration. The efficiency of the engine at high temperatures. When you add more heat, I might stand corrected, but I do believe you increase the chance of premature detonation (I hate knock knock jokes). There's a point which I would imagine the efficiency of the engine would decrease before 126 deg C. But those air density calculations are right
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Old 10-02-2001, 11:53 PM   #10
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Quote:
Originally posted by Really_Soaked
There's another varriable you've got to take into consideration. The efficiency of the engine at high temperatures. When you add more heat, I might stand corrected, but I do believe you increase the chance of premature detonation (I hate knock knock jokes). There's a point which I would imagine the efficiency of the engine would decrease before 126 deg C. But those air density calculations are right
That's a good point. It would probably be detonating like a banshee well before the intake temps could reach 126C.
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Old 10-03-2001, 11:59 AM   #11
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That's true with the detination thing, but that's not what I was looking for. It could actually detinate at any temp, more likely though at higher temps agreed. With high enough octane, retarded timing and proper fuel the intake temps should be able to go quite high. I don't want them to, I was just curious as to the relation between temperature and efficiency. More of a technical explanation with figures and numbers, not a simple "colder is better."

Chad
93 1.8T
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Old 10-31-2001, 05:12 PM   #12
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I found what I was looking for and thought I'd share it.

According to "21st Century Performance" by Julian Edgar "...the peak power to improve by 1 percent for every 4 degrees C temperature decrease of the intake air temperature."

So going by the previous 67C temp. If I got it down closer to ambient, say 30C, that should equate to about a 9.25% increase in power, or about 10-15hp on my motor.

Chad
93 1.8T
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Old 11-01-2001, 12:28 AM   #13
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I have watched the OBDII intake air temps in 4th gear pulls on the highway and noticed the following temps with VF22 BB turbo and JCS 5x18 intercooler.

0 psi 80 degrees outside 125 degrees F cruising
7 psi 80 degrees outside 96 degrees under boost
10 psi 80 degrees outside 115 degrees under boost

0 psi 55 degrees outside 90 degrees F cruising
10 psi 55 degrees outside 75 degrees F under boost

14 psi on 70 degree day was 140 degrees

This last one was in a 5th gear pull to 5,000 rpm. This is also when we think I crushed the #4 piston before getting dyno'd, and when the first IC leak started. Then the piston let go at 12 psi at the drag strip and all we could get the rest of the night was 8 psi, with two tiny holes blown out in the IC. Don't try this at home! I guess we just wanted to test the limits of the bottom end and IC, eh? Yeah, I know

On the Dyno, on 70 degree day, external intercooler temps stayed 82 degrees at 8-10 psi boost and climbed less than 1 degree over the 20 second pull. We did not look at OBDII intake air temps on the dyno. For the Dyno the IC was pre-treated with an IC water sprayer hooked up to the windshield wiper fluid, but the IC was dry by the time the test was conducted.

Couldn't figure out why the temps are hotter under cruising than under boost, until we decided maybe it is the dual throttle body fuel injectors cooling the intake charge as they spray in there between the TB and the MAP sensor. I assume the intake air temp sensor is built into the MAP sensor?

Larry
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Old 11-01-2001, 02:53 PM   #14
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Quote:
Originally posted by ImprezaRS dot com
...
Couldn't figure out why the temps are hotter under cruising than under boost, until we decided maybe it is the dual throttle body fuel injectors cooling the intake charge as they spray in there between the TB and the MAP sensor. I assume the intake air temp sensor is built into the MAP sensor?

Larry
www.ImprezaRS.com
That's very interesting data.

Your theory about your extra injectors cooling the intake charge sounds reasonable enough. The intake temp sensor is colocated with the MAP sensor mount on the 2000s (on mine, at least).
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Old 11-04-2001, 12:31 PM   #15
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One reason why the intake temperature might be more at cruising is the actual temperature of the intake manifold itself. If you feel it after the car has been running for a while, you'll notice it's quite warm to touch. This could be heating the air. As you run higher boost, the intercooler is doing it's job and the high amount of air that passes through the intake runners is not heated as much as it would be with less air flow.

I'm working on making some phenolic spacers for my car to solve this and hopefully lower the intake temps. Has anyone else messed with this?

Chad
93 1.8T
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Old 11-04-2001, 03:14 PM   #16
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whered you get a thermometer to stick in there?
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Old 11-04-2001, 03:40 PM   #17
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Dabbler: You are right about the equations. The law which concerns the matter can also be written as:
p*V/T=constant, where p is pressure, V volume and T temperature in Kelvins. If you do the math you'll get the same results.
This simple law is valid very well in this pressure and temperature range.


Oh, and Dabbler I checked from your website that you have installed your turbo to your NA ej18. I have a some question about similar install and I have posted a topic to this forum. Nobody has answered , but maybe you can answer the questions I have asked there.
Check out the topic, I'd appreciate any information.

Well, this was off the topic...

Sorry about the poor English, haven't wrote much lately since I'm in the army currently.


tony
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Old 11-05-2001, 12:51 PM   #18
the Dabbler
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Quote:
Originally posted by tonytiger
Dabbler: You are right about the equations. The law which concerns the matter can also be written as:
p*V/T=constant, where p is pressure, V volume and T temperature in Kelvins. If you do the math you'll get the same results.
This simple law is valid very well in this pressure and temperature range.
Yes, the Ideal Gas Law:

P*V=n*R*T

P = pressure
V = volume
n = # molecules of gas
R = Universal Gas Constant of the appropriate units
T = temperature in some absolute scale

As you say, quite applicable for everyday sorts of calculations.

Quote:

Oh, and Dabbler I checked from your website that you have installed your turbo to your NA ej18. I have a some question about similar install and I have posted a topic to this forum. Nobody has answered , but maybe you can answer the questions I have asked there.
Check out the topic, I'd appreciate any information.

Well, this was off the topic...

Sorry about the poor English, haven't wrote much lately since I'm in the army currently.


tony
To clarify, the site you mention is subachad's (and very nice, I might add). He has quite a bit of experience with EJ18 turbo setups.
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Old 11-05-2001, 01:21 PM   #19
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Quote:
Originally posted by Andrew
whered you get a thermometer to stick in there?
Are you asking what kind of thermometer it is? If so, I would imagine subachad is using a thermocouple and voltmeter arrangement. That would probably be the easiest and most reliable.
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Old 11-06-2001, 12:26 AM   #20
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For the temperature measurements, I stuck a thermometer under the hood temperarily. The wire is too short to make it into the car, so all my numbers are from the max/min function built into the probe. Here's a link to where I got it, but you might be able to get a similar unit from Radio Shack or Fry's.

http://www.autospeed.com/cgi-bin/bro...?category=579&

Some of the measurements were from the probe being stuck into the intake tract, whereas others are from the probe stuck in the intercooler.

I'm going to try the extra injector thing pretty soon. I have a few extra injectors, so all I have to do now is figure a way of mounting them, getting a pressure switch, and wiring them into my factory ECU. I'll put this on my site, once it's done.

Chad
93 1.8T

P.S., since the site link is down, here's another

http://208.129.213.117/subachad/intake_temperatures.htm
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