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Old 05-16-2008, 05:03 PM   #1
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Default High ambient temps = lower boost ???

This is not for my Miata, it is for my Subaru. So this is a general turbo question on boost control.

I'm tuned for 21psi from gt3076r using a Hallman MBC with the heavy spring in it.
Yesterday my outside temp on the dash read 95*F, and I was only hitting 15-16psi max. Tonight when it was cooler out, I tested and was hitting 20-21 like I am supposed to be. Did a couple more pulls on the way home and hit about 19-20 pounds.

Why are my boost pressures so low when it is hot out? Shouldn't the ball/spring in the Hallman be reacting at the same boost pressure no matter what the ambient temp is?

Maybe there is something to do with air density that has to do with the boost controller responding, but I can't think of it if there is.

-Nick
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Old 05-17-2008, 04:50 PM   #2
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Anyone have any ideas?
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Old 05-17-2008, 10:20 PM   #3
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Most factory cars do this, I believe as a form of knock control. But idk for sure
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Old 05-17-2008, 10:53 PM   #4
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Yeah, if your motor is knocking, it is going to lower the boost. More heat means more likely to knock.
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Old 05-18-2008, 11:58 AM   #5
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Did you get it figured out? Give us something. If we are going to answer your questions, we like a little feedback.
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Old 05-18-2008, 01:39 PM   #6
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I am a subaru / evo / skyline tuner

the ecu in a subaru has temp compensation maps on timing / boost (but that boost wont do anything since the mbc is in place) the scoob has its intake air temp sensor in the MAF
so if intake temp is greater than x value - the ecu reduces timing - by reducing timing this may impact your boost - I have seen many cars overboost in lower ambient temps - than when they were tuned - during the heat of the day
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Old 05-18-2008, 02:02 PM   #7
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So, it goes off of temperature not knock? I was under the impression that it would retard timing when there was any kind of knock going on.
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Old 05-19-2008, 04:09 AM   #8
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it will pull timing based on knock / knock severity as well

what model car is it - let me look in the ecu and post up the images
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Old 05-20-2008, 03:44 AM   #9
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also hot air is less dense so the turbo would have to spin up more to get the same ammount of air in there ... just a thought.
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Old 05-20-2008, 04:55 AM   #10
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I think I may have figured it out.... my boost gauge is not accurate at all. I also blew the motor, pretty sure its rings or ringland for cylinder 2.... not sure why, but thinking it might be due to overboost since I was unable to accurately monitor my boost levels (boost gauge was reading low and not consistent).

For the sake of figuring out if this is possible and why, I will answer all the questions though.

Quote:
Originally Posted by J.T. View Post
Most factory cars do this, I believe as a form of knock control. But idk for sure
It has a MBC, factory boost controller is sitting in a box.

Quote:
Originally Posted by 3barboost View Post
I am a subaru / evo / skyline tuner

the ecu in a subaru has temp compensation maps on timing / boost (but that boost wont do anything since the mbc is in place) the scoob has its intake air temp sensor in the MAF
so if intake temp is greater than x value - the ecu reduces timing - by reducing timing this may impact your boost - I have seen many cars overboost in lower ambient temps - than when they were tuned - during the heat of the day
I have a blow-thru MAF, so the temps it is seeing is post-intercooler so they shouldn't be too bad.
The car was also tuned at night, we finished up at about midnight so ambient temps were pretty low.

Quote:
Originally Posted by 3barboost View Post
it will pull timing based on knock / knock severity as well

what model car is it - let me look in the ecu and post up the images
07 STi.... Brought it to my tuner and we figured out the motor was on it's way out (smoking from oil fill/dip stick as well as my catch can). Because it wasn't going to hurt anything, we tuned it with a meth setup I had recently hooked up and made 352whp (stock STi reads 220whp). I got home and did a compression check and got these results: 135, 75, 134, 136. So I made 352whp with **** *** compression numbers in one cylinder, I wonder what I will make once its all fixed (forged pistons, comes stock with forged rods and crank).

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Originally Posted by akaryrye View Post
also hot air is less dense so the turbo would have to spin up more to get the same ammount of air in there ... just a thought.
But the turbo should EASILY be able to do this. It's a GT3076r, it barely starts breathing at 21psi.
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Old 05-20-2008, 09:03 AM   #11
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How the hell did you toast that motor so quickly? You just beat the **** out of on a poor tune?


I had a friend w/ an 05 making 485/485 on a FP Red on a stock motor w/ bolt-ons (3" up pipe, downpipe, headers, etc.)...for like...20k miles or so...toasted the motor when Hydra got pissed at his meth and cut injection @ 29.5lbs of boost. It still made in the 300s with a compression of like 60 in the #4 cylinder. I have a new found respect for that motor.
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Old 05-21-2008, 04:39 PM   #12
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You know what is strange, my miata did the "exact same thing". "Exact same thing" being that, once after boosting the car for like 10 minutes of driving/ hot ambient temps. , the boost started to drop.

It was tuned for 12 lbs. , but as the car started to get hotter the boost would drop lower and lower. Eventually it would settle around 8 lbs. which you all know the difference between 12 and 8 lbs. is going to be pretty dramatic, so it really sucked.
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Old 05-21-2008, 04:52 PM   #13
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Quote:
Originally Posted by miatamania View Post
How the hell did you toast that motor so quickly? You just beat the **** out of on a poor tune?


I had a friend w/ an 05 making 485/485 on a FP Red on a stock motor w/ bolt-ons (3" up pipe, downpipe, headers, etc.)...for like...20k miles or so...toasted the motor when Hydra got pissed at his meth and cut injection @ 29.5lbs of boost. It still made in the 300s with a compression of like 60 in the #4 cylinder. I have a new found respect for that motor.
I know the motor should have been able to handle the power I was giving it, but as I said a couple posts back....

Quote:
Originally Posted by crashnscar View Post
I think I may have figured it out.... my boost gauge is not accurate at all. I also blew the motor, pretty sure its rings or ringland for cylinder 2.... not sure why, but thinking it might be due to overboost since I was unable to accurately monitor my boost levels (boost gauge was reading low and not consistent).
It did have a tune, but no overboost protection since I don't have a standalone and the oem MAP sensor only reads to like 22psi or something like that.

Quote:
Originally Posted by ak47bravo View Post
You know what is strange, my miata did the "exact same thing". "Exact same thing" being that, once after boosting the car for like 10 minutes of driving/ hot ambient temps. , the boost started to drop.

It was tuned for 12 lbs. , but as the car started to get hotter the boost would drop lower and lower. Eventually it would settle around 8 lbs. which you all know the difference between 12 and 8 lbs. is going to be pretty dramatic, so it really sucked.
Thing is, there wasn't a huge power difference. I guess I should have taken this as a hint that it was actually hitting target boost and it was just my boost gauge ******* up, but unfortunately I didn't connect the dots until it was too late.
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