Boost advice? - North American Motoring

Boost advice?

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Old 07-10-2018, 09:04 PM
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Boost advice?

Hi guys,

I was wondering what can cause boost to go over target then massively taper afterwards? I am thinking wastegate but not sure what would cause the boost drop.

Probably have a bad turbo since the dealer did not swap it during engine failure...

Target boost is 21psi. Actual spikes to 23-24psi then tapers to about 14psi.

Any ideas where to start?

Thanks.
 
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Old 07-11-2018, 12:09 AM
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Our small stock turbo's most likely can't keep up with that amount of boost. Gotta think of the turbo as an air pump. If it can pump a higher volume of air than the motor needs you create positive pressure (boost). Since our turbos are small, they can't consistently produce a volume of air our motors need at higher RPM's which is why you get the big drop off. The engine is evacuating more air from the cylinders faster then the turbo can refill them. The drop off is also to prevent your turbo from reaching zero resistance and literally over spooling and destroying itself. This is the point where if you want to be able to produce more boost and hold it, you need to upgrade to a bigger turbo. The waste gate will only help in maintaining boost up until the point your turbo starts to over spool.
 
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Old 07-11-2018, 12:47 AM
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Originally Posted by Andrew Park View Post
Our small stock turbo's most likely can't keep up with that amount of boost. Gotta think of the turbo as an air pump. If it can pump a higher volume of air than the motor needs you create positive pressure (boost). Since our turbos are small, they can't consistently produce a volume of air our motors need at higher RPM's which is why you get the big drop off. The engine is evacuating more air from the cylinders faster then the turbo can refill them. The drop off is also to prevent your turbo from reaching zero resistance and literally over spooling and destroying itself. This is the point where if you want to be able to produce more boost and hold it, you need to upgrade to a bigger turbo. The waste gate will only help in maintaining boost up until the point your turbo starts to over spool.
Thank you for the info! Makes sense.
I guess the real issue is figuring out the boost spike.
 
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Old 07-29-2018, 05:45 PM
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temperature also has a huge factor. remember our tunes are typically set by changing the set torque values. so this means that the car/ecu will boost the car until the torque target value is achieved. on cold days, i don't go over 17PSI. on warm days, i'm between 18-20psi, and on REALLY HOT days, i'll hit 22psi. that's our timing being advanced or pulled back and yet the ecu is still boosting to achieve the target torque value.

on race fuel (4 gallons 101 octane, rest on 93) on a really hot day, (93 degrees) i was sitting at 16-17PSI...car ran great.
 
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Old 07-30-2018, 03:32 PM
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Interesting... so is there a check factor or something that comes into play to keep you from blowing up an engine on a hot day? Max boost regardless if it's hitting the torque target? A hot day in LA valleys is often over 100, and if I'm headed to the desert for some reason, could be 110 or more. Thanks.
 
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Old 07-30-2018, 06:03 PM
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Originally Posted by J_L View Post
Interesting... so is there a check factor or something that comes into play to keep you from blowing up an engine on a hot day? Max boost regardless if it's hitting the torque target? A hot day in LA valleys is often over 100, and if I'm headed to the desert for some reason, could be 110 or more. Thanks.
i don't know the answer to that. what i can say is from driving the car on the track and with the increased boost, i'm more comfortable modulating the throttle vs just mashing the pedal. if you go full throttle too early (full load) you can certainly spike the boost. i tend to modulate the throttle, finding the balance between enough throttle pressure until i get over 4000-4500 RPM at which point i safely go WOT.
 
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Old 08-15-2018, 06:55 PM
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What boost pressures do you guys see close to redline?

Also, what peak boost pressures do you guys see while tuned?

Figuring out if the taper I experience is normal due to turbo limitations or something else.

Thanks!
 
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Old 08-16-2018, 12:34 AM
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Are you using the stock turbo? I gathered that you are from your post, but just wanted to make sure.Just to be clear, I don’t have a good idea as to what the problem may be or how to figure that out. I’m just answering your other question.
As far as the boost goes, How quickly are you losing the boost pressure? Is it hitting 22 (or whatever you hit) then starting to drop almost right away? And how quickly does it drop? Are you sure you don’t have a boost leak?
On a 3rd or 4th gear pull, with a JCW turbo, my target is 22psi at 5K RPM, and it will hold there as long as I hold my foot in it, all the way to the red. Even under a steady throttle though, the boost will go up or down depending on the engine load. If I am going along on the highway or something, if I give it like half throttle, the boost will jump right up, but does start to decrease almost right away as the load on the engine changes even though I haven’t changed the throttle position. Could this be what you are experiencing?
 
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Old 08-16-2018, 10:01 AM
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4th Gear Boost Drop

Having trouble uploading 3rd Gear. Will do soon.
Similar effects with you tuned folks? Or out of the ordinary?
 
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Old 08-16-2018, 10:02 AM
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Old 08-16-2018, 10:55 AM
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Hmmm. Interesting. I think someone who know a lot more than me will need to chime in, but just my gut feeling is boost leak. Have you checked all your hoses and connections?
 
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Old 08-16-2018, 11:42 AM
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Originally Posted by MrGrumpy View Post
Hmmm. Interesting. I think someone who know a lot more than me will need to chime in, but just my gut feeling is boost leak. Have you checked all your hoses and connections?
Did a smoke test at a shop and found nothing. Maybe it’s an issue that’s exposed with pressure. Will need to do pressure test.

Unless it’s just the turbo running out of breath at that boost level. I’d like to know what other people see with tunes.
 
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Old 08-16-2018, 12:26 PM
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check timing at the point youre losing boost PSI.. could be ECU kicking in
*see if timing gets pulled back or goes negative @ point boost drops
 
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Old 08-16-2018, 12:29 PM
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that seems expected by me. i've had 2 different tunes and both of them taper the boost after peak. by redline you're down between 15-17psi (at least for me). i say 15-17 as what i mentioned before regarding boost at varying temps.

IMO, that is normal.
 
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Old 08-16-2018, 05:51 PM
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I don’t know.... maybe the turbo is just running out of breath or the ECU is lowering the target after you hit peak, so that it doesn’t run out? Maybe someone who is running the stock turbo with a tune can chime in. Has it always done this or is it a new thing? I have a little bit bigger turbo on there now, so my car won’t behave quite the same...the tune probably makes a big difference as well. What kind of tune do you have...manic?
 
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Old 08-26-2018, 03:33 PM
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You have hybrid turbo?
 
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Old 08-26-2018, 10:03 PM
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I had to replace my turbo so bought one with a 40mm compressor. It’s basically a JCW turbo from JMTC
 
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Old 08-27-2018, 03:05 AM
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Any performance gain vs stock one?
 
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Old 08-27-2018, 09:01 AM
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Yes there is. If you’re just running the factory tune, you can run a higher level of boost, or keep it around the stock range. It makes an improvement right away, because with a bigger turbo, it doesn’t have to work as hard to provide the same level of boost, and at the same time, it’s moving more cfm of air at that same boost pressure. More air = more HP :-)
with a tune it’s nice because it (still) doesn’t have to work as hard as the stock turbo, and it doesn’t want to run out of breath at higher rpm’s.
 
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