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Old 05-12-2021, 10:54 PM   #1
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HKS Turbo hitting 15psi?

Hi all, was hoping someone could explain or shed some knowledge on my current situation. Recently installed an HKS GTIII-RS turbo kit on my 2015 BRZ. The wastegate spring is at 12psi which is high for stock engines but I know that boost isn’t what kills our engines, it’s the torque. The kit works on stock motors but can also be paired with HKS stroker kit.

Car is tuned on 94octane, making 300whp and 240tq. The tuner was able to bring psi down to 10psi by adjusting preload, set boost cut at 15psi so it doesn’t creep and also set rev limiter to 6500. I also asked to keep the torque low in the lower rpms. Support mods include oil cooler, vented hood, clutch, pcv catch can, boost/oil press/afr gauges. Stock engine, stock injectors and fuel pump. Car does not get tracked just daily and some spirited driving.

My concern is that the car does build boost up to 15psi on the kit, then cuts at 15. I know I’m not making a whole lot of torque early on, however I am worried for knock due to the compression + high boost.

Is 15psi peak boost on 94oct a bit much? Or does it really come down to tune/torque?

Thanks!

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Old 05-13-2021, 04:53 AM   #2
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if wastegate is 12psi and you get 15psi it s fair if you have concern
Do you have a cat after the turbo, and can you link a log of your car ?

yes 15 psi is too much for stock compression and petrol, knocks are too wild and car gets detuned and runs risky
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Old 05-13-2021, 09:40 AM   #3
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How are you measuring the boost? Are they limiting RPM because it creeps at higher RPM? Unfortunately, this is an age old problem with internal wastegate on vehicles that have a completely catless exhaust- do you still have a cat in your frontpipe?
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Old 05-13-2021, 10:13 AM   #4
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How are you measuring the boost? Are they limiting RPM because it creeps at higher RPM? Unfortunately, this is an age old problem with internal wastegate on vehicles that have a completely catless exhaust- do you still have a cat in your frontpipe?
Limiting rpm because it does creep higher past 7000. I have a prosport boost gauge to measure boost! No cats in the front pipe. Should I get one?
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Old 05-13-2021, 10:15 AM   #5
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if wastegate is 12psi and you get 15psi it s fair if you have concern
Do you have a cat after the turbo, and can you link a log of your car ?

yes 15 psi is too much for stock compression and petrol, knocks are too wild and car gets detuned and runs risky

No cats, Iíve been meaning to do some data logs, will probably do so over the weekend. My Afrs have been running fine, would I be able to notice there if it starts to run a little risky ?
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Old 05-13-2021, 10:44 AM   #6
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Limiting rpm because it does creep higher past 7000. I have a prosport boost gauge to measure boost! No cats in the front pipe. Should I get one?
I meant where is your reference? Like where did you install the T fitting for the line? Is the prosport gauge use a digital sender, or is it analog (does a vacuum line run to the gauge itself?)
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Old 05-13-2021, 04:23 PM   #7
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I meant where is your reference? Like where did you install the T fitting for the line? Is the prosport gauge use a digital sender, or is it analog (does a vacuum line run to the gauge itself?)
Boost is tapped into brake boost, t fitting to boost, then digital sender then to gauge.
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Old 05-14-2021, 02:05 AM   #8
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Unless I am really confused about preload, you can't lower boost below your spring pressure by reducing preload. If you are removing preload then you are allowing there to be less pressure on the wastegate door. You could add so much length to the rod that the door stays wide open, and as you tighten the rod down, the door would shut more and more. If you leave the door cracked a little then you will bleed off exhaust, so the turbo would require more exhaust gases higher in the rpm band to overcome the bleeding gases to spin the turbo, so in essence, you have increased turbo lag and the boost threshold higher in the rpms. If you do that then it is possible the turbo would see less boost if the engine couldn't spin the turbo enough, but I am assuming your turbo isn't so large where such a situation is possible, nor do I believe your tuner would set the rod to have the wastegate always slightly cracked. In this scenario, the rod still won't open more until boost pressure exceeds the wastegate pressure, which is still going to happen at spring pressure or 12 psi. Some springs will crack at a lower boost level and then open at their set level.

This shows a wastegate cracking open at 10psi, but then opening fully at the spring pressure of 12psi. Some springs will not crack until boost pressure is hit. External wastegates with a EBC can allow someone to run pressure to the spring to keep it closed until the last possible second. This guarantees boost hits wastegate pressure, but often there is a boost spike because there is a delay between wastegate activation and boost control, so having a spring that cracks first can be good.





Do you have a data log to share? How is it that your tuner achieved a boost level of 10psi max, but now you are hitting 15psi? What changed? Are you hitting a boost spike or is it boost creep or is it sustained 15psi? If it is something new then the most likely problem is that your vacuum line from the turbo to the wastegate came off, has a crack, has a hole from getting burnt, etc. Make sure that hose is good.

If your problems were always there then maybe he added too much preload. If someone kept adding preload then they would compress the spring more and more and more. The spring could get so compressed it would be incompressible, or it would only allow the wastegate to crack open, so there would be boost creep on top of a higher boost level, or it would just take more boost to open the wastegate, but it would open all the way, so boost would be higher, but there would be no creep.

Boost creep is when the wastegate is too small for the turbo, so the wastegate initially opens and maintains boost, but as RPMs build, exhaust gases continue to build enough to spin the turbo more, so boost rises.


Here is boost spike.
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Old 05-14-2021, 11:48 AM   #9
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Cliff notes:

Preload can’t reduce boost below wastegate spring pressure; it can only raise it above spring pressure. At best, rod length could be set to allow the wastegate to be cracked, but this will only raise boost threshold and turbo lag.

You have boost cut set at 15psi, and you are seeing this max boost, which means without the fuel/throttle cut, you could possibly see even more boost. The most likely scenario is the wastegate isn't opening. This is most often caused by a leak from a cracked, damaged or popped-off vacuum hose going from the turbo compressor to the wastegate. Make sure that is secure.

If the hose is solid then determine if this is a boost spike, boost creep, or if it truly is that the turbo is spinning freely to 15+ psi. A spike could be from routing the vacuum line from the intake manifold to the wastegate instead of directly from the turbo compressor housing or from excessive preload. Too high of boost is from a wastegate door sticking shut from mechanical damage or very excessive preload. Boost creep is from an undersized wastegate for the engine/turbo, which is highly unlikely because this is an off the shelf kit.
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Old 05-18-2021, 11:24 AM   #10
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Cliff notes:

Preload canít reduce boost below wastegate spring pressure; it can only raise it above spring pressure. At best, rod length could be set to allow the wastegate to be cracked, but this will only raise boost threshold and turbo lag.

You have boost cut set at 15psi, and you are seeing this max boost, which means without the fuel/throttle cut, you could possibly see even more boost. The most likely scenario is the wastegate isn't opening. This is most often caused by a leak from a cracked, damaged or popped-off vacuum hose going from the turbo compressor to the wastegate. Make sure that is secure.

If the hose is solid then determine if this is a boost spike, boost creep, or if it truly is that the turbo is spinning freely to 15+ psi. A spike could be from routing the vacuum line from the intake manifold to the wastegate instead of directly from the turbo compressor housing or from excessive preload. Too high of boost is from a wastegate door sticking shut from mechanical damage or very excessive preload. Boost creep is from an undersized wastegate for the engine/turbo, which is highly unlikely because this is an off the shelf kit.
I wish I knew a bit more about how the tuning aspect worked this is what my tuner said!!
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Old 05-18-2021, 12:53 PM   #11
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I wish I knew a bit more about how the tuning aspect worked this is what my tuner said!!
https://www.hksusa.com/application/f...59012-00en.pdf

HKS says the boost actuator is set at 89kPa, which is 12.9psi, but there is some overboost that gets the car above that, as seen in their graph of the catless setup, which is your setup. So like DarkPira7e said, you are seeing raised boost levels because of the decrease in backpressure and subsequent better flow leading to a gain in boost. According to the catless graph, the car might be seeing around a peak of 95kPa, which is around 13.77psi. As you can see from the second graph, the modification of the rod reduces the boost initially by semi-cracking the wastegate and bleeding exhaust, but the turbo still wants to pop the wastegate fully at spring pressure, so that is why boost slowly raises to within around 5kPa or 0.7psi of its potential, which is around 90kPa or spring pressure.

Quote:
Loosening the nut up to maximum 3mm to decrease the initial boost pressure
In short, the mod the tuner did reduced psi for much of the RPM band, but your catless frontpipe added that boost back to the top end, as if you were running a stock frontpipe without the mod. Your best bet would be to run the stock front pipe with the mod, but you will still be getting to a peak of 85kPa or 12.3psi according to the graph.

I don't know why they call it a 90kPa (13psi) vs 75kPa (10.8psi) peak setup, when the results clearly show that the mod with cat still gets the car to 85kPa (12.3psi).

If you want less boost, while keeping the frontpipe, or if you want less boost even with the stock frontpipe and mod, then your only option is to replace the internal wastegate with a replacement that has a 8psi spring or something. Discuss this with your tuner.

https://www.rallysportdirect.com/par...tor-7psi-black
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Old 05-18-2021, 06:20 PM   #12
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HKS says the boost actuator is set at 89kPa, which is 12.9psi, but there is some overboost that gets the car above that, as seen in their graph of the catless setup, which is your setup. So like DarkPira7e said, you are seeing raised boost levels because of the decrease in backpressure and subsequent better flow leading to a gain in boost. According to the catless graph, the car might be seeing around a peak of 95kPa, which is around 13.77psi. As you can see from the second graph, the modification of the rod reduces the boost initially by semi-cracking the wastegate and bleeding exhaust, but the turbo still wants to pop the wastegate fully at spring pressure, so that is why boost slowly raises to within around 5kPa or 0.7psi of its potential, which is around 90kPa or spring pressure.



In short, the mod the tuner did reduced psi for much of the RPM band, but your catless frontpipe added that boost back to the top end, as if you were running a stock frontpipe without the mod. Your best bet would be to run the stock front pipe with the mod, but you will still be getting to a peak of 85kPa or 12.3psi according to the graph.

I don't know why they call it a 90kPa (13psi) vs 75kPa (10.8psi) peak setup, when the results clearly show that the mod with cat still gets the car to 85kPa (12.3psi).

If you want less boost, while keeping the frontpipe, or if you want less boost even with the stock frontpipe and mod, then your only option is to replace the internal wastegate with a replacement that has a 8psi spring or something. Discuss this with your tuner.

https://www.rallysportdirect.com/par...tor-7psi-black
Holy crap thanks for finding this info for me. I will likely be doing this, just gotta ask to tuner. Adjusting the plate position in conjunction with the catted front pipe should fix my overboosting issue according to the manual. Another dumb question would be, would the front pipe have to be HKS or just any other brand with a cat ie: invidia, avo, lachute, berk etc. Thanks a million!
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Old 05-18-2021, 10:14 PM   #13
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Holy crap thanks for finding this info for me. I will likely be doing this, just gotta ask to tuner. Adjusting the plate position in conjunction with the catted front pipe should fix my overboosting issue according to the manual. Another dumb question would be, would the front pipe have to be HKS or just any other brand with a cat ie: invidia, avo, lachute, berk etc. Thanks a million!
No problem.

It sounds like your tuner attempted to do this mod, but maybe he didn't do it enough. Realistically, the mod won't change the top end much; it will make the biggest different in the mid-range, but that could be enough to protect things, if that is your concern.

The two graphs were comparing the stock frontpipe with a catless pipe, which in all likelihood, is also a larger diameter pipe. If you want the lowest possible torque then you would put the stock frontpipe back on and do the plate mod too. Any combination of larger diameter frontpipe or higher flowing catted front pipe will give you results that are between these two graphs. You could use any brand, but again, you can see the results from the catless frontpipe and no mod (highest torque/psi) and the catted frontpipe and plate mod (lowest torque/psi), so any frontpipe that is between those two extremes will be between those two torque/psi values, which may be not much different. Any catted frontpipe will likely be a larger diameter pipe than the stock frontpipe and the cat will be a high flow cat.

Personally, I would keep the stock frontpipe for emissions and noise reduction, but you may want to remove the cat for your own reasons like sound.

Your other option is to get a new internal wastegate with a lower spring like at say 8 psi, and then you could get a EBC to increase boost to anything you desired. You could do boost by rpm or boost by gear. Typically most EBC will do 2-3x the wastegate spring, so that would cover you beyond your needs. More money though.
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Old 06-02-2021, 05:44 PM   #14
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No problem.

It sounds like your tuner attempted to do this mod, but maybe he didn't do it enough. Realistically, the mod won't change the top end much; it will make the biggest different in the mid-range, but that could be enough to protect things, if that is your concern.

The two graphs were comparing the stock frontpipe with a catless pipe, which in all likelihood, is also a larger diameter pipe. If you want the lowest possible torque then you would put the stock frontpipe back on and do the plate mod too. Any combination of larger diameter frontpipe or higher flowing catted front pipe will give you results that are between these two graphs. You could use any brand, but again, you can see the results from the catless frontpipe and no mod (highest torque/psi) and the catted frontpipe and plate mod (lowest torque/psi), so any frontpipe that is between those two extremes will be between those two torque/psi values, which may be not much different. Any catted frontpipe will likely be a larger diameter pipe than the stock frontpipe and the cat will be a high flow cat.

Personally, I would keep the stock frontpipe for emissions and noise reduction, but you may want to remove the cat for your own reasons like sound.

Your other option is to get a new internal wastegate with a lower spring like at say 8 psi, and then you could get a EBC to increase boost to anything you desired. You could do boost by rpm or boost by gear. Typically most EBC will do 2-3x the wastegate spring, so that would cover you beyond your needs. More money though.
So fast forward, added the stock front pipe which did add some backpressure. Car was running a little too efficient ahah, with catless front pipe, catless downpipe, aftermarket overpipe and basically straight N1 exhaust. Lowered the boost pressure to 10 by adjusting actuator rod/plate positioning. So now, it doesnít go over 10psi all the way through the power band!!!! Dynod just shy of 300whp and kept torque low at 220ft/lbs, but all in all sheís running good. Fingers crossed.
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