Boost Spike vs. Boost Creep the real deal

Darkiller · 09-22-2006, 10:19 AM

Since these two terms are very commonly used in describing turbo/engine behavior, yet they are not always understood correctly, I wanted to attempt to clear them up.

People often mix the two up, or even interchange them, when they are in fact two VERY different things.

Boost Spike: Boost spike is when the boost level initially "spikes" up to higher than the preset boost setting, and then quickly settles back down to where it should be. As most people with turbos know, once the boost pressure in the intake starts to rise, the rate at which it rises quickly increases until the pressure is increasing at a phenomenal rate. This means that, if your boost is set at 12 psi, when it reaches that point it will be increasing so quickly that it will go higher than 12 psi and then drop back down once the boost control system can correct it, which is within a half second or so.

Some causes of spike are bad boost controllers (only ball-and spring type MBC's should be used, and only proven electronic boost controllers should be used), long boost source or wastegate activation hoses, and the lack of any boost controller at all. It's basicially an effect a t slow response time of the boost control system.

Boost Creep: While boost creep also refers to an unwanted rise in manifold pressure, its cause and effect are totally different from those of spike, as is the way it manifests itself.

As you know, boost pressure is controlled by the wastegate, which allows exhaust gasses to bypass the turbine wheel. In effect, it creates an alternate route for the hot exhaust coming out of the motor to take, which means that any gas passing through it will not spin the turbine wheel.

Now, if this wastegate cannot flow enough to bypass the required amount of exhaust, then that means that too much of the gasses are going to go through the turbine wheel, meaning that it will have too much energy imparted on it (it will be spinning too fast). As the excess exhaust gas amount gets greater and greater, the turbine wheel spins faster and faster, and the boost level rises.

Creep happens ANY time when the wastegate cannot bypass enough exhaust gas to keep the boost under control. This can happen when the wastegate is too small in diameter, or when the design of the wastegate doesn't allow it to open enough, or when the wastegate simply doesn't have a good enough flow path to divert a lot of exhaust. It can also happen when you increase the amount of exhaust coming out of a motor (running more boost/airflow, making more power).

Since this tends to get worse and worse as the engine speed rises (more cycles per second is more conducive to more exhaust gas, to a point), that means that one will see the boost climb to the preset level on the boost controller, and then it will gradually creep up past that line to a "minimum" given the circumstances.

That minimum can be 2 psi above the set boost level, or it can be over 30 psi, depending on how the wastegate is designed, how big it is, the car's setup, and more.

It is also important to not that you certainly can have spike AND creep at the same time, which would result in the boost level jumping up, settling to the preset level, and then slowly climbing back up again as you approach redline.

thx to KPT4321 at dsm tuners

***Insane06STi*** · 09-22-2006, 10:21 AM

Good info, add Boost cut and boost leak - effects/symptoms.

RdRacer301 · 09-22-2006, 10:27 AM

Ahhh.... good ol' boost creep. As night temp drops below 60 during this time of the year, I have noticed a slight increase in my boost level... approaching 20psi (1.4xbar). I am not looking forward below freezing temps.... or,I may have to port my WG.....

Great info, BTW.

**nandanrp** · 09-22-2006, 10:36 AM

This should have been posted in Engine, Power, & Performance. Please post in the right forum next time. Thank you though for taking the time to write this up; I added it to the Engine, Power & Performance threads of interest sticky.

PuffOfSmoke · 02-16-2007, 11:13 AM

Quote:

Originally Posted by Insane06STi

Good info, add Boost cut and boost leak - effects/symptoms.

what is boost cut?

**tyler_sti** · 06-18-2007, 10:48 AM

You're referring to fuell cut I belive. When the engine cuts fuel, due to over boosting. NOT GOOD!

**WC LATHE WERKS** · 07-18-2007, 04:22 PM

very good post, Im happy I went with a VF43 turbo on my WRX because of the new wastegate design (even though my puny 2L probably wont push enough gas to creep)

M3 Fanatic · 11-29-2007, 06:11 AM

A simple boost creep thought from a noob.

IF I understand the theory correctly, ambient outside temp is the leading cause of boost creep.

So wouldn't one of those short ram intakes that draws all that engine bay ambient temp air help eliminate it in cold weather? kinda like a preheater for a diesel!

Eric

fullerton · 11-29-2007, 06:27 AM

Quote:

Originally Posted by M3 Fanatic

A simple boost creep thought from a noob.

IF I understand the theory correctly, ambient outside temp is the leading cause of boost creep.

So wouldn't one of those short ram intakes that draws all that engine bay ambient temp air help eliminate it in cold weather? kinda like a preheater for a diesel!

Eric

I think you could have a point, but I am not sure how effective a HAI would be in warming/expanding cold air enough to negate any creep. Especially at high speeds/boost when the intake has very little time to heat the large amount of incoming air. Most of the problems with a SRI/HAI stem from sitting and baking the static air under your hood in traffic and then trying to get on it.

BUT- boost creep is not entirely temperature dependent. If you can't flow enough air around the compressor in cold, chances are the same thing will happen (possibly to a lesser extent) in the warm.

Tortfeasor STi · 11-29-2007, 07:29 AM

Quote:

Originally Posted by M3 Fanatic

A simple boost creep thought from a noob.

IF I understand the theory correctly, ambient outside temp is the leading cause of boost creep.

So wouldn't one of those short ram intakes that draws all that engine bay ambient temp air help eliminate it in cold weather? kinda like a preheater for a diesel!

Eric

Boost creep occurs in the upper rpm range in higher gears. If you are doing the speeds that requires, the under hood air is going to be close to ambient as the engine wont have enough time to heat it as well. A lot of air moves through the engine and radiator, so when going at highway speeds, the air of a SRI is not going to be all that warm. A better cure is proper tuning and porting the waste gate in the worst cases.

boostedsti06 · 12-27-2007, 11:44 AM

so am i getting this because it is like 46 deg here at night ??? i have an 06 sti with cobb ap ver 2 ACN91 map and a single catted perrin turboback ??? is it ok to get once in awhile

09-22-2006, 10:19 AM	#1
Darkiller Spec C Club Car: 2006 STI BLUE Fav Mod: Fp20g and protune Location: 1000 oaks, SoCA Posts: 1,505 Join Date: Dec 2005 Trader Rating: (0)	Boost Spike vs. Boost Creep the real deal Since these two terms are very commonly used in describing turbo/engine behavior, yet they are not always understood correctly, I wanted to attempt to clear them up. People often mix the two up, or even interchange them, when they are in fact two VERY different things. Boost Spike: Boost spike is when the boost level initially "spikes" up to higher than the preset boost setting, and then quickly settles back down to where it should be. As most people with turbos know, once the boost pressure in the intake starts to rise, the rate at which it rises quickly increases until the pressure is increasing at a phenomenal rate. This means that, if your boost is set at 12 psi, when it reaches that point it will be increasing so quickly that it will go higher than 12 psi and then drop back down once the boost control system can correct it, which is within a half second or so. Some causes of spike are bad boost controllers (only ball-and spring type MBC's should be used, and only proven electronic boost controllers should be used), long boost source or wastegate activation hoses, and the lack of any boost controller at all. It's basicially an effect a t slow response time of the boost control system. Boost Creep: While boost creep also refers to an unwanted rise in manifold pressure, its cause and effect are totally different from those of spike, as is the way it manifests itself. As you know, boost pressure is controlled by the wastegate, which allows exhaust gasses to bypass the turbine wheel. In effect, it creates an alternate route for the hot exhaust coming out of the motor to take, which means that any gas passing through it will not spin the turbine wheel. Now, if this wastegate cannot flow enough to bypass the required amount of exhaust, then that means that too much of the gasses are going to go through the turbine wheel, meaning that it will have too much energy imparted on it (it will be spinning too fast). As the excess exhaust gas amount gets greater and greater, the turbine wheel spins faster and faster, and the boost level rises. Creep happens ANY time when the wastegate cannot bypass enough exhaust gas to keep the boost under control. This can happen when the wastegate is too small in diameter, or when the design of the wastegate doesn't allow it to open enough, or when the wastegate simply doesn't have a good enough flow path to divert a lot of exhaust. It can also happen when you increase the amount of exhaust coming out of a motor (running more boost/airflow, making more power). Since this tends to get worse and worse as the engine speed rises (more cycles per second is more conducive to more exhaust gas, to a point), that means that one will see the boost climb to the preset level on the boost controller, and then it will gradually creep up past that line to a "minimum" given the circumstances. That minimum can be 2 psi above the set boost level, or it can be over 30 psi, depending on how the wastegate is designed, how big it is, the car's setup, and more. It is also important to not that you certainly can have spike AND creep at the same time, which would result in the boost level jumping up, settling to the preset level, and then slowly climbing back up again as you approach redline. thx to KPT4321 at dsm tuners This ad is not endorsed by this member. Please register or login to hide this ad.

09-22-2006, 10:21 AM	#2
*Insane06STi* Nanned ® Location: Where the left lane is th Posts: 17,349 Join Date: Oct 2005 Trader Rating: (9)	Good info, add Boost cut and boost leak - effects/symptoms. Last edited by Insane06STi : 10-07-2006 at 06:55 PM.

06-18-2007, 10:48 AM	#6
tyler_sti Gold Member Car: '07 CMG STI Fav Mod: Not knowing how to "spell" hah Location: los angeles Posts: 3,737 Join Date: Aug 2006 Trader Rating: (15)	Re: Boost Spike vs. Boost Creep the real deal You're referring to fuell cut I belive. When the engine cuts fuel, due to over boosting. NOT GOOD!

07-18-2007, 04:22 PM	#7
WC LATHE WERKS Authorized Vendor Car: 04WRX /STi Swap Fav Mod: TiC AST Coilovers!!! Location: Tri-State, USA Posts: 1,309 Join Date: Nov 2005 Trader Rating: (90)	Re: Boost Spike vs. Boost Creep the real deal very good post, Im happy I went with a VF43 turbo on my WRX because of the new wastegate design (even though my puny 2L probably wont push enough gas to creep)

11-29-2007, 06:11 AM	#8
M3 Fanatic Junior STI Driver Car: 05 Bone stock silver Fav Mod: Cobb Stage 1........only mod Location: Connecticut Posts: 69 Join Date: Nov 2007 Trader Rating: (0)	Re: Boost Spike vs. Boost Creep the real deal A simple boost creep thought from a noob. IF I understand the theory correctly, ambient outside temp is the leading cause of boost creep. So wouldn't one of those short ram intakes that draws all that engine bay ambient temp air help eliminate it in cold weather? kinda like a preheater for a diesel! Eric

09-22-2006, 10:27 AM	#3
RdRacer301 S204 Racer Car: 05 OBP STi Fav Mod: My Right Foot Location: Rockville, MD Posts: 2,538 Join Date: Oct 2004 Trader Rating: (1)	Ahhh.... good ol' boost creep. As night temp drops below 60 during this time of the year, I have noticed a slight increase in my boost level... approaching 20psi (1.4xbar). I am not looking forward below freezing temps.... or,I may have to port my WG..... Great info, BTW.

09-22-2006, 10:36 AM	#4
nandanrp Indiatr0n in yo IWDPL Car: 05FTW OBP STi Fav Mod: Brum Brum Location: downtown Chicago Posts: 48,830 Join Date: Jun 2004 Trader Rating: (11)	This should have been posted in Engine, Power, & Performance. Please post in the right forum next time. Thank you though for taking the time to write this up; I added it to the Engine, Power & Performance threads of interest sticky.

12-27-2007, 11:44 AM	#11
boostedsti06 STI Driver Car: 06 OBP STI Fav Mod: perrin exhaust Location: arizona Posts: 264 Join Date: Oct 2006 Trader Rating: (0)	Re: Boost Spike vs. Boost Creep the real deal so am i getting this because it is like 46 deg here at night ??? i have an 06 sti with cobb ap ver 2 ACN91 map and a single catted perrin turboback ??? is it ok to get once in awhile This ad is not endorsed by this member. Please register or login to hide this ad.