Boost Creep Issues? Then please read on .... :)

Toby@ToyTuning.com

Banned - DO NOT BUY FROM TOYTUNING
In Brief:

Boost creep - When you set a pre-set amount of boost (say 10psi max) you expect it boost to 10psi and stop right? Well this isn't always the case. A turbo will naturally boost 1-2 psi more than what you set it on initial spool (when you put your foot down) and will then slowly settle to your pre-set boost. In this example the boost is set at 10psi, when you first accelerate the turbo will boost to 11psi and will quickly settle back to 10spi through the rev range, this is common physics of any turbo charger.

Boost creep in this example would mean you accelerate, the boost goes right up to your pre-set amount (10psi) and continues to boost more than you have set it. There is no set amount, some may creep to 14psi, some may creep to 24psi, it depends on many variables. This is why they call it boost creep, the boost is "creeping" past and beyond your pre-set limit. There are several causes and remedies but let's first understand why.



Understanding:

To make it easy I will use a glass of water as an example. Read the first sentence to understand, then read the second sentence where I have noted what part of the car I am referring too in brackets.

Sentence 1: Imagine pouring water into a glass with a hole in the bottom, the water slowly drains away. Pending on the size of the hole you can poor more water in and it will still drain out the hole. Imagine pouring water into the glass really fast, you will get to a point where you are putting in more water than the hole can physically drain and the glass will slowly overflow even though there is a hole in the bottom, this is boost creep.

Sentence 2: Imagine pouring water (air) into a glass (turbo) with a hole in the bottom (wastegate), the water slowly drains away. Pending on the size of the hole (wastegate) you can poor more water (air) in and it will still drain out the hole (wastegate). Imagine pouring water (air) into the glass (turbo) really fast, you will get to a point where you are putting in more water (air) than the hole (wastegate) can physically drain and the glass (turbo) will slowly overflow even though there is a hole in the bottom, this is boost creep.

That should make it quite easy to see what is happening. In essence too much is coming in and not enough coming out. You can either enlarge the hole in the glass (port the wastegate), you can pour less water in (restrict the airflow) or you can chuck the glass away and get a 2 pint pitcher (New Turbo/Bigger Wastegate e.g. external)

Turbos are simple in design, all they do is compress air. You have to imagine an engine as a big air pump, the more air you can get in (more oxygen) the better and faster it will be. A turbo just compresses the air molecules (oxygen) meaning you can squeeze even more air into the engine, simple yeah? In practice yes, but in real life there are lots of variables that can effect a turbo and it all needs to come together as well balanced equilibrium (that just means a perfect balance).



The Stock Setup:

Now lets see what can cause this frustrating uncontrollable boost. The stock CT9 (the stock turbocharger on a Starlet GT or Glanza) is quite small, hell it doesn't need to be very big as they are light cars! When Toyota designed it, they didn't make it with us in mind. They made it as good as it needed to be with the stock equipment supplied with the car from factory. Often people say that the CT9 is rubbish, or the stock intercooler is rubbish, this is not true, they are perfectly suitable for what the car was made for.

Turbo's have wastegates, this is what opens and let's some of the boost out, if they didn't have wastegates the turbos would boost forever as there would be nothing to let the air out, they would boost continually until the turbo blew up (not literally but it's not good!) Now the stock wastegate can only do so much. In stock form they are 28mm in diameter, it's not very large but more than adequate for a standard Starlets boost pressure. If Toyota had wanted you to run 20psi then they would have designed it differently but this is not the case.



Possible Causes:

Then we come along and want to make the car faster, we make a few changes and suddenly the boost goes mad and it won't settle where you want it to. Here are a few examples of what we can do to mess this careful balance up:

- Reducing back-pressure

This can be in the form of an uprated exhaust manifold, decat (removing the stock catalytic converter) and a wider exhaust system. It could be just one, or it could be all three. When you reduce the back pressure this allows the turbine wheel in the turbo to spin faster as the gases are being exhausted quicker as there is less pressure resisting against it. This in turn means the compressor wheel is now spinning faster and sucking more air in.

Confused? This is how a turbo works. If you look at a turbo, where the air goes in is the compressor, the part that compresses the air (where you stick an air filter/induction kit on). If you took the stock "16V EFI Pipe" off (inlet) and looked in the turbo you would see the compressor blades. At the other end is the turbine wheel, there connected and one piece. The turbine wheel is spun by the wasted exhaust gases that come out of the engine. Just like on a normal car you have an exhaust system, except on ours the gases are put to use and directed towards the turbine which is what spins it. This spins the compressor which sucks the air in through the inlet.

So you can see what is happening, we are removing the catalytic converter (restriction) which increases the rate in which the exhaust gases flow, we are increasing the diameter of the exhaust throughout with an uprated exhaust system (say 2.5inch) which again increases the flow. You could do one, or the other or both. Same goes with the manifold, if you use an uprated unit that is less restrictive the gases are simply flowing faster, this is causing the turbine wheel to spin faster which obviously spins the compressor faster.

Now let's go back to the wastegate. The wheel in the turbo is now spinning faster than normal, more air is coming in at a faster rate. But shit, what about the wastegate? We now have more air coming in but the wastegate is still only 28mm in diameter so how will it get rid of the excess air? Answer is it won't, we now have too much going in and not enough coming out, this is boost creep.

- Air Filter / Turbo Inlet

We have just learned how reducing the back pressure in the exhaust can cause uncontrollable boost. The air filter is a similar problem. The stock inlet on the CT9 is pretty long. Not only is it long but there are lots of bends. The stock airbox is also quite restrictive. No it's not rubbish, it's fine for what it was made for but we are looking to increase performance not run a stock car!

Now most people will want to get rid of the stock airbox and use a better open ended filter/induction kit (Apexi, HKS etc etc). Not only does this allow more air in but it sounds better as you can hear the turbo "sucking". Most will put an uprated air filter at the end of the inlet track. You are still using the stock inlet so there are still plenty of restrictions. Ideally you don't want an open filter like this in the engine bay as it only sucks hot air in (this is why the airbox is enclosed with a cold air feed), these filters are meant to receive cold air as they are open but this is a story for another day!

The next step up is relocating the filter behind the headlight, again you are still using most of the stock inlet tract and there are a few restrictions. This doesn't mean you won't get boost creep but there is still a less chance, not that you should be gambling for this. Next up from this is removing the stock inlet altogether and relocating the filter to the front bumper. This is great for cold air but now you have removed all the restrictions from the inlet. This means the turbo can now suck in ample air quickly. You have reduced the length of the intake, the air has to travel a shorter distance and there are very few bends to slow it down therefore it moves at a faster rate.

The problem is the wastegate is still the same size so again the turbo is taking in more than it can get rid of, as we all know by now this is boost creep. Let's make the wastegate bigger you say? Before we jump the gun let's see what the pros and cons are now we know why it's happening.



Remedies / Solutions


- Port The Wastegate

This simply means making the wastegate bigger, porting it out. Now this seems the simplest cure, but there is a problem. Anyone who is experienced with the stock CT9 knows that wastegate cracking is a common problem. There is a huge amount of heat around the wastegate, the turbos are small, this means that they heat up quickly or disperse heat badly. They are fine in stock form but when you run more boost they can crack very easily. Even running stock boost can crack it over time, for e.g. consistent high speeds were the turbo is working hard, like on a motorway if you floored it for an hour at top speed (not that you should be doing this). The constant spinning of the turbo means it soon gets very hot, turbos generally operate between 1000 degrees C to 1500 degrees C so it's a lot of stress for a small turbo.

This means that the stock wastegate is an inherent weak point, yes you can port it but you have to be careful as if it's already badly cracked then your not going to help it. If there are only minor "hairline" cracks then it should be okay. Don't take this as gospel truth but a little porting should be okay as long as you get a professional to do it and you don't go mad. I can't say 0.1mm will do it, I can't say 2mm will do it, each car is different and will need to be tested, just make sure you get the metal filings out as this won't help either.

So now all we are doing is increasing the diameter of the stock wastegate hole thus allowing a greater volume of air to be expelled due to the increased amount of air coming in.


- External Wastegate

The wastegate we have been referring too is in an internal one which is stock on a normal Starlet and normal CT9. An external wastegate is simply that, a wastegate which is external and not internal. Common sizes used on the Starlet are 38mm, as you can see 10mm bigger than the stock wastegate. I won't explain the benefits or the drawbacks, all you need to know is the wastegate is a lot bigger so you can simply expel more gases. The only way to use one on a CT9 is by welding the stock wastegate shut and installing one on the manifold, it's a complicated process and needs to be done well to work well. All we are doing is replacing the small wastegate with a bigger one.


- Restrict the Inlet / Exhaust

As we know by now, relocating the filter and reducing back pressure can cause boost creep as there is simply too much coming in and not enough coming out. We can restrict the inlet somehow, either by covering some of the filter (preventing some of the air coming in) or by using more bends, more bends will slow the rate of air.

We can also create a restriction in the exhaust, either by using a smaller diameter system or by physically putting a restriction in there, like a bung with a smaller hole restricting the amount of air flow which in turn will slow the turbine wheel down on the turbo. You could do both, you could do one, there are not definite fixes each application is different so you will need to test regardless, no one car is the same as they say.


- HKS Actuator or Similar

The actuator is what controls the INTERNAL wastegate on the CT9, obviously if you have an external one you can disregard this. The normal CT9 actuator can become week over time, purely as it's just a spring opening and closing the wastegate. Like a normal spring, if you pull it backwards and forwards enough the spring will loose tension.

There is a common myth that a HKS actuator will solely cure boost creep, it wont. Due to the stronger spring inside it, it will open and close the wastegate quicker than an old worn spring in the stock actuator. This will help to bring on the boost faster but the wastegate is still the same size. It may give it a better chance purely as the wastegate will open quicker but if the hole is physically too small then it won't stand a chance. It may be on the limit and could cure the problem for the occasional Starleteer who is only suffering from a small amount of creep but 9/10 it won't. If you really think about it a worn or weak actuator will mean loss of boost as it can't keep the wastegate shut under boosting conditions therefore the gases are flowing straight out of the turbo and therefore loosing boost.

Whilst they can sometimes aid the process they are not a remedy alone, in this case the aforementioned scenarios will need to be addressed.



Conclusion

There we have it, whilst each car is different there are many variables to take into consideration that we haven't discussed. Temperature for example, the colder the air the denser it is, this means more oxygen which in turn means greater compression of air (what a turbo does), again this will effect the the problem and is why a lot of people are fine in the summer, yet when winter dawns (for us UK lads) the problem suddenly arises. The same can be said on the motorway as there is more cold air available when compared with driving around the city streets.

There is no definitive size of exhaust, inlet tract, air filter, wastegate, each application is different. There is no surefire fix, each method will need to be evaluated to determine which method is best. This isn't just a Starlet problem, it's common on all turbo cars. Starlets just happen to have small turbos and Toyota didn't expect us to run 5-10 psi over what they used from the factory.

This isn't made to scare you but to teach you why it happens in simplified terms. Great gains can be had from removing the restrictions, there are huge performance benefits you just need to be careful when selecting your parts. A particular manifold won't prevent it, a particular exhaust or air filter won't prevent it, each application is different. Some may experience it, some may not. At least if it happens you can read this and re-evaluate the situation to resolve it! :)

We are only a PM or email away if you are still unsure and are always prepared to help whatever the case may be.

Best Regards,

Toby@ TT
 
Last edited:

TurboDave

Member +
It sounds a bit rushed to me Toby, you could ave spent spent some more time on it!

Na a very good explaination mate.

Dave

:)
 

Toby@ToyTuning.com

Banned - DO NOT BUY FROM TOYTUNING
No problem guys, that's one common question sorted!

If there are any more then let me know and will do my best to make it easy to understand. :)

Tobs
 

GTti

Member +
It's a shame that no matter what you type this will still be ignored :p (That's down to the admins to enforce though)

Good job mate, in the few years I've been on these forums (Almost 4?!), you've gone from being pretty passive into the spokesperson for this site.
 

Toby@ToyTuning.com

Banned - DO NOT BUY FROM TOYTUNING
Thanks mate, I don't pretend to be a space engineer but with over 5 years of Starlet modding under my belt, the forces that are Kevin (20 yrs working on Toyota only) and his father (who used to own a Toyota dealership!!) we make a bloody good team. We live and breath Toyota :)

The main thing is that five years ago I was a customer, I bought the wrong parts, I made mistakes and learnt from them. After 2 years of building what hopefully will be the maddest, baddest Starlet there has ever been I like to think I can really offer some benefit to our customers and members.

I have bought pretty much anything ever made for the Starbo, re-introduced some of the rarest parts (pretty much kept Zep Racing afloat and that's not a joke) and have built up amazing relationships with our suppliers both in the UK and Japan.

We hope that in 2008 you will really get a flavour of what we can do with demo car no1 and no2. We could not have done it without you guys so we thank you all very much for your continued support, how sweet but oh so true! :)

T
 
Last edited:

sMARTINside

Member +
Wow Toby, what a complete write up! It confirms many weird incomplete concepts that I had in my brain, since the last year! You have put the light in my mind... Thanks a lot! I never ran my 4E with the top mount intercooler. «i installed a front mount right away on my Tercel. I never had problems with boost creep. Does a front mount help to buffer the boost peaks coming out of the turbo, by having a larger volume of air to compress before pressure rises to the intake manifold?
 

hardcoreep

Member +
Here's the problem Toby. Boost Creep happens when the boost is SET. Boost on the 4E isn't SET. The FACTORY SOLENOID is not set at a FIXED boost level, but at a duty cycle. This means that if I hooked up a Garrett T3 turbocharger to such I'd still get 10psi, even though the CFM (the actual volume of air through the turbo) has changed. So creep can't happen in a system where boost isn't set.

The reality is boost pressure is dynamic and you will always have variations due to external conditions. As you pointed out turbo boost pressure is related to its efficiency thus the more air it can move out of the exhaust housing the more boost pressure it can create. Removing a restriction will naturally cause boost pressures to go up because the turbocharger can now move more air (ie you need to pour faster).

Boost Creep occurs when the wastegate flow has been severly compromised and no matter what you do boost pressures keep rising significantly. The big factor of boost creep is no control of the boost level at any position. If boost tapers off to its natural curve then you don't have boost creep, you've simply just created a more efficient boost situation.

There is no way a CT9 at 15psi is going to experience boost creep. The Starlet has a very restrictive intake/exhaust system an simply removing these will increase boost levels as the car offers no such control vs. lets say a Subaru Impreza which has an ECU mapped with solenoid duty cycles. I don't care what turbocharged vehicle you own, if you add an intake and exhaust, just by sheer physics boost pressures would rise. That's a fact of turbocharged life. If you can't control that rise by PROPER METHODS then that's boost creep.
 
Last edited:

hardcoreep

Member +
Here's an example:

The Mitsubishi Eclipse is one of the most famous cars for this. The 2nd gen car has a very small turbocharger thus freeing up the system allows the 2litre engine to unleash its full power. The exhaust housing is too small for the application over stock settings thus the wastegate port can't flow fast enough to stall the exhaust wheel. This means under load boost continues to increase with rpm. It never tapers off. Its best described as an very long boost spike. The car is set for 10psi at MAX, however boost creep will see 16-18pi+

Adding a boost controller to this car actually makes the boost creep worse as you're trying to restrict the already small wastegate. I you set the boost on the EBC to 5psi under load you might see 10-15psi, THAT's boost creep.

Which brings me to my next point, running boost, and peak boost. I find this with people all the time. If you've ever mapped a turbocharged vehicle you know that the turbo has a peak boost level, and a 'flat' boost level. A basic compressor map will tell you this. Its is airflow through the engine that will determine where on the map, in terms of shaft speed, where the turbocharger operates.

So what happens is that the person never takes his car to redline to find the full peak level, or experiences a load situation where s/he hits peak shaft speed. So they drive around at 10psi and the first time they floor it they see 12-13psi they start screaming there's a problem.

I think the problem is one of knowledge and expectations. UK GT owners, for whatever reason, believe that the car comes running at 9psi and that no matter what you do to the car it will stay running 9psi. I'm sure if you slammed an air fliter on a Escort RS Cosworth boost pressures would rise as well.
 

Toby@ToyTuning.com

Banned - DO NOT BUY FROM TOYTUNING
Indeed it comes back to what we've said really, that's why I didn't even mention EBC's as it will only cause more confusion. The wastegate is the depicting factor in all these examples mentioned.

Don't forget you have stated that you can't get boost creep with 15psi on a stock turbo, this is with a standard setup only. Using a filter and exhaust with little restriction will soon cause boost creep issues as I'm sure you are aware even on 15psi.

Some members may be doing what you are saying and thinking they have boost creep when in fact it's natural turbocharger physics but if you read what we have written in full it actually mentions everything you said :)

T
 
Top