Actuator Options
Turbo Mini Forums

Well we have been running a 15psi actuator on the TT for years, and no matter what we did we couldn't make over 15psi (bleed valve fully open etc.).

It was suggested that the wastegate was being blown open, but I never bought this idea. The actuator we had was so stiff thst you could only get it on by pressurising it with a foot pump.

Anyway, to prove a point we managed 18psi on the new engine (same turbo) using a standard Metro actuator (4.5psi).

However, I was unhappy with the boost control, I felt it was struggling to catch up and boost was wobbly at best. So I now have a 10(ish) psi actuator, which means the boost controller will have less work to do.

Additionally I have a 1.5mm restrictor feeding the boost controller, I may reduce this to 1mm, as we had no boost spikes (one thing at a time though).

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I think that we are suffering from the tendency of most manufacturers to give a simple label to something that is more complicated than originally meets the eye.

So, a "15psi actuator" will give 15 psi of boost, but it very much depends on the spring rate and pre-load as to how it achieves that.

Assuming a linear spring rate (more on that later), then the force exerted by the actuator can be determined as so:

Force ~ (spring rate * movement) + pre-load + (boost/actuator area)

So you can achieve the force required to get 15 psi boost by either a high pre-load and a low spring rate or vice versa and anything in between.

How the actuator behaves when you try to increase the boost will depend on the spring rate. If you have a low spring rate, then you will need to put a lot of extra pre-load per psi of extra boost.

Ultimately, when the engine is under boost and the actuator is modulating, if it has a low spring rate then it is more likely to be overcome by the exhaust back pressure.

Back to spring rates, the pictures of actuator springs that I have seen show them as being conical. If my memory serves me correctly, then I think that means that the spring rate is not linear. Just to complicate things.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

From that it would seem logical that the lower the spring rate the finer the control will be needed to maintain a steady boost.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Perhaps calling any actuator 15psi is just wrong.

Ive tested a few xx psi actuators and most do crack open at a lower rate, and are fully open at a higher rate.

The simple reality is they are not linear.

And then EGBP operating on the flap and it's linkage with the actuator will play a part. As will inneficiencies or simple realities within the engine itself.

So there really are a multitude of factors.

Boost control in theory is a very very simple thing.
The reality of it though is far from simple !

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Very educational gents - thank you.

The exhaust back pressure can be at least 15 psi higher than boost on average.

At some parts of the engine cycle, it can be 40-50 psi higher depending on the cam and manifolding.

So if the ratio of areas of the actuator diaphragm and the wastegate port is too low, then the wastegate will bounce if not get held open in some cases.

EDIT: ratio upside down.

Edited by Paul S on 23rd May, 2011.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Yes, actuator springs are variable rate:

http://www.mitcalc.com/doc/springs/help/en/springs.htm

http://www.forgemotorsport.co.uk/content.a...product=FMAS048

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I guess you need to consider the pressure on both sides of the wastegate, it's surely never zero on the exhaust side.

Also, the length of the wastegate lever will have a bigg effect on what force is holding the valve shut..

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

So...

whose going to measure the diameter of an actuator diaghram and wastgate valve hole and run through my calcs then.

Well volunteered Joe :)

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I've got a spare GT17 actuator. Would this be a good candidate to chop up as so many of us ate using them?

In the 13's at last!.. Just

just measure the outside of the can,

thats close emnough

Didn't we want to find out if there's a tapered spring in there too?

In the 13's at last!.. Just

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Well, the data comes from a simulation of a GT17 on a 998, so I would not expect it to be causing too much of a problem.

You can still get the exhaust pressure below boost whilst the valves are on overlap and still have an average pressure higher than boost.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Monitor actual EGBP. You really dont want to be seeing more than 2:1
Unless you are building a torquey low boost setup with very fast spool.

But if you intend to make power, you need to try and keep EGBP lower, especially if the cam has some overlap.

The last thing you want is massive pressure pumping back into the cylinder instead of fresh charge during that overlap.

That's why mild cams with no overlap work so well in even badly designed boosted setups. They can tolerate high backpressure better.

But it will be far better not to have that excess pressure in the first place, although it will hurt low end spool.

It's just finding the right compromise

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Stevie, I could not agree more.

I got this bit a little wrong:

Force ~ (spring rate * movement) + pre-load + (boost/actuator area)

Thinking about it, as the actuator rod is held under boost:

(spring rate * movement) + pre-load = (boost/actuator area)

The resistance against movement by the wastegate is

(spring rate * movement) - (boost/actuator area)

Edited by Paul S on 24th May, 2011.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I think you're going into it too much.

A 15psi actuator will give you a good starting point for say 12-13psi with plenty of scope upwards.

And similar will apply to most. Using an ecu to control boost will give you lots more room to play though. I'd say use the softest actuator you can get away with.

That will allow you to turn the bosot down, but also to map boost with rpm to soften the hit if need be. It will help traction, the clutch and the gearbox.
It may not feel as nice, but there's a good chance it will make the car faster and more reliable.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

The GT17 actuator is around 49mm

In the 13's at last!.. Just

Right. I had a fiddle with my actuator last night. Went from half a hole to a hole and a bit. Set off this morning and ....

25psi stuttering/fluttering as before with the bleed valve wound right in. So before melting something, I removed the bleed valve from the equation and plumbed the actuator signal straight into the take of on the compressor. This gave 6-7 psi.

So something is up with the plumbing to my bleed valve in the car. All I can think is that the hose a bit small (1-2mm ID) compared with the piece I used to today (3-4mm ID). This could have the same effect as a restrictir perhaps?

I've not tried boost above 15psi yet because the bleed valve is so much less sensitive now that it's fitted in the larger ID pipe. I'll report back once I have.

In the 13's at last!.. Just

Hmmm, now I can't bleed it to any more than 15psi.

The only think that is different is the ID and length of the pipe.

In the 13's at last!.. Just

Ah, Interesting!

sounds like you now need the restrictor trick,

my mony is on the pipe being kinked behind the dash

And you're correct. The pipe was squashed between the engine steady and bulkhead. I've un kinked it and put the bleed valve back onto the original pipework now and can bleed it off to 20psi :) No fluttering stuff going on either :)

In the 13's at last!.. Just

Sweet!

next stop switchable boost!

Edited by Joe C on 1st Jun, 2011.

bit late in spotting this one, but yeah the std gt20 actuator gives 15psi...i thought it was 17psi at first but my gauge over reads.