adjustable boost ?

[ficke]

One of the modifications that was done to my car by a previous owner was the installation of an adjustable boost control knob,or as translated from the German invoice 'steam wheel' . I wondered what the necessity is of this control? It would seem to me you adjust to safe level for your engine then forget? right? but I am finding out that that level changes all the time, in the AM I get almost a full BAR of pressure so I adjust down to the .8 BAR I feel is safe for my engine. Then in the after noon I will not make .6 BAR so I adjust up.
On a cool night I stopped for fuel, the intercooler must of heat soaked, I did not adjust but noticed that the boost pressure was down for ten min. or so of running?
What do others do with this control?

[jsveb]

It is a manual boost control. I got one just like it, even in the same position, on my 930.

Mine is locked down and the boost limited by the waste gate at 0.8 bar.
If you decide to play with it beware it can blow up the engine ie blown rings.

I would make sure to monitor afr a long with this. I mainly kept mine just because it is cool and old school.

[turbo ride]

The other name for this is the "Schrapnel Knobben" :-)

[81Kremer930]

Originally Posted by turbo ride

the other name for this is the "schrapnel knobben" :-)

nice!

[gumba]

Like every else, fun in moderation.

[mooney265]

see that allen screw on the side - - use it

mine is locked at 0.95-BAR.

[Miguel Antonett]

If you're reading the OEM boost gauge, it is not "that" accurate, it is more like a ball park indicator and with time it gets worse.. it could show variations that are not there; on the other hand, heat sink is an issue and it does affect boost levels. If you really want to monitor boost levels get a better gauge.

The boost control knob - lock it or get rid of it.

[ficke]

Thanks for the reply's.
What I really want to know though is how much is your boost effected by weather conditions? Mine is a lot. Many say to lock down the boost control knob to a pressure, OK I like that, do you do that on a cool damp morning and then on a hot dry after noon do you make full boost before redline? or do you do it on a hot dry afternoon and have massive full boost by 3500 rpm in the AM? It would seem the safe way is on a cool damp AM and give up boost for the other times. Is that what you do?
by the way I am reading my boost levels from a VDO gauge installed where the clock used to be, I believe it to be accurate. Thanks all for the help.

[jsveb]

In theory ( correct me if I am wrong) the boost is controlled by the waste gate spring only. This spring is not affected by the cold and dense air. The dense air does however make it easier for the turbo to generate the pressure, boost, but the waste gate should just dump it the way it always does.
However, I think it is commen that on the first boost, so to speak, you can make the boost pressure spike above the set limit, usually set by the wg spring, but in your, and my, case also by the manual boost control - however not lower than the spring.

The temperature change makes a significant change in available power.

Performance wise the engine is directly affected by density altitude. Density altitude is again directly affected by temperature. The density altitude goes up 120 feet per one degree increase celsius.
This is a huge impact on performance

Right now the temperature here in lower Alabama is down to about 0 C. In the summer we see 40 C. This 40 degree difference adds up to a difference in density altitude of 4800 feet.
If we then compare this to the fact, that at about 18000 feet in standard atmosphere conditions the ambient pressure has been reduced to half of sea level pressure, 29.92 Hg or 1013.25Hp, the drop is not quite linear, but close enough, we will be able to guesstimate what the effect actually is.

A SWAG based on the above should give us a performance incr/decr of about 15 percent.

This is also why we, a specially on a modded car with out efi, should monitor afr. It will run leaner in cold winter temps.

[cole930]

Yeah !!!! What Jesper said.

Cole

[JakobM]

without a EFI tuned map for higher boost I would never use a manual boost valve. on an oem 930 setup, for me, it is way too big of risc running lean/hot and having damaging detonation. It is something from the past, many people have blown pistons and valves on that account. you can drill a holes in the wg hose and you get same effect. today you can visual control your tuning. yes you can try display afr but a "driver/owner" is usually not an experienced tuner, hence no clue what happens when turnining the valve e.g. inlet temp., ignition, egt, etc. all "horisontal" ignition curve after manual distributor/boost retard has set in. too risci. I dont like to run a 930 with a doctors/tuners telescope on ears otherwise when doing tuning/mapping

[jsveb]

Jakob, I am not arguing against efi. Simply because it makes sense, efi that is.
But, the 930 itself is from the past. So old school, to some, makes sense.
There is generally no sense in the manual boost controller, but I actually want to use it as a safety precaution. I got a waste gate with a 0.6 bar spring in it. So I can dial it down. Valet mode if you will.

I also believe if
We are realistic and keep our goals within the capabilities of CIS, we don't need to monitor all these variables, its safety is Inherently built in by the Porsche engineers.
This is only true for a balanced and fully operational CIS.

[ficke]

Jsveb, thank you for the reasoned and thoughtful response. Yes the spring will release pressure at the same amount every time. but under different atmospheric conditions that pressure is built at different RPM,s there fore changing the power band.
For Street driving or people who have 'point and shoot' driven style this is not an issue.
But for people who drive closer to the edge, carry more speed in turns, when the power comes on is very important especially in a rear engine car.
Porsche had these boost knobs installed on factory built race cars, they were not installed to give more power at a dramatic moment in the race because every moment is dramatic and the engine should be at maximum safe power through out the race.
No, I believe they were installed for driveability, to adjust boost to meet the changes in weather conditions, to maintain a predictable power deliver.
In so many words that has been my question.

[JakobM]

jsev, you can not dial a wastegate boost down with a boost/bleed valve
minimum boost is always determined by the spring, that is why you should always pick a wg with a low spring rate (requires low turbine back pressure) - and then "bleed" either manually or electronically to your desired boost levels in order to go back on low boost. you can never regardles efi, cis etc. bleed down a wg. The minimal boost level is always determained by the spring (if you dont bleed air through a blow off valve), where as the upper boost level is controlled by "fooling" the wg.

a bleed valve is very far from the process of changing a spring, the operator needs to know to dial very very carefully knowing what to look and hear out for. most people dont know what detonation sounds like, what afr should read or what damage detonation makes. detonation still happens regardless "safe" afr readings. a boost valve do not have an upper limit like changing a spring - it can let you go +2 bar instantly when bleeding air. Almost whenever I have come across a manual boost valve on non-efi setup either the current owner or the past owner has felt for the "try" and went too far on turning on the valve. I had a 930 seller at one point with a cis track car with a boost valve mounted on the cage in the front. he told me he normally did 1 bar, he had tried 1,5 bar on track but felt the car did not run well on that level so he went back on 1 bar. He new nothing what the engine just tried tell him and how he messed up. I instantly lost all interest on that car. without his story I would still have calculated with an engine tear down just seeing the valve.

[jsveb]

Jakob, regarding WG spring setting the lower limit, that is actually what I tried to say in my post #9 above and also referred to when I mentioned my 0.6 bar spring since this is lower than stock on the 930, hence the term " dialing it down.".

Ficke, regarding available engine power:

I am fairly certain Porsche indeed used different power levels at different times. The drivers were able to dial it up if needed. Also today teams use qualifying trim/modes and race modes on the engine management systems etc.

But you are right, it is important to know when the power comes online, especially in a rear engine vintage-style (read: lag) high performance car . And, those characteristics do change some with environmental changes.

[JakobM]

ficke, your boost reading should be the same regardles air temperature, density etc.
the reason is that .8 is .8 bar regardles how many air molecules you get "packed" in the .8 bar (cold or hot air)
a WG works on pressure difference under vs. above wg actuator membrane, not difference in air molecules
the pressure at sea level warries only little (wheather) around 100 kpa / 14.7psi / 1 bar and warries equally both under and above the wg actuator membrane as back pressure is build on top of the atmospheric pressure

This has nothing to do with the fact that an engine runs with much more power from .8 bar boost with cold air (more molecules) than .8 bar boost with hot air (less air molecules)
The difference to note is "equal" pressure with changes in amount of air molecules

if your boost levels is changing, then something is wrong in the setup. could be a loose hose or connection. As a start I would unplug the bleed valve hoses and connect as oem. if you still have unstable readings, and your hoses is intact, then the wg it self may needs to be looked at, or check your manifold/turbo/wg/inlet flanges for loose fit

[JakobM]

Originally Posted by jsveb

Jakob, regarding WG spring setting the lower limit, that is actually what I tried to say in my post #9 above and also referred to when I mentioned my 0.6 bar spring since this is lower than stock on the 930, hence the term " dialing it down.".

I then misunderstood you I saw your post as an intention to turn down below starting point. so you are bleeding to e.g. .8 bar and then turning down to starting point .6 bar. nice setup

[1986 930]

My 930 has a RUF Variable Boost. It has the same .6BAR Spring and when you dial in all it can give, it is .9 BAR with a stock IC. For some strange reason the orig owner paid many $$$ to do a RUF conversion but did not go with the RUF Intercooler much to my dismay. So with a .6BAR Spring your available range is .6BAR - .9 BAR

I am still not sure why you would want to adjust it down but you can. I guess if you are running a bigger spring, then it might be good to not have it making some crazy boost, like 1.5 BAR or something similiar all of the time. That generally ends with all of the above horror stories. I guess a vairable boost would be handy if you wanted to maitain a specific presure at various elevations, ie, mountain driving, much like old recip aircraft did with turbos to help with maintaining HP at altitude. I guess if you live at the foot of the mountains and go driving up the hill a lot, it would make more sense.

For street driving you can leave it dialed in at .9 with no ill affects. I do not track/race mine so that could be different. How the boost comes on is also a factor of how you apply throttle when you are driving. There is a pretty good variation with how the boost comes on depending on when you get on it. My stock turbo had a hard time getting .9 BAR except for an acceleration run up the freeway on-ramp and almost never in 1st gear. With a K-27 HFS + B&B Headers/Exhaust, I get .9 BAR pretty easily and it holds all of the way to redline.

You will find that a cold, dry, day is what your 930 likes the best. Like others have said, the weather conditions will effect how your fuel injection systems gets that presure in the exhaust and how the air cools your engine and its components. Typically that is what you are feeling in the "seat of the pants" when your car seems to run better in certain conditions.

[RarlyL8]

I am not a fan of adjustable boost on a CIS 930. What is the point? Set the engine up to max safe power and lock it down. CIS is a static system which you can set to optimal AFR for a specific boost level. If dialing in more boost you are in no-man's land, so AFR must be set to accommodate max boost. If so it is no longer optimal at lesser boost levels unless more controls are added.

What I would use adjustable boost for is the opposite. To dial it down for when someone other than myself is driving the car. This is where a TiAL waste gate with 0.2bar spring comes in handy.

As for the odd weather related boost figures you see boost is boost. Hot humid weather will make the turbo lazy and increase lag but full boost will still be reached long before redline. If not there is a problem somewhere.

[mooney265]

Just took the car out for a spin with the cold front fully in and boost set at 0.95BAR. All's I can say is my car can still scare the b-jesus out of me!!

Did a couple 2nd and 3rd gear pulls to red line and got my adrenaline flowin!! Woo-Hoo!!! Not too sure if I'll be calmed down by bedtime [10-hrs from now]...

In keeping with the original post, I didn't crank up the boost for more Nor, did I even consider it...