BOOST DROP AT WOT
12-11-2004, 07:33 AM
#16
Instructor
Yes, the factory manual states pressure drop starting at "higher rpm" and reaching 1.52 bar at 5800 rpm for the 951. For the 951S, it states that full pressure (1.75bar)should remain until 5800 rpm, and only then start to drop.
My 951S starts dropoff a bit earlier than that - it would be nice to improve it ........but first, I need to find the reason for the occasional cutouts.
Tommy
My 951S starts dropoff a bit earlier than that - it would be nice to improve it ........but first, I need to find the reason for the occasional cutouts.
Tommy
12-11-2004, 11:29 PM
#17
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Not quite.
Try taking the control hose off the wastegate or try to clamp it off. it will HOLD pressure, the spring is NOT WEAK it is the cycling valve that allows the boost pressure to open the wastegate. This is programmed into the DME. Have you not noticed that as you get on the throttle you get an increase in boost, then it falls off, then take your foot off the throttle, and mash it to the floor again, it will again temporarily overboost as it was designed.
The stock wastegates will hold all of the pressure that the exhaust can produce without bleeding any off, it's the DME that allows the CV to control the level of boost by providing pressure to the wastegate control side making it open. Remove that signal and THEN you'll see what the turbo can really boost to. The K26/6 will overboost the car at anything below redline. Yes, the k26/6 is not efficient at the higher rpms/flow rates, and will fall off, but it's still capable of overboosting the engine (that's another set of posts).
In summary, the wastegate is not a weak point, the fact is that it's working as designed, and fully capable (with the correct control signal (air line pressure)) of staying fully closed to provide full exhaust flow to the turbo at all rpms. The newer twin port wastegates allow for better control (since they are using the air (boost) pressure to open and close the wastegate, versus the stock single port - which only uses boost pressure to open the wastegate and spring pressure to close it), but the stock wastegate is perfectly fine for holding boost.
That is all I'm saying.
Dal
Try taking the control hose off the wastegate or try to clamp it off. it will HOLD pressure, the spring is NOT WEAK it is the cycling valve that allows the boost pressure to open the wastegate. This is programmed into the DME. Have you not noticed that as you get on the throttle you get an increase in boost, then it falls off, then take your foot off the throttle, and mash it to the floor again, it will again temporarily overboost as it was designed.
The stock wastegates will hold all of the pressure that the exhaust can produce without bleeding any off, it's the DME that allows the CV to control the level of boost by providing pressure to the wastegate control side making it open. Remove that signal and THEN you'll see what the turbo can really boost to. The K26/6 will overboost the car at anything below redline. Yes, the k26/6 is not efficient at the higher rpms/flow rates, and will fall off, but it's still capable of overboosting the engine (that's another set of posts).
In summary, the wastegate is not a weak point, the fact is that it's working as designed, and fully capable (with the correct control signal (air line pressure)) of staying fully closed to provide full exhaust flow to the turbo at all rpms. The newer twin port wastegates allow for better control (since they are using the air (boost) pressure to open and close the wastegate, versus the stock single port - which only uses boost pressure to open the wastegate and spring pressure to close it), but the stock wastegate is perfectly fine for holding boost.
That is all I'm saying.
Dal
Originally Posted by hosrom_951
Dal Heger: I did not say that the stock wastegate is bad, i said that the spring is weak. Porsche MADE IT THIS WAY, even BRAND NEW 944 Turbo Wastegates will still bleed off. They are designed that way 
12-11-2004, 11:36 PM
#18
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Dal: Not to start anything, but in that case if you upgrade the turbocharger (which would be capable of holding boost to redline) and bypassing the CV and getting a set of chips, you would still have boost dropping at upper rpm range. At least that is what was dicussed here, and ended in replacing the wastegate with a better unit (Tial, LR Dual Ports etc..) to hold to boost to redline.
12-12-2004, 04:25 AM
#19
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To answer a question asked a while back: Yes the DME can cut-off fuel. There should be a significant difference in the way this feels however, compared to a CV cut-off. If the DME cuts fuel it will be more like a total choke of the engine. No fuel, no more boosting pressure, its much more drastic than less boosting pressure.
I believe Dal has hit the mark here. The wastegate will hold whatever pressure it is given, thats what causes overboost which can happen in any car. In old age, the spring can become weak, and may leak off exhaust pressure that should normally be held perfectly fine. Thats what shims are for, shorten the travel of the spring, increase spring pressure. If the boost will not rise past 1.2, the CV valve has failed. The valve is a normally closed solenoid switch, so when non powered it will pass pressure to the WG by default, limiting the boost severely.
The problems with the smaller hotside is not as focused on their ability to move larger volumes of air, as it is on how fast the turbo would need to spin in order to achieve the same CFM. Modding your car to allow a smaller hotside to hold boost close to larger one would be severely endangering the turbo.
I believe Dal has hit the mark here. The wastegate will hold whatever pressure it is given, thats what causes overboost which can happen in any car. In old age, the spring can become weak, and may leak off exhaust pressure that should normally be held perfectly fine. Thats what shims are for, shorten the travel of the spring, increase spring pressure. If the boost will not rise past 1.2, the CV valve has failed. The valve is a normally closed solenoid switch, so when non powered it will pass pressure to the WG by default, limiting the boost severely.
The problems with the smaller hotside is not as focused on their ability to move larger volumes of air, as it is on how fast the turbo would need to spin in order to achieve the same CFM. Modding your car to allow a smaller hotside to hold boost close to larger one would be severely endangering the turbo.
12-12-2004, 01:10 PM
#20
Race Director
danno here...
"The wastegate will hold whatever pressure it is given,"
Uh no... try the clamping-the-hose test and gently roll on the throttle and try to maintain 15psi boost in the mid-range. Then gradually give it more and more gas as RPMs increase. At some point, you will have it completely FLOORED with the hose to the wastegate clamped off, and you'll still have a dropping boost in the upper-RPMs.
There is an unregulated max-flow boost-curve in all turbos. A lot of the Honda guys run their turbos full-steam without ANY boost-controller or wastegate at all. Ends up with 35-45psi boost in the mid-range that drops off to 20psi by redline. Then they just map fuel & ignition to match this particular boost-curve. Just scale it down to fit our cars. The wastegate then is to cut off the peak of the boost-curve, like clear-cutting a mountain. But it cannot make the turbo generate more boost than its max-flow rate.
"The wastegate will hold whatever pressure it is given,"
Uh no... try the clamping-the-hose test and gently roll on the throttle and try to maintain 15psi boost in the mid-range. Then gradually give it more and more gas as RPMs increase. At some point, you will have it completely FLOORED with the hose to the wastegate clamped off, and you'll still have a dropping boost in the upper-RPMs.
There is an unregulated max-flow boost-curve in all turbos. A lot of the Honda guys run their turbos full-steam without ANY boost-controller or wastegate at all. Ends up with 35-45psi boost in the mid-range that drops off to 20psi by redline. Then they just map fuel & ignition to match this particular boost-curve. Just scale it down to fit our cars. The wastegate then is to cut off the peak of the boost-curve, like clear-cutting a mountain. But it cannot make the turbo generate more boost than its max-flow rate.
12-12-2004, 01:42 PM
#21
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Actually, Dal Heger,
also, not quite,
The wastegate spring IS weak; however on purpose by design. This allows the exhaust back pressure to contribute to the opening of the wastegate. Therefore, it's the COMBINATION of the pressure signal allowed by the CV via the KLR(not the DME), and the back pressure that opens the wastegate.
So yes, if you clamp off the pressure signal, boost will rise dramatically; but at a lower level than you may think the exhaust pressure will force open the wastegate no matter what. At what boost this will happen depends on the effeciency of the exhaust housing.
So, there is a real engineering reason why the #8 housing, BY NATURE, will have less of boost drop off; and yes, Porsche programmed this into the engine management to insure this will happen in order to keep the exhaust back pressure in check.
If you increase the flow capability of the turbo (intake, exhaust, or both) you will have an increase of boost without changing a single thing with the boost controls.
also, not quite,
The wastegate spring IS weak; however on purpose by design. This allows the exhaust back pressure to contribute to the opening of the wastegate. Therefore, it's the COMBINATION of the pressure signal allowed by the CV via the KLR(not the DME), and the back pressure that opens the wastegate.
So yes, if you clamp off the pressure signal, boost will rise dramatically; but at a lower level than you may think the exhaust pressure will force open the wastegate no matter what. At what boost this will happen depends on the effeciency of the exhaust housing.
So, there is a real engineering reason why the #8 housing, BY NATURE, will have less of boost drop off; and yes, Porsche programmed this into the engine management to insure this will happen in order to keep the exhaust back pressure in check.
If you increase the flow capability of the turbo (intake, exhaust, or both) you will have an increase of boost without changing a single thing with the boost controls.
12-12-2004, 02:33 PM
#22
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Dal Heger,
As Danno and Tommy already pointed out here, you are not quite correct.
First of all, as Tommy pointed out, the DME doesn't control the CV, the KLR does. The DME does handle the fuel cut overboost protection which is based on air flow/rpm limits. The KLR also has an boost checking feature that tries to lower boost to something like 0.2-0.3 bar when error are detected.
Second the stock wg spring IS VERY WEAK! It was designed that way on purpose. I've tried the clamping-the-hose test (actually the removed-the-hose test) and at 4000 rpm I got 1.45 bar but at 6000 rpm I only got 0.8 bar. This was with a rebuilt wg and a K26/8.
The point here is that with an aftermarket wg, a shimmed stock wg or a stock wg with a different spring you can get better boost at higher rpm but the gain will not be that great as the turbo is out of it's efficiency range. Change to a bigger turbo and boost will go up in the higher rpm range even with a stock wg.
Tomas
As Danno and Tommy already pointed out here, you are not quite correct.
First of all, as Tommy pointed out, the DME doesn't control the CV, the KLR does. The DME does handle the fuel cut overboost protection which is based on air flow/rpm limits. The KLR also has an boost checking feature that tries to lower boost to something like 0.2-0.3 bar when error are detected.
Second the stock wg spring IS VERY WEAK! It was designed that way on purpose. I've tried the clamping-the-hose test (actually the removed-the-hose test) and at 4000 rpm I got 1.45 bar but at 6000 rpm I only got 0.8 bar. This was with a rebuilt wg and a K26/8.
The point here is that with an aftermarket wg, a shimmed stock wg or a stock wg with a different spring you can get better boost at higher rpm but the gain will not be that great as the turbo is out of it's efficiency range. Change to a bigger turbo and boost will go up in the higher rpm range even with a stock wg.
Tomas
