|12-18-2004 12:13 AM|
I honestly think this is just due to significant variation in power output. If Saab had listed 230 crank hp for the 2.0 turbo people would be upset! It doesn't reach 230 every time you accellerate in certain kinds of weather. Saab just listed a number that is reasonable for all conditions, instead of just some.
At least, that's what I think. No proof either way.
|12-17-2004 10:20 PM|
|12-16-2004 01:06 AM|
|SaabTuner||Quick Summary/Explanation: Choke flow is a volume per second limit, rather than a mass per second limit: Ergo, the mass has to be compressed to fit into the volume limit to pass through an orifice during choke-flow. That compression is done by the engine, and is how you get excessive backpressure and EGT.|
|12-16-2004 12:47 AM|
Because between mach 1-2 or so the shock-plane is normal to the flow it should be able to discharge out the wastegate, at least in part.
The second paragraph might be better said like this:
The engine will discharge a given mass of exhaust gas at any particular load point. As choke flow is reached the total-pressure required for that mass to flow through the 05 cm^2 opening will increase. Since the dynamic, or velocity-pressure cannot increase to a value above mach ~1 the static pressure must increase.
Static pressure is pressure the engine will "see" as backpressure.
It also means that, once choke flow is reached, your 3" exhaust system will do very little, if anything to help exhaust flow out of the engine. That might explain a lot of people's findings with the 05 cm housing!
Man, I'mma nerd. Why don't you guys have a :nerd: emoticon!?
|12-16-2004 12:12 AM|
The two movies are run with the same boost,ecu,injectors and intercooler.
Movie #1 (Right click , save target as..)
T25 turbo with stock exhaust.
Movie #2 (Right click , save target as..)
TDO4HL-15T #6 , modified with ported wastegate and a stronger actuator.
3" downpipe and exhaust.
|12-15-2004 10:33 PM|
Unfortunately the transient flow characteristics through turbine housing are extremely complicated and difficult to explain without testing. I agree with your last statement Adrian. I believe the Ventury effect keeps the wastegate closed under nominal conditions. Only under extreme conditions, when the exhaust gas flow is beyond the ability of the turbine to flow the gas, will the wastegate be forced open. When the exhaust gas flow becomes larger than the flow through the turbine then the flow will stall and the dynamic pressure will become static pressure.
If the exhaust gas flow leaving the engine in greater than the turbines flow capability then there are only two options. The exhaust gasses will backflow into the cylinders and decrease power until the flow drops. Or conditions change inside the turbine housing and the wastegate is forced open.
With my current turbo setup and 6000ft elevation I can run my turbo without any boost control. I can disconnect the MBC and still have a smooth boost curve. I believe it is the combination of small turbo and wastegate being forced open that allows me to run this setup.
|12-15-2004 10:00 PM|
|12-15-2004 09:40 PM|
|stevehayes01||Ok its clear to see who the mechanical engineers are in this crowd!! Everyone else take one step back!!!!!|
|12-15-2004 07:59 PM|
|12-15-2004 07:50 PM|
|Tweek's Turbos||ERP, are you saying that if you push over 19psi you should have an uprated wastegate???? I'm lost.|
|12-15-2004 07:38 PM|
I respect your oppinion, ERP, but I'm not completely convinced that's a problem.
The ventury effect should lower the pressure against the wastegate because it is normal to the flow direction. Via the Bernoulli equation, high velocity gas has a low static pressure in the direction normal to the flow. However the Bernoulli equation only works for incompressable gasses, which means gasses well under the local speed of sound. The speed of the gasses near the wastegate is borderline ... though at the end of the nozzle it's reached mach 1.
On an exhaust with the stock manifold, the "bends" in the manifold will convert a lot of that velocity pressure into static pressure, which can "make it" out the wastegate.
On an exhaust with tubular headers, Dr. Boost (from the Saab factory) has noted that there are sometimes "problems" with the wastegate. Sometimes he says it cannot dump enough pressure! Quite the opposite from dumping too much. This is because the velocity pressure is maintained, and very little static pressure exists to "turn" at a right angle out the wastegate.
Do I make any sense?
Here's a page from the Glenn Research Center at the NASA website on static and dynamic (velocity) pressure: http://www.grc.nasa.gov/WWW/K-12/airplane/dynpress.html
Only static pressure can make it out the wastegate.
|12-15-2004 04:03 PM|
Adrian, I agree with your theory about exhaust gas flow through the turbine housing. But I want to throw in another variable. The pressure difference across the wastegate flap. As the exhaust gas flow velocity increases so does the pressure. At some point the pressure difference between the exhaust housing and downpipe will be large enough to force open the wategate and divert part of the exhaust gas flow.
The wastegate flap area is ~0.80 in^2 and the wastegate I tested took about 15 lbs to open the flap. This means it would only take a pressure difference of 18.75 psi to force the wastegate open. Seems really low. But even if this value is off by several factors it is still likely that the wastegate being forced open would be enough to limit the exhaust gas flow to subsonic speeds. Of course the flowrate through the wastegate is extremely important in this assumption so it should be calculated.
Once the wastegate is open it creates lots of turbulence. Which is where a larger diameter downpipe would become beneficial and an external wastegate becomes ideal.
|12-15-2004 02:09 PM|
it really was the point... i am not sorry that this thread took the turn that it did though because i have learned a bit more.... and that is the best part of forums... is what u leanr.. see i spell new things everyday....
|12-15-2004 02:06 PM|
T7 has an "optimum map". I actually have seen the "optimum" knock-retard map, though I have been asked not to share it. (And I will honor that request.) I do not want to lose the trust of the person who shared it with me. I do not want to make a big deal about that either.
I cannot be specific, but I think I can safely share some very basic non-numerical information. I cannot tell you the actual values, but this is useful even without the actual values ...
I use 91 octane and in cool weather I usually have the "optimal" spark map with only a cooler thermostat an watter-wetter as "mods". Cali 91 octane, according to all the Subaru tuners, acts like 89 octane everywhere else. I think it is safe to assume that 93 octane will produce an "optimal" map 99% of the time except perhaps in 95F+ weather.
The engine will run the "optimal map" as long as there is no knock, and no heat-soak. Even on 100 octane race fuel the engine will lower boost after a couple hard runs. I believe this is to prevent long-term damage from a high EGT. An exhaust won't change this since the ECU cannot directly measure the drop in EGT from the exhaust.
Because the engine does not usually have any trouble with knock, especially not on 93 octane, I don't believe the mods will allow any more power. T7 will not run unlimited boost until it knocks. Having run 100 octane fuel several times I can tell you that T7 generally runs as much boost on 91 octane (read: more like 89) as it does on 100 octane. Mods won't change that.
Does that make sense?
p.s. Rico, definitely right about T7 cars being underrated for the most part. Was that really the point of this thread though? You said in the beginning about T7 being more responsive to mods. Most of the comments I made were to try and show that those increases were probably just random. My car increased several horsepower each time I ran it.
|12-15-2004 01:58 PM|
|stevehayes01||Nope we dont! We have 91 but I will add in a gallon or 2 of 104 Race gas when I go so that should give a little bit more balance to it. We do have more consistent weather than most places with less adverse conditions and such. I usually run octane boosters in my car so I can atleast get to that awe inspiring 92Octane. I ran 104 Race on my C900 and I was able to pull 19PSI on a modded APC unit so I have faith that race gas will do a little for the T7 to equalize it a bit.|
|12-15-2004 01:54 PM|
|ricot83||which brings another question steve.... u guys dont have 93 octane in ca right? because i believe thats what i was using on the dyno|
|12-15-2004 01:45 PM|
Ok point taken Ricot! It is killing me having to wait until January to get the Dyno done on my car!! I want to know what it is putting down so I know where I stand. Sorry for high jacking your post man!
One more question Adrian....
If you run 110 Octane Race Fuel the T7 will produce more power because it see's less knock or chance of knock right?
Simply stated that is the whole basis of the T7 is to keep in the "Safe" zone of operation and run right on the edge of that boundary right?
So the higher the octane the less chance of knock the higher the T7 should allow the HP output?
This is derived from the theory that the T7 will only let the car run until there is a chance of Knock so it would make sense that the higher the octane the less knock potential there is so the more HP that would be derived.
|12-15-2004 01:10 PM|
|ricot83||ok the point of this post has nothing to do with the 7hp gain, but more so with the stock numbers , also i am happy to hear that with a 6cm2 housing i can flow better cuz thats what i have.|
|12-15-2004 12:51 PM|
I don't think it would throw a CEL with a few mods. If it was on low octane, like here in Cali, an exhaust and an intercooler would probably do T7 a world of good on our hot summer days! It'd be like having higher octane.
If you put an exhaust on T7 you have to wonder why the ECU would allow more air. T5 doesn't "see" the air, which is why you get more power. A Subaru's boost control system doesn't "see" the extra air either. (It's fuel mapping does, but not boost control.) But the T7 boost, fuel, and ignition computers all "see" the extra airflow. If they are designed to allow more airflow, what stops it from just running more boost while completely stock?
More importantly: The throttle is the mechanism that stops the engine from running too much boost (mg/c) when stock. How does that change with an exhaust mod? Why wouldn't the ECU close the throttle just like before when it reaches that same value?
See what I mean? With any other turbo car you get horsepower because the part of the ECU that controls the air going into the engine doesn't "see" extra airflow. Trionic 7 does see it. If Trionic 5 could "see" the extra air, it would stop it also!
Anyway. Someone with some mods needs to get a data-logger to see how much airflow is getting into their engine! That'd be a lot more putatively accurate than a dyno-run since all our MAF sensors are calibrated more or less the same, whereas dynos are not!
|12-15-2004 12:05 PM|
I still do believe that the SAAB T7 is capable of more than what everyone says or expects without throwing a CEL or reducing power output based on mods.
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