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03-03-2007, 12:22 PM | #201 |
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Some very interesting discussion going on here. I'm interested to see what happens when some of the people gathering this data get access to a dyno to test their results and see what actually makes power.
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03-03-2007, 02:19 PM | #202 |
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The more the Mass Air Flow, the the higher the possibility of more power.
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04-23-2007, 08:24 PM | #203 |
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Guys,
I have a wicked stg 2.5 to try out on this Subaru. I will keep everyone posted. It would take a couple of months. http://socob.bb/forum/viewtopic.php?t=315 Hint, I will betesting from 0-52 |
04-24-2007, 01:53 PM | #204 |
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Subscribed. This is one of the most informative threads I've seen on any forum. I will try to collect some more data to contribute when I can find some free time.
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04-25-2007, 01:22 PM | #205 | |
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The whole point of tuning the AVCS is to trap as much clean air into the cylinder as we can without pumping it into the exhaust. When tuning the stock ECU, set AVCS to Zero and scale the MAF so Actual AFR matches mapped AFR. Airboy showed that adding AVCS increased cylinder load, but what about AFR. The ECU calculates the fuel based on MAF flow, if the flow goes up the AFR should stay the same as mapped since your MAF scale shouldn't change. If you start seeing leaner AFR, then you are pumping fresh air into the exhaust. But is pumping alittle bit of air into the exhaust to get a fresh cylinder charge optimal? Maybe only at lower manifold pressures, once boost goes up I'd like to trap as much of that air in the cylinder. It is too bad the AVCS turbo engines don't also have a AVLS system as well. It would be nice even if the AVLS was only on the intake side, at higher rpm it would be beneficial to have a longer duration and even close the valve further after BDC to cram as much air into the cylinder under boost. Writing this post gives me ideas for custom cam grinds.... |
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04-25-2007, 05:11 PM | #206 | |
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And I definitely agree that more MAF doesn't necessarily mean more power, because MAF is exactly that, Mass Air Flow. It doesn't know where the flow is going, just that it's going past the MAF sensor. |
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04-25-2007, 05:24 PM | #207 | |
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as far as "wasting" clean charge air out the exhaust port, i don't think that's too much of a concern... perhaps in an NA build to a greater extent, but not here. the reason is that exhaust gas backpressure is higher than boost pressure the entire time that the engine is really making power. the only effect that will cause the charge air to push out exhaust air is due to momentum/inertia. i would gladly give up a little bit of charge air that is "first" in the chamber since it will be highly diluted with exhaust gasses anyway. jm2c ken |
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04-25-2007, 05:27 PM | #208 | |
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exhaust gas backpressure is always higher than boost pressure, often higher than 2x boost pressure, and sometimes with overspun and/or inefficient turbos higher than 3x boost pressure. |
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04-25-2007, 05:32 PM | #209 | |
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04-25-2007, 05:37 PM | #210 | |
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04-25-2007, 06:13 PM | #211 | ||
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look at the big power high rpm cams... huge duration. how do they make power? by taking advantage of these inertial effects to pump air in and push air out. their NA VE goes up. by your reasoning they are throwing away power because they're wasting clean charge air. when have you EVER heard of VE being a "bad thing?" Quote:
my stance: you move more air through the engine and you have set the stage to make more power. every bit of reading i've ever done wrt tuning engines is focused on maximizing VE. if MAF goes up for a given MAP, MAT and RPM then you have just increased VE. |
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04-25-2007, 08:10 PM | #212 | |
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You keep trying to compare a N/A engine to a Forced Induction one, I never said exhaust scavenging is a bad thing. BUT, without an increased duration to compensate for air lost scavenging you are going to lose VE. Even in a boosted engine you are going to need a bigger duration at high rpm which is why it would be nice if subaru added AVLS to the turbo engines. My Stance: Adding AVCS at high rpm will net lower power even if tuned for. |
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04-25-2007, 09:32 PM | #213 |
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Exactly. If the air is going out the exhaust port, it isn't going into the cylinder. If you have a finite amount of time to pump air into the cylinder, then if air is going out the exhaust port it's that much less air that stays in the cylinder. As Master says, if you could also increase duration to compensate for this so you wound up with the same net amount of time pump air into a cylinder with a closed exhaust port (so you're actually pumping air into it, not through it), then maximum scavenging would be a good thing, because it would give you the highest VE. Unfortunately, we can't change duration, so we can't compensate for the wasted air that's pushed out the exhaust port. Therefor, at high RPM's, where there's a lot of air velocity, it's more efficient to rely on that to clear exhaust out of the cylinder rather then overlap. At low RPM's, it is not. Hence why Subaru put AVCS on the car in the first place.
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04-25-2007, 10:49 PM | #214 | |
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Bottom line, more airflow is more power potential. Unless you screw up the spark and/or fueling significantly, you're gonna make more power... |
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04-26-2007, 05:39 AM | #215 |
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04-26-2007, 01:48 PM | #216 | |
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THe problem is that your stance is not correct. I really hate to see someone make such a huge mistake in blanket statements. Its like you worked this out in your head and confirmed to yourself you where right and now thats Gospel. C |
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04-26-2007, 02:09 PM | #217 |
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What's wrong with his statement? You'll certainly never drop below 100***37; VE, but you will still loose a bit of VE. Of course it also depends on how you measure VE as well.
Again, we really need someone to put up some dyno graphs comparing various AVCS advance's power curves to really be sure of what is going on. |
04-26-2007, 02:17 PM | #218 |
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THe fault with his statement is that its theory.
C |
04-26-2007, 03:04 PM | #219 | |
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tell me how you would even MEASURE how much clean charge air is "lost" out the exhaust port during the incredibly short duration of valve overlap? you can't. nobody can. so how on earth would you expect to even have an issue with the "definition of VE" if after 100+ years of ICE engineering, testing, and development, there's only one way to measure it? there is no question here. VE is, was, and will be how much air IS going into the engine vs. how much air SHOULD BE going into the engine. you don't have to listen to me. clark is here telling you you're flat out smoking crack, and he's got an infinite amount more experience tuning avcs than i do. |
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04-26-2007, 03:11 PM | #220 |
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I've done some more AVCS testing at the drag strip this past week. I need to do some more testing, but I think I've at least come up with an efficient way to test for real results (i.e. trap times in addition to airflow/map/VE calculations previously discussed).
I will actually post my results as soon as I can reasonably convince myself of the effects rather than just tell people they're wrong... Last edited by Freon; 04-26-2007 at 03:17 PM. |
04-26-2007, 04:19 PM | #221 |
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Of course VE is the amount of air that is in they cylinder vs what should be. But you can measure that based off the intake, off the ambient air pressure, off the intake but based on the density of what air is at 20*c rather then what it actually is at that moment, etc. All of which give you different VE's, and one of which gives you a VE that actually changes with boost. That was what I meant when I said "depending on how you measure VE"
Also, quite obviously either the amount of air being lost out the exhaust port, or the fact that it sucks out some of the charge fuel, at high RPMs actually matters, otherwise running large amounts of advance at high RPM's would be a good thing, which obviously it isn't based on data presented throughout this thread. I'm not saying that no advance is best, that's obviously not true, but I'm saying that it's probably only a small amount just to give that little boost to scavenging, because otherwise you start loosing too much air or too much fuel. Last edited by AruisDante; 04-26-2007 at 04:30 PM. |
04-26-2007, 04:38 PM | #222 |
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You can measure net VE if your MAF scaling table is accurate. Even if it isn't that great you can still calculate relative changes to VE before and after changes to your AVCS map. It's certainly a valid method, even if it won't be the only thing you'll want to look at.
This will not tell you if you are getting reversion or if you're passing clean air through to the exhaust side. You'll have to watch knock activity, WG duty, EGT, etc. to make better guesses on those. Even a road dyno isn't a bad technique. Again, it may not necessarily be accurate, but as long as it is repeatable and consistent, you can note if you have gains or losses. |
04-26-2007, 08:31 PM | #223 |
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Good conversation everyone. Nice
Freon, I cannot wait to see the results. One of my client will do it himself again with my help and we will have fresh data. |
04-28-2007, 04:49 PM | #224 |
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For what it is worth, you can measure how much is blown thru on overlap...
There are a ton of variables, but it all boils down to a reduction in exhaust temp, and an increase in torque... The air that gets blow thru, will be a lot cooler, reducing the exhaust temp... I did an experiment on mine for a 200-300 degree temp drop as a guide... My advance map now tapers off from boost onset, following a curve, to about 6k... I found that there is a direct relation to the Boost level and VE of the engine to get what I would call the right amount of blow thru... I did all of these tests on the road in, 4th gear, with a logger, and a g-force meter to confirm when acceleration dropped off from to much blow thru... I discovered real quick, that too much blow thru is a bad thing... I obviously lost power with too much blow-thru... I would be curious to see if what I did shows a difference on a dyno, it did show that it accelerated faster, but that might not show on a dyno with a different acceleration rate... |
04-28-2007, 05:03 PM | #225 | |
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i'd love to see a situation in which more MAF is measured, everything else stays the same, yet the rate of acceleration decreases. i also wonder how it comes to be that the egts go down. certainly the uncombusted charge air is obviously a hell of a cooler than once it's fired-off, but it also has a nice ripe AFR as it has already been fueled yet not ignited. i would imagine that air fuel mix would be more than willing to combust once it hits the fiery environment of the exhaust manifold. |
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