Whats the ideal AFR for a NA subaru? for a Turbo Subaru? i think i am runing a little lean, i have about 14.5 AFR at low throttle cruising. this is from the front O2 sensor. shouldnt this be closer to like 13?
I run 14.7 under light throttle cruising and the egt's look sililiar to everyone elses so I guess I'm doing alright and so are you
light throttle cruising = closed loop the engine is under virtually no load when you are cruising, so your ECU will target a stoich (14.7:1) AFR. Totally normal. Turbo Subarus, depending on the quality of gas available and the desired tradeoff between safety and peak power, ideally get tuned for around 11.0:1 or so. People with crappy gas or who are being super safe might go for high 10's, people with race gas or water injection will be much leaner, into the mid or high 11's.
ahhh. i didnt know that stioch was all the way up there at 14.7:1. i know feel much better about my readings. whats the difference between closed loop and open loop, besides the fact that it is easier to tune in open loop? about the boost thing. when you are doing a light cruise (ie <30% throttle) but are at higher RPMS arnt you running boost? since the turbo is spooled?
Not necessarily, usually spool is induced by more load and therefor more air running through the engine. The more air, the more spool. It is very easy to get full boost while in 5th gear at decent rpm's because when you press the throttle, it requires more power (more air/fuel) to get the car moving faster. I hope this makes sense (it kinda makes sense to me )
OK, I have no idea what this tune is for. Top speed, road racing, drag racing, etc. That being said, most turbo cars will be able to do 12:1. This is on a low timing map, which will produce more horsepower than torque. What I do is start with a specific max timing and tune the AFR to the max without detonation. There are MANY ways to tune, I'm just adding mine into what you do for tuning, as an alternative to show how many ways there really are to tune. Closed loop is controlled by the oxygen sensor inputs and will try to keep the car @ 14.7 on it's own using fuel trims and oxygen sensor trims. Open loop runs on a set of specifically tuned maps and will never differ from what it's maps are set for. The <30% throttle thing really depends on how you're applying it and what kind of turbo you have. My stock wrx will spool at ~10%-15% (I'm only talking few lbs since it wasn't specified where you're actually considering "spooled.") On the other hand, I need a good 40% on my Eclipse or it won't be spooling at all. This is in 1st gear that I'm talking about since that wasn't specified either. :shock: The more air the more spool??? Haven't heard that one yet. I'd say the more spool the more air, since the turbo's moving toward it's efficieny range. I guess you could say that spool is load induced, but that's because it usually ties into <10%-<50% (depending on turbo) throttle. It's very easy to get boost in 5th because when you're in 5th you're usually at highway speeds (me anyways, 60+mph) and the rpms are already wayyy up there (~3k.) And where does the stock turbo spool at? ~2800rpm. If the WRX had a 6th gear like the STI, it would be harder to to spool it right away, because the rpms are so low. Consequently, you'd need to push the throttle harder with this imaginary 6th gear for it to spool.
i am slightly confused. i thought that because you are runing at highway speeds and say around 3k RPM, and the turbo spools @ 2800rpm as you stated. doesnt this mean that at 3k rpm there is adequate exhuast flowing through the turbo to give you boost at your light throttle cruising speed? or are you saying that at 3k rpms at light throttle there is not ample exhuast flow to generate boost and the amount of exhuast flow is determined more by the throttle then by the RPMs?
What I'm saying is, CRUISING you won't have boost. This is based on what I think about when "cruising" is constant speed, not going up, not going down. But, once you add that ~15% to throttle, there will be boost there. Sorry if I am explaining things badly, I just woke up and can't get back to sleep =/. BTW: of course hills affect this, so I'm only talking about a flat road.
yikes! Sorry about that one . The way I always saw it was that you let more air into your engine through the throttle plate while under throttle. This in turn allows more air to enter and then exit the engine. This extra air will provide more spool on the turbo which will then create positive pressure after the turbo (boost). Now the pressure is forced through the throttle plate into the engine and the cycle is repeated with better results due to the increase of air volume(a quicker increase in boost than before). I think I might be a little crazy thinking this, but it sorta makes sense to me. Maybe I'm just getting down to the nitty gritty and really worthless things :cry: I need to go back to bed :lol:
There is no such thing as Oxygen trims. :roll: It is short term fuel trim and long term fuel trim. Also long term trim is applied to open loop fueling, so it is not s set number every time.
Load is something you need to consider. An engine under no load builds no boost. It is the load on the engine that causes exhaust temps to rise and exhaust velocity to build due to the ECU delivering more fuel and more air to compensate for the load that is on the engine. If you approach a hill at 3000rpms @ 10% throttle, and you begin your climb at 3000rpms at 10% throttle, you cannot expect the engine to remain at 3000rpms with a 10% throttle input because the load on the engine has increased. Load on the engine is a force that impedes the motion of the engine output shaft. OK, an engine at 3000rpms @ 10% throttle with the forces of wind drag and tire drag acting upon it is at a cruising load levels. This of course is theoretical and the numbers wont exactly match what you might be seeing, but for the sake of example, this is where we are at. You are travelling 75mph in 5th gear @ 10%throttle input and 3000rpms. You will have no boost at this time as your engine is at the theoretical zero load, or the point where little to no throttle is required to continue the forward motion of the vehicle. The forces of drag from wind and road are overcome by the power to the output shaft. Now you come to a 15% incline in the road. You do not apply any throttle. The force of gravity is now acting against the forward motion of your car and is thus applying a negative force in an opposing rotational direction to the output shaft of the motor. This is load. If you remain at 10% throttle, the added load will continue to impede the forward rotational force of the output shaft. This will slow the car down until it reaches the point where 10% throttle is enough to overcome the new load. For theoretical purposes, we will say that 2000rpms is where 10% throttle will equate to a 'no load' situation when climbing a 15% grade. If you wanted to return your engine speed to 3000rpms, it would require more throttle input to overcome the negative load that has been applied to your engine's output shaft. Let's say you apply 20% throttle to your engine. The engine is now being asked to overcome the negative forces applied to it and increase its speed to 3000rpms. This creates more load on the engine. In the situation before, when you began the climb up the hill, you didn't ask your engine to try to overcome this load, you simply let the negative load slow the car down. By increasing the throttle and asking the engine to overcome the load you now see the engine begin to work. The exhaust temps will rise because the engine is now fighting this negative force to reach the no-load condition again. This increased exhaust flow and heat is what will spool the turbo. The turbo then applies positive air pressure to the engine...of course timing and fuel control would now play a role, but this is simply the basics. The engine reaches 3000rpms and you are still at 20% throttle. The turbo will stop spinning as the engine is comfortable at 20% throttle @ 3000rpms on a 15% incline. The 20% throttle is enough to overcome the forces of wind/gravity and tire drag that are being applied to it. This is why, at x rpms you can have zero boost, but at the same rpm level with more load you could be at full boost. I am tired, so that may be long winded and hard to follow. I'll condense it later if I feel so inclined.
lol, what a monster of a post! i get teh just of it though. i know understand that the spolin of the turbo is more relaint on throttle input (which controls exhuast output) then on just RPMs.
You will be hard pressed to find many street driven turbo Imprezas running 12:1 AFRs. I don't know if it's something inherent to the engine design or what, but very rarely have I ever heard of people tuning (under maximum load mind you) leaner than mid 11's to 1. And never on pump gas. You are right, there are many ways to tune, but I don't know why you would start from a timing figure plucked out of thin air and then achieve a final AFR for that amount of advance. Your maximum amount of advance will vary greatly on the flow characteristics and volumetric efficiency of the motor. Seems to me that it makes sense to optimize your AFR curve first and then work towards MBT while avoiding detonation. But this is totally an armchair opinion.
Most places will do just that, set boost (or get real close), set afr, and adjust timing from there. MBT is going to be different on every motor (based on volumetric efficiency), so starting there is a bad idea. Then the whole time you are doing this, you need to watch your egt just to make sure that you don't have a melt down. So basically, there is no BEST afr for turbo's or na. Most na stuff I would guess that you can shoot for 12:1, but again, you need to watch your egt. Russ
if you were running thar rich (12:1) on NA, shouldnt you have enough extra fuel coming to keep things cool? you would think that for instance runing at stioch (14.7:1) would be running alot hotter since you have less liquid fuel coming into the combustion chamber to keep the engien cool?
