Hot air causing knock?
As I said Chris ... DLL shows identical power comparing the summer and winter tunes. you are correct ... I gained no power, but the winter tune was much further from the knock threshold in the summer.
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I'd say its the higher humidity and lower density of air in the summer.
I hate to bring this up again ... but I can run more aggressively in the summer than the winter. I know it goes against most of what we know, but I've seen it 2 years straight on my own car.
I just re-tuned about a week ago and I'm running about 2º advanced across the map from my winter tune with 0 knock. Boost is ECU controlled and PSI based and I didn't change those maps. I was at 2º at peak boost and 10º at redline ... I'm now at 3º-4º at peak and 13º at redline.
Last summer I ran 26 psi and had no issues. As the temps dropped knock started to occur. 7-9 counts on a drag run. I pulled timing, knock went away and my traps and ETs got better by about .4".
A few of us actually had a pretty big discussion on this a while back ... the possible differences between denser air at the intake and denser air after the intake (better FMIC). Even stranger still, my upgraded FMIC (which should have a similar effect as cooler temps) also allows me to run more timing. So the data is contradictory. We tossed around the idea of winter blended fuel being a partial cause.
I will say that DLL shows basically identical power curves when comparing the less advanced winter tune and the advanced summer tune ... so you do make more power in the winter with less timing. No issue there.
I just re-tuned about a week ago and I'm running about 2º advanced across the map from my winter tune with 0 knock. Boost is ECU controlled and PSI based and I didn't change those maps. I was at 2º at peak boost and 10º at redline ... I'm now at 3º-4º at peak and 13º at redline.
Last summer I ran 26 psi and had no issues. As the temps dropped knock started to occur. 7-9 counts on a drag run. I pulled timing, knock went away and my traps and ETs got better by about .4".
A few of us actually had a pretty big discussion on this a while back ... the possible differences between denser air at the intake and denser air after the intake (better FMIC). Even stranger still, my upgraded FMIC (which should have a similar effect as cooler temps) also allows me to run more timing. So the data is contradictory. We tossed around the idea of winter blended fuel being a partial cause.
I will say that DLL shows basically identical power curves when comparing the less advanced winter tune and the advanced summer tune ... so you do make more power in the winter with less timing. No issue there.
i agree, colder air can cause knock.. I believe, because its causes the motor to run more aggresively...
The next time I log my car I should do a airflow vs boost comparison and see how much the air density differs ... if my thoughts are correct, we should see a bit less airflow at the same boost level. This makes sense since the faster moving molecules in the hot air should build more boost with less flow.
The next time I log my car I should do a airflow vs boost comparison and see how much the air density differs ... if my thoughts are correct, we should see a bit less airflow at the same boost level. This makes sense since the faster moving molecules in the hot air should build more boost with less flow.
Also, as mrfred noted, in summer, even at the same relative humidity, you have much more water in the air than in the winter. Depending on temperature, as much as 10x or more in the summer. 100% relative humidity in the winter doesn't mean the same amount of water in the air as 100% relative humidity in the summer. Here is a little chart to help explain that:
Last edited by l2r99gst; Jun 13, 2008 at 08:10 AM.
By airflow, I hope you mean mass airflow and not Hz. In that case, just do a load vs boost comparison. You will see that your load will be lower in summer for the same boost pressure than in winter. Remember that load is a mass airflow/rev.
Also, as mrfred noted, in summer, even at the same relative humidity, you have much more water in the air than in the winter. Depending on temperature, as much as 10x or more in the summer. 100% relative humidity in the winter doesn't mean the same amount of water in the air as 100% relative humidity in the summer. Here is a little chart to help explain that:
Also, as mrfred noted, in summer, even at the same relative humidity, you have much more water in the air than in the winter. Depending on temperature, as much as 10x or more in the summer. 100% relative humidity in the winter doesn't mean the same amount of water in the air as 100% relative humidity in the summer. Here is a little chart to help explain that:
J/KSuper on point on this one. Relative humidity != Absolute humidity. I'm nearly certain that you can run more timing in the summer for two reasons.
1) abs. humidity is higher on average causing lower burn rates (think WI)
2) AFR = constant, boost = constant, air density goes down, which means for the same knock threshold (peak cylinider pressure timing) you can add timing.
I would guess #2 plays a more significant role.
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I am seeing the same thing. In winter my car was knock happy at a lesser boost level than I am in now. But even in 110 degree weather, I barely see any knock even during continuous pulls. Timing same for both. Even though the AFR richened a bit, but I leaned it back to 11.3-11.5 without any adverse effects. Will confirm my findings this winter.
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I have to agree with touring bubble, the timing is more forgiving in the summer time. Except if you are trying to run too much boost as it will heatsoak the intercooler much more quickly on a 100* day. As far as the engine goes it's always relatively the same temp. The thermostat controls that. It's the air temp and density that are changing.
colder air is more dense right...
denser air is more prone to explosion/ignition
that or maybe the ECU is running less timing on hotter days than what the map suggests.... so then we think we can put more timing in...
denser air is more prone to explosion/ignition
that or maybe the ECU is running less timing on hotter days than what the map suggests.... so then we think we can put more timing in...
A MAF sensor measures air density / temp by design. A map sensor on the other hand just reads pressure and uses an IAT sensor to compensate for specific air density/temp. This is over simplified as most MAF based systems incorporate weather variation correction tables as well.
Last edited by Mr. Evo IX; Jun 15, 2008 at 07:30 AM.
Mrfred helped me figure this out when I had my boost control issues late last year ... timing follows the "corrected" load value which is the same load the boost follows before Mrfred's boost control mods. This is the same reason fuel doesn't correct for the thinner summer air as it's load values are not temp/baro corrected.
Also, to he airflow/load comments above ... My corrected load increased by about 15-20 from winter to summer. I'm not sure how the mass air flow calculations work with load values, but maybe someone could chime in ... I would think an increased corrected load value would mean more air flow, but I'm not sure ... it seems backward from what I would have thought.