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Few observations while doing long distance road trip

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Heyo, currently on a road trip from Perth to Brisbane then to Tasmania, more details on that are in the off-topic thread here:

but few things ive noticed as ive been keeping an eye on fuel economy and how the car behaves in general (oil/water temps, pressure. volts etc) so I can spot if anything is off and one thing ive noticed is the car needs slightly more throttle and actually sits on 1psi of boost to maintain 120kph but after about half an hour or so needs a little less throttle and will sit at about -1.5psi (GTR with ITB's hence the low vac reading at cruise)

Engine oil gets up to temp well before this happens so im thinking maybe gearbox and diff are getting up to temp and reducing load on the engine? Ive tried comparing apples to apples as much as possible as in, flat road, same temps and ignoring the times its abit hilly etc and seems consistent... I also get the most km from a tank if ive already been driving for awhile then fill up and start counting km's.

Just something I found interesting... Another reason why a car that only does short trips would get worse economy.

Another thing I found interesting is since its ITB its a 4D tune with TPS as the main fuel load... but if I hold the throttle at exactly 20% and am cruising along on a flat road then a large hill comes up and I dont move the throttle the engine is (I would have thought) being subject to increased load... but as far as the tune is concerned its at the same load???

Is this accounted for in any other way? MAP does not seem to change all that much... RPM does drop from a large hill but this may actually cause the engine to fall into a leaner AFR target area making things worse? altho Timing would normally be lower

Not that Id think you would be very close to the knock limit at these low throttle positions, But it would be the same situation at higher throttle positions... except maybe you wouldn't loose rpm but maybe just climb rpm slower....

I guess its the equivalent of suddenly adding a large amount of weight to the car... im just thinking how does the tune account for the extra load even tho you would be at the same throttle positions...

As I write this its occurs to me that the actual air ingested by the motor doesn't change so you would need the same amount of fuel for a given throttle position and rpm... But it does seem like you can be at different amounts of actual "load" on the engine but be at the same TPS & RPM (depending on weight / incline) so would you need different timing / AFR target?

I understand AFR target load input relates to MAP so maybe that sorts it out?

So is timing actually... so yeh maybe ive just had too much time on the road and too much time to think haha.

The situation you describe is more obvious in a naturally aspirated engine with itb's. If you map the fuel delivery solely off TPS with no background comp for MAP and then tune on the dyno everything looks fine. On the road however we can get a situation where the manifold pressure signal changes a little based on the gradient of the road - For example going up a slight incline at 3000 rpm and 20% TPS vs flat road and same RPM/TPS. The manifold pressure increases slightly as we go up hill. As a result of this the air mass in the cylinder changes and we tend to see the AFR move slightly lean.

Unless I'm dealing with an engine with very large cams that gives a very erratic vacuum signal, I'll typically use a background fuel comp (load = MAP) even when I'm tuning on TPS. This means that the ECU will take care of fluctuations in MAP at a fixed TPS for the most part and we end up with more a more stable AFR. With the way your GTR is mapped, this is already accounted for in the fuel model and is a requirement for the multi throttle turbo fuel model to work - Essentially if we double MAP, the ECU doubles the injector pulse width.