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Highway mileage terrible post-protune

Cobb Accesstuner 
6K views 22 replies 8 participants last post by  8cd03gro 
#1 ·
Before adding my full fueling, exhaust and intake goodies (increasing volumetric efficiency on both intake and exhaust side), I was getting 23-24mpg on freeway cruise. After parts and dyno and road protune, I'm getting 20-21, verified against refueling gallons and mileage.

Car seems to be running a little more vacuum at -5psi than before, but the AFRs are in the low 14s as opposed to around 14.7 for off-boost cruise. IDCs are sitting at about 10% (1000cc size), whereas before with the stock injectors they were running 10% at the same speed and load. So it seems to be running excessive fuel for cruise.

I was hoping cruise mileage would improve incrementally, but the car seems to be just dumping fuel into the engine off-boost. On boost, it downs fuel like it's going extinct but that's normal.

How should I go about trying to fix this?
 
#5 ·
Honestly >20mpg I a jealous. I am lucky to get 15 in a perfect setting. Also depending on geographical location and winter this could lead to higher oxygenated fuel content and burning off. If you are comparing pre October fuel averages, wait until April or so to retest.

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#7 ·
I don't follow the logic. Just because the car is plumbed better and makes power more effortlessly does not mean it does so more efficiently. It still requires a certain amount of explosion to keep the wheels turning the same speed up the same incline against the same headwind.

Side note, other than not spraying fuel at all, what is the lowest usable duty cycle an injector can utilize in practice? Is it possible to do 1%? 5%? I know these are possible in theory, I'm just thinking out loud - 10% seems pretty low, maybe less than that would produce pretty inconsistent results which would create a rough ride?
 
#8 ·
At warm idle I see 1% injector and it's perfectly smooth.

I'll try to explain a bit better. It takes a certain amount of energy to keep the car at a certain speed (all drag forces). It also takes a certain amount of energy to run the engine (engine friction, compression, and pumping losses). Better VE (volumetric efficiency) is less work on part of the engine (getting air in and out). You still have pumping losses due to the throttle plate (pulling a vacuum takes energy). The engine operates most efficiency at wide open throttle and low RPM (minimum pumping losses and friction).

It is common that big turbo guys see better cruise mileage than stock turbo guys because the turbine is just about the biggest exhaust restriction. They have less exhaust side pumping losses. Of course you're going to suck more gas when making power because they're making a lot more power.

I think the main issue is that my car is running too rich under steady state cruise. I'm not using any more power to maintain a certain speed. If anything I'm using less from the mods I put on. By Purple STI's logic, I'd be getting better mileage by using the smallest injectors one could put in.
 
#9 · (Edited)
Not what I said at all, simply implying your 1000cc injectors can pump nearly twice as much than stock. You had it protuned, which is going to usually net in higher power + cleaning up ots maps or stock. Also with increased injector size your tuner is more comfortable pushing your car safely. Therefore power will increase by increasing flow, that will also require additional fuel. A protune does not make your physical engine more efficient, only reengineering the block, heads, rotating assembly etc will actually net that kind of change. And in my case cruising with a larger turbo, have used both 30 and 35rs rotated I have not got better gas mileage. You can only dial down the pump and injectors so much before you are taking a very big issue of running lean.

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#11 ·
I'm not a physicist, but in the real world an increase in volumetric efficiency does not necessarily lead to improved fuel economy. I hear what you are saying that the losses from running the engine should be lower while providing the same output.

Realistically I think you could see an improvement if, say, the increased volumetric efficiency lead to a better torque curve, so you could ultimate run an even higher overdrive gear without bogging down the engine. But without messing with gearing, I don't think it's going to make a practical difference.

I'll agree with you, I think if you were at 10% IDC at stock then you should be able to operate off of less than that now, but that's what makes me wonder if there is a practical lower threshold (at least given a certain injector size) to IDCs. That's good to know about the 1% at idle, but perhaps when the engine is under load it is a different story?

Hopefully someone else with a similar setup can chime in on the numbers they see at cruise...
 
#12 ·
I'll agree with you, I think if you were at 10% IDC at stock then you should be able to operate off of less than that now, but that's what makes me wonder if there is a practical lower threshold (at least given a certain injector size) to IDCs. That's good to know about the 1% at idle, but perhaps when the engine is under load it is a different story?
You make a good point and that is a question that would be nice to have answered. The only time I see more than ~1% IDC at idle is when it's a cold start, and even then it's 3-4% until the engine starts warming a bit.
 
#13 ·
Hmm. Maybe not then, maybe it just needs additional fine-tuning. Have you mentioned your concerns to your tuner? I wouldn't harp on the fuel economy thing, I would just ask if they have insight why you're seeing the same IDC under the same conditions as stock etc, they might be willing to take a look at it?
 
#15 ·
No I didn't think so. I just meant they only have x amount of time with each car, it's not like they can fine tune every single aspect of the car in one session. I doubt they do too many 6th gear runs on the dyno anyway :lol: It's kind of amazing what they can do in a few hours, really.
 
#16 ·
Lol, I've done mods on tons of cars and motors, owned a shop... Basic performance mods like reducing exhaust restriction and advancing timing always increase efficiency and fuel economy, if not there's something wrong.

Tuning generally increases efficiency as well, but I'd look at timing on this one... I'd guess your advance curve should have been increased for more advance but if the timing is retarded vs before that's a problem. AFR should be right at 14.7 at cruise as well. IDC should be about half of what it was before.

IMO, there's definitely an issue if you're losing that much fuel economy, tuning needs to be fixed.
 
#17 ·
I've also usually found better mileage after adding a turboback and tune.

I have heard though that going to bigger injectors will tank your economy numbers. I'd bet that its one of those things where the tuner doesn't want to run below a certain IDC at cruise (kinda like how many measuring instruments aren't the most accurate in the lowest or highest part of their range).

Whether or not that's something that can be tuned out... I couldn't say. But if you were at 10% IDC before and under the exact same conditions you are still at 10% IDC than I would agree you are pumping more fuel than ideal.
 
#19 · (Edited)
So it seems to be running excessive fuel
I was hoping cruise mileage would improve incrementally, but the car seems to be just dumping fuel into the engine off-boost. On boost, it downs fuel like it's going extinct but that's normal.

How should I go about trying to fix this?
I understand and agree with all your posts in this thread. What are you asking, cause you know the car is running rich and low 14s is rich and not helping anything and the answer is tuning.

IDC1000s didn't hurt your milage on their own, and high efficiency means better mileage. You correctly argued this. Low duty cycle alone isn't causing it either. It's just not the right duty cycle. Besides your mileage isn't going to change by changing idle a tiny bit. It's the cruise that's rich and that will decrease it.

Out of curiosity you are tuned MAF?
 
#20 ·
I understand and agree with all your posts in this thread. What are you asking, cause you know the car is running rich and low 14s is rich and not helping anything and the answer is tuning.
Just bouncing my train of thought off of you guys to see if I was on the right track before asking the tuner if it's a legitimate performance issue that might need to be looked at.

I appreciate everyone's input.
 
#21 ·
I've been tuning some mileage back into my RSTi after a safe protune. I noticed that any roll into throttle immediately put the car in open loop and would drop me from 14.5 to 11.2:1. This is at 1.3 load and below.

My air fuel was mostly adjusted by maf calibration, so I've been recalibrating the MAF to agree with the afr table. This way I can more easily ramp the fueling richer from 14.5 to 11.2 in an easy way. I've also put some cl/ol delay back in ~750ms to keep the car from dumping fuel on any slight throttle stab. I'll be keeping near stock AFRs below 3000rpm.

I've already seen one mpg come back, hoping to get one more.

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#22 ·
Update, took my car back to my shop for a tune check and oil cooler hookup.

End result: smoke tested the car, which yielded a vacuum/boost leak on the intercooler piping. This was due to the use of T-bolt clamps supplied with the Grimmspeed hose kit. The old kit uses a two-piece collector, which is now discontinued.

Their new kit is a single cast piece with high torque hose clamps. Swapping them out ASAP.
 
#23 ·
-5psi is way too little vacuum, you've got a leak.

Update, took my car back to my shop for a tune check and oil cooler hookup.

End result: smoke tested the car, which yielded a vacuum/boost leak on the intercooler piping. This was due to the use of T-bolt clamps supplied with the Grimmspeed hose kit. The old kit uses a two-piece collector, which is now discontinued.

Their new kit is a single cast piece with high torque hose clamps. Swapping them out ASAP.
Nevermind :tup:
 
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