Thread: Bad MPG question

Hi guys, recently I have put a newly built motor in my NX and have been getting much worse mpg than I used to. The new motors compression is dropped to 8.5:1 and I have been driving it na (breaking in/solving any issues I have before I boost the s#*t out of it) for about 3 weeks now. I have troubleshot mostly everything in regards to the fuel system including pump, lines, regulator, injectors, and filter. At this point I'm currious if the dropped compression could be the issue with my gas mileage. This past Tank of gas I got 163 miles and that was it (and no I havent been beating on it any worse than I normally did lol). I just feel that that bad of gas mileage is due to some prolbem I can't figure out, and any ideas/person experience or advice you could give me would be greatly appreciated! Thanks for the help in advance!

Sounsd like you're running pig rich, which means you shouldn't be driving it in it's current state, let alone beating on it (if only slightly). Hopefully this is not your only means of transportation.

You need to be careful with those shiny new bearings. I'd begin with an oil change so you don't wreck them from what I can only presume is watered-down oil.

1) Do you have a wideband O2 sensor installed?
2) What are you tuning/datalogging with?
3) Have you checked for vacuum leaks?
4) Leaky injector/stuck open injector? (You said you checked your injectors but...)
5) New lower injector seals/o-rings used for the build?
6) Do you have a wideband O2 sensor installed?

Lower comp. pistons are not your problem here. 160 miles to a tank is no bueno!

I do not have a wide band O2 installed, im running a completely stock ecu, i smoked it and dont have any vacuum leaks, my injector o-rings are brand new, and with my injectors they were firing fine but couldnt tell if they are stuck wide open or not.

Using MPG as a means of measuring engine performance is a little bit "after the fact". I hope you can find some money in the budget to properly monitor your AFR's, especially if you're going to boost it. This is a necessity!

What engine and ECU? Also, when you say newly built engine, do you mean OE re-built or built for boost?

I do have a wide band already just no gage lol and with this mpg thing its not about the performance its just that something is wrong ya know, and its cp pistons, eagle rods, .5 over, vvl hg, arp's, acl bearings, ported polished bla bla bla completely stock ecu (nismotronic on the way) and thats basically it

First using the per tank measurement is very inaccurate. On my 16 gallon P11, the needle is on empty and light comes on at 11 gallons one day, 12 gallons another day. Best way to do it is to fully fill up the tank, then reset tripodometer, drive, fill up the tank fully again and take the miles and divided by gallons filled.

Going to 8.5 CR you are loosing a lot of efficiency. Every combustion cycle now gets much less energy, you now have to make up for it with more timing. Aka retune is a must, though you can cheat with just advancing the base timing.

Yup thats exactly what i did, gas light came on at 163 mi then i filled which was 11.3 gallons which is roughly 14.4 mpg.... Thats pretty bad lol especially when i used to average around 25-27 when driving normal. Now ive gone through a little more than half of that fill up and im only at 114 mi and i know im gonna hit around the same number for this fill up as well

What ecu are you running? Also did you replace the spark plugs?

Its a completely stock b13 ecu, and i put brand new ngks in it less than a month ago when i had no choice but to start driving it again

Vadim, I agree with your statement about less combustion efficiency but I did not think that was his only problem so I did not mention it off the top. I figure a freshly built engine with proper ring seal should be seeing atleast 300 miles per tank under normal circumstances. I just can't see a 3-4 degree timing difference making up for a 50% drop in MPG?! The OP could (as you said) bump up his mechanical base timing to say 18-19* (assuming it was already @ 15* /with min. 91 fuel) to compensate for the lower comp. pistons but I just don't see this doubling his fuel efficiency. Now, if the timing was set incorrectly to begin with and was at say 10* base with 8.5:1 vs 9.5:1 pistons, then you are getting closer to the solution.

Vadim, I'm guessing that the DET ECU would only have more timing than the DE ECU, at base idle and under light load and while out of boost, Is this correct? Under boost circumstances the DET ECU would have lower timing in the boost rpm range vs the N/A DE ECU in that same rpm range. So is it safe to assume that the OP would see the biggest gains (while using the DE ECU with 8.5:1 pistons) below say 3500 rpm, if he were to bump up his mech. base timing?

I experimented with anywhere from 10-20* timing on my VE with the stock DE RR ECU and while I noticed a big jump in performance from 10 to 18*, I did not see a massive jump in MPG. Just my personal experience here.

OP. As Vadim mentioned, try bumping up your base timing and monitor the difference. Also, when you changed your plugs did you change the plug wires/cap and rotor?

I think the best bet is going to be putting that snazzy new JKTuning ECU in there. This will allow you to monitor your AFR's, inj. duty cycle, ECT, timing and all of your sensors for proper function/voltage ranges.