my last gas mileage thread for a while (m20 vs m50) I promise.

[Kozworth]

True that. I just need to find a happy medium between mpg and torque that i want. I might consider swapping diffs.. im just super indecisive. Atleast i want to work on my e30.. with my vws i wanted to sell them instead of actually tinkering with them when something arose haha

[nando]

1). Never, ever go off your OBC. Calculate mileage based on the odometer and fill up.

There's nothing wrong at all with the OBC, unless you've screwed with the engine electronics. mine is within a 10th of a MPG of my hand-calculated values, and I don't even have a factory ECU and my injectors are almost 4x larger.

it's a myth that the OBC is inaccurate. Probably started with people doing injector swaps with no correction to the electronics or software. Now people just repeat what they read on the internet 5 years ago..

FWIW, I also agree, that doing a swap under the impression of saving money is the wrong idea. I'd figure a realistic budget to be in the range of at least $1000-$1500 (unless you do an everything from the junkyard, barely holding together with bailing wire and zip ties swap). That will only go up with mods, unforseen parts replacement, and the more maintenance you do, plus the condition of the engine itself.

There's more to it than just swapping the motor itself, and if you don't replace all the wear/maintenance parts, you're just asking for reliability issues down the road (maybe not a good idea while in college).

Then figure how much gas you can buy with that swap budget. It's quite a bit, even at $4 a gallon. and you'd be swapping a known quantity, well maintained and reliable motor for something new and unknown that will likely have teething issues, and if you do it really cheap, reliability problems down the road. It's a risk that I can't see being worth it.

and you really don't know that the M50 would get significantly better fuel economy - you might go from 22-23, to 24mpg. that would take forever to pay itself back. heck, even a larger increase, say 27mpg, could take years to pay off - you'll probably be long graduated, have a new job, a wife and kids, and maybe not an E30 any more.

a diff swap makes more sense. 3.64, 3.55, 3.38, and 3.25 are all available in medium case. a good used diff can be $300-$500. you can rebuild one in the $800 range (depending on core cost). The risk is lower and the payback would be sooner.

up to you..

[nando]

The OBC and econometer will be off if there is a chip. Both are driven by injector pulse width, which changes when a chip has been installed. To find out what the true mileage is, fill the tank & record the mileage each time. After several tanks of gas you will have a good number for average mileage. If your commute is 90%, or better, highway driving the number will close to best possible mileage. But if a good bit of the time the car is driven in an urban setting the number will be closer to the "city mileage".

a chip does NOT throw off the OBC. the only thing that will throw it off is a base fuel pressure change, or an injector size change. Injector size and fuel pressure are the constants - the OBC is looking at VSS and injector PW as the variables. Since it knows the constants, it always knows how much fuel flows for a given increase in PW (linear relationship). the chip makes zero difference.

[BergerNJ]

Some of the guys on here report breaking even on cost of the m50 when they sold the m20....some guys spend almost 2 grand for a good m50.....


Installation and what nando mentions, the unforeseen maintenance, or replacement of parts will definitely drive that number way up. I only know a little bit on the m20 and m50s, but I do know this investment will not pay out when referring to a few searches of mpg threads.


go for a taller or "longer" diff.....and then go through the normal maintenance on the m20.....squeeze every last bit out of it. Make sure everything is in order. When was the last time that motor had the head off and was known to be working perfectly? Every part under vacuum is perfect?

Once you have everything in order, you will have yourself a properly maintained e30 with records to prove, you will be keeping the sale price of the car up when normally it goes down with inattention or abuse.

Also, correct me if I'm wrong, when a motor is making more power, it becomes more efficient as long as you don't drive hard on it? So if you had a chip tune, yes mpg will go down, but isn't that only when he drives hard? if he babies it through the gears and is strictly shifting according the tq curve he should get better mileage right?

They make tires that are also better for mileage, open your exhaust up a bit?

I'd rather go crazy in weight reduction and storing all the torn out stuff in my house or garage, than doing a motor swap based on theory and hope.

[Kozworth]

Im glad Nando chimed in, because everyone was chipping in false ideas about chips and obcs. Like i said, I have another post that many people have throughly explained the obc and how it cannot be off unless the injectors are leaking/changed for a different size. Reguardless, I have been getting around what the obc has said for my avg mpg.

I believe the chip only effects gas mileage since people decide to whomp on it. If you drive the car normally w/ or without a chip, it should be about the same. Just, what is the point of chipping it when youd drive it the same?

I do see in similar eyes with how a swap is inefficient. I have a lot of manhours into maintenance and my m20 is pretty tip top for the most part. Ive just seem a lot of various threads reguarding guys getting 28-30mpg with an m50. Even me going lightly would not see remotely close to that. And with a 3.73, I dont see how anyone could really get much over 25 if that on highway.. Ive read tons of posts from many sites on the m20 and I know basically a low 20 mpg is about average combined.

[tinkwithanr]

While having a chip by itself won't kill your gas mileage, it is not true that it won't effect it at all. One of the ways they make more power is to change the fuel mixture to run a tad on the 'rich' side. This allows the engine to produce slightly more power, but obviously also effects the fuel consumption. If you really want to get the most out of your m20 I would change the diff to a lower ratio and take the chip off your ecu and see what that gives you. Even if you don't change your driving style I would be you would see a decent improvement.

[nando]

A chip should only increase fuel at wot, not during cruise. And most of the power comes from reoptimizing the ignition timing for premium fuel, not from a small change in afr.