fuel economic engine choice

[josh_s13]

so i recently picked a z31 for a daily driver, and the engine will probably need 800+ dollars within the next year or so. I dont really want to dump that much money in an old vg30e. i have been thinking that there may be some other relativley low cost alternatives. In my opinion near 30mpg from 87 octane with around 200 to 250hp would be near ideal.
I had thought that a ca18det with a rasied compression, retarded timing, and a lot of intercooler would give me a chance at this.
What are you guy's thoughts on this?

[Def]

You can get an NA SR20DE for super cheap. Toss an OEM SR mani and a T25 with an eBay IC if you want a little more power. I'm not sure it'd work on 87, but it might at like 6-7 psi and pulling timing in boost.

The only negative of the DE for the ultimate fuel economy setup is that the 5th gear is .838 vs. the .759 of the S13 DET trans. But I'm sure you can trade that with somebody with a DET trans (heck, I'll do it!).

The CA is going to have to be worked too hard to give over 200 HP to run on 87 octane IMO.

[josh_s13]

Wouldn't the lack of oil squirters on the sr20de hurt the chances of running the lower octane successfully? Maybe decking the head on a sr20det would yield better chances, although I would assume that mid 20mpg would be all that I could get.

[josh_s13]

For what it's worth, I still have coilpacks, injectors, and intercooler from the rb20 that I had. I think these parts are interchangeable with the CA18.

[Matt93SE]

A properly tuned VG30E-T will get ~30mpg and they're dead-nuts reliable.

..And they came in the car...

.. And you can make 400+hp on stock internals if you want to turn up the wick..

[Def]

No issue with lack of squirters for a DD.

I don't see a VG30 getting 30 mpg in a z31...

[Def]

Wouldn't the lack of oil squirters on the sr20de hurt the chances of running the lower octane successfully? Maybe decking the head on a sr20det would yield better chances, although I would assume that mid 20mpg would be all that I could get.

An SR20DET would probably do high 20 mpg if driven right in a Z31. On the highway at about 80-85 mph with really draggy aero mine would do about 27-28 mpg with the 4.36.

A DE will have higher thermal efficiency with the higher CR, and if you gear it long enough it'll get pretty freakin' sweet gas mileage. Way better than a VG30.

[josh_s13]

An SR20DET would probably do high 20 mpg if driven right in a Z31. On the highway at about 80-85 mph with really draggy aero mine would do about 27-28 mpg with the 4.36.

This gives me high hopes, especially with no aero and 3.7 gears (3.54 gears available in some other models)

The first tank of gas yielded 19.6MPG with a mix of easy city and highway driving. Part of the tank was on a bad O2 sensor, and the whole tank was on a couple dozen other broke things.

I assume that the vg30 is also a bit heavier than some of the smaller turbocharged options. I figure that's worth something in the fuel mileage equation.

Without trying to open a huge can of worms what are the major differences in the sr20de and sr20det aside from the obvious turbo assembly? Also, any idea of how much valve to piston clearance the det has on the stock cam?

[Matt93SE]

No issue with lack of squirters for a DD.

I don't see a VG30 getting 30 mpg in a z31...

You haven't hung around on the VG forums much, have you?

Myself and many other VG Maxima guys routinely get ~27mpg on cars with basic bolt-ons.. add a proper ECU tune for lean-burning while cruising and you can easily top 30mpg at highway speeds. I've seen it done several times, even on turbo cars.

[Def]

This gives me high hopes, especially with no aero and 3.7 gears (3.54 gears available in some other models)

The first tank of gas yielded 19.6MPG with a mix of easy city and highway driving. Part of the tank was on a bad O2 sensor, and the whole tank was on a couple dozen other broke things.

I assume that the vg30 is also a bit heavier than some of the smaller turbocharged options. I figure that's worth something in the fuel mileage equation.

Without trying to open a huge can of worms what are the major differences in the sr20de and sr20det aside from the obvious turbo assembly? Also, any idea of how much valve to piston clearance the det has on the stock cam?

The other thing is friction - more cylinders generally equals more friction.

Piston to valve clearance is fine for any even remotely streetable cam you can put in the engine. Stick with stock cams tho'. That MPG was also with 264/264 cams which generally decreased mileage due to a greater amount of pumping losses and overlap with a bad intake/exhaust pressure ratio at high manifold vacuum conditions.

[Def]

You haven't hung around on the VG forums much, have you?

Myself and many other VG Maxima guys routinely get ~27mpg on cars with basic bolt-ons.. add a proper ECU tune for lean-burning while cruising and you can easily top 30mpg at highway speeds. I've seen it done several times, even on turbo cars.

Going leaner than 15.3:1 causes additional fuel consumption on gas.

Yea, maybe with a tail wind and drafting you can get 30 mpg on the highway with a VG30, but the gearing is generally aggressive for the displacement to be a really efficient engine, and it's got a decent amount of internal friction.

Anything can happen, but the SR20DE will be superior given the right gearing.

[josh_s13]

I think the big plus with the vg is the hp/dollar factor. With the old parts i have lying around i may be able to get 350+bhp for around 1000-1500 dollars. Based on observing the fuel range display, 30mpg could be doable in the vg, with a steady 60-65mph.

The sr would definatly end up costing more. Maybe around 3 grand once all is said and done. So about 1500 dollars more for about 5 more mpg and 100 less hp. With how much i drive the 5ish mpg would cost about 275 less per year in fuel.

[josh_s13]

It may also be feasable to run a secondary small fuel cell filled with race gas that could do a boost dependent fuel mix with 87 octane.

[Z28ricer]

Or you could just put a stock SR20DET in, with a fmic, which shouldn't be hard at all to get 30mpg and 250rwhp out of, on premium fuel.

If you drive 1000 miles/mo, you're looking at $8.50 per month difference between regular and premium. It may look annoying seeing the price is higher at the pump, but c'mon I spend more than that in a week in tolls sometimes.

[Matt93SE]

Going leaner than 15.3:1 causes additional fuel consumption on gas.

Yea, maybe with a tail wind and drafting you can get 30 mpg on the highway with a VG30, but the gearing is generally aggressive for the displacement to be a really efficient engine, and it's got a decent amount of internal friction.

Anything can happen, but the SR20DE will be superior given the right gearing.

Factory tuning on these cars is fairly rich- somewhere around 12.5:1 last time I had my Maxima on a dyno. Getting that to even 13.5:1 at low load will get you near 30mpg with that change alone and is still completely safe off-boost.(27mpg/12.5*13.5= 29.2mpg)

With the R200 in those cars, you can also go to a 3.9 or whatever final drive you want to slow the thing down. They also have the same gearing as the KA and SR, so they have a tall 5th gear.

Sure, a 4 cyl will give you better economy on average, but there are many things you can do with the VG to make it more efficient. The costs to do that with what he has on the shelf are less than the cost of dropping in an SR, so you can buy a LOT more gas with that $1500.

Something to note here:
Fuel cost for 15,000 miles per year at $3.25/gal
30mpg = 500 gal = $1625
28mpg = 535 gal = $1741
27mpg = 555 gal = $1805
25mpg = 600 gal = $1950

So 25mpg to 30mpg is $350/year difference in fuel cost.
Given the anticipated remaining life cycle of the car (let's say 5 years, MAX), you're looking at $1750 difference in fuel costs from 25 to 30mpg.

There's the immediate cost of material to drop the SR into the car. That's not to mention the labor to do all of the custom work- engine/trans mounts, driveshaft, wiring harness, etc etc etc. There's several thousand in labor and another thousand or so in material to do all of that work.

So he's much better off leaving the VG in there and getting even 25mpg than he is to pay several thousand bucks to install a smaller 4cyl engine that's only slightly more efficient.