90-95 D21 4x4 fuel mileage and aerodynamics thread

[HRH]

EDIT: SEE POST # 12 REVIVING THREAD.

 

What are you getting for gas mileage in your 90-95 D21s with KA single cams? Particularly extended cab 4x4s.

 

I'm getting shitty mileage in my 4x4 extended cab. In my previous 96-97 standard cab hardbodies, I got right around 21-23 mpg, and that was redlining it everywhere, with 215 wide tires. Of course it was obd-II versus obd-I.

 

The ECU says everything's fine, though it appears as though it's been running too rich. Put in a new 180 degree thermostat (good quality Japan) and a new coolant temperature sensor last night. Went on a 45.8 mile test run tonight, hit a little wind on the freeway, and did a few sessions of full throttle up to about 85ish mph (4x4s don't like that, even with a camper shell.) Ended up getting 17.6 mpg.

 

If you read the other thread, the mileage before that after replacing the restrictive driveshaft carrier bearing was 15.2.

 

The mileage before that with the restrictive carrier bearing was 14.7.

 

It appears it might be getting better, and I did throw in another can of Seafoam today just in case the injectors are dirty. That's my next thought, seeing as the ECU says everything is fine, and the Datsun guru says the injectors could be dribbling enough to not set a code, but not atomizing and that would suck mileage. Definitely a thought, but the thing runs pretty damn good.

 

Anybody got a 4x4 ext. cab with a KA getting decent mileage?

[kiznook]

25-27 in my 91 2WD KA 5speed, 235/75/15 w/ 4.62 pathy rear. about 3000-3500 RPM @ 60 :(

 

I would say your milage is pretty normal if not high. I know a guy that gets 15 at best in a standard cab 4x4

[alabama_lowlife]

i get in the area of 18 on both of my 97's. both 4x4 extended cabs with KA's and 5 speeds

[Braden]

18 to 21 mpg for those is common, a friend of mine has one he says he gets 18!!!! hell my dads new tundra ext cab long bed 5.7 gets 16!!!

[BEEBANI]

Yeah, I was getting 18-25 in my '97. Mostly 18 when I was running 33x12.5-15 BFG's on it. The opened up exhaust and k&n filter with intake helped a lot.

 

Here she is in all her glory

 

 

 

Edited November 6, 2009 by BEEBANI
added pics

[HRH]

I talked to another Datsun guru today, a friend of my regular Datsun guru and he mentioned an issue with hardbodies that is particularly prevalent.

 

Apparently the ground wire going to the MAF sensor was assembled crappily and so they will not ground properly which will lead to bad inputs and crappy mileage. Says adding a redundant ground is the best way to fix it. Will definitely try that today as I remember reading about that from a while ago, just jogged the memory. Not an issue on 96+ trucks I don't think, obd-II got rid of some of those problems.

 

Anyway, between that ground wire issue and the stock exhaust, which I'll cut out soon, I'm hoping to get back up into the 20s. I'm only running 30x9.50/15s for pete's sake. Not much tire.

 

Beebani: Did you need to do a lift to fit the 33s?

 

As far as the exhaust, have any of you ever looked into that tiny muffler right before the actual muffler and after the cat? On the last hardbody, removal of that piece gained a full second quicker in 0-60. Inside you'll see a perforated tube, which cuts off flow by probably more than half. Not sure if it was noise, emissions, or what, but damn those things suck. I never did test gas mileage with it on, but I'm sure it's something.

Edited November 6, 2009 by HRH

[BEEBANI]

No, I just cranked the torsions up in the front. Had to trim the bottom corner off of the front fenders near the bumper for turning.

[HRH]

Alright, pretty much got all the issues sorted out regarding the crappy gas mileage.

 

Found a few tech tidbits, and some common stuff I should have looked at first! When your gas mileage sucks, you might want to actually remove the air clean and have a look! :eek: I found mud had snaked it's way into the throttle from my last 4x4 outing. I had removed the snorkel tube and airbox to free up engine space, but forgot the only air inlet was the existing original inlet in the airbox.

 

As you might guess, this small hole has to suck a major amount of air when you're full throttle through a puddle! Yeah. So to not only increase airflow, but lessen the sucking ability of only one area, we hogged out the airbox. Now the sucking pounds per sq. inch are much less (good technical term there! :D), which results in better airflow, and less chance of violently sucking mud and crap down the intake.

 

We also greased the bottom of the filter, which I realized I neglected to do, though I did it on every previous hardbody.

 

I also cleaned off the mud from the MAF, which was probably affecting mileage as well.

 

*********TECH TIDBIT**********

 

I contacted another Datsun guru and found a common problem of MAF grounding issues on 90-95 hardbodies. The MAF ground is poorly connected, and over time will contribute to uneven idling and poor fuel mileage. The answer is a redundant ground. On mine, there was also a shielding ground cable wrapped around the wire loom going to the MAF. Seeing as both went to ground and it was just hanging out there, I soldered it to the white (middle) ground wire of the MAF, and ran a redundant ground as shown in the photos.

 

Lastly I played with the new welder and after LOTS of birdshit welding, have a fully functional exhaust, and a new universal cat. Found a hole in mine on the upper honeycomb. Don't point out the birdshit. I know. Penetration was good, prettyness was not. And I wasn't welding with gas as I hadn't bought any yet and wanted to do it right away. Next time will look better.

 

So after opening up the intake and exhaust, I'm getting much better mileage, I'd say somewhere in the 20 mpg or slightly better, which is fine for a 4x4 pushing all that weight around. Haven't calculated yet, but the gas guage is staying full a lot longer and mileage traveled indicates about that.

 

I'll post pictures in a second here:

[HRH]

 

 

 

 

 

 

 

 

 

 

 

 

 

[datzenmike]

I've also heard that there is a separate ground wire for the exhaust manifold. This keeps it solidly grounded instead of through the rusty bolts. The good ground is needed to keep the O2 sensor accurate. If there isn't a wire run one to the head. Won't hurt.

[HRH]

Yep, there is one from the back of the head to the firewall. Good call! I think my crappy mileage days are over. Who said fuel injection is easier? :D Shit with carbs I would have known I was getting shitty mileage due to a metered fuel leak. :D

[HRH]

NOTE THE DATE / Reviving Thread: 3-31-11.

 

As many of you have noticed, I finally finished the KA-Z24 motor build for the hardbody. Left me with a 9.2ish to 1 compression ratio Z24 block with KA head. Cam is now .5 degree advanced. Motor is on about mile 600 or so, just switched out to regular oil from running 30wt non-detergent for the break in period. The new motor is quiet and hands-down better than the previous two boat anchor KAs with oiling issues.

 

Now that I've solved that problem and I'm finally starting to have some confidence in my D21 again, I've turned my attention to maximizing the mileage out of the truck. I'm currently getting 18.8-19.9, haven't hit the 20 and above mark though pretty darn close. Motor is still breaking in too, though.

 

 

Currently running 4.375 gears (stock) with 30x9.5/15 BFG ATs. Switched out from the Hankook MT RT03s which were bad ass mud tires, but also decreased mileage due to tread and softness by about 2 mpg. Next available gear ratio is 4.62 out of a Pathy, with a limited slip, which is highly desirable, but I'm almost considering not changing it due to having a little extra power now to pull lower rpms on the freeway.

 

While I fully understand the D21 XC 4x4 is a heavy pig, I'm determined to get it as close to 2wd mileage as possible.

 

That said, we have a few options:

 

1. More power.

2. Lower gears to turn the 1.2" taller tires.

3. Aerodynamics

4. Lightening the load (weight reduction)

 

Now, we've gone and done option 1 with clearancing the head and increasing the compression ratio. I should have around 150 hp now, versus the stock 134. Still not a lot, but good and within my modest budget.

 

Lower gears would help, but will also increase rpm on the freeway. I may have enough extra power to compensate for the gearing change due to larger tires. Jury is still out on this one, keep kicking it around. Whole rear ends are rather spendy. I can do it, just have to commit.

 

Aerodynamics is by far my favorite option. I got some .060 aluminum Diamond plate that I'm going to cut into underbody panels that will attempt to box up the frame and prevent air tumbling underneath the truck. I suspect this will help a lot. Still not cheap, a big sheet of that was $75, and hopefully will be rigid enough to resist vibrating with the wind. We can always through some angle iron across the top side of it to cure wind ripples. The overall height of the truck is certainly tall, but not all that much taller than a 2wd hardbody. Another thought is to remove the lift blocks from the body and return it to 2wd height. Not sure I want to go to that much effort though, and worry about the 30s hitting the fender under full compression also. Not to mention I do take it offroad and the body is low enough to need to be careful in deep puddles.

 

Another thing on the aerodynamic front. I had an early hardbody hood with the louvers on my '96 2wd that I autocrossed. I have a strong feeling that the early hood had a golf-ball-dimple-like effect of breaking up the wind surface at the front of the hood, thus increasing the aerodynamic profile of the louvered hood. Everyone started shaving those louvers out in the mini trucking scene, and it was probably costlier to produce, which is why I imagine they ditched them after '91 I think. If anyone has any information on that, I'd love to hear it.

 

Lightening is always good. A header might lighten my front end by 10 pounds, but that's really just an excuse to buy a header. :) One thing I have noticed is the earlier D21s with Z24s had R180s and H190s as opposed to C200s and H233Bs. Right there that's 100 pounds if not more. I know my 720 XC 4x4 weighed in around 3600 pounds. The D21 has extra bed weight with the internal walls but drivetrain weight is probably the main difference with it weighing in at around 3800 pounds.

 

 

One thing I may do is get some tubing and make a scale model wind tunnel to test my theory. I need to find some j spec models then of 2wd and 4wd XC hardbodies. Haven't looked into that yet, may check ebay Japan to see if they're available.

 

This is pretty much all I thought of today while working at NAPA. Isn't ADHD a bitch? ;)

 

Oh, and for piece of mind: On the fed gov site regarding fuel mileage, it lists most of the KA powered D21 4x4s as getting 16 city, 20 highway, which seems to be fairly accurate. The 2wds mostly get 20/25, which is accurate from my dealings with two previous 2wd hardbodies.

 

The standard cab 2wd I had weighed in around 3100 pounds if I remember right. Extracab is probably another 100-200 pounds.

[mike]

I was curious to see what your mileage would do with the new engine. For the cost to regear it would probably take 10 years to see the gains of MAYBE 1 or 2 MPG at best... considering that it didnt go down.

 

As far as less rolling resistance with the BFGs, you would see a night and day difference again just switching to a tall skinny all season radial. You should see if you can find somebody who would let you borrow some 265 75 16s (almost a 32 but skinnier) in the least aggressive tread possible. I actually noticed a drop in mileage when I went from basic Toyo ATs to the BFGs... They were cheap but I am not a fan...

 

Also what sort of oil are you running in your diffs/t-case/tranny? Switching to synthetic in my old V6 Taco that was lifted on 35s got me about a 1 mpg gain (17mpg).

 

Just out of curiosity what do the VGs get in the 4x4?

 

For your belly pan idea... dont buy new stuff, youre just going to get it dirty... Go check out the recycling yards, most will let you buy it by the pound. The place on rosewood just off of Francis (about the 1500 block) actually stocks usable stuff on shelves... usually sheetmetal but occasionally they have diamond plate.

[HRH]

Well already bought the diamond plate. Aluminum was why it was so expensive. I got the thinnest stuff they had.

 

The oil in the diffs hasn't been changed yet. Don't know if the previous owner did the 100,000 mile service or not. 149k now. Transmission is 75-90 Royal Purple.

 

The VG 4x4s get about 14-18 according to the gov site. I know a buddy of mine with a WD21 (Pathy) who normally gets 21 though. Still, about the same, crappier in town.

 

And yes, I could go to pavement tires. In fact, I debated lowering the 4x4 to 2wd height, and putting 215/60/15s back on. I like 2wd ride height hardbodies, but then I wouldn't have any clearance for off-roading.

 

Really it's an aesthetic choice. I know the tires I have aren't the best for mileage, but for once I care about looks just enough to find another way around it. 4x4 hardbodies just look sad with 235/75/15s, and even sadder with big tires with passenger car tread. Like a railroad truck, the stock wheels all sunken in the giant fenderwells. ;)

 

And the cost to regear would actually be pretty miminal considering if I can find the right Pathy at PNS, it's all of $100 to grab both diffs and it's a center section, so no set-up. Pull axleshafts, slide in the new diff, and go. But there haven't been any pathys at PNS, and the trucks never came with LSDs. So if I'm going to bother to regear, I'm going LSD or nothing.

[HRH]

Alright, fixing to upload some pictures of the first aero plate after lunch. Okay I lied, couldn't wait, here you are! Making a second after lunch back near the gas tank.

 

 

 

 

 

 

 

 

[HRH]

Alrighty then! Have a bunch of pictures, the whole project came out really nicely. I'm curious as to how much a difference it's going to make. Less than a few pounds has been added onto the truck. The panels should be rigid enough to resist wind buffeting, but I'll check for cracks after a few highway trips.

 

 

 

 

 

 

 

 

 

 

 

[Sealik]

Still waiting on that scale model wind tunnel.... ;)

What did you plan on using for smoke....?... :D

[HRH]

Well, I actually wasn't planning on using smoke, but a scale attached to the front of the model, super sensitive, and then go by the least amount of poundage. Haven't made it yet, not sure I'm going to if I can get to my target mileage without it. And I know I could further improve the aero with a tonneau cover, but they're quite expensive and I like being able to throw motors/dirt/gravel/whatever in the back at any given time. Maybe I'll cap it off with stylish plywood before the trip to Canby! :D

 

I did take it on the freeway today and it seemed to help quite a bit at speeds, but that may have been my imagination. I'm going to fill it up before the work commute tomorrow, that will be a much better test. 30 there, 30 back.

[Sealik]

Well, I actually wasn't planning on using smoke, but a scale attached to the front of the model, super sensitive, and then go by the least amount of poundage. Haven't made it yet, not sure I'm going to if I can get to my target mileage without it. And I know I could further improve the aero with a tonneau cover, but they're quite expensive and I like being able to throw motors/dirt/gravel/whatever in the back at any given time. Maybe I'll cap it off with stylish plywood before the trip to Canby! :D

 

I did take it on the freeway today and it seemed to help quite a bit at speeds, but that may have been my imagination. I'm going to fill it up before the work commute tomorrow, that will be a much better test. 30 there, 30 back.

 

 

Ya...just being somewhat facetious.

Figured to 'birds' with 1 bong..... :lol:

Anywho....that mileage sucks.

One of 4x4 720s got 29.5 MPG with the Weber....highway miles only

[HRH]

Yeah, the 720s were quite a bit lighter than the hardbodies. Hell, my 720 XC 4x4 with the quickie rebuilt Z20S motor was pulling 22 mpg with a Weber. It had NO power though. Barely got out of it's own way. The hardbody has lots more balls. Still, yes mileage should be better. Just did a long trip to pick up a free wheel balancer! Woo hoo! One of the guys I worked with got it when a local guy sold his shop, and he doesn't have much use for it. Older Bear machine, but still computer spin balancer. Needs the internal battery for memory replaced, but that shouldn't be too difficult. I'm just stoked to have a balancer for the cost of free99. The truck seemed to get great mileage on the way there and back too, even with around 400 pounds in the back. (Lots of old lead weights in boxes.)

[HRH]

Okay, so the first round of testing is in. Previous to this tank, the best mileage I could pull was 19.9 mpg. During this tank of 190 miles, mileage corrected to 202.35 at correction factor of 6.5% due to larger tire size, I got 20.24 mpg! This includes two days of tailwinds, two days of headwinds, some calm, then a headwind coming to work today. I filled up right before work after I got off the freeway from the headwind fun. Given that, had I not had that headwind I'm betting I might have hit 21, or at least 20.5.

 

So the aero plates definitely helped. Maybe only .5-1.5 mpg, but that's certainly worth it. I'll get my $75 out of material costs during a summer of driving, no problem.

 

Still planning on re-gearing. I think this will help the most as it will make the tires easier to turn, and going to a 4.62 ratio from a 4.375 with 1.2" taller tires, will bring me within .003 of the factory ratio/tire size. That will raise my rpm slightly, but the lower gearing will also make it easier to move my heavy pig down the road. Still should be a plus on the mileage. May not get to 25, but I'm thinking I might be able to get to at least 23 with a re-gear. We shall see.

[datzenmike]

.5 to 1.5% it all counts and best of all it's built in and does not go out of tune nor do you have to drive differently.

[HRH]

Alrighty then! I went by Pull and Save Saturday after work and did some binocularing on the rows. Found one more pathy that was just put out. So figured what the hell, better go check. BINGO! 4.62 LSD. Stealthily, I didn't mention anything to anyone except a few ratsun gurus so as not to have someone else beat me to it! Okay, not many people are really looking for 4.62 Pathy diffs, but still. Loaded up everything last night, got up early and got down there right as they opened. 2.5 hours later, I was heading home with both diffs!

 

The only downside is the rig had 228K on it. Rather a lot, but it was an automatic and the interior was better than my truck actually. It was blue, one of the only reasons I didn't take the seats, they were freakin' perfect! So it looks like it was well taken care of. The diff looked beautiful, still need to clean her out and inspect the surfaces for any nasties, but I think they'll work just fine.

 

The only other thing I'll need to do is swap the flanges on the front diff so I can use the 4cyl 4wd halfshafts. Shouldn't be too hard I think.

 

 

 

[HRH]

So one other thing I just remembered, I have an ABS sensor in the snout of my H233B diff. Need to measure the diffs, I suspect the older style may be slightly shorter. Hopefully there's enough driveshaft to accommodate the difference. Aside from that, I did some checking on the hoods today since I have Ray's 86.5 D21 here for a warranty heater core. Not too happy about that, but oh well.

 

Back to the hoods, debating the aero effects of the old hood with the three louvers. The old hood is also flat. The new hood without the louvers has a strengthening bulge at the forward center, which would slow down the air, perhaps making a cushion for fast-moving air to sit on, or lessening the turbulence at the lower edge of the windshield. I have a feeling the old hood is still more aerodynamic, but perhaps not? Anybody have a good theory on that?

 

 

 

[HRH]

Last tank was 20.85 mpg. Have not switched axle ratios yet. Finding with the current setup, 2600-2750 rpm is the ideal range for best mileage.

 

That's mostly highway driving, small amount of city from highway to my house and a few errands.