How to make your CRX gas efficent!

[straub]

What's up Crew?

I was curious, can we modify our cars to get the best gas mileage out of them? If so, what are some of the ways to get better gas mileage? I'm getting ~235mi/tank...

Beside easying up on the pedal!

Thanks a million!

90' Si

[Dave_Darling]

First: Make sure the car is in good shape. When's the last time you did a tune-up, replaced the spark plugs and wires, etc?

Second: Air up those tires. Forget what the spec on the car says, go up to the "MAX PRESSURE" printed on the tire sidewalls. That reduces your rolling resistance, which means better fuel economy.

Third: Use a 5W30 or possibly even 0W20 synthetic oil. (I run 5W30 "dino" oil; I may go to 5W30 synthetic next time.)

Fourth: Slow down. The air resistance goes up with the square of your speed, so it takes four time as much energy to push your car at 80 MPH as it does at 40 MPH. Slowing down to the speed limit on the freeway will gain you quite a lot.

Fifth: Get into top gear early. Any cruising speed over 35 MPH, I'm in 5th gear. If I am accelerating at speeds as low as 40 MPH, I'll stay in 5th gear as well!

Sixth: Never use full throttle--don't go over about 3/4 throttle.

You can start going really wild on car mods, but they will only give you incremental gains. You can poke around on ecomodders.com for pics of two Civics with significant aero modifications (user "basjoos" has one of them). But driving techniques will get you more than aero mods on their own will. Most especially SLOWING DOWN.

BTW, my last tank was about 36 MPG, 8.5 gallons used for about 300 miles. I was disappointed; my previous tank was just under 45 MPG (over 400 miles on the tank). But The Boss asked to drive my car for a few days during this last tank, and she's a real leadfoot...

--DD

[thorasic]

You could change to an HF transmission but those tall gears are not good for anything but economy. I have driven all three transmission types and I prefer the DX since the gearing allows more relaxed driving than the SI and the rpms are significantly lower at highway speed while still giving you some passing ability.

I have heard that it is possible to change the 5th gear out of an HF into the SI tranny to create a hybrid which can accelerate quickly and has that tall 5th for long distance economy. There is a write up about it somewhere online.

[straub]

Thanks Crew for the update!

Dave_Darling,

Wow, 0ver 400mi! That is awesome. Are you driving an Si model? What do you mean: Never use full throttle--don't go over about 3/4 throttle?

Thanks a million!

[Dave_Darling]

My car is a 1990 Si, just like yours. I used to get right around 30 MPG for a 10 gallon tank. I think that long tank was right around 430 miles?

What I mean by the throttle is not to push it all the way to the floor. Don't push the pedal down more than 3/4 of the way, even when you're accelerating.

Obviously you do what you have to in an emergency, though!

One thing I forgot (in part because I take it for granted now)--don't move the throttle up and down. I have some friends who cannot seem to keep a steady throttle position, they're always speeding up and slowing down. That uses more fuel than steady cruising.

...There are more techniques, like driving as if you didn't have any brakes so you leave lots of space in front of you, and timing lights so you don't have to stop and then accelerate again. But the biggest thing is to slow down.

--DD

[straub]

Did you install a new tank? My tank is only 8 gallons. Can you add a larger tank?

[NozHayr]

I have a stock, 225K 1990 Si and get a consistant 35 MPG.

Once my gage needle is "fully" in the red, I fill up to the top (full filler neck) and the pump reads 9.25 gallons. This always happens at 325 +/- 3 miles.

I'm often curious to see how far below "red" on the gas gage that I can travel, but have never had the nerve.

I'm a pretty mellow driver and shift at 2750 - 3000 for the most part (old age). All my miles are city miles.

FYI, just for your reference!

[Kwicko]

You could change to an HF transmission but those tall gears are not good for anything but economy. I have driven all three transmission types and I prefer the DX since the gearing allows more relaxed driving than the SI and the rpms are significantly lower at highway speed while still giving you some passing ability.

I have heard that it is possible to change the 5th gear out of an HF into the SI tranny to create a hybrid which can accelerate quickly and has that tall 5th for long distance economy. There is a write up about it somewhere online.

[Kwicko raises hand...] Yes indeed. I'm one of the guys who used to build these hybrid transmissions. The one I have in my own CRX uses ZC gearing for 1 thru 4, mated to an HF 5th gear and an Si final drive. The ZC gears are a little shorter than the Si and DX gears (first gear is the same on all of them, actually) for better acceleration, and the HF 5th becomes what I call a "true overdrive" - it's best used for cruising and saving gas, not for accelerating. This combination won't give you the same 5th gear speeds as an HF transmission, though - the Si final drive is considerably shorter.

The car this is in is a '91 CRX DX. That's a big part of why I chose to use the HF 5th gear - I liked being at around 3600rpm at 80mph. The hybrid transmission is pretty close; 80mph is right around 3650rpm. The only problem I didn't foresee is that I've been driving Si models for quite some time, so I unconsciously tend to cruise more by engine note than by the speedo reading. As such, I tend to look down and find that I'm doing 85-87mph! 80 won't necessarily get you a ticket around here, but over that definitely gets some attention.

By the way, in case anyone didn't know this, the only difference in the Si and DX transmissions is the final drive; all other ratios are exactly the same. The DX uses something like a 3.77 or 3.8 final drive ratio, while the Si uses a 4.25 final. There are some Japanese transmissions that used an even-more-aggressive 4.43 final drive, and someday I'd like to get my hands on one for my next "uberHybrid": ZC 1-4, HF 5, 4.43 FD, and Honda limited-slip diff (also from certain JDM transmissions).

Okay, so that's the basics and background. What about the results? Well, if I flog the living crap out of the car for an entire tank of gas, all city driving, all with the A/C on, I'll get the mileage down to 22 or so. If I just drive normally, it'll be above 30mpg for the same commute. On the highway is where the transmission really shines - I've done highway treks where I've hit from 42 to 46mpg without really trying to conserve.

Other economy tips: 13" steel wheels with 70-series tires. It's not really aerodynamics at work here, but rolling resistance. Less rubber on the road equals less friction between the car and the road surface. It takes less fuel to keep the car rolling on the narrower tires. Of course, grip, handling, and stopping distances will be adversely affected, but the ride will be a little smoother...

I've been playing around with some aspects of "hypermiling" lately in my Accord, trying to see how far I can push the mileage. Shifiting before 2000rpm, coasting as much as possible, shutting off at longer lights, keeping the speed down, etc. It's working, but it's a whole new way of driving. One thing to keep in mind: Drive your mirrors! I do my darndest to make sure I'm staying out of the way while I'm doing these things.

How's it working? Well, I filled up last Sunday, I've put over 90 miles on the car since, and the gauge is just now below the "Full" line. This is kind of my "beater" car - it's old and not really well-tuned, and it generally gets a little less than 20mpg around town. If I can up that to 30mpg, I'll consider it a success.

For more - and better - info on the hybrid transmission idea, look to our own Mista Bone. He's probably forgotten more about Honda transmissions than I've ever known. When we were putting these together, my capacity was more as idea guy and guinea pig in chief, and logistical support (finding a source of ZC transmissions and HF 5th gears). Jobs, lives, and families cropped up, and we kind of lost interest, so we gave up on building them.


Mike

[Kwicko]

Also, with the whole "hypermiling" idea, you don't even use 3/4 throttle. You barely even breathe on the throttle. I'm just brushing it with my foot, and like I said, shifting before 2000rpm whenever possible. Obviously, things happen that require quick action, but as much as possible, just keep off the gas.

Needless to say, acceleration can be measured in geological time. The Accord goes from zero to sixty in mere minutes, crawling on its hands and knees.

The number one rule I observe when trying to do these experiments is to simply STAY OUT OF THE WAY. Just because I'm content to poke along in the slow lane doesn't mean everyone else should have to, and few things are more annoying than that jackass doing 35mph in a 60 zone. All things are relative, so stay out of the way and conserve what gas you can, when and where you can do it safely.

Your Mileage May Vary...

M

[cleanpigpen]

that sucks that your boss didnt put gas back in your car, and downshift too is better than nuetral

[Kwicko]

that sucks that your boss didnt put gas back in your car, and downshift too is better than nuetral

Good point; idling has the engine turning at 750rpm or so, while engine braking cuts fuel flow...

[vwduud]

that sucks that your boss didnt put gas back in your car, and downshift too is better than nuetral

Good point; idling has the engine turning at 750rpm or so, while engine braking cuts fuel flow...

I've been wanting to ask this question. So you're saying down shifting while coming to a stop, clutch out (engine engaged), so that the engine is acting to slow the car down "cuts fuel flow"?

Is this strictly a function of fuel injected vehicles?

[bobski]

Is this strictly a function of fuel injected vehicles?

Probably. On EFI setups, if the ECU picks up that the throttle is shut and engine is above a certain RPM, it simply stops firing the fuel injectors. I don't see how fuel flow could be completely cut on a carb'd vehicle.

Going back to the original premise, yes, coasting down in gear uses less gas than idling in neutral, but it also slows you down more.
Consider this situation: you can see a light a little ways ahead that will be red by the time you get to it. You can either A: shift into neutral and get some ridiculously high FE (well over 100 mpg) or B: stay in gear, using no fuel at all. Which one you choose should actually depend on your speed and how far away the light is.
- If the light's a quarter mile away and you're only doing 40 mph, coasting down in gear could bring you to a stop before you reach the light. You would have to stay on the throttle for a while to cover some of that distance, then coast down. Shifting into neutral to cover the whole distance could be a better option.
- On the other hand, if the light's only a 10th of a mile away, and you're doing 60, conserving your momentum isn't really an issue, so coasting in gear is the better option.

Actually, you could get the best of both worlds if you could trick the ECU into cutting fuel flow regardless of throttle position. Then you could open the throttle all the way and eliminate a good chunk of that pesky engine braking while the car's momentum keeps the engine spinning. I guess you could rig up a fake throttle-shut TPS signal, but the ECU might throw a code when it sees the throttle shut in deceleration, but atmospheric pressure on the MAP sensor. So you would need a fake low-vacuum signal for the map sensor as well.

[SeeOurWrecks]

However that wears the clutch down, changing out brakes would be cheaper and easier. How about coasting in neutral with the engine off?

[stnapycnaf]

However that wears the clutch down...

Not necessarily, rev matching would bring the engine and the transmission to the same speed, eliminating clutch wear from having to grip and engage the flywheel like normal.

['9445]

Actually, you could get the best of both worlds if you could trick the ECU into cutting fuel flow regardless of throttle position. Then you could open the throttle all the way and eliminate a good chunk of that pesky engine braking while the car's momentum keeps the engine spinning.

I'm not sure how much this might help. I tried to see how much difference this would make in my civic on a long steep hill I regularly go down. I shut the ignition off while coasting down the hill in 5th, then tried opening and shutting the throttle. Couldn't feel it start to pick up any more speed with the throttle open rather than closed.

[bobski]

How about coasting in neutral with the engine off?

Well, you either put wear on the starter once you come to a stop, or wear on the clutch by bump-starting the engine when you're close to stopped. Also, you need to be mindful of your brake booster's limited vacuum reserve.

[mossin_90]

Kwicko is your DX, DPFI or MPFI?
Mine is bare stock DX, I'm thinking about the mini me set up just because everyone seems to agree that the multiple injectors saves more gas than the DPFI.

can someone explain the relationship between a higher final dirve ratio and HP Vs lower HPs and lower final drive. or even a higher HP engine with lower final drive.

BTW
by final drive you mean the differential gearing ratio ?

wouldn't a higher final drive, drives at higher rpms when driving in 5th than a lower final drive. same 1-5 gears ratios

Ive noticed that the HF has the lowest final drive, I wonder if I can use it in my DX

ohh I almost forgott, my local dealer parts man said HF came only 4sp...huhhh

[bobski]

can someone explain the relationship between a higher final dirve ratio and HP Vs lower HPs and lower final drive. or even a higher HP engine with lower final drive.

Well, technically speaking, HP stays the same no matter what your gear ratios are. Torque on the other hand changes drastically with gear ratio, and torque at the wheel is what actually moves the car forward. Changing the gear ratio trades torque for RPM; It's like moving the fulcrum of a lever around... You trade travel at one end for force at the other. Torque is a measure of force, HP is a measure of work (force * distance). HP is more or less torque * RPM - it's indicating the rate of delivery of the available torque.
Say you had a big spiral spring (like you find in old wind-up clocks) and you wound it up. You might get several hundred foot-pounds of torque, but when you try to use that torque, you might only get a few shaft revolutions before the spring becomes unwound and no longer applies force. The spring would have high torque, but low HP.

A higher final drive ratio gives the engine better leverage over the wheel, so you get higher wheel torque in a given gear and therefor faster acceleration. The trade off being that the wheel can't spin as fast in top gear due to the rev limit of the engine. Increasing the final drive trades top speed for acceleration. As with many automotive systems, there's a practical limit. If you use some ridiculously high gear ratio that has you redlining at 5 mph, it will take you longer to spin the engine and flywheel up than it takes to accelerate the car. You would have enough wheel torque to spin tires at idle, tow semi trailers and pull tree stumps out of the ground, but your acceleration would suck.

Looking at that the other way around, a lower final drive would trade acceleration (wheel torque) for top speed (wheel RPM). I've been talking about top speed being a function of the engine's rev limit, but you can also look at the the lower final drive as reducing the engine RPMs for a given vehicle speed. The slower the engine spins, the less energy is wasted on friction and so the better fuel economy you'll get.
The functional limit here is the engine's available torque. Our engines don't have much displacement to work with, so if you take the RPMs too low, the engine may not have enough torque (or be outside it's efficient range of operation, killing FE) to maintain speed against wind resistance and other drag forces. The same effect can be seen in higher performance cars - I recall reading a few years ago that the then current model Corvette reached it's top speed in 5th gear, despite having a 6 speed transmission. The 6th gear ratio was meant for cruising... Once the engine's torque was cut down by the lower 6th gear ratio, there wasn't enough torque at the wheel to maintain max speed. Point being that there's a limit to how long you can make your gear ratios.

BTW
by final drive you mean the differential gearing ratio ?

Yeah, the final drive is a matter of the ring gear around the differential and the gear that meshes with it (the pinion gear I think it's called?).

[edited in a few minor corrections]

[mossin_90]

wow that was interesting

Thanks a lot for the info Bobski

[cleanpigpen]

restarting the car from nuetral would cause the same amount of clutch wear than down shifting (either you would have to come to a complete stop to not destroy the starter or clutch start it above 10mph)

sorry bobski i missed yours, Disregard this^

[Kwicko]

Kwicko is your DX, DPFI or MPFI?
Mine is bare stock DX, I'm thinking about the mini me set up just because everyone seems to agree that the multiple injectors saves more gas than the DPFI.


ohh I almost forgott, my local dealer parts man said HF came only 4sp...huhhh

My DX isn't exactly a DX any more. It's got the MPFI setup, but while we were doing that, we also tossed in a D16A6 Si engine that I had lying around. It was a known-good engine, since I'd personally removed it from my wife's '89 Civic Si when it got wrecked. The engine always felt particularly strong (a "Wednesday engine" maybe?), and it never burned or leaked, so I knew if anything happened to the rest of the car, I wanted that engine! As luck would have it, the car got wrecked, so I parted it out and saved the engine and transmission. The transmission got a full rebuild with the new gear ratios, all-new synchros and bearings, etc., and the engine just got dropped straight into the DX while we were changing over to the MPFI wiring setup. Once all the parts were gathered, the actual work - pulling the engine & transmission, wiring the MPFI, and dropping in the new drivetrain - took the better part of a day. And a week later, we drove it from Austin to Orlando for a CRX Meet. It was flawless.

And your parts guy needs to go back to school. I don't know of any CRX that came with a four-speed, unless it was an automatic. There WAS a version of the Civic that had a 4-speed. My first Civic was an '84 model with a 1.3-liter 4-speed. Talk about slow... but it was still an awful lot of fun, and it was the car that put me onto Hondas.

As for your other questions, Bobski answered them far more eloquently than I ever could...

M

[mossin_90]

Thanks for the info as well
I think that the guy doesnt know the least about crxs.

My first honda was a '90 civic, 4spd manual HB if memory serves me well, wordless to say I regret selling it, but back then I wasnt thinking about saving gas.

[HorsemenTech]

Wow I feel special my 90 X dx gets about 38-40 miles to the gallon mix of city inter-city driving between 35-55 miles an hour. I ran 1 tank from in the neck full to right below red line empty and had 455 miles on a 12 gallon tank. When I filled back up I still had just shy of 2 gallons in the tank. Odd thing is that is with a severly slipping clutch. Anybody near Dayton jumpy to help change it?

[sxysweed]

The number one rule I observe when trying to do these experiments is to simply STAY OUT OF THE WAY. Just because I'm content to poke along in the slow lane doesn't mean everyone else should have to, and few things are more annoying than that jackass doing 35mph in a 60 zone. All things are relative, so stay out of the way and conserve what gas you can, when and where you can do it safely.

Problem down here is that EVERYONE mashes on the gas at every light, they drive SUV's, and they drive at 80mph in almost all lanes. Makes it hard to hypermill..

I roll to every stop, sometimes shutting the engine off, really saves the gas. Also, I shift at 2500rpm. Better take off, but doesn't use too much gas. But on the highway, I sit at about 73mph, and that drives it down a bit. Still, I average about 30mpg.

I have a question, how does the ECU decide how much gas to give? Becuase I'm wondering if 'Engine Breaking' takes less gas than just neutral and rolling, or even shutting the engine off and rolling?
EDIT: Didn't see this was answered...

But what's a good gear to go into for engine braking? Cuase 5th feels too light, doesn't slow it down enough..