Scott1982

Well,
I know a lot of people have asked questions regarding how to improve gas mileage on their cars - I was/am in same boat. I have a car with 144K miles, runs like a champ but it really consumed a lot of gas - 170 miles to the tank (at least as far as I dare go). This is a stock car - all rats nest present and accounted for!

I read, read and read more...

Here is what I did and I absolutely give credit to the folks that created these concepts... I should point out that I just got over 280 miles on the tank (no other changes, same “normal driving”, to work and kids events on weekends) and I expect to hit 290. I should point out, my “new” RX-7 in 1983 bested at 320 miles per tank on a trip to Albany.

I first used Sterling’s procedure to adjust the idle mixture and went toward the lean side (just a little rough). I originally set mine up after the carb rebuild so the car ran real smooth, even 400RPM (warm)... However, it really stank. I should note that my cat is present but I believe PO removed contents. This adjustment made it so that it was bearable to work around the car while it was idling.

I have included the procedure here to save search:

Carburetor Idle and Mixture Tuning (Sterling)

Idle SPEED screw...located under the linkage on the right side, very low, and hard to see. Requires a very long flat blade driver.

Idle MIXTURE screw...located dead center driver's side of the throttle body (cast iron lower part of carb).

Run engine and get it warm. Fuel behavior is subject to temperature. Tuning a cold engine does no good when it becomes warm.

You are tuning the IDLE only. Tuning the idle has no effect on anything over 2000 rpm.

Adjust the idle SPEED to anything below 1000 rpm.
If you cannot get below 1000 rpm, you have a vacuum leak...a cracked hose, dried-up gasket, that sort of thing- It's letting air come in to the idle circuit, and bleeding off the signal to the fuel source.

Next, adjust the MIXTURE screw by turning it clockwise till it wants to stall. This is leaning the idle mixture.

When it starts to "hunt", or stumble, turn it back 1/2 turn.

Redirect your attention to the idle SPEED screw, and turn it counterclockwise until you are idling at 750 rpms.

Return to the idle MIXTURE screw, and adjust it the same way as before.

Check the idle SPEED again, and if necessary, repeat both steps.

You’re done.

Timing - a big deal

I again did a lot of reading (including the FSM) and was initially perplexed that the timing mentioned for the distributor did NOT match what I saw with my timing light. For example, it mentions full advance at 1750RPM of 10deg... Huum, I was seeing 20 at about 3500RPM - well the distributor moves at half engine RPM.

After measurement with the timing light (oh yea, ignition sparks on this car really do get your attention), the following table was put together. Now, I added data from Peejay (big gas mileage improvements - search!), the commonly recommended 24L/16T and finally what I set up (which was intended to be the 24L but 14T (I kept the stock spread). I thought people might find this interesting – the curves are not exactly correct as shown as the timing is supposed to start increasing @ ~1000RPM’s but I figured close counts. Peejay (search - he has put a lot of good information out) runs with mechanical advance on at all times – I will likely be trying that soon, but I wanted to baseline where I was and I am thinking it is time for the 2nd gen coil mod after maybe one more tank (and I finish the maint on my other car) and maybe a fuel pressure mod to 2.5 psi.

PS - In simple terms - I adjusted my timing 5 degrees advance from stock!!!!! (There are many threads that show how to mark your pulley - I followed them!)

ray green

iTrader: (14)

Great improvement but you may still have a way to go, I routinely get 350 miles+ per tank with my stock 85 GSL (23-24 mpg - 402 miles on one tank is my personal best, running on fumes when I pulled up to the pump).

One big difference from stock in my case is no cats, I have a bonez replacement pipe, so this probably helps a lot. Also most of my driving is interstate commuting (ie 70-80 mph), not a lot of stop and go.

Still I'd like to do a little better so I will take a close look at your suggestions and try them out over the next few weeks to see if they work for me.

Thanks!
Ray

Scott1982

Well you are doing better than I ever did when my first car was new and that 320 miles on one tank was all highway miles! I do have a replacement pipe but wanted to do this one step at at time (and I am due for inspection this month... although, this year I am exempt).

Scott

tallbozo

iTrader: (5)

I just got 375 miles to a tank. I had the carb rebuilt,mechanical secondaries, full emissions removed, full catless exhaust, k&n intake, NGK plugs and wires. When i first got it i was getting 250 miles a tank, but since i've done these mods, it has drastically improved. Keeping your timing and maintaining your carb and ignition will save you some extra MPG's

Scott1982

Is that all highway - I am driving mixed (which is hard to quantify but my driving is consistent from week to week). Either case, I don't think there is any chance I can hit 375 without doing more even on highway. You prompt me to do more... Now if there wasnt a check engine on the wife's honda or the body work on my daily driver to do...

Scott

t_g_farrell

iTrader: (1)

Good info. For most folks getting bad gas mileage is usually caused by poor maintenance and a poor tune. Usually they fall into two categories; the all stock full-rats nest category or they're in the after market carb and header category.

If you have a full rats next like you have then all the hoses and all the solenoids have to be present and working with no wiring issues and the carb has to be tuned to spec. Then as you and Peejay have done you can tweak the timing and mess with the distributor to up your mileage.

If your in the aftermarket carb and header group then a good tune on the carb is required plus using vacuum advance. I'm in this category and I get ~25 mpg with my setup:
- 48mm RB Dellorto DHLA carb
- RE perforance exhaust
- 2GDFIS
- vacuum advanced from the manifold
- timing set to RB specs for the Dell (L 2 BTC T 19 ATC @700 rpm)
- dual MR2 electric fans (no thermo control yet)

Before adding the vacuum advance and the electric fan I was sitting at around 20 mpg. Each of those mods got me back about 2 mpg or so which was a side benefit because I was really doing them to free up a few ponies which the butt dyno confirmed. Maybe someday I'll get it dynoed.

Rx-7Doctor

iTrader: (13)

I have everyone still beat except peejay on MPG with my SE. With the personal best of 31 MPG. :-)

But we can't count peejay against me because he has a 12A.

ray green

iTrader: (14)

Doc, you wouldn't be pissin' on the bushes, would you?



Let's have some details on how those SEs are so much better than 12As at gas mileage.

Rx-7Doctor

iTrader: (13)

Lol!

I believe that a contributing factor to my mileage is the higher elevation where I am.

While I was still in CA I did testing as stated in my main fuel economy thread. I got 26 MPG before I removed the stock fan set up. When I installed the electric fan and did a retest of similar driving I got 27.1 MPG still in CA.

My best mileage before I used Acetone was 29.1 MPG. My best mileage after I used Acetone is 31 MPG.

Still have a couple more things to try then record the results from each experiment.

Then I plan on installing the MegaSquirt on the Se and see what I can get. :-)

For those who may still not of read my thread-

https://www.rx7club.com/1st-generation-specific-1979-1985-18/fuel-economy-report-since-recent-mods-se-656700/

Siraniko

iTrader: (13)

Its not the car!!!! Its the loose nut behind the wheel.

dbragg

iTrader: (7)

get a 2nd gen coil, itll make a world of difference. that and an exhaust it can breath better with will help a lot.

vipernicus42

iTrader: (4)

I get about 23.5mpg according to google (I get 10km/L) mostly highway miles.

That's with a Sterling, the Racing Beat "Street Port" exhaust with the dual long primaries, a Mallory Comp 70, an e-fan and stock ignition.

That's on 87 octane (the "cheap" stuff here, if you can call $1.25/L cheap) with a stock 12a pushing 220,000km that does 7.2kg/cm2 on all faces of both rotors... running stock timing with an ISCRacing airbox and K&N air filter.

Not bad I'd say. I'm going to give it a tune-up, double check the timing and see what I get on the way back from Kitchener once my suspension is in. I get the feeling that I could get MUCH better if I actually took the time to jet the carb (which I haven't yet, but hope to do before the end of the month)

Jon

twinkletoes

iTrader: (17)

Geez. I'm getting around 200 to a tank with city and highway driving on my 83 GSL. I have a 13b GSL-SE engine with a sidedraft mikuni 44 and rb exhaust. Should it be that bad?

tallbozo

iTrader: (5)

its mostly mixed driving. I had the accelerator pump spaced out to get more fuel in when the secondaries kick in, but that killed my gas mileage so i went back to stock spacing. And I think keeping your foot out of the throttle helps alot too. No more than half throttle to be exact. I get better mileage than my friends civic, and he really hates that

Scott1982

Added 2nd gen coil (2GDFIS) - actually don't notice much difference either in idle (mine was really good anyway) or in mpg... Another 2 tanks and the bypass pipe will go on in lieu of the carb. I should note that the driving I do is to/from work - mix of city/hwy. My daily driver, 95 BMW 525 (2.5L 6 cyl) gets 22.5mpg on this drive so I need to at least beat that as my goal!

Scott

ray green

iTrader: (14)

Bypass pipe replacing the cats should have a big effect, you will instantly gain power and this will translate to improved gas mileage. At least of all the mods I've done this was the big winner by far. This includes the 2nd gen coil, which smoothed my idle and acceleration, but didn't have a significant effect on mileage, and a full racing beat exhaust system, which is really nice but still no better than the stock header and muffler with the cat replacement pipe.

Most of my driving is the 110 mile daily commute into Atlanta and I stack up well with your BMW at 23-24 mpg, so give that 7 a chance, it's way more fun in traffic!

TheRX7Project

iTrader: (3)

Anyone in favor of conserving fuel, raise your right foot.

bliffle

Unfortunately, in CA we have to pass a visual check for stock configuration as well as the usual smog sniff test. So it's hard to do some of the mileage mods. About all we can do is modify idle settings and spark advance. Maybe changing settings before and after the smog test.

I drive moderately and get about 16 mpg. But when my 83 was new I always got about 22-24 mpg, so the changes to the CA fuel have had a big effect.

I hate to advance timing too much for fear of detonation and risking Apex seal deterioration.

j9fd3s

iTrader: (3)

i managed 25mpg on my 79 without doing more than turning a couple of screws. car is 100% stock. the emissions stuff doesnt really affect mileage, except that the cat cant be too lean without melting. on that car, idle mixture has a huge impact on mpg. the 81+ dont seem to be as important

detonation isnt really a worry, but being on the safe side is always good, set it to 22btdc L...

LizardFC

iTrader: (1)

Appreciate the write up. I'm gonna give this a shot after I fix my brake issues. (Stopping is even more important than gas mileage right now!)

My bone stock 85 GSL typically gets around 320+ to the tank, with maybe 350 being max. That's with an idle of 1500RPM and really awful alignment. Still haven't figured out what's going on with the idle, but this car has been screwed with a lot by previous owners.

Scott1982

The bypass will go in after this tank of gas (200 more miles). I want to see how well just the coil change does first. Thanks for the input.

Scott

Rx-7Doctor

iTrader: (13)

I just got 26.9 MPG with the tank before this last one with combined city/highway driving and higher than normal speeds on the 1 way trip of 160 miles. An average of 75MPH.

The return 1 way trip at speeds of 60-65MPH I just filled up (165 miles) the mileage was at 30MPG. :-)

For those that have not read my mileage thread this is on my 85 SE.

Scott1982

You still using acetone? or is this without? 30MPG is awesome and is what I am hoping to get to for an upper bound. I am wondering if this is possible for carb (not discounting PJ).

I have yet to be able to do any amount of highway mileage. Just using my to from work baseline which really clobbers my daily driver from 30mpg on highway to 22.5, which really doesn't help anyone else for direct comparison.

Scott

peejay

My current 13B is pulling down a whopping 20.8mpg. That .8 means a lot to me, if I saw under 20 then I'd freak.

But then it's also a new engine... still pretty tight... doesn't cruise well at all.

Rx-7Doctor

iTrader: (13)

Yes I am still using Acetone.

Last trip I took I got 33.5 MPG. That is the second time I got that mileage.

Next I am shooting for 35 MPG.