Okay, so these days I've found myself concentrating on engine care and fuel efficiency issues. Having been out of a job for 5 months, and now I'm making only half of what I used to make ... I can't afford to be as heavy-footed as I used to be. I also happen to work about 50 miles from where I live ... so 500 miles per week really takes it's toll at the pump. I'm running the 87 Octane ****-water ... because it's best for my wallet in these lean times ... and I have no turbo to please ... but I put some 93 in every once in a while.
I've been using premix in addition to my OMP (just in case, because I had to do a little ghetto rigging to make it work with the Dell'Orto and I'm not sure how much oil is getting in there) ... and the other day I got to thinking ...
Doesn't the premix (especially when you use as much as I do) lower the effective octane rating of the fuel? I have no idea what goes into RON and RMS calculations and all that jazz ... (last time I've used RMS was AC circuits in college Physics ) ... but seriously, I don't know what goes into calculating octane, but I started thinking about how this affect gas mileage and was wondering if any of the resident nerds could shed some light (peejay, manntis - that was a jab at you guys just playing around)
Yeah ... so what are the facts or you could just share your thoughts? However, I figured it could be useful to all of us ...

 

Ok, I don't know the calculations either, but I have a couple of hints gleaned from the book "How to Modify Your Mazda RX-7".

Three things that I found that may or may not help:

1. This one has more to do with power and racing but...the author describes two schools of thinking when it comes to timing and what octane to run. One school was to run increased spark lead with high octane fuel and the other school, that of which the author subscribes to is to run conservative spark lead with low octane fuel. The author states that in both 1985 and 1986 IMSA Camel Lights championships were won running low octane fuels (although I cannot find the page right now it is on, I think later the author states it was 86 octane that was run).

2. Run 10w-30 or 10w-40 instead of 20w-50. This will show a slight increase in mi/gal.

3. Premixing with mineral based oils will lower the octane of the gas. The race engines talked about in this book were running a 100:1 fuel-to-oil ratio which comes out to about 6-1/2oz of oil per 5 gal of fuel. A very rich mix which the author warns is not appropriate for race engines that are not as highly stressed. Basically do not run a premix that high in your street engine

Also, if your car is doing a majority of sitting which as you state above, it is not, the author recommends using a synthetic oil in the premix instead of mineral based since a mineral based oil will settle out of solution. Also the synthetic does not affect the octane as much as the mineral based oils do. The synthetic based two-stroke lubricant is what one of the contributors recommends in the book but on the other hand in the same paragraph another contributor stays with mineral based since the synthetic leaves a residue on the plugs due to incomplete burning of the lubricant and the plugs tend to foul quicker when the synthetic is used.

BTW, factory recommended oil flow rate from the OMP (oil metering pump) is 2.2cc in six minutes at 2000rpms. in the 12A.

Hope this all helps.

 

One tube or both?

~T.J.

 

Yeah run the lowest octane you can on non-turbos. Premix for "street" and you'll be all good!

 

Both and the stock OMP can only be set to a minimum of 150:1 meaning it could not even get down to 100:1 if you wanted to mix that rich.

 

actually, thanks, yes it does help ... quite a bit. based on what you've said you read, i've gone way past overkill and into "the downright, ******' ridiculous" with the amount of premix i've been using. so, i will run this tank out and fill up with out using any the next time. then, i'll try to start using a consistent, more reasonable, amount from now on.

it's just that i've been tracking the gas mileage for the past two weeks and there has been 2 MPG change, and the only thing that i can think of as being different, is the amount of premix i've used (because it varies depending on how much i make - oh, and i mix with mineral oil) ... so i wanted to know if the fluctuation in mileage is a "normal" thing ... or if i should attribute it to the changing combustion characteristics (based on the lowering of the octane due to premix)

it's all become a little pet project now. i'm going to track it for a few months and make a graph of it to chart changes ... to be honest, i'm already impressed with it. 22.6 MPG on a car with pretty big Dell'Orto, stock exhaust (with leaks), in dire need of a good tune-up, shitty tires and 30wt oil ... i don't think that's too bad. it's a damn sight better than i had expected ...

 

I'd like to see that graph when you're finished. Will you be using some type of spreadsheet for it? I agree with you in that the amount of pre-mix is modifying your burn characteristics. If you're gonig to graph it out I'd suggest starting with a fresh set of plugs. I think as they foul your mileage goes down too.

BTW if you'd like to look that book up the ISBN is:

0-89586-383-9

Mike

 

i tried buying that book and i was told it was out of print

and sure, you can see the graph when it's done. unfortunately, i still live in the stone age (i don't have a digital cam )and will probably do it the old-fashioned way with a pen and paper. however, i can always transfer it to a spreadsheet afterwards

 

Another thing they recommend in the book that has nothing to do with gas mileage but everything to do with lubrication is not to use a single weight oil. It will actually thin out more with heat than a multi-weight oil does.

Yup, the book is out of print. I bought mine on Ebay. When I bid on it, I did not know the original price was only $12.95! I ended up winning the auction with a total payment including shipping of $42. Plenty of action during the auction as it is a rare and sought after book for 1st gen collectors. Lots of neat stuff in it, my favorites being the extreme measures the race teams do to actually lighten the motor.

I'd like to put a dellorto on mine once I get a few other things fixed. Gotta love the look of a sidedraft carb

 

Why are you using 93 octane? Everything I've read indicates normally aspirated rotaries are happier with the lower 87 octane fuels... am I overlooking something?

 

LOL, actually the high-octane fuel is ****-water. Octane rating is the anti-detonation rating, which means that high-octane fuel takes more heat to detonate, which is another way to say that it is "watered down". The power rating of fuels is their specific heat value, which is usually given in BTU's/pound or BTU's/gallon. Pump gas has basically the same specific heat (approx. 18,700 BTU/lb), regardless of the octane rating. Racing fuel generally has more BTU/lb because is uses more powerful ingredients. Save your money and stick with the 87 octane if your engine isn't knocking.

The oil may change the octane rating very slightly, but I don't see where this makes any difference. An easy rule of thumb for pre-mix is one ounce of oil per gallon of gas. You should disable your OMP so that you don't use too much oil.

BTW, it's RON, MON, and PON, with the formula being (RON+MON)/2 = PON, aka (R+M)/2 as seen on the sticker on the pump. RMS is for electronics.

I lost mine.

 

as dumb as this will sound, i think you did GREAT buying it for $42 bucks. i think i may start looking around for it on e-bay (or whatever other auction-type sites i can find) whenever the dollars can be applied at bit more liberally.
as for the Dell'Orto, it sure does look nice. i've learned to like it a lot more than i did a few months ago, but i'm still not a HUGE fan! the only thing i'm waiting on is to start experimenting with jetting and such ... again, i will have to wait until i get back on my feet. right now, my only goal is to keep the car happy and running as efficiently as i can. a full tune-up and exhaust is my next goal.

you know, i ask myself that same question every once in a while, when i'm at the gas station and find myself thinking ... "damn, i can't wait to 'til i can start buying 93 again" ...
the only thing that i can offer is i just think 93 out of habit, because i don't put anything less in my MR2 Turbo, and so i guess at some point i had started to just put it in the Rx-7 just out of mindless habit. before i got the MR2, i used to only put the 93 in the Rx-7 when i went on really long road trips. however, my finances have me using the 87 in the Rx-7 (the MR2 has been parked since i lost my job ) ... so hopefully i'll just keep using the 87 in the Rx-7 even after i get the MR2 back on the road.

i never really thought of it that way, but i guess it makes sense. i always thought of it as measuring volatility. so you've taught me yet another fact today. i just want to say thanks for the octane info. and yes, as i just said to manntis, i intend to save the money and keep on using the 87 in the Rx-7 ... it does the job just fine.

 

oh ... and i plan to stop using the premix whenever i solve the OMP issue i'm having. i'm guessing that my problem rose from the fact that i'm running an SE motor with a Gen II lower manifold. i honestly don't know. basically, the rod that actuates the OMP in conjuction with the throttle linkage does not allow me to connect it to the carbie. i had discarded a lot of my Gen II 13B parts, and even IF i still have a front cover, it would be in NY, not here with me (FL) ... so i had to do something.

i used a clothes hanger to make a rod (insert heartless laughter and ridicule from other forum members here ) and i'm not sure how effective it is at getting the job done. so i'm running the premix to cover all bases. so as soon as i can get my hands on the right part, i'm going to stop using it.