Xyntax

For the skeptics, exit this thread and go bother someone else . This thread is for the open-minded

Anyone seen that RX-8 Tech CD which was made in Flash? Someone was kind enough to share it here (go to multimedia section). Anyways, there was something there that most SDAIS diagram scanned didn't mention: the increased airflow at 3250 rpm and the opening of secondary injectors at 3750 rpm.

I've read this months ago but recently I got curious about exploiting that rpm gap. I figure that the reason I have been getting bad mileage (avg 15mpg) is that I am cruising at the wrong rpms. I have tried granny shifting and spirited driving, but nothing gives the best result. So, since 2 nights ago, at 3/4 tank of fuel I started driving it like a normal M/T car. I no longer cruise 45mph at 6th gear. Here's the keypoints in my new driving experiment:

- Shift at 3750 - 4000 rpm
- Cruise at 3000 - 3500 rpm

It seems to me that the RX-8 engineers put some thought into a perfect daily-driving rotary car. 35mph is the most common speeds in the street right? That's 3500 rpm at 3rd gear. At certain highways, 45 - 50 mph is the average, thus 3500 rpm range at 4th gear. Freeway, 65 - 70 mph, also at the 3500 rpm range at 6th gear. It just makes sense that they made the breathing system in this car so that it is both efficient at cruising speeds and quick if you need to accelerate. I'd like to call this 3250 - 3750 rpm zone, the "Cruising range".

Try cruising at 3500 rpm at any gear, what happens when you step on it more? It goes quickly to 3750rpm and above right? Just when more fuel is injected into the chambers. I find this driving style really fitting with how the RX-8 is designed.

:D Now for the surprising results:
Granny shifting this "sports car" and cruising below 3000 rpm really bored me to death. I get to drive my Solara faster than my 8 (what a shame) So, I'm crossing my fingers that this driving style will get me out of the bored-hole. I started making driving style changes at 3/4 tank right? I had around 60 miles on my trip meter. Now, I have the needle sitting on the half tank line with 140 miles already. Don't tell me that is normal for me because I drive 95% city and I "normally" get 90-100 miles on the half tank line. The last time I saw the needle sitting at the half line with 140 miles on the trip is when I was driving back and forth to Monterrey on a weekend.

If this experiment provides consistent results for me, this is really good news for those who daily drive the RX-8 within the city most of the time (ie. brothervoodoo)

Oh, I have M flash with 6500 miles on the odo with 40 psi on all tires.

ps. this kinda reminds me of the members here that get really good mileage yet they never granny-drive. Maybe this is why.


... I will post an update when I refill the tank.

shelleys_man_06

I'm not a skeptic. Half of the time I drive under 5000 rpm. My gas mileage is always about 22.35.

Xyntax

shelleys_man_06, I have tried driving that way too. I had the lowest mileage driving that way, 14 mpg. Around that time, I just wanted to drive the 8 like how I feel. Turns out, I only improved the mileage if I drove at least 50% on the highway/freeway.

I will not dismiss that driving style though, because I only did that at 1 tank and it was before I got the M flash.

brothervoodoo

There's a lot of good information about fuel injectors and various threads that discuss RPM vs. port opening, etc...

Try some searches using the following search terms "SDIAS", "tertiary", "fuel injectors".

You'll get good hits discussing the first port opening at 3750...

shelleys_man_06

Actually, this car enjoys being pushed hard. Having to drive this car at low-rpm is not only boring, but it can seriously kill the gas mileage. I forgot to add that I normally drive on the freeway so I can get to school. Otherwise, my fuel tank drops more than Mary Kate Olsen's weight. I hear the 13B-MSP enjoys chowing down on fuel at idle or low-rpm. Is this true, not only for the RENESIS, but for piston engines as well?

Xyntax

After my next fillup, I'll try to raise my cruising range at 3750 - 5000 rpm. I'll update this thread when I get my results.

From now on, screw granny shifting!

zoom44

my s-dais thread specifically makes mention of the port opening and the injectors coming on line at 3750. and i can tell you for certain that cruising the highway at 4k uses more gas than cruising the highway at 3500. at 80mph i made 22.5 mpg. at 65 i made 24mpg

Xyntax

Right. This is where I read about the port opening at 3250 rpm and injectors at 3750 rpm.

http://chicano-mail.com/Bideos/renesis module.zip

It was a snatched out Mazda Renesis Training Module. I always thought the port and injectors come on at the same time and at 3750 rpm. Would this mean that the range between 3250 - 3750 rpm produce a leaner mixture then? Could anyone with A/F logs confirm the changes (if any) please?

Xyntax

I have viewed the module stated above and took some screenshots of the part where it mentions about the 3250 - 5500 rpm's increased airflow. The diagrams that are commonly posted on this forum never mention that range, instead it focuses more on the 3750 where the secondary fuel injectors coincide with the opening of the secondary ports. Well, that is true, however it fails to mention that the secondary intake ports open at 3250, 500 rpms before the secondary fuel injectors come on.

Here's a reduced-in-size screenshot of the module talking about the less than 3250 rpm range:



And here's what happens at above 3250 rpm:



... take it for what its worth.


EDIT: I searched on the forum for "3250 rpm" and I found out that a lot of members here have mentioned about the RX-8 getting 90% of it torque from 3250 rpm and up. The increase in airflow might contribute to that.

r0tor

iTrader: (1)

as i recall its a certain rpm AND at least XX% pushed down on the throttle for the secondaries to open... not just 3750rpms

Phlash

40 psi?? Good lord...does that not beat the crap out of you???

Xyntax

Phlash69, it's not so bad. It kinda feels more like a sports car now :D both in maneuverability and feel.

You really won't feel much difference if you just drive around San Jose (smooth roads). As for potholes, let's just say you can almost measure how deep the pothole was with the way the car bangs.

Tony Orlando

Not to hijack your thread, but you are going to be mad when your tires are bald in the middle, while the outsides look brand new. I run 40 front, 38 rear when I autocross , but isn't 40 a little high for daily use? Since you're riding on partial tread, the car will feel more "darty", but also follow any imperfection in the road. What's the point?

Xyntax

You're probably right Tony, but this is just one of the things that I test out on my 8. I will be reducing it to 32 - 36 psi some day. I'm trying to get used to the 40 so that when I reduce it back to the normal range, I would be able to tell the difference between them.

zoom44

according to the yamaguchi book both happen at 3750. i dont have a copy of that module to watch. anyone want to send me one? it could be a typo in one or the other. i would be very suprised however to find that kind of mistake from mr. yamaguchi. i will try to get clarification

StealthTL

iTrader: (1)

During his early development, Canzoomer posted that the mixture went really lean between 5k and 6.5k......

Well, anything for scientific research, I used one whole gas fill, cruising over 5k, shifting at 6500.

That was FUN!

Didn't do a damn thing for my mileage, but really fun!

S

Maolin34

Ok....with an open mind, I tried the to restrain myself and shift below 3750 for an entire tank of gas. Cruising on the freeway at about 70 to stay below 3750(In the SLOW LANE).

I have 2700 miles on my 8, always use premium fuel, and have been averaging 12-13 mpg.

When maintaining below 3750rpm I came up with 15.53 mpg. A slight improvement, but not worth the pain of NOT hearing the Renesis work. On a fresh tank, I drove how I wanted to drive. Shifting wherever I wanted, staying in a lower gear to be in the powerband, etc....the result, a suprising 14.9 mpg. WTF? I consider the possibility that it will loosen up a little as it breaks in further....but why such a small difference? Here are my theories....

1) My poor gas mileage has been during the very hot and humid days of July. I did not do much night driving when it was cooler. I must say it was very hot inside the car while driving, and even after the car had been sitting in the shade of my garage. The engine generates alot of heat. In the cooler days lately(70's instead of 90's) the car has more punch(As you would expect) and seems to run smoother. I fear hot weather may play a small part. (small).

2) The secondary injectors coming on line at 3750 should be governed by two things, RPM and throttle position. If the injectors come online at 3750 regardless of throttle position, it is a tremendous waste of fuel...this is, of course, an thought not knowing the injectors duty cycles at various rpm. The secondary injectors could come online, but the split the work....the primary could go to 50%, while the second maintains 50%....but why bother? If rpm is the switch for the second injectors, Mazda made a mistake in my opinion. Tell if you know more.

I have more, but I will add in later....try and keep this post from becoming a book.
*** MOD EDIT: Sorry, no street racing stories. ***
I love this car!!!! Querks and all...

Xyntax

Sucky news y'all, I just filled up 13 gallons at 200 miles (usually its 210 - 220) I had 9K runs at the last quarter tank, but still...

I was using 91, now I'm gonna do it with 87.

...crossing my fingers.

Tony Orlando

I drive 50 miles each way to work, mostly on the FL turnpike. I drove the last two tanks with the 3250-3750 method while cruising, and was rewarded with no extra mileage. My fuel light always comes on between 230 and 240 miles.

Back to redlining it.

legokcen

Wow. 40 psi. Maybe that's my problem. I keep it around 32 psi. Averaging 20 gpm, so I'll try 40 for a while.

takahashi

Very true...

High psi helps, the way Xyntax drive is conservative and I use that too. It is more quiet.

My set up is the same except I am a F flash from Oz and 39psi 4 corners.

I have 12L/100km with 70% freeway and 15L/100km in traffic.

A tank last me 450-500km.

It used to be more before and I found after a few month/ 5000km the mileage improved

JeRKy 8 Owner

I haveknown about this little "HAPPY MPG ZONE" on the tachometer for a long time now --since last September in fact. Never helpedme at all before or after the M flash. Perhapsmy car is just "special" like that.

Xyntax

I'm on 87 octane now and just crossed the half line. I have 180 miles on my tripmeter by now. What happened last time was that after the halfline, the needle went down to the 1/4 line faster and then back to normal to til the empty light turned on.

I wouldn't be mentioning this on the forum if it wasn't anything unusual for me. It is just rare to see anything more than 110 miles on the half tank with 95% city driving. First it was 140, now 180.

About the 40 psi, I think I read it on a different thread and it was a post by Taka :D

Chrisbert

I cannot find that Flash file. Can you link the thread please?

Also, can our car be throttled manually from under the hood or is it totally electronic?

Thanks,
Chrisbert

JohnO

Maybe I'm missing something here. All this talk about getting worse gas mileage at 80 than at 65 has less to do with whether the secondary injectors are operating and more to do with the amount of power required to cut through the air.

The power required to run down the road increases as the cube of the velocity! If you are driving steady state at 60 MPH and it takes 20 HP, doubling your speed to 120 MPH takes 8 times the power: 160 HP (I have no idea what actual HP is required at 60 MPH). Neglecting losses in the engine and transmission (which probably increase linearly with RPM), operating the car in 6th, 5th, 4th or 3rd at a steady 60 MPH all take the same horsepower (and therefore gasoline burned per minute).

What consumes gasoline is the amount of power you tell the engine to produce to accelerate the car or cut through the wind (by pushing down on the right pedal). If you accelerate slowly and smoothly and keep your speed down, you will get better gas mileage than if you drive like I do...

I've got 8400 miles and get around 13-16 MPG. When I loaned my car to my mother & father (who were out here visiting) to drive throught the Sierras, they got 25, 19 & 24 MPG on the 3 tanks they used. Luckily I wasn't in the car with them - BORING!

What I'm saying is that HP=gas, MPG is related to throttle position (throttle down => MPG down), and you are in control of your gas mileage.