I was wondering something about the Renesis. As stated many times the 2nd injectors turn on when the Renesis goes beyond 3750 rpm to make extra power or help with power. (I might be wrong why it turns on extra injectors but stay with me)

Since there is a new ECU now could there be a way to tell the RX8 not to turn on the 2nd injectors in 6th gear?

Could this boost the highway fuel economy numbers significantly?

Just a thought...

 

I don't know the right or wrong way to tune the engine for economy. But certainly a programable ECU would be a valuable tool to do this.

Of course, you'll be sacrificing performance. For the price of buying and installing an ECU, plus tuning time etc. How much gas could you reasonably save?

 

Well, I thought that if the ECU gives a good boost in NA HP and maybe increase your highway fuel economy to boot...that would make the ECU even more appealing.

It would be like the ECU had a switch for performance, economy, or performance with highway boost. Something like that...

We don't use 6th gear for power anyway so I thought maybe we could at least turn off that feature for 6th gear so that in the highway we could hit higher numbers which in turn would increase fuel economy overall.

For those people who drive the highway and city it would really help them out (maybe) since I figure that those people who have gotten 26-28mpg in the highway (here at the rx8club) must have gotten that by staying under the 3750 rpm 2nd injector point. (Maybe they were significantly under 3750 rpm...but I'm not sure)

Just maybe...there might even be a way to do something to the current ECU or some other mod that could make this happen without the ECU.

If it could increase your overall fuel economy by 2mpg overall or 3-4 mpg in the highway, the ECU would pay for it self...well, after many many years...LOL!

 

IMHO: A lot would depend on the lubrication requirements and the programming of the oil pump. If you are running 4k+ rpms and the af is too lean, without a corresponding increase in the oil injection, there could be trouble.

 

The second injector is putting in more gas to keep the air fuel ratio. More air at higher rpm needs more gas to keep the ratio. If it doesn't come on, you'll lean the mixture out.... don't know how much, but it will be lean. Could go past the stoicheometric point and be too lean.

 

Thanks for the input...I"m always learning on this site.

Hmmm, so you would have to take care of a few things to make this work.

(1) Cut off the fuel ejector
(2) Cut out some air (not sure if that would even work)
(3) Make sure it has enough oil

It's almost like you would have to have a reduction in air to make it work but what type of problems would that present?

Again, reducing performance in 6th gear is really not a concern because in the real world we don't use 6th gear for performance but "could" add significant fuel economy gains.

Well, I'm just writing down my thoughts...while I know a good amount about cars it's just the basics. To be realistic, if it could have been done it probably would have been done.

 

if you are just looking for better gas milage why not put a set of 4:11 gears in the back. They should drop the rpm's about 800 rpm's so while going 75 mph in 6th gear instead of turning 3750 rpms you would be at just under 3000. You would be sacrificing some acceleration due to the little taller rear end. Cost it would be cheaper than the ecu + tuning

 

Yes...but I want it all....this is America aint it? LOL

 

Changing the gearing will really help. My Fd has a 4.10 in the rear. On the highway while cruising at 76mph, my rpm's are at 3k. Last year on my trip to seven stock, I got 26mpg. My engine had about 88k on it at the time.

I think the injector staging on the 8 is both load and rpm related like in my Fd. 3750 rpms is the transistion point for the 8, however if the rpm's are below 3750 and enough throttle is applied to cause the primary injectors duty cycle to reach 50%, the secondary injector should come on line. On the Fd this is done to smooth the transition and it keeps the primary injectors from over heating so they last longer.

Believe it or not, you guys can put a boost/vacuum gauge on your 8 and monitor the engines vacuum while cruising. The more vacuum you see on the gauge while cruising, the less throttle that is being applied(therefore less gas is used). It's a good way to monitor how much throttle is being applied while driving. On my car with the cruise control on, you can clearly see less vacuum as soon as the car starts going up hill because more throttle is being applied.

 

The air is dependant on the volume of the engine and the rpm it is running at. If you remove the air, you do not have the compression that is needed to supply power. This is like a un-turbo if there is such a thing. Taking air away will make it less efficient.

 

Ok I'm getting really curious about changing the gear ratio in 6th. Does anyone know how much this would cost, and/or who would do it?

 

I see...

I thought I knew pretty much everything about the RX8, but what type of rear end do we have now?

So, my suggestion has the following problems:

(1) At X rpm the engine asks for X amount of air to make X amount of HP
(2) If we reduce the fuel level the a/f mix becomes to lean which can cause: (??xxx??)

The problem has more to do with the 2nd issue than the first (so I think) because at 6th gear we don't need alot of HP to cruise. If we had 125HP in 6th gear cruising at 70mph it would not matter if that became 100HP at 70mph. BUT, if in turn that reduces efficiency I might actually reduce not only HP but fuel economy also.

In regards to the 2nd issue if the a/f level becomes too lean we have (??xxx??) problem which we have to care about.

Help me understand...is that right?

So, that's why Tigster is telling me that the real workable way would be to change the rear end so the rpm's would drop. Problem with that...we reduce acceleration in the RX8 which I know none of us want to do.

In conclusion, Mazda didn't do it for a reason and I'm talking crazy without knowing the details of how all of this works. LOL

Thanks...at least I learned something new.

 

We were talking about changing the gear ratio in the rear differential of the car. I am sure you could change just sixth gear but that is alot more labor intensive and more expensive than just changing out the rear end. The down fall for changing the rear end is you will be a little slower, but your mph in each gear will rise a little.

 

they arnt talking about changing out 6th gear in the transmission that would be very expensive, if you could even get a new 6th gear with a differnt ratio that would work in this tranny.

they are talking about changing the REAR END (FINAL DRIVE) gear that is in the differential. The stock on this car is 4.44 I think (correct me if im wrong). The Transmission is used to change the RPM that the engine outputs. The rear end is used to change the rpm that the tranny out puts.

Changing the rear end gear would give you higher top speed (if you lower it, assuming your making enough power to go ___ speed, say changing to a 4.10) or give you better accel (if you raise it, say 4.88)

Fuel economy would be incressed/decressed based on the fact that at 70mph (for example) you engine RPM would be higher or lower than current.

The lower the ratio on the rear end the slower the car will accel (since the final drive is affecting all gears) one of the ways this car gets away with having lower power (same with other mazdas, such as the miata) is by using a high rear end like 4.10, 4.33, 4.44, ect.

The same idea is why cars like the corvette can make 400hp and still get high 20's on the highway, they have enough power they can use a rear end like a 2.89 or whatever, meaning that in 6th gear at 80mph they can be turning something like 2100rpm.

 

AH HA....So Mazda could have done something different with 6th gear, I knew it...LOL!

OK, if I can fault them for something is making the 6th gear near useless. Let's be honest here most sports cars or when and if you take your 8 to a track you won't be using 6th gear that much.

So, if they put in 6 gears why didn't they give us a lower 75mph cruis rpm...why Mazda, why!!! They built this car knowing about the 3750 2nd injector thing yet foolishly thought that we all drive at 70mph on the highway when it's more like 75.

Thanks for your response...at least I learned something from all of this.

 

Yeah, they did the same thing on the Miata... the 6th gear on the 6MT was barely any taller than 5th gear on the 5MT. They need to make a bigger gap between 5th and 6th on their cars.

If I could change the gear ratios on this car, I'd make 1-4 shorter, 5th would be the same, and 6th would be taller.

 

EXACTLY!!! I've noticed that too! The change in rpm from 5th to 6th is laughable...why did Mazda do that? What is the purpose of doing something like that? Heck…the RX8 might have hit 26 or 28mpg in the highway if they would have made 6th gear usable and maybe even gotten 20 in the city by the EPA standards.

This is too strange…there must be reason, worth it or not…there has to be a reason why they did it.

 

As stated in other posts, the extra injector is there to keep the air to fuel ratio right. Cut off one injector and the other can't put out enough fuel to keep up.

If you reduce the air, you reduce the amount of power being made by the engine. At cruise, your probably only making 15hp or so, just enough to keep up with air resistance. Reduce the hp and you slow down, period.

There already is a device to manually control the air flow into the engine. It's called the throttle, and it's connected (more or less) to the long pedal on the right.
--R

 

HA HA...what evER, but where have you been? Take a look at my latest rant...lol :p :D

Explain that one...lol

 

Gear selection seems mostly a matter of taste. If I were changing the gear rations, I'd lower 4rth a bit, and maybe lower 5th a little, but leave 1,2,3 and 6 alone.

If you were to do a 60-90 mph run in 6th you'd be able to use a calander to time it. Imagine how slow it'd be if 6th were higher!
--R.

 

The "closeness" of the ratios is partly mechanics and partly tradition. Close ratio gear boxes like the 8's do not drop the revs significantly so that you keep the engine in the powerband. The widest ratio change is between 1 and 2 and then the ratio changes between each successive gear gets smaller and smaller. Believe it or not, if they put a wider ratio change between 5 and 6 everyone would be complaining about how weird the shift from 5 to 6 feels.

As long as I am spending other people's money, the solution to the cruising speed rpm is a nifty device that I had on a couple of vehicles in the past. An overdrive differential. Yes, a 2 speed differential. Borg Warner and ZF used to make these. My Fiat 124 Spyder came with it standard. I added it to my Volvo 1800 (the original weird looking station wagon sports car hatchback) These cars had 4 speed transmissions and you used the short differential gear for accelerating and flipped the switch (they are electrically actuated) for the tall cruising gear when you were on the highway. This is the absolute no compromise solution. Keep your nice short gear for acceleration and have a nice tall gear for the highway.

 

beachdog...you nailed it, but now how to convince Mazda to do such a thing on a car that has low sales volume. Hmmmm...

 

going back a bit, i think the two main concerns to running too lean of a mixture are cat failure and engine failure. neither of which i'd like to induce on my car

 

It's not quite no-compromise. The compromises are:
1. Increased weight
2. increased cost.
3. increased mechanical complexity (more things to break.)

If it doesn't improve handling or make the car faster I'd leave it off.
--R.

 

It's no compromise for the issue of the cruising rpm. You get nice low rpm for cruisng and retain nice short gearing for acceleration. Heck, you could even go to a 5.x ratio and get some nice quarter mile times.

As far as weight, not sure how much heavier, but when I did the changeout on the Volvo, it couldn't have been more than 20-25 lbs. In an independent suspension vehicle like the 8 it is sprung weight and very low so it actually helps lower the cg and make the 50/50 distribution more true (since the 8 is closer to 51/49 anyway).

As far as cost, I began by saying as long as I am spending other peoples' money. It would probably be cheaper than rebuilding the trans with a custom 6th gear ratio.

As far as complexity, I had the Volvo for a couple of hundred k miles and the rear never broke. The Fiat ate engines so I never got to find out how reliable the diff would be.