rrrx7

I have a 88 rx7 with auto trans that never sees over 3800 rpm ! I wanted to get feedback on an idea to improve the fuel mileage on this beauty which is why i drive it (not to race it). Anyhow here is the idea: change both primary and secondary fuel injectors to about 250 cc size for better atomization AND have both primary and secondary come on at all rpms (provide secondary with same signal as primary). Will this give better fuel economy and punch at low rpm. Also if the fuel burns better than original 1988 fuel injectors one can get replace heavy duty cats with more free flowing cats thereby adding more overall power ? I will appreciate all feedback !

lduley

iTrader: (1)

its not good for a rotary to see under 3800 rpm, it does more damage than good honestly

diabolical1

iTrader: (3)

welcome to the board.

the short answer is no. it's not as simple as getting smaller injectors.

the long answer is research.

for example, you can increase fuel pressure and get injectors with a better spray pattern, but at the end of the day, you'll still be stuck with the stock tune. you will make better progress with changing that. however, it's not really about changing one thing. you will likely have to change many. there are quite a few other things that you can do to help improve MPG, from tires to exhaust to weight reduction. some of it may appeal to you and some of it will not. that is why i say research.

for example here's a fairly recent thread that touches on the topic: https://www.rx7club.com/rotary-car-p...r-mpg-1084573/

rrrx7

Thanks ... but i have 2 rx7 s one with 177k miles and one with 70k and both are driven under 4k rpm with no problems. Both are na engines. I was thinking going to 4 250 cc injectors might not require any tinkering with existing stock setup but give better punch and emissions at lower rpms.

diabolical1

iTrader: (3)

assuming you don't damage the engine outright, your MPG will likely decrease with such small injectors. i honestly can't say if the engine will even run with injectors just over half the size of stock. for the sake of argument, let's say it does, you can bet it's going to try to achieve the same "burn" that it was programmed for with the 460s and you should be able to imagine the hell that that's going to play.

rrrx7

Yes thats why i was going with 4 250 cc injectors having secondary ON all the time similary to primary injectors. This wud be total of 1000 cc thru out rpm range and i never go over 4k rpm anyway.

rrrx7

I meant having secondary s go on n off at same time as primarys..

diabolical1

iTrader: (3)

i understand what you plan is, i just don't happen to agree with it. however, it's your dime as they say. you're going to do what you feel you must and i suppose there's nothing wrong that, in theory. incidentally though, you do realize that in your scenario of absolutes (and it doesn't really work like that), you're technically still using 80 cc more than stock, right?


as i said, if fuel economy is an issue or project for you, there is a lot of information around here:
https://www.rx7club.com/2nd-generati...faq-fc-494667/ - post 6 has some links, but there are many more threads around the board in other sections.

BLUE TII

iTrader: (9)

You have some throttle plates behind the 2ndaries and some progression on the throttle opening (primary plate 1st then 2ndaries).

So you will be injecting into dead air stream that won't make it into the engine if you fire the 2ndaries with the primaries.

You can go around changing the throttle body design, but this is all done by Mazda specifically for throttle response and mpg.

Not saying you can't improve on Mazda, but go about it a better way.

You can lean out the stock injection a whole bunch with a piggyback, plug and play or stand alone ECU.

Mazda had to run the car really rich for emissions and reliability of cat.

On my turbo car with 720cc primaries the rotary is happy cruising at 3-4% duty cycles, 16:1AFRs, 1,000C EGTs spewing eye watering NOx levels.

rrrx7

Thanks for the inputs 2 all. I understand now why the throttle body has to be different than stock. Are the replacement 460 cc fuel injectors say from denso any better than the 1988 460 cc injectors the car came with in terms of atomization like say more holes etc ?

rrrx7

bluetii is the diagram kind of what u meant. Although it appears as the secondary injectors are shooting gas into same air as primaries except upsteam.

diabolical1

iTrader: (3)

the 2 streams don't mix until they hit the chamber. that diagram is 2D (obviously, i know) but if you actually look at your car you will see that the secondary rail only feeds the secondary runners. they are separated.

BlueTII can speak for himself, but i think what he was basically trying to tell you is that your idea of firing all injectors at the same time while using the stock intake setup (only difference being your 250cc X 4 injectors) wouldn't work as you think it would because you have the throttle plates physically blocking airflow to the secondaries for a certain amount of time before they open.

if this is because of what i said earlier, don't sweat it so much. i was just throwing out ideas at that point. besides, as i said, you'll still be stuck with them following the factory ECU's commands at the end of the day. tuning is where you'd make the biggest strides.

that said, i'm hell-bent on trying some Rx-8 injectors [S1 secondaries, S1 automatics or S2s] whenever i get my FC running. they should be an improvement over OEM, i think. i know a few (maybe 2 or 3 that i've read) people have talked about using them, but i can't recall seeing where anyone actually did yet.

rrrx7

Thanks diabolical1. I might give my idea a rest for now !!!

Injectors should have lots of improvement since 1988 days is what i was thinking too.

BLUE TII

iTrader: (9)

BlueTII can speak for himself, but i think what he was basically trying to tell you is that your idea of firing all injectors at the same time while using the stock intake setup (only difference being your 250cc X 4 injectors) wouldn't work as you think it would because you have the throttle plates physically blocking airflow to the secondaries for a certain amount of time before they open.

Yes.

rrrx7 Thanks diabolical1. I might give my idea a rest for now !!!

Injectors should have lots of improvement since 1988 days is what i was thinking too.

Yes, you could just refit your primary injectors with more modern injectors of the same CC and see a small improvement in driveability and cruise mileage.

One huge problem with rotary gas mileage is Mazda had to tune the cars to run really really rich and then pump in air to the catalytic to convert all the hydrocarbons. Reason rotaries have to run so rich for emissions is to avoid NOx emissions that happen when they are run leaner and until very recently could not be converted with a catalyst.

You can create your own lean burn rotary if you don't care about smogging and get better gas mileage. Mazda has documented plenty of tricks for lean burn. After you get lean burn you start to come up with the rotaries other faults concerning gas mileage such as poor combustion sealing grid, long intake air/fuel travel path, high internal surface area, poor thermal efficiency, etc.

Better to get a daily and fix the rotary up for making power in my opinion.

Evil Aviator

You have created a very dangerous condition. If the engine is not revved over 3800rpm on a regular basis, the secondary injectors do not activate, and very hard deposits develop on the air bleed and clog it up. Once clogged, if you ever try to accelerate past 3800rpm (such as getting onto a freeway ramp) then the engine will run lean, causing major damage that may blow the seals. If you are in a bad situation then you could end up wrecking your car when the engine blows.

If it were my car, I would immediately stop driving it and send the fuel injectors out for cleaning and "rebuilding".

This is a better idea: Contact a good rotary engine tuner in your area. Tell him that you want him to install and tune a standalone EMS with Injector Dynamics fuel injectors. This will allow for far better control of the fuel delivery, resulting in better gas mileage and power. It is common to exceed 30mpg at 70mph cruise with such a setup. The brand of the standalone EMS doesn't really matter as long as the tuner can meet your goals.


Yes, you can replace the original cat with a high-flow cat as long as it isn't against your local or state regulations. However, there will be very little power increase between a new OEM cat and an aftermarket high-flow cat, although there may be a significant increase if your current cat is severely clogged.

rrrx7

Thanks bluetii n aviator for useful info.

RotaryEvolution

iTrader: (30)

if you really want better mileage the best method would be an SAFC and a wideband to tune the fuel ratios, but don't expect a night and day difference. you will spend almost as much as you will save over several years time, not to mention effort.

but as others said the automatic cars are already stressing the engine by not letting it run off the carbon that builds up inside the engine.

rrrx7

thanks lyger. Wonder if all the carbon build up in engine will also show up in oil. My oil looks clean, a golden color n its been 2000 miles since last oil change.

rrrx7

Is it possible to change rx7 auto transmission to shift into next higher gear at 4k rpm. Are these shift points programmable on 88 rx7.?

RotaryEvolution

iTrader: (30)

nope, in fact the transmission came from a piston engine car and wasn't designed for the rotary cars nor was it modified to suit them which is why the gears are so out of whack in the auto cars.

carbon deposits in the engine usually doesn't show up in the oil as darkening, blowby is an indicator of that. you can offset the carbon buildup by doing semi regular hard runs with the car or doing decarb treatments on it with water. both, along with premixing 2 stroke is the best recommendation for longevity with an automatic rotary car, they tend to die at lower mileage due to the above factors.

i can't comment on how to tweak the auto transmission to perform better, they're actually rather rare to come across and of all that i owned none were automatic or had anyone ask me to do anything for them.

rrrx7

thanks lygear now i m wondering what mazda piston engines were designed to use this "rx7 auto transmission" ???

Sgtblue

iTrader: (16)

It's likely you'll never own your RX7 long enough to realize any monetary benefit from doing any of the 'ideas' discussed for the modest improvement in mpg. If you can't enjoy the car as it is, consider another car.

rrrx7

i have enjoyed this car everyday since 1988 when i drove it off the showroom. Most reliable car ever !!!

RotaryEvolution

iTrader: (30)

fairly certain it was used in some of the rear wheel drive cars such as the MPV and 929, perhaps even the automatic B series pickups. even the FDs are sluggish with an auto transmission and 260hp at the crank because of the backward gearing in the auto.

what sgtblue alluded to is the engine doesn't really tend to come alive until after 5k RPMs, rotaries are even less efficient engines at lower RPMs as even miniscule throttle changes result in overly rich conditions to keep the engine happy to accelerate. it's not a very economical car for low RPM use when compared to say my honda insight that can virtually idle at 1000RPMs at 35mph in 5th and get 130mpg.

you can get over 25mpg with a way to tune the ECU but it's like flushing money to save money, a programmable ECU will cost a couple grand. a piggyback wont give as good of results and would cost about $500 with a wideband and might net just over 20mpg strictly on the highway, city mileage in a RX7 will always be in the toilet. i spent a lot of time tuning my turboII and the mileage in city is abysmal still.

rrrx7

No problem. After all feedback i m happy with my rx7 the way it is !!!! A babe magnet