jjwalker

I was thinking about this yesterday...

All the guys without an OMP and running premix, do you guys avoid engine breaking since there is no fuel/oil being sent into combustion chamber?

I was also wondering if even with an oil metering system, if oil starvation of the apex seals is still an issue. I live in a hilly area of Dallas (yes, there are a few hills in DFW) and I engine brake mostly down these hills, running anywhere from 2000, 3000 rpm while coasting down in gear instead of eating my brakes up. My engine only has 2000 miles on it and I don't want to chew it up from oil starvation of the apex seals.

From what i have read in the FSM and other posts about the oil metering system, the metering in S5 cars, such as mine, is controlled by the position of the AFM. Correct me if this is wrong. So, my thought is, when I am coasting my AFM is shut, and the engine is being delivered an amount of oil sufficient for idle, but not really 2-3k rpm while I am coasting in gear.

Thoughts?

arghx

iTrader: (3)

According to Mazda's 1990 SAE paper "New Technology employed for latest 13B Rotary Engine" the OMP control logic on the s5 is based on airflow, rpm, and water temperature.



On the s4 it is of course based only on the position of the mechanical linkage (which is determined by throttle opening angle).

There's another section in the paper where Mazda discusses the lubrication requirements for the engine:



Basically in high rpm, high airflow conditions the apex seal needs the most lubrication to keep the temperature down. At low airflow the lubrication requirements are a lot less. The housings can also retain an oil film to an extent anyway. That still leaves this a somewhat open question, but after looking at Mazda's graphs I wouldn't be that worried if I were premixing only (which I'm not).

jjwalker

Ah, I missed the part stating engine RPM, I guess that answers my question.

As you said though, the question of when only premix is applied, its a bit more open ended.

lastphaseofthis

iTrader: (8)

Why would your AFM close if your engine is at 2-3k rpm? because the thottle is? hah like the AFM gives a **** what your thottle is doing, it's still open, and still letting air in.

as far is break-in, i've been told by an infamous builder himself, that he puts engines on a stand with no OMP and no PREMIX. to break them in for a few hours before shipping them to customers. and i asked what about the wear on the apex seals, to which he reply'd
" lack of oil on the seals only increase the speed at which the seal's wear into the housing and make better compression sooner"

jjwalker

Yes, I know air is still getting in, but it does close. I was curious before I posted this question and removed my air boxe and observed the AFM and its position. I revved the engine and when the throttle slapped shut, so did the AFM. Obviously this is exactly what happens under decel since the engine is under load, but the test is sound enough.

beefhole

iTrader: (29)

I've been wondering the same thing as I recently enabled fuel cut on my haltech. I'm going to keep the mechanical omp in place, but not connected just for low load/off the gas conditions. I also will run premix.

FelixIsGod29X

iTrader: (50)

Lack of oil on the seals only increase friction which = heat and undesired wear.

sharingan 19

iTrader: (16)

Good question. Great answer.

Hazard15301

iTrader: (5)

Yea... that was my guess. I can see why he would think that, but I thought to myself as soon as I read it, Wouldn't that cause an ungodly amount of heat and warpage? When you think of apex seals and no oil, its just painful.

Puck

iTrader: (1)

The fuel being fed to the engine would lubricate the seals to some extent, although, not as well as oil would (obviously). It's not like you are spinning it round and round without any lubrication at all.

farberio

iTrader: (7)

I have a haltech, fuel cut, and I premix.

It doesn't bother me because my fuel cut is usually for a very brief period, always at very low loads, and the housing coating does retain some oil.

rx-7 obsessed

i have to ask who the builder is that sounds like the stupidest idea ever? yes they probably would break in sooner, but the motor would also BRAKE sooner

ericgrau

iTrader: (1)

I would think with no fuel the old oil wouldn't be burned off either, at least not for the duration of that hill. As for the AFM reading when the throttle is closed, it should be lower even though the AFM isn't closed. Less air is flowing.

But I don't engine brake regardless, at least not in low gear, because of the wear the high rpms are supposed to cause. IIRC your brake pads are cheaper than your clutch or your brake pads are cheaper than your tranny or some statement like that. I also try to save down-shifting in general for passing and I try to rev match when I do, because of the wear that also causes.

alex91n/a

iTrader: (2)

I would assume that if "engine braking" was harmful to our cars, and it's a rather common thing for people to do. I'm almost positive Mazda would have A. Fixed it so it wouldn't cause damage to your engine. Or B. They would have told people not to do it. Sorta how rx7's need about 1 qt of oil added every.. About 1000 miles. If it were really bad I'm sure it would be common knowlege among rotary people by now. Now cars running premix is a different story and I have no idea about that one

farberio

iTrader: (7)

That's awesome for a track motor, the more your engine brakes the longer you can step on the throttle.

Now, if you mean the engine will BREAK sooner. Well...I personally doubt it. Premixing is required, but there is a common misconception that you will blow up your car the instant you forget to do it. That's just not true. Some extra wear...yes, zomg damage...no.

Gene

iTrader: (1)

Brakes are cheaper sure but when you ride them down a long downhill and arrive at a sharp bend with them cooked you'll be sorry.

alex91n/a

iTrader: (2)

Yeah that's why u don't rid the brakes all the way down u js give them a little break every few seconds, and if worse comes to worse u can engine brake... Idk maybe people have alot less common sense then I thought...

Aaron Cake

iTrader: (1)

Remember, the chrome rotor housing coating is porous, so it is designed to retain oil. Even though fuel injection is cut during decel, there is virtually no load on the engine and all surfaces are still lubricated through this retained oil. However if I was tuning a race engine that would constantly be operating at high RPM, I would probably turn off decel fuel cut or at least keep the injectors open a little to provide some lubricating premix.


Probably not enough airflow during engine braking to keep the flapper open. There is an idle bypass in the AFM that runs past the flapper (why we have the variable resistor).

I know the posts you are referring to. It is clear that this builder, while popular and talented, doesn't understand some of the fundamental mechanical processes happening within the engine. Specifically, in this case, the difference between "bedding in" and "wearing out".

rx-7 obsessed

thank you for proving my point