Obviously detonation is somthing to be avoided... but since 100 octane fuel is common in Japan... does anyone have any idea if we are gonna have any problem with the lower grades of fuel in various places?

Here, regular is 95octane and Super is 97 or 98.

I also heard that it might on some cars be necessary to reset the ECU somtimes since it will only ever lower it's settings for low grade fuel... never recover with a higher grade used...

Has anyone got any idea, both in general and in how it applies to rotaries?

Blue, I think I read somewhere that a rotary engine doesn't need a high octane fuel. In fact, the article stated that the lower that octane the better. I'm not sure where I found that . . . I'll do a little search and see what I can come up with.

I put regular 87oct in my 87 RX-7 and I put 91 premium (Yay for California, 91 is as high as we go!) in my 93 RX-7. The FD needs the high test gas.

Not that I would put diesel in my Rx-8 but can rotary engines run on diesel? Is there a special rotary diesel engine?:confused:

I would want to stay as high of an octane possible, one ping on a rotary and you break an apex seal, which means rebuild. I hope that it is less of a problem on the 8 though but with higher compression rotors it would seem like it would be more of a problem.

Originally posted by fritts
I would want to stay as high of an octane possible, one ping on a rotary and you break an apex seal, which means rebuild. I hope that it is less of a problem on the 8 though but with higher compression rotors it would seem like it would be more of a problem.

OK, I'll go with that, but if it's got to run on 91 octane in California, and regular is 95 here... I guess I can't go wrong???

To address an earlier concern reagarding the ECU settings - Mnay modern cars have an electronic knock sensor, and this adjusts for lower and higher grade fuels automatically.

How would this work in a rotary though? I'm not exactly sure if you can retard or advance timing in a similar manner to piston engines. Any experienced rotor heads want to have a crack at that? Thanks.

Originally posted by red_base 95
To address an earlier concern reagarding the ECU settings - Mnay modern cars have an electronic knock sensor, and this adjusts for lower and higher grade fuels automatically.

How would this work in a rotary though? I'm not exactly sure if you can retard or advance timing in a similar manner to piston engines. Any experienced rotor heads want to have a crack at that? Thanks.
What does a knock sensor do? My car has been getting really bad gas mileage as of late, and I know the check engine light is on because of a bad knock sensor.

Could this be a culprit? And how much do they run to replace?

Thanks!

Originally posted by Hercules

What does a knock sensor do? My car has been getting really bad gas mileage as of late, and I know the check engine light is on because of a bad knock sensor.

Could this be a culprit? And how much do they run to replace?

Thanks!

Well, presumably the knock sensor detects pinking or pre-ignition... so if it's failed then the ECU may just retard the timing as far as it can to be safe... which is likley to play havoc with your performance... (And probably mileage too)

Originally posted by red_base 95
To address an earlier concern reagarding the ECU settings - Mnay modern cars have an electronic knock sensor, and this adjusts for lower and higher grade fuels automatically.

How would this work in a rotary though? I'm not exactly sure if you can retard or advance timing in a similar manner to piston engines. Any experienced rotor heads want to have a crack at that? Thanks.

My issue is not with the ECU retuning for lower grade fuel, but I had heard that on some models it will never return to normal... it never return to the setting for the higher grade fuel without a reset.

Anyone confirm or deny this?

On all the cars I've owned the timing comes back pretty quickly after a knock.

Just to clear a few things up:
1) NA rotaries LOVE low octane. When it was available, I knew people running 85 Octane (about 80 US RON) and getting great performance. The shittiest fuel the better. My friends with NA rx-7's all try and fill up on the worst fuel they can find :)

2) One ping does not equal no motor. It's a common misconception largely held by people who've never detonated a rotary. My 60,000km motor has had about 4 seperate knocks one of them so sickening my first reaction was to see if it could still idle (on-the-fly compression test). 3 of the knocks were my fault 1 of them came from bad fuel.

Think of it like a piston motor, except when detonation get bad enough to lunch head gaskets on piston motors, rotaries can begin to loose apex seals.

BTW a failed knock sensor in most programs = safety mode. You'll get very conservative timing (read:bad mileage)

HTH

-pete

Originally posted by Mazda man
Not that I would put diesel in my Rx-8 but can rotary engines run on diesel? Is there a special rotary diesel engine?:confused:

That's a good question. It would probably come down to whether they could design the combustion chamber to get high enough compression (~20:1). If you're asking if you could put diesel in an ordinary rotary and expect it to run, no.

Originally posted by Macabre


That's a good question. It would probably come down to whether they could design the combustion chamber to get high enough compression (~20:1). If you're asking if you could put diesel in an ordinary rotary and expect it to run, no.

Here is a link to a diesel rotary engine (http://www.rotarypowerinternational.com/PR020714.htm)

Originally posted by rpm_pwr

1) NA rotaries LOVE low octane. When it was available, I knew people running 85 Octane (about 80 US RON) and getting great performance. The shittiest fuel the better. My friends with NA rx-7's all try and fill up on the worst fuel they can find :)



So the 8 will run beter on lower octane fuel, right?

well, we can't know until the data from the factory comes out on recommended fuel ratings... i DO know though that econo boxes that are DESIGNED for like 87 octane fuel (yay Canada!! :p) NEED TO HAVE 87 octane fuel, otherwise you end up with dirty everything and heavy emissions as 92 octane just won't burn as well in a lower compression motor not designed for it...

looking at the RX-8's RENESIS, and GUESSING (completely; for those who don't know, i'm just a 19 year old econ major in university with a car fetish), because of the 10.*:1 compression, and 9k RPM redline, i'd wager something around 89 or 90 octane recommended...

what's weird though is that i read that stupid article in SCC about the 350Z, and they dyno'ed it with "shitty" 91 octane, and SOMEHOW got a gain of like 1.5 peak hp with another run on gas in the high 90's, or even 100 (can't remember...)... is that bullshit, or just an over active knock sensor??

Originally posted by BlueAdept


Well, presumably the knock sensor detects pinking or pre-ignition... so if it's failed then the ECU may just retard the timing as far as it can to be safe... which is likley to play havoc with your performance... (And probably mileage too) How much to replace it do you think? For the part only, I get labor done cheap from some guy :)

Modern engines are tuned to be pretty close to the edge, mostly for emissions and fuel economy reasons. That's why you don't get much in the way of gains from off the shelf "chips" in normally aspirated engines any more until you start to modify them in other ways - they're tuned aggressively from the factory and even adaptively. On a warm summer day with California's 91 octane, it's no suprise that the Z they tested performed marginally better with a higher anti-knock-index fuel.

Any idea WHY..... you can say they love low octane... but is there a reason, other than just the design doesn't suffer some of the problems requireing higher octane in piston engines...

Is there a reason wny higher octane fuel is bad?

I would think that mazda would take full advantage of the use of high octane fuel so that they could advance the timing on the engine and make more power. I was looking over some info on the NA rotaries and found that people advance the ignition timing and then use high octane fuel to make more power, especially down low. Sure enough at stock timing though, the NA use as low octane as they can due to the ease of combustion. My question would be since they have increased the compression on the Renesis and because of the availability of high octane fuels to make power will Mazda use it to make more power. If they have set it up for low octane gas then there will be performance chips available to take advantage of increasing the ignition timing.

Originally posted by BlueAdept
Any idea WHY..... you can say they love low octane... but is there a reason, other than just the design doesn't suffer some of the problems requireing higher octane in piston engines...

Is there a reason wny higher octane fuel is bad?

low octane = faster flame front. Having thought about it I think I agree with fritts - they may go for agressive timing in search of those last few HP and thus you would have to run high octane. If they go conservative then low octane all the way.

Basically the closer you get the engine to grenading the more power. Lower octane = faster flame fronts (more chance of pre-ignition) = more powerful burn. Large ignition advance = more combustion pressure = better burn, but you need high octane gas because of the risk of detonation. It's a matter of wait and see what they do.

-pete

Originally posted by wakeech
...what's weird though is that i read that stupid article in SCC about the 350Z, and they dyno'ed it with "shitty" 91 octane, and SOMEHOW got a gain of like 1.5 peak hp with another run on gas in the high 90's, or even 100 (can't remember...)... is that bullshit, or just an over active knock sensor?? It's neither. 1.5 HP is well within the margin of error for those kind of dynos. Especially since the runs were obviously not done back-to-back, but they would have had to burn through the rest of the 91 Octane, and then some, before they could be sure that they had mostly 100 test. The 1.5 HP measurement basically means that nothing changed.

---jps

oh, :D okay sweetness... i had a feeling those guys at SCC were kinda full of it on that, as they are with so much else... :rolleyes:

Originally posted by Sputnik
It's neither. 1.5 HP is well within the margin of error for those kind of dynos. Especially since the runs were obviously not done back-to-back, but they would have had to burn through the rest of the 91 Octane, and then some, before they could be sure that they had mostly 100 test. The 1.5 HP measurement basically means that nothing changed.


It was actually 5hp, or almost exactly 1%. Still within margin of error, but regardless, with modern engine management you will usually get a hair more power with slightly higher octane. You'll also usually get a bit more by simply resetting the ECU, because you'll be running on the "best case" maps even with less-than-best fuel, until it re-learns what kind of crap you're running through it and picks more appropriate tuning.

I agree completely with the idea that a few HP is complelely within the margin of error on the dyno. The dyno error, heat soak, and other issues make measured differences of a few HP meaninless.

As an example, I had my FD RX-7 on the dyno, and in back to back runs the measured power was different by 10 HP.

If they reset the ECU between Dynos on the 350Z it could have very well been running on a more advanced timing map. Which resulted in the extra horsepower.

well, all in all, it amounts to "not the fuel"... good enough for me. :)

and ya, rpm_power, your reasoning seems correct (on the more powerful with the lower octane)... i just wonder how much of a difference it would make...

Originally posted by rpm_pwr


low octane = faster flame front. Having thought about it I think I agree with fritts - they may go for agressive timing in search of those last few HP and thus you would have to run high octane. If they go conservative then low octane all the way.

Basically the closer you get the engine to grenading the more power. Lower octane = faster flame fronts (more chance of pre-ignition) = more powerful burn. Large ignition advance = more combustion pressure = better burn, but you need high octane gas because of the risk of detonation. It's a matter of wait and see what they do.

-pete

This is actually a common falacy. Higher octane fuel does not have a slower burn. It has more resistance to pre-ignition. Octane rating and burn speed are not related. Additives can be added to fuel that increases the burn speed without affecting octane. Nutec is a race fuel that, while extreamly corrosive, has a very fast burn rate - the faster burn rate will give an extra 10% power.

Don

Originally posted by R1_Kilroy


This is actually a common falacy. Higher octane fuel does not have a slower burn. It has more resistance to pre-ignition. Octane rating and burn speed are not related. Additives can be added to fuel that increases the burn speed without affecting octane. Nutec is a race fuel that, while extreamly corrosive, has a very fast burn rate - the faster burn rate will give an extra 10% power.

Don

OK, but why would the timing have any way to control problems caused by too low octane fuel? If it's only pre-ignition that's the problem, why would sparking later make a difference?

pre-ignition cannot be cured by timing retard, but detonation often can. Pre-ignition is when a hot-spot ignites the mixture before the spark fires. Detonation is caused by uncontrolled combustion burning unevenly and hitting everything in the combustion chamber with a pressure wave from the explosion. By lighting the spark later, the fuel/air is more evenly compressed when it's ignited and burns in a controlled fashion that can by properly absorbed by the piston or rotor. The downside of timing retard is it's still burning while the piston or rotor is on its way into the exhaust cycle so some of the energy is wasted. Fuel with a higher anti-knock index is less susceptable to either because it's volatility is lower.

Originally posted by Macabre
pre-ignition cannot be cured by timing retard, but detonation often can. Pre-ignition is when a hot-spot ignites the mixture before the spark fires. Detonation is caused by uncontrolled combustion burning unevenly and hitting everything in the combustion chamber with a pressure wave from the explosion. By lighting the spark later, the fuel/air is more evenly compressed when it's ignited and burns in a controlled fashion that can by properly absorbed by the piston or rotor. The downside of timing retard is it's still burning while the piston or rotor is on its way into the exhaust cycle so some of the energy is wasted. Fuel with a higher anti-knock index is less susceptable to either because it's volatility is lower.

OK, I understand pre-ignition... and incorrect timing... I've tuned a lot of engines, so I know that noise well! But "Detonation" is not somthing I've come across... perhaps becuase most cars I've owned ran on 98+ octane even the lowest grade we get is 95...

I understand the concept pretty well, being an engineer at heart, but can you (anyone) explain what causes detonation, and what that feels/sounds like... hopefully how to avoid it should be pretty self explanatory from there!

Originally posted by fritts
If they reset the ECU between Dynos on the 350Z it could have very well been running on a more advanced timing map. Which resulted in the extra horsepower.

Do you, or anyone here know what it takes to reset an ECU?... or otherwise what it would take for the ECU to learn that you now have better fuel than you did yesterday...

On most vehicles it just a matter of unhooking the positive terminal of the battery and rehooking it up.

Originally posted by BlueAdept


OK, I understand pre-ignition... and incorrect timing... I've tuned a lot of engines, so I know that noise well! But "Detonation" is not somthing I've come across... perhaps becuase most cars I've owned ran on 98+ octane even the lowest grade we get is 95...

I understand the concept pretty well, being an engineer at heart, but can you (anyone) explain what causes detonation, and what that feels/sounds like... hopefully how to avoid it should be pretty self explanatory from there!

First off, let me say everything I know relates to reciprocating piston engines.

Now.

Detonation and pre-ignition are different terms for the same thing. Both mean ignition of the fuel/air mixture prior to the spark-plug initiated flame reaching the that portion of the mixture.

You asked a great question - why does retarding the ignition solve the problem. This is counter-intuitive - what good does it do to spark the mixture later when it is already igniting before the spark.

The answer is fairly complicated so bear with me...

Ping, pre-ignition or detonation occurs under high-load circumstances. This can be translated as low revs, high gear and/or ascending a hill working against gravity. This type of situation results in very high combustion pressures (imagine starting a bicycle in the highest gear - it is very difficult to pedal and you really have to work hard).

This high combustion pressure causes extreme heat in the combusion chamber. These "hot spots" cause pre-ignition of the mixture.

For the sake of argument, imagine a combustion chamber as a clock with the spark plug at the 12 o'clock position. Imagine piston Top Dead Center (TDC) as the 10 and 2 position. The reason you get more power from spark advance is the flame takes time to start at 12, move to the 11 and 1 position, then the 10 and 2, then 9 and 3 and so on.

If you advance the spark ignition to fire when the piston is at the 9 and 3 position, the explosion should arrive at the TDC position sooner than it would have if it fired at the TDC position. This results in more power. Obviously, if you advance the timing too much you can have the explosion hitting the piston crown as it is still ascending. This can result in, most comonly, pitted pistons and broken piston-ring landings.

Now, why does retarding the ignition timing reduce knocking. It has to do with leverage (the charge has an easier time pushing down the piston once it has started to descend) and the fuel/air charge sits in the combusion chamber longer, providing a cooling effect. Downshifting when you hear knock will raise your engine revs (less load) and increases your mechanical gearing advantage (less load).

Remember, the knock sensor can only vary ignition a few degrees. So just because a vehicle has one, does not mean it will not suffer from pre-ignition. This situation is critical with turbo-charged cars.

Now, I am out of breath!

Ciao,

Don

"It is the impact of the resulting shock waves on the engine's parts that produces the metallic ping or knock that is heard. To sum up, we can say that the treatment of the unburned charge causes changes in its nature, possibly resulting in auto-ignition."

long winded description: http://link.sandiego.com/scripts/wheelbase/message.idc?passin=297

Just for the record, I heard my first pinging yesterday. At a speed of ~ 84 mph, 6th gear on a flat freeway. Going off the gas and keeping it @ 80 mph, the ping was gone. Back up to 84, the ping came back.

The sound is very low and I wouldn't have noticed it if the radio had been on.

I've driven faster than that before and didn't hear a ping. Up there it is explained that low revs (= higher load on engine) could cause it. Seems opposite to what I observed.
Anyway.

I attribute it to the fact that I was running cheap Costco 87 gas. I had run out of gas 2 days before (road trip) and filled up - emergency in unknown territory - at Costco <rolling eyes>. Normally I buy 87 octane gas from Union 76 or Chevron.

As the pinging occurred, I then stayed at 80. Since my tank was almost empty anyway, I filled up at the next station - Chevron 93 octane.

Further testing didn't produce any pings at speeds up to 110 mph.