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What Your Cockpit Gauges Won't Tell You

By David Lane

The Question

A respected 3rd gen RX-7 driver took his well-modified car to a race track. He usually ran it on race gas for track work, but none was available that day. He "cracked an apex seal" with the car running strongly at 14 psi and no audible knock. He had ordered an Air/Fuel ratio meter (Halmeter--it was on back-order), and lamented that its presence in the car at the track would have alerted him to a lean condition--which would have averted the disaster. Something didn't sound right about that, so I set out to discover if it was true.
It was more than just an academic question for me. My own car has an after-market turbo. If some hidden gremlin can kill an engine without leaving a trace, and without showing itself on any of the usual cockpit gauges, I want to know about it. The search for the answer involved the sometimes complex relationships between hardware (gauges), software (fuel), and dufus-ware (that would be the drivers). This essay proceeds in that order.
Before jumping into it, I need to thank the many people who have contributed their opinions, guesses, equations and personal experiences. I was also fortunate to be able to question Corky Bell, Peter Farrell, and John Pizzuto on these issues. Larry Mizerka's posts covered some of the academic aspects of the subject, and he has been an excellent source of information. In presenting my findings, I need to emphasize that some of it is still speculation, bumping into the edges of generally available knowledge.
The challenge is to put what I found into some practical setting that is actually useable by someone glancing at the car's gauges while flying down a back road or going for a personal best lap on a race track. Also (as I remind people so often) I have no scientific background, so I must be able to place what I have learned into some sort of logical construct, intuitive truth, or practical application before it makes sense to me. I admire and respect people who recognize truth in numbers and equations. Unfortunately for me (and my frustrated past math teachers) I am not able to join them.
This post will take the form of an essay. It runs about nine printed pages. Nevertheless, I hope it makes for interesting reading. There should be some surprises here for the more sophisticated reader, but I have made a point of including enough basics so that those new to the subject won't get lost. Confused maybe, but not lost. Welcome to the club.

Oxygen Sensors

All fuel injected RX-7s (and most modern cars of any kind) use a sensor in the exhaust pipe to help the car's computer adjust how much fuel to inject. Volvo introduced it in 1976 in conjunction with a three-way catalytic converter under the name of "Lambda sond." Usually, the computer only consults the sensor ("closed loop" mode) under cruise conditions when it is looking for maximum efficiency. The sensor--called an oxygen (O2) sensor--puts out a voltage when the oxygen content of the exhaust gasses falls below the norm for the atmosphere. The voltage range is from 0 to 1 volt.
Here is how the oxygen sensor functions. Oxygen in the air is consumed when fuel burns, so increasing the amount of fuel to a given amount of air (a richer mixture) will deplete a greater part of the available oxygen. The O2 sensor in the exhaust pipe responds to this by putting out more voltage. Engineers look for 14.7 parts air to one part fuel (14.7:1) as an ideal mixture for cruising. The O2 sensors in our cars, therefore, are designed to be most sensitive in that range (14.1:1 to 15.1:1). The 14.7:1 ratio is variously referred to as "Lamda 1" or "stoichiometric." Roughly 85% of the voltage output range of an O2 sensor occurs within this limited range of A/F ratios. (This information comes from eyeballing a graph supplied by the Halmeter people.)
The sensitivity of the O2 sensor in this range is good news for cruising down the highway because it enables the system to do a great job of producing maximum mileage and minimum emissions. It is bad news for us performance types because we need a much greater percentage of fuel in the A/F mixture to extract maximum safe performance out of our cars. For this reason, our computers ignore the readings from the O2 sensors (shift to "open loop" mode) and refer instead to pre-programmed "maps" for fuel delivery when we put our collective feet to the floor.
During acceleration, you need an air/fuel ratio somewhere around 12.5:1. (This came from the Halmeter lit, and may not be exactly right for rotaries.) Unfortunately, the graph shows that a typical oxygen sensor's voltage variation is very limited in that range. A secondary flaw is that the sensors on our cars are sensitive to heat. They don't put out anything meaningful until exhaust temperatures reach 360C (680F), and anything over about 900C (1650F) becomes problematic. This is interesting since a modified turbo rotary engine can routinely see exhaust gas temperatures in excess of 900 degrees Celsius. In other words, just when we (dufus-ware) are most interested in finely tuned information, the O2 sensor is shrugging it's shoulders and saying, "Weeelll, I think we are possibly dealing with maybe around this amount of oxygen, but it's kind of hot in here and I wasn't really calibrated for that, so I wouldn't want you to quote me."
Some manufacturers provide electrically heated O2 sensors to bring them up to operating temperature sooner. Performance-minded drivers have noted that heated sensors are much less sensitive to leaded fuel (high octane racing gas). However, above about 600 degrees Celsius, a heated O2 sensor has the same weaknesses as any other.
NOTE #1: List members have told me that they think the O2 sensors are heat-sensitive within their operating range; that the reading at a specific air/fuel ratio will vary with exhaust gas temperatures. I have not come across information to confirm or discredit this, so I merely pass it on.
NOTE #2: I have heard people state that our cars are in closed loop mode during idle. This is unlikely to me, since exhaust gas temps are low at that time, and I doubt that the O2 sensor would be reliably within its operating range.
NOTE #3: Stock RX-7 oxygen sensors can be replaced with less expensive units. I have heard from Tri-Point that the stock sensors are particularly consistent over time and temperature variations, and they recommend them when working with after-market engine management systems, some of which stay in closed loop more of the time.

Air/Fuel Meters

It is possible to monitor the output from the O2 sensor using a volt meter between the sensor and ground. Even people with non-fuel injected engines can install an O2 sensor in the tail pipe and use the readings for tuning purposes. If memory serves, most people are looking for about .82 volts under maximum acceleration. Since the O2 sensor is relatively insensitive in that part of its range, the reading is best used in conjunction with other indicators to find that perfect mix. A more elegant choice is to buy an Air/Fuel meter.
The first problem in talking about A/F meters is that there are two classes of instruments out there. Laboratory grade A/F meters are costly, and not meant for permanent installation. They are accurate in all ranges, and not bothered by changes in exhaust gas temperatures. Thus, their sensors can be temporarily placed in the tail pipe. These instruments have digital read-outs, and are the ONLY reliable way to do fine tuning on an engine if you are looking for absolute information about high performance A/F ratios.
Inexpensive Air/Fuel meters express the output voltage of the car's O2 sensor as an A/F ratio displayed with LEDs. These meters come in several shapes and sizes. The primary flaw in their usefulness is that they cannot be more accurate than the information coming from the O2 sensor. However, they are small enough to mount permanently on your dashboard, bright enough to glance at when driving assertively, and cheap enough to be a worthy addition if you start messing with your car. They respond very quickly, alerting you if your latest "improvement" causes lean running, or if something in the fuel supply system malfunctions.
What the inexpensive A/F meters won't do is to give you a high resolution, accurate reading of a performance oriented A/F ratio. The range of most of these meters is about 16:1 on the lean side, to about 12:1 on the rich side, and most of the LEDs are unmarked. Two factors become obvious here: First, if you are looking for an A/F ratio near 12.5:1 an A/F meter of this type will be close to the end of its range (where the information coming from the O2 sensor is least sensitive). Second, if the "rich" side of your A/F meter only contains five or ten LEDs above stoichiometric, you won't have enough resolution to see very small changes. While these limitations seem to damn inexpensive A/F meters to near uselessness, that is not the case. The information they give you, when combined with other observations and experience with the car, can be very valuable--especially if you have some manual control of your fuel delivery system via the typical add-ons (additional injectors, boost dependent fuel pressure regulators, fuel computers, etc.) associated with after-market turbos, or with upgrades for stock systems. The resolution on the meter is consistent with the quality of information coming to it from the O2 sensor. So, even though the absolute readings may not be reliable, seeing if the reading stays constant as revs and boost build is very valuable. As we will see later, the value of this has a lot to do with which generation of RX-7 you are driving.


There are several excellent sites on the web about gasoline, so I will limit my comments to octane and its effect on knock. One of the reasons the original question (less octane equals leaner running?) was so hard to answer is that you are unlikely to find two fuels that differ only in octane--especially when you are dealing with racing gas. The notion several people had was that higher octane fuel gives off more energy. Thus, if (for example) a teaspoon of higher energy fuel explodes, it will burn more oxygen. This will result in less oxygen in the exhaust. The O2 sensor will develop more voltage and the A/F meter will give a richer reading. It certainly seems logical.
The problem is that octane is simply a measure of a fuel's resistance to knock (more on knock later). There is no implication I could find that by changing octane alone you would also make a fuel release more or less energy. So, if everything else was left alone, and you put a bottle of octane enhancer (like 104+) into a tank of 93 octane fuel, your A/F meter would not change its reading.
In the real world, high octane racing fuels are denser and pack more energy in each "teaspoon." A lab grade A/F meter will display the difference as a richer reading. Thus, there are at least two advantages to racing fuel. You have access to more energy at the same fuel flow, and the extra octane will allow you to run at higher boost levels. An off-the-cuff comment by someone in the know was that you need to raise the octane rating by three points to accommodate one additional psi of boost. This echoed another comment--that a 3rd gen running at 14 psi on pump gas could get as high as 18 psi on the highest octane race fuel. Don't try this at home, kids.
The Question: Would our friend have been able to see a lean condition on his Halmeter if he ran his "race gas tuned" car on pump gas? The answer I got was most likely not. A lab grade instrument would have shown it (this was actually observed by Peter Farrell), but stock O2 sensors are pretty numb in that range of richness. This, combined with the grossness of the LED readouts, makes it highly doubtful that a dash-mounted A/F meter would have flickered any differently than normal--much less shown the kind of difference which would have been interpreted as an "alert."
NOTE #4: I came across an interesting factoid from the "Reference Library" section of www.lubrizol.com which stated that cars need additional octane as they age due to the build-up of deposits in the combustion chamber. These deposits take up space, which effectively raises compression. This explains the knocking I have observed on a number of aging cars I have owned. All responded positively to higher octane fuel.

Exhaust Gas Temperature Gauges

The next question, of course, is whether or not an Exhaust Gas Temperature gauge would have helped. Many swear by them because they get an "absolute" reading of temperature. On the positive side, EGT gauges are not subject to the non-linearities of an A/F meter. If you can repeat the same scenario, you should get comparative readings. So, for instance, if you note the EGT reading after doing a full throttle run from 3000 rpm to red line at 10 psi, then do the same thing at 12 psi, you should be able to see the difference on an EGT gauge. The same can be said for altering your fuel mix, installing a bigger intercooler, and maybe even changing your timing (retarding timing results in higher exhaust temps). Further, there are known parameters out there for exhaust temperatures with rotary engines. Mazdatrix notes in its catalog that full race engines run between 900 and 954 Celsius (1650-1750F). They also say that they observed a '89 fuel-injected pro SCCA car that was happiest at 773 Celsius (1425F). Because of the wide range of "best" exhaust temperatures, anyone who assumes his or her engine is happy based solely on the number appearing on an EGT gauge is taking a risk.
This brings us to the "down side" of EGT gauges. The temperature reading is influenced by the location of the probe: usually on the manifold, but sometimes aft of the turbos or even further aft than that. A second problem is the response speed of the unit. A race car, running "full out" on a track, has plenty of time to develop a stable exhaust temperature. Running on the street, most of us can't tell whether the gauge is registering the real temperature, or if it was just on its way up there when we had to let off the gas to keep from ramming the nice person in the SUV who pulled out in front of us. My point is that for street use, you have to get used to what the EGT gauge is doing, and be aware of differences when you change something on your car. A third area of concern is that the EGT gauge (like the A/F meter) will tell you when something has changed, but neither will tell you exactly what it might be.
NOTE #5: After an autocross run, I checked the "peak-hold" feature on my EGT gauge, and it was only reading about 775 Celsius. Normally on the street I see about 825C after spirited driving. The water temperature gauge showed that the car had heated up by almost 10 degrees (F) during the run, so it is likely that the EGT gauge did not have time to come up to temperature. Had I leaned my A/F mixture based solely on what the EGT gauge was telling me I would have been taking a risk.
The Question: Would an EGT gauge have saved our friends engine? Again, probably not. In real life our track driver (who also used the car on the street with pump gas) would have been accustomed to those typical readings. Maybe the readings would have been a little higher when he went to the track, but as long as the gauge was reading in a reasonable range, he would have had no absolute way of interpreting the added heat as something which might have damaged the engine--unless he heard knocking--which brings us to:

Pre-Ignition and Knock

Detonation; Knock; Ping; Pre-ignition. You hear these terms mentioned all the time, so we might as well straighten them out. Let's get pre-ignition out of the way first. Nothing mysterious about it. The A/F mixture (intake charge) explodes before the spark plug fires. You would figure the intake charge would have to get pretty hot to do that, and you would be right. The pressure from a high compression engine is enough to generate that kind of heat. (In fact, diesel engines are designed to fire on the heat from compression alone.) Higher octane fuel is the antidote, so in general, a higher compression engine will need higher octane fuel. Cramming more intake charge into the combustion chamber has the same effect as raising compression, so in general, the higher your boost, the higher the octane requirement to avoid pre-ignition. Finally, premature inflagration (I just made that up) comes more easily if the intake charge is hot when it enters the engine. This is why larger intercoolers add a margin of safety in forced induction engines--at least until you turn up the boost.
Another cause of pre-ignition is a hot spot in the engine. Maybe some of those carbon deposits are glowing red hot. Maybe the spark plug itself is hot enough to ignite the mixture before firing. This is almost certainly the case if you have ever experienced a car that kept trying to run after you turned the key off.
The more tricky term is "knock." Although most of us prefer to talk about "detonation," it turns out that "knock" is the correct term as used in automotive texts. "Detonation" is actually slang, and "ping" is not a well defined term at all. That having been said, I will stick with the term "detonation" for this discussion.
Detonation differs from pre-ignition in that it occurs AFTER the mixture starts to burn. Normal burning involves a flame front--a relatively slow, controlled explosion--which marches along in a calculated fashion. As you would expect, normal burning raises the pressure in the combustion chamber. Sometimes this is enough to get the last bit of intake charge (called the "end gas") so excited it explodes before it is supposed to. It is a very hot explosion, on the order of ten times the heat of controlled combustion.
But there is more to it than that. If you graph the amount of pressure in a combustion chamber during normal burning, it shows a relatively smooth event. The occurrence of detonation shows up as a sharp spike on the graph--a sudden shock wave if you will, with pressures on the order of several thousand psi. The duration and strength of the explosion is too fast to contribute to the rotational output of the engine. Like a slap in the face, the full impact must be absorbed within the combustion chamber itself. Damage is most likely to occur at the weakest points--namely the apex seals. Piston engines designed for high stress situations can have the piston rings further away from the crown of the piston. The only choice for a rotary owner is very expensive apex seals--but even then, there is no such thing as a detonation proof engine. The shock of repeated detonation will eventually weaken anything it can, and the heat generated will take care of the rest.
The question: How would this show up on standard gauges?
This is where we get into speculation. I don't know of anyone who has purposefully run a rotary engine to destruction through prolonged knock with the intention of seeing what the gauges read during the process. I assume Mazda has experimented with something similar over the years, but I did not run into any data.
In practice it hardly maters. It all happens so quickly that any hint of knock must be accompanied by getting off the gas. Yet even this is not simple. What happens if your exhaust is too loud to hear the knock? And what happens if, say, only a very small amount of the "end gas" detonates? Would you see the results on a gauge? Could you hear it? Could it damage your engine if it was allowed to continue? How about the specter of pre-ignition combining with detonation? Answers to most of these questions would include so many qualifying statements ("...it's possible that maybe under certain circumstances...") as to be of little use, but we can still deal with knock in a direct manner.

Knock Sensors

Third generation turbo RX-7s and second generation T-IIs come with knock sensors integrated into the electronics. Knock sensors use a microphone--usually on the rotor housing or intermediate housing. The mike feeds electronics which are tuned to recognize knock from the engine. Once identified as knock, the computer intervenes by retarding the ignition timing. Why does this work?
Gasoline engines are usually set to fire the spark plugs before the combustion chamber reaches its smallest size (maximum compression). On a piston engine maximum compression is when the piston is at the top of the compression cycle. The same happens on a rotary relative to the position of the rotor in the chamber. In other words, the mechanical compression cycle is not complete at the time the plug fires. Thus, during combustion, the total pressure in the chamber is a combination of the remaining part of the mechanical compression cycle plus the pressure from expansion caused by the burning fuel/air mixture. If you delay the firing of the spark plug, more of the mechanical compression cycle will have passed at the time the intake charge is lit, so the overall amount of pressure (and heat) in the combustion chamber reduces. This reduction in pressure should be enough to ease the tendency for the end gasses to detonate. The more you retard the spark, the more relief you get from detonation.
Those with after-market turbo kits can add a knock sensor. J&S makes one that intercepts the firing signal going to the leading plugs and delays it in proportion to any knock that is sensed. The unit is very sophisticated, and can identify which rotor face is associated with the detonation. It then retards the spark to that face only. If more than one face is involved each face is treated independently. Owners of highly modified factory turbo cars can also benefit from such a device since the range and capabilities of the stock knock sensor may not be enough to fully protect an engine that is exceeding factory output.


Just as air/fuel meters fall into two major classifications, so do RX-7 people. And this fact alone is a major contributor to difficulties when discussing the value of the gauges and meters which are the subject of this document.
Owners modifying 1st and 2nd generation non-turbo cars are often working with fairly gross devices for enriching fuel. They might have an adjustment or two on the fuel pressure regulator, a few knobs and buttons on a controller for additional injectors, and maybe even some sliders on a gadget that modifies the computer input from the air flow meter. The engine management computers on these cars have no idea what manner of gizmos are being bolted on, and while the fuel injected models may be able to sense greater air flow to the engine, they will either run out of fuel trying to keep up or go into fail-safe mode. Intrepid power junkies that we are, we immediately try to disable anything that gets in our way, and we hope to extract as much power as possible without running into detonation.
Owners of non-fuel injected cars can mess with carburetion--changing the type and size of the carbs themselves, and also the jetting.
For this group of 1st and 2nd gen owners, the information given by an inexpensive A/F meter and an EGT gauge can make a huge difference. The resolution of the instrument is not all that special, but neither is the ability to make very fine adjustments to the system. We realize that we are playing a dangerous game with engine life, so we generally try not to get too near the theoretical limits. This "head room" is our only safeguard to make up for the inherent slop in our ability to control critical engine functions. Without a full fledged after-market engine management computer (Haltech, Electromotive, Motec) it is the best we can do. Those of us with T-IIs have a little more to work with, but bringing a stock T-II to 3rd gen levels of power (and more) requires many of the same kinds of compromises and risks. Again, headroom is the best safety solution.
Third gen drivers have an entirely different perspective. The complex engine management computers and stock knock sensors on these cars make it possible for them to run safely without much headroom--thus the much higher power output. If additional power is desired, improvements to hardware are necessary. Unfortunately the stock fuel maps can only accommodate so much, after which upgrades to the fuel management system are necessary. These upgrades must stay within the already close tolerances of the stock computer. Nothing short of a lab grade A/F meter will do the job. Maximum effort cars run within a very narrow band of safety, so tiny changes in critical systems, unlikely to be displayed by dashboard meters, can easily account for the difference between a happy engine and a dead one.
Does that mean that a dash-mounted A/F gauge is useless for a 3rd gen? It depends on how far you are going with it. Certainly, people doing their own experiments with intake and exhaust are far better off with an inexpensive A/F meter than with nothing. It will tell you in a general way if you are exceeding the stock system's ability to provide enough fuel, and it will be quick enough to indicate whether those boost spikes you may be seeing are accompanied by a lean mixture. EGT gauges are similarly useful for reasons already discussed. With both, you may be able to ascertain whether your after-market chip is keeping up with the latest eight-inch diameter extractor exhaust tip you bought--the one that has tunable back pressure because it incorporates a modified Jet-Ski drive unit which sucks the gasses out of the exhaust at a rate synchronized with the car's engine.
However, if you are trying to push the envelope with one of these engines, you are going to be working in pretty dangerous territory and, as they say, without a net. You will need all the help you can get.

The Answer

It seems clear that a Halmeter and probably an EGT gauge would not have "sounded enough of an alarm" to barge into our friend's consciousness and cause him to sense danger. Possibly when our friend couldn't get race gas, it was not enough to cause anything obvious, but it effectively removed what little headroom he had. The car was just running too close to the edge. There is a reasonable chance the exhaust note was too loud for him to hear an occasional "tick tick" under load which, while not a full-fledged popcorn sound, is still associated with knock. Even more likely, the knock was not severe enough to be audible under the best conditions. This scary thought has been confirmed by the number of times people see the indicators light up on their knock sensors without any other indication that something is being stressed. Maybe the car had been driven hard previously, and it just picked that particular day to give up the ghost. We will never know for sure.
I was hoping we would get a report when the engine was torn down, but the car has been sold and the new owner is putting a Mazda rebuilt engine in it, so the old engine will be out of sight when dismantled.
The only after-market device I know of which might have alerted him to an impending problem is a knock sensor. It is very easy to watch the display activate on pump gas, but fall quiet on race gas. It is equally easy to see the display come alive at one boost setting, but fall blank again if you back off, or if you add fuel to your mix. The best news is that you rarely, if ever, hear detonation in the process--and even then it should fall silent after a single ping. Yes, it is possible to have a little bit of knock which is not going to reach your ears, but is going to do its work on your engine.
While we might tend to spend time pondering the meaning of a subtle difference in an A/F meter or a EGT gauge, human nature is to respond quickly to the character of a knock sensor that is unexpectedly dialing in ten degrees of spark retard. Driver optimism ("It's probably nothing important."), in one form or another, is certainly one of the leading causes of engine failures. For those of us looking to push the envelope in any generation RX-7, investing in an A/F meter, EGT gauge, and Knock Sensor, is money well spent--especially when taken as a percentage of the cost of the overall project. We could also talk about water temperature gauges.....but that's for another time.
Best wishes,
David Lane  dlane@peabody.jhu.edu

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