Avoid turbo-induced disaster with a wide-band oxygen sensor

J.G.
By J.G. Pasterjak
Dec 8, 2023 | Turbocharging | Posted in Shop Work , News and Notes | From the May 2021 issue | Never miss an article

Photography by J.G. Pasterjak

A turbocharged engine is a double-edged sword. More power is readily available through more boost, but so is more disaster. 

So, even if you are vaguely considering turbocharging an engine, plan to at least monitor the other factors that will be affected by the additional air. Most notably, this means keeping an eye on how much fuel gets mixed into that combustible fog being rammed into your cylinders.

The relationship between fuel and air in your engine is multifaceted. On one hand, you have something called the stoichiometric ratio, which is the ideal mixture of air and fuel that, when combustion occurs, results in no leftover fuel or its catalyst (oxygen, in this case). 

[Turbo boost is good, but too much can be bad]

For typical gasoline, on Earth, where air is about 21% oxygen, that ratio is 14.7 parts air to 1 part fuel. Of course, different gasoline blends and elevations affect this number slightly, but that’s the baseline.

But that 14.7:1 ratio doesn’t tell the whole story of max potential. See, while you may be burning all of the fuel at that ratio, secondary effects could be robbing the engine of maximum power. Complete burning of the fuel means less atomized liquid fuel in the combustion chamber, which means less cooling of interior combustion surfaces, which means more energy lost as heat, for example.

Now, every engine is different, and most make maximum power at some ratio that is not the ideal 14.7:1, but this ratio is only attainable through dyno testing and precise monitoring. If you have a modern car, the monitoring part is usually as simple as slipping a connector on the OBD port and using some software or a hardware logger to read the data being sent to the onboard computer. 

But our 1991 Toyota MR2 Turbo has no such modern accoutrements, and the factory oxygen sensor that sends exhaust data to the onboard computer is extremely low-resolution.

Time for Some Sensorship

The solution is a proper wide-band oxygen sensor, which usually means the installation of an accompanying in-dash gauge or monitor to keep track of the numbers. But we found an easier solution that installs in minutes from Performance Electronics. The company’s PE-Wideband kit consists of a Bosch sensor that connects to a Bluetooth-enabled controller you can monitor via an app on your iOS or Android device. 

The setup just requires a power and ground connection, so all the wiring can stay under your hood–or engine cover, in our case. The unit also has CAN inputs and outputs and a 0-5v output to connect it to dedicated gauges or your programmable ECU.

Installation on our MR2 was a snap, as we’d just installed a TCS Motorsports downpipe that we had the foresight to order with an extra oxygen sensor bung already installed. We simply removed the plug, installed the new sensor, and plugged it into the unit. We were up and running within 30 minutes. The kit also comes with a weld-in bung made of mild steel if you don’t already have an available and willing bunghole. Stainless bungs are available as well.

The app is easy to use, has real-time monitoring and some basic logging features, and reads AFR either as ratio or lambda. Typical AFR shows ideal as 14.7:1, with lower numbers–meaning more fuel per air mass–indicating a “rich” condition and higher numbers indicating a “lean” condition. Lambda expresses ideal stoichiometric ratio as 1, with readings lower than 1 indicating rich and higher than 1 indicating lean. 

So now that we can easily monitor our air/fuel ratio, what do we do about it? Well, first off, it’s good to know that our engine is properly metering fuel.

At wide-open throttle, the onboard ECU richens the mixture to around 11:1, which means a lot of unburned fuel–but also a lot of safety margin for not producing excess heat or preignition, which can quickly kill an engine. As we add boost and open airflow, ensuring the fuel mixture is within the safe zone will be additional insurance against premature disaster.

But if we ultimately want to make the most power that we can for a given configuration and boost level, we’ll need a way to precisely adjust that fuel mixture at various load levels. Unfortunately, unlike more modern ECUs, the MR2’s vintage computer is not easily reprogrammable, so we’d need to install an aftermarket engine management computer. For now, though, this setup is a welcome upgrade.

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Comments
SV reX
SV reX MegaDork
12/8/23 9:29 a.m.

An "available and willing bunghole"?  You guys are a riot. 
 

That's a good article. Thank you. 

Msterbee
Msterbee Reader
12/8/23 10:32 a.m.
SV reX said:

An "available and willing bunghole"?  You guys are a riot. 
 

That's a good article. Thank you. 

alfadriver
alfadriver MegaDork
12/8/23 10:33 a.m.

I would encourage people to get a data logging system, so that you are not logging with your eyes. You should be driving. 
 

And even on a dyno, a logging system will show you precisely where the issues are instead of trying to juggle a/f, engine speed, and some load measurement. 

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) UltraDork
12/8/23 1:50 p.m.

I like for the ecu to just do all the work for me and kick into failsafe mode if AFR isn't in the appropriate window.

codrus (Forum Supporter)
codrus (Forum Supporter) GRM+ Memberand PowerDork
12/8/23 1:59 p.m.
alfadriver said:

I would encourage people to get a data logging system, so that you are not logging with your eyes. You should be driving. 
 

And even on a dyno, a logging system will show you precisely where the issues are instead of trying to juggle a/f, engine speed, and some load measurement. 

Agreed, an AFR gauge is at best a warm fuzzy, it's not really useful by itself.  AFR data logged along with RPM, manifold pressure, etc is what you need for tuning.

As far as "failsafe" goes, IME widebands are considerably less reliable than things like fuel pumps and injectors.  I favor tuning the car with the wideband (on the dyno) and then turning off the automatic provisions so that the computer is using the tuned, known-good fuel map.  It can monitor the AFR and turn on a big red warning light if it thinks things are going askew, but it should not silently "fix" the fuel mixture.

 

alfadriver
alfadriver MegaDork
12/8/23 2:19 p.m.

In reply to codrus (Forum Supporter) :

It would be really nice if MS had an on board provision for a Bosch sensor. Then I would trust it more. 

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