"Turbo, turbo everywhere!"
Determining the Optimum Air/Fuel Ratio
With the Evora ready for tuning, it was the air/fuel ratio that we were most interested in as this is usually what goes out the window when you've modified any major components that can alter airflow. If you're new to tuning, the easy way to think about this is as follows: Consider a trombone played by a musician in a jazz club. When he plays with the slide pulled completely in, he achieves a higher pitch than when the slide is fully extended. This is because the length of the slide changes the resonant frequency of the wavelength of air. The same holds true in an engine's induction system. If you change anything between the intake-air filter and the exhaust collector, you may potentially alter the frequency of the air-flow wavelength and therefore affect the accuracy of the EFI system as far as the airflow modelling is concerned. The EFI system must be able to accurately determine how much air is entering the cylinders otherwise the fuel and ignition calculations may be far enough off that performance and efficiency suffer. In the case of our Evora here, the cold-air intake piping is of a larger diameter than the factory system and far less restrictive, so we expect this to alter our air/fuel ratio. The exhaust, on the other hand, is only different well behind the catalytic converter and header collector, so it will only affect the mass of air able to move through the engine, not necessarily the frequency. In other words, if we didn't install the cold-air intake, the exhaust modification wouldn't likely require any changes to the EFI calibration. But we did modify the induction system, so let's get on with showing you how much of a difference correcting the EFI calibration can really make.
On our first dyno run, we saw that horsepower and torque had only increased by 16 HP and 13 lbs/ft. Pretty dreary considering the effort, but it showed the new exhaust was up to snuff. We made sure to note any deficiencies with the EFI calibration and logged the air/fuel ratio data so that we could analyze it. We witnessed the following:
Air/Fuel Ratio vs. Engine Speed
RPM 3,000 4,000 5,000 5,500 6,000 6,500
AFR 12.7:1 12.9:1 13.5:1 13.2:1 12.6:1 12.4:1
At peak torque output, between 4,500 and 5,500 RPM, the engine was running lean, and at the upper end of the power-band the engine was running richer that necessary. We corrected the air/fuel ratio to obtain the following result:
RPM 3,000 4,000 5,000 5,500 6,000 6,500
AFR 13.0:1 12.9:1 12.8:1 12.8:1 12.9:1 12.8:1
With the above fuel mixture we experienced no knock retard (running 93-octane pump gas) and definite jump in power across the board, but also a much bigger increase from 4,500 RPM on up through the rev-range. The end result was a gain of 24 hp and 23 lbs/ft. You can see below how much the AFR made a difference at peak torque and top-end. MET
corroborated with Chris Stellas in the fabrication of the exhaust. Chris came up with the idea of running a short-length, low-restriction muffler as a straight-through setup instead of running twin Helmholtz style resonators, which was the original plan, just to see what would happen. The fear was that it would be far too loud, but as it turned out, the Evora sounded amazing! The OE muffler isn't terribly restrictive, but it is big and quite heavy compared to the small assembly that Justin welded up and bolted into the car. Below you can see just how much room is opened up once the factory unit disappears. It was enough that it got Dan Maslic trying to tuck a turbo installation into the Lotus before Chris and Justin got back from lunch. That didn't work out as time was far too short. Chris eats as though he's dieting and Justin mows through his lunch faster that a John Deer on lawn duty. Regardless, you can see below just how much room there is in the Lotus for a custom exhaust once you ditch the OE muffler.
You won't find many people around here who don't love the Lotus Evora. In fact, we're huge fans of this car and pretty much anything with a Lotus badge on the bonnet (yes, we're honouring the Brits here...with our use of the word "bonnet" and the "u" in honouring). But enough of that. We got our hands on this car and were asked to install a cold-air induction system as well as fabricate a custom exhaust system for the little beast. Justin Haswell installed the cold-air-intake system and then