You Can Tune a Car, but Can You Tune a Fish (Shark)?
By Andrew Olsen
If I’ve said it once, I’ve probably said it twice; tuning a car is not just installing the hardware. If you’ve been a regular reader of my little sandbox here on the P4, then you know I am not a Porsche purist. I have a disease that causes me to take a perfectly good Porsche (my particular flavor seems to be 928’s) and then add a whole bunch of engine modifications that cause it to make crazy horsepower.
This latest project is no exception. The Euro 928 has been a great exercise in playing with 16 valve 928s. This car has a 4.7 liter V8 with Euro heads and US cams and a 1.7l positive displacement roots-type blower. One of these days I will get it to the dyno and we will see what kind of horsepower it really makes, but my guess is about 375HP at the wheels.
The key to any modification like this is making it reliable. Of course the hardware needs to be up to snuff, and for many people they think that’s where it ends, but that’s not it. In fact, I would say that’s just the tip of the iceberg. The majority of the work comes in the tune, especially if you’ve never done it before.
Since my last installment, I have been very busy tuning this shark. It’s amazing what a proper tune does for a 928. I’m very lucky that a really smart guy in the U.K. who also happens to like 928’s has way too much time on his hands and decided to decode the ECUs that are in the 1985+ and later 928s. He developed a piggy-back computer and a software application that allows us to make changes in real time and see what happens. He affectionately calls his contraption the SharkTuner. It’s not cheap, but it’s a lot cheaper than buying a new 928. We have two in the Detroit Metro area.
The SharkTuner allows me to modify the injector size, pulse width, and all the fuel maps contained in the ECU that controls the fuel. On the spark side of things I get to control spark timing for idle, warm up, cruise, acceleration, and wide open throttle (or WOT). The main requirement to make this work is you need to have a laptop and a wide-band oxygen sensor. The wide-band will tell you exactly how rich or lean your car is as you tuning and is a crucial piece of equipment for a modified car.
I started out with the stock fuel maps, mostly because I didn’t know where else to begin. The map is basically a 2-dimensional table based on RPM and engine load that tells the ECU how long to pulse the injectors. But because I’m using injectors that flow at least twice as much fuel as the stock injectors, I began adjusting the injector size until the engine started to read about 14.7 air/fuel ratio (AFR) on the wide-band. This is what is commonly referred to as “stoich.” Look it up if you don’t know what that means. You will learn something cool.
Generally, you want about 14.7 AFR for idle and cruising. For acceleration and WOT you want the mixture to richen up to low 13.X and with a supercharger or turbo, you want low 12’s when you’re all full boost. Then once you get the fuel roughed out you begin focus on the timing. Because my motor is a bit of a Franken-motor, there is no good stock baseline to work from. I used the stock ignition timing maps for a Euro anyway, but I no longer have the Euro cams, so who knows what I really should be using. The rule of thumb is that when you have a boosted motor, you want to retard ignition timing about 1 degree per pound of boost. In my case I make about 7 pounds of boost. So that would mean I should retard ignition timing by about 7 degrees from where it is now at WOT. So I started with the stock map and I pulled out about 12 degrees to be extra safe. After all, I’m still just tuning.
Then I went back to the fuel side of things and started getting the fuel exactly where I wanted. One thing I noticed is that the engine has to be fully warm to properly tune it. That doesn’t mean the temp gauge reads warm, that’s water temp, not engine or oil temp. Fully warmed means the engine is fully heat-soaked and the engine oil is at operating temperature. This requires you to tell the wife you need to drive the car for at least 30 minutes before you can even tune it. It’s a wonderful excuse to have to drive your Porsche.
Doing this solo took several hours to get the fueling mostly dialed in, but it took several weeks with many little tweaks to gain confidence that the fuel map that I had created was robust enough for driving regularly. I’m still not 100% sure it’s perfect, but I’m confident that it’s good and that it won’t harm anything.
Then I turned my attention back to the timing map to see if there was any room for improvement there. I played around with it, and when I add timing, the car is fast. REALLY FAST! But because I don’t have active knock monitoring, I cannot tell if I’m damaging the motor. I am thinking about ordering a system that measures knock but at this point in the year, I think I will dial it back a bit and leave it where it is for now.
When all the tuning is finished, I get to put my super-geek hat on and burn a 4k EPROM. When was the last time you played around with an EPROM burner? After burning the EPROM, I installed the chip in the ECU. Then I re-installed the ECUs in the car and took it for one last test-drive.
I have about 500 miles on the car now and I have to say it’s pretty nice. So far, there are no leaks. I still have an issue with the clutch dragging when it’s cold, and I might have a caliper that’s dragging. But all in all, I’m very satisfied with the car. Next spring I plan to take it to a number of DE events and really enjoy the car the way it was meant to be enjoyed.