How many years have you been tinker-tuning your Corvette only to have to keep popping the hood for more tweak-tuning? Can't quite get the carburetor in a proper state of tune? Rough running, misfire, hard starting hot and the darned thing falls on its face when you mash the pedal. These repeated underhood visits are beginning to wear on your patience because they just aren't netting results. It's time for a plan that gets results.
Modern technology has brought good things to classic C1, C2 and C3 Corvettes in the form of drop-in electronic ignition, improved carburetion, bolt-on fuel injection, single-wire high-amp alternators, disc brakes, overdrive transmissions and professional dyno tune shops. There are dyno tune shops from coast-to-coast and border-to-border. Anyone can invest in a chassis dyno and hang up a shingle and call themselves a tune shop though dyno manufacturers typically won't sell you a dyno until you've been properly trained on their equipment. It takes more than a sign and an expensive dyno to be a great tune shop. It takes a tremendous amount of education and experience to be a great tuner. Savvy tuners understand the dynamics of internal combustion engines. What's more, they have to be computer programmers, too. They have to possess insight into what electronic engine control does and how to fine-tune it and get happy clients back on the road.
You would be surprised how many tuners and repair shops will not touch anything with a carburetor, which leaves plenty of classic Corvette buffs out in the cold without a prayer. If the professional tuner is working with carburetion and old-fashioned point-triggered ignition systems, they need to understand the dynamics of how carburetion and ignition work together to help turn heat energy into rotary motion and power.
Consistent air/fuel mixture and proper ignition timing are the keys to performance. Due to the very nature of internal combustion, timing and mixture are everything to how an engine performs. Starting an engine and getting it to idle is the easy part. Getting an engine to perform in linear fashion as rpm increases takes a more in-depth understanding of how the engine comes off the carburetor's idle circuit and onto the power circuit. Performance needs to be seamless from idle into power as rpm increases. What's more, the ignition system has to keep pace with rpm.
Internal combustion is not as simple as "suck-squeeze-bang-blow." Fuel and air do not "explode" in the combustion chambers as we are so often told. In fact, an explosion is exactly what we don't want. When atomized fuel ignites as the piston approaches top dead center (TDC), it ignites in something of a quick-fire across the top of the piston. The flame front travels across the piston dome at incredible speed. Heat from the light-off is thermal expansion, which acts on the piston, connecting rod and crankshaft journal turning linear motion into rotary motion and power.
We want the air/fuel mixture and ignition timing to function in such a way that both travel side-by-side with engine rpm. It takes time for fuel and air to ignite. Although ignition and light-off happen quickly, there's still the element of time in fractions of a second. This is why we don't light the mixture right at top dead center. We start the light-off before top dead center (BTDC) so that by the time the piston reaches top dead center ignition of fuel is well underway and all that heat energy works effectively at making power. We want the most cylinder pressure possible from the light-off.
When we say "before top dead center" we're talking a specific number of degrees of crank rotation before top dead center. At idle, ignition happens at roughly 6 to 12-degrees BTDC. However, this just won't cut it at 6,000 rpm. At 6 to 12-degrees BTDC it would be nearly impossible for an engine to reach 6,000 rpm. This is why we light the mixture well before the piston reaches TDC above 3,000-3,500 rpm. This gives the fuel and air time to ignite and act on the piston by the time it reaches TDC. Although 6 to 12-degress BTDC may seem like a lot, the piston is just a pinch shy of TDC at idle. By the time the engine rpm passes 3,500 rpm, ignition timing should have reached the maximum of 36-degrees BTDC, depending upon fuel octane and what you have for mechanicals (meaning compression, chamber design and cam profile). This is known as total timing or the maximum acceptable amount of advanced spark timing.
The are reasons why the total timing is typically 36-degrees BTDC at roughly 3,000-3,500 rpm. Anything greater than 36-degrees BTDC can result in spark knock (known as detonation or pinging), which can potentially damage the engine from a light-off that comes too early on compression/ignition stroke. The deadliest combination is a lean fuel mixture and early ignition timing. This drives combustion temperatures skyward. Spark knock is very hard on an engine and can cause piston, ring and ring land damage, not to mention a blown head gasket and shattered spark plug insulators. If a pre-tune spark plug inspection shows tiny bits of aluminum on the firing tip, this is a strong indication of detonation because those bits of aluminum came from elevated temperatures that melted portions of the piston dome. Another issue, especially if an engine has never been apart, is carbon build-up in the chamber, which increases compression and creates hot spots that can cause a premature light-off and deadly detonation.
Professional tuners with extensive training and experience understand how fuel and air react in the chambers. They know instinctively by sound, feel and even exhaust aroma what's going on in each chamber. Of course, they need diagnostic equipment to help give them insight to what's happening inside. Great tuners begin not with a wide-open throttle blast, but a lot of methodical inspection and troubleshooting beforehand. They will spend the better part of a day pulling spark plugs, doing a visual inspection, replacing suspect parts, and examining the vehicle from bumper to bumper. It is dangerous to put a vehicle on a chassis dyno without knowing its overall condition. The best tuners will tell you anyone who dyno-thrashes a Corvette without a thorough inspection is courting disaster.
There are tune shops and there are professional tune shops. The professional tuner will have extensive education and training on the subject and have a methodical approach to each vehicle that comes into their shop. The professional tuner will ask important questions because the last thing they want is engine damage. They will want to know how you drive your Corvette most of the time, then take the time to confirm your Corvette's state of health by checking all of the details before they even get started. The professional tuner first performs a road test to confirm performance and to get the engine good and hot before they go to work.
A good professional tune isn't just about the engine's condition, but overall vehicle condition. If you have lousy brakes they're not going to want to take it for road testing when dyno-testing is complete because it is important to be able to stop. What's more, no respectable tune shop wants to dig your driveshaft out of the wall because the universal joints hadn't been replaced since the Vietnam War. Engine mount failure would also be a catastrophic experience. Your Corvette must be healthy before it goes on the dyno.
Ray McClelland of Full Throttle Kustomz is a full-service professional tuner. He and his team are fully capable of tackling just about anything that comes into the shop. Tuning is Ray's area of expertise. However, before he can tune he likes to know a vehicle's overall health is solid before he puts it on the dyno. His routine begins with a road test to determine if there is a driveability concern that needs to be duplicated that he may not be able to duplicate on the dyno. "I try to do all of our tuning on the dyno because we have an eddy current brake dynamometer, which allows us to actually drive a vehicle on the dyno to simulate street driving with various loads and conditions. Not all chassis dynos and tune shops can do this," Ray tells us.
Once all road testing is complete, Ray inspects the engine, driveline and brakes. The engine gets a spark plug reading, compression check and cylinder leakdown test. He then checks for vacuum and exhaust leaks, which will certainly affect performance. His ignition timing checks include base and total advance timing along with a look at the rate of the curve. What this means is, ignition timing must travel in a linear fashion with rpm. The carburetor float levels are also checked. The accelerator pump shot is checked for volume and spray pattern. Secondary throttle response is also checked.
"Oil and coolant condition are checked along with an examination of the fuel system, including the condition of the fuel," Ray stresses. "The engine and driveline are inspected for oil leaks and any other irregularities." Accessory drive belt, accessory condition, engine mounts, exhaust system and a host of other elements are inspected that will confirm if the engine is healthy enough to tolerate a dyno test. Your engine's current state of tune must also be established before it goes on the dyno.
Once Ray confirms vehicle health he places it on the dyno and begins with idle air/fuel ratio checks and adjusts as necessary, then moves to normal driving, running it through the gears and feeling out the driveline in "vehicle simulation mode" on the dyno. He listens for noises and feels for abnormal levels of vibration. When normal operating temperature is reached, Ray will put your Corvette through its paces, monitoring the air/fuel ratio at cruise and if all appears to be OK at that point he will perform a throttle "tip in" pull, which is basically cruising down the road at around 2,000 rpm at part throttle, then he goes to wide-open throttle to verify that the air/fuel curve enrichens the way it should.
If the air/fuel ratio doesn't roll properly with rpm and goes lean, which can severely damage an engine, it is caught before any damage can occur. "This is one of the most critical mistakes tuners make," Ray comments, "I have personally witnessed tune shops run a vehicle at wide open throttle, then go back to a customer and tell them they need a tune because it is running excessively lean or worse." He adds with a chuckle, "Then, they will hand the customer pieces of a piston skirt." He talks about these tune shops in stunned disbelief because it is all so unnecessary if you know what you're doing.
"If the air/fuel mixtures richens up properly with rpm, I will then perform wide-open throttle pulls beginning at 2,000 rpm, with each following pull going 1,000 rpm higher than the previous pull. Then, on up to 4,000 rpm and higher until the engine's redline is reached, making necessary adjustments along the way," Ray tells us, "This is very critical to keeping your engine safe throughout the tuning process. All too often tuning mistakes are made and engine damage happens far quicker than a human brain can respond and come off the throttle."
Once this phase of tuning is complete and all repairs are performed, the vehicle will sit overnight. A cold start inspection will be performed the next morning, then, there will be a series of full-throttle pulls to ascertain engine tune once the engine gets to operating temperature, with Ray documenting air/fuel ratio throughout the rpm ranges. Once Ray is confident of his findings on the dyno, the customer gets a report on what should happen next. Vette
Full Throttle Kustomz
Summit Racing Equipment