Today, let’s start off with a classic, in this fine example of a ‘76 corvette.
In 1975, GM proclaimed that they now had “a more efficient corvette”. This meant the addition of catalytic converters, electronic ignition and the mandated use of unleaded gasoline. In ‘75 GM nixed the 454 due to increasing federal regulations, meaning in ‘76 you had your choice of the the L-48 with 180hp or the L82 with 210hp.
Our customer was referred by someone who recently purchased one of our GM LS-x dyno tunes. While we are quite good at all things EFI, we also do know a thing or two about carburetor and distributor adjustment. A few years back, this customer had an engine rebuilder go through his L-48 equipped C3. After a fresh performance rebuild, long tube headers and an MSD ignition box this customer was told he now had a motor that was pushing 400 horses. However, after all was said and done the customer felt as though the car just didn’t have the punch he felt it should.
Getting fed up with his lackluster performance, he decided that he needed a professional opinion on ways to get his car packing the punch he desired. Soon he found his way to our doorstep, and said those magic words we love to hear: “Make my car go Faster”. Those of you who follow us regularly know that that asking us for more power can lead to wild outcomes. While we would love to sell him a belt driven blower sticking through the hood, or better yet, hilborn injection on a destroked, high revving, 13:1 compression motor, we insisted to first see if his motor and trans were running optimally. As with any performance modification we do, we checked the entire vehicle top to bottom.
Almost immediately we noticed an issue: the vacuum advance for the ignition was hooked up incorrectly. This means less timing advance at higher RPM. Before routing the lines correctly, we strapped this beauty to the dyno so we could document any and all changes to performance. While we would have loved to report back that all 400 horses were found alive and well, the “heartbreaker” Dyno Dynamics dynamometer had other things to say. Only 199hp were finding their way to the rear wheels. Can we do better?
Switching the vacuum advance correctly quickly showed us a slight loss of power. What? How could this be?! Were we wrong? We checked timing and found this engine was very far advanced. Unless he was running race gas or had an extremely advanced cylinder head, this was far too much timing. When the car wasn’t reaching its intending timing advance, it seems as though someone thought to just crank it up.
A quick lesson on timing. When adjusting timing, much caution must be taken to make sure that you reach the intended timing window. Ignition timing, simply, is the point of time in degrees of rotation where the spark is commanded and combustion begins. Ideally you want this at the point that will give the piston the greatest downward force during the combustion stroke. Ignition timing advance will add incremental increases to power for each degree used until the window is reached. Once inside this window, the advances in timing will still produce hp but the gains will not be as significant as those made previously. Eventually you will see wheel horsepower plateau with each advance; this is the edge of the timing window. Once you go outside this window you fall into the territory known as the “knock threshold”. Knock or “pinging” is bad; in fact it’s downright destructive. It is ignition at an inopportune time that otherwise interferes with the normal movement of the crankshaft. This breaks pistons, rods, blocks, and any other part of the engine that is unfortunate enough to be on the receiving end.
Now you might think that in order to get the most power and efficiency, you would get timing into this window and put it right to the edge, just outside the knock threshold. This may be true for people running stable exotic racing fuels and e85, it is not true when it comes to gasoline. When running pump gas, the best spot for timing is just above the point where the gains in power start to become less pronounced. We do this as a safety precaution against gasoline’s poor cooling properties. With temperature, gasoline loses octane effectiveness. Therefore, in a hot engine, the knock threshold will creep into the timing window, shrinking your safety margin. So going as far as you can with timing may result in more power but you could run into a potential issue when things get hot.
Now back to the corvette. This particular engine was on that very edge of the timing window. The point in which you start to lose power from knock. Pulling it back 3 ½ degrees netted us a cool 18hp. Not bad for an engine designed almost 35 years ago. Well, the owner now has now driven the car and while pleased with the gains, he wants more. Who can blame him? Most of the newer model cars we have in here do at least 180 hp. For reference, 180hp is what a stock hyundai genesis puts out to the rear wheels on our dyno. Looks like we might get to do that hilborn injection after all.