Why Ignition Timing is the key to your engine operating properly: Understanding Engines Part Trois (that's 3, but all fancy like in French ooh-lala!)

Why Ignition Timing is the key to your engine operating properly: Understanding Engines Part Trois (that's 3, but all fancy like in French ooh-lala!)

So you want to jam more llamathrusts into your engine and you've swapped a month's worth of cursed gypsy gold for the biggest boomstick you can get from your friendly online parts store. But your engine won't work and nobody wants to come to your birthday party... what the deuce went wrong?!

Before you huck all your fairy bread into the backyard in a fit of rage, you need to calm your farm, check all your paperwork, and realise The Internet lied to you. Watching Marty and His Royal Dudeness, Tony, cam the LS3 in Black Chops IN THIS EPISODE will give you many of the answers you seek.

Every brand new cam should be sold with a piece of paper detailing all the important specs the engine needs to run with this new piece of hardware. I've included the cam sheet which came with the VCM #883 cam I had installed in my Holden Commodore ute several years ago, as this an older version of the piece of paper Marty is holding in THIS video. 


All engines rely on a combustion cycle, and the key element of this is getting the perfect timing of when you ignite the mixture of fuel and air in the combustion chamber, releasing horsepower, dorts, and all sorts of internal combustive happiness. If you don't explode your air/fuel mix at the right time you either waste horsepower, or your engine doesn't run at all and you end up a SadBoi.

As the engine spins, the pistons race to the top of the cylinder and squash the air and petrol to its perfect mix, so you need to work out when to fire the spark plugs. While ECUs are very smart pieces of technology we still need to tell them the correct time to fire the spark plugs and Unleash The Llamathrusts.

Ideally you want to be firing the spark plugs a fraction of a second before the piston gets to the top of its travel, making the most efficient combustion. Because this is happening *before* the piston gets to the top of its travel engine nerds called this "advance". 

Too much advance and you're firing an explosion as the piston is still coming up the bore, and too late an explosion and you don't get the benefit of all the nicely squished and mixed air and gasoline. Every engine is (broadley) different in its exact timing requirements, but the principles of getting your timing bang-on is the same. 

In the above photo Tony The Legend is holding the bottom wheel from the timing chain on Black Chop's LS3. The spikes he's pointing to are the teeth which the timing chain attaches to, and that chain is what keeps the crankshaft and camshaft spinning together in time. The slots on the inside of the sprocket are keyways which slot into another piece (see below, to the left) that rides on the snout of the crank, and the various keyways allow for an amount of timing adjustment.

The timing chain can come in single or dual-row, depending on how hard you're going to treat your engine and how spicy the combo is. You can also see four raised sections on the cam wheel (the big one on top) in the above pic - these are hones for the cam sensor to read off and make sure the cam wheel is spinning in time with the crankshaft (which has its own tone wheel). This way the ECU knows the engine is all spinning in time together.  

In the above photo you can see there is a dot on the bottom of the cam timing gear, which is pointing to a small mark on the crank sprocket. These two marks need to be pointing together, but don't assume everything is sweet just going by this, as you really need to "degree the cam in" to make sure everything that goes up and down, and around and around, is happening in sync. 

For this, you need some wire, a dial gauge, and a cam wheel. It's also a bloomin' good idea to have a Tony, because he's an absolute gun of a bloke.

To "degree a cam" you use a dial indicator (which measures lift) to find Top Dead Centre (TDC) on cylinder 1, which is when the piston is at the top of its travel and Peak Squish is attained. Don't sweat if you don't have a dial indicator as they're expensive, specialised pieces of engineering equipment, so make friends with someone who has one (and knows how to use it) and ask them to come around and show you how to use it on your engine.

Once you have a dial indicator bolted to the block you install the cam degree wheel onto the crank snout, and bend a piece of wire pointing to TDC on the wheel.

To find true TDC you need to note the numbers on the wheel at 100-thou BEFORE TDC and 100-thou AFTER TDC, then split the difference to find the middle point. This is important because as the crank swings through its rotation the piston actually sits at the top of the cylinder for a split second, known as "dwell time".

You need to account out how many degrees this dwell occurs for, because degrees on the wheel, can be correlated to timing of the engine and lift of the cam. Too far advanced and your engine can detonate, or not advanced enough and it will be sluggish.

As Tony explains, "A general rule of thumb is, for every cam degree you advance your timing, you gain four-thousandths of an inch worth of cam-lift. Also, note the camshaft spins at half the speed of the crankshaft, so one cam-degree is two crank-degrees."

The lads then double-checked the cam lift using a special tool which simulates one of the LS7 hydraulic roller lifters the lads are using, in conjunction with a dial gauge, to measure how far up and down the lifter is travelling.

By doing all this Marty and Tony worked out they needed to remove the timing chain and adjust the crank sprocket four degrees advanced to put more cam-timing in, so the engine would work at its best potential performance.

This is where the cam spec sheet comes back into play as it tells you how many degrees of lift you should be seeing at TDC. If your engine falls within the range of measurements listed, then you're right to keep going putting your engine back together safe in the knowledge you're not going to have issues with a sluggish, low-power car or something that is going to smash valves into pistons. 

At this point maybe go grab a cold drink/tea/biscuit/frozen gojiberry protein ball (hi MOOG) because I'm throwing all sorts of science and words and information and stuff, and I don't want your brain exploding.

Leave a comment

Please note, comments must be approved before they are published