Technology behind a timing key

[buster024]

After the endless threads lately about a timing key, I finally decided to make the big $20 investment and pick one up.

With that being said, I'm trying to envision what is actually happening inside the engine. I'm guessing that this key retards the piston from hitting TDC until after detonation now? Obviously this is adding heat issues (minor), so it must be creating higher compression. I feel dumb for asking, but I just can't picture what's actually changing my piston stroke.

[bdiddy72187]

I'm not 100% sure but I believe it advances it 6 degrees to better time the actual combustion of the fuel mixture. Stock, the combustion takes place when the piston is well on its way down, but with the key it occurs at just about TDC. like I said not sure but I think I'm right. :)

[HondaRaceReady]

I'm not 100% sure but I believe it advances it 6 degrees to better time the actual combustion of the fuel mixture. Stock, the combustion takes place when the piston is well on its way down, but with the key it occurs at just about TDC. like I said not sure but I think I'm right.
Yeah, stock 400ex's fire late, so some of the combustion pressure is wasted. With the timing key in, it fires just slightly after top dead center. Any closer to TDC and it could detonate.

Its pretty simple actually. You are moving the flywheel from the stock position advancing the ignition 6 degrees by doing so. The flywheel is controlling when the spark occurs, thats why you see all the wires and the coil inside and the flywheel has magnetic parts. By making the spark fire sooner the flame front is actually changed which makes it better than high compression to a certain degree. So the explosion occurs sooner. With that happening it has more pressure and the piston is higher up so it has a longer way to go down with force. Its easier for me to explain in person with my fist as the piston and my arm as the rod. I probally just confused the heck out of you. lol

[Dangerous400EX]

Simply put all the key does is change when the spark plug fires, making it fire a nanosecond faster. Nothing is changed regarding the engine mechanically

[GPracer2500]

Originally posted by buster024
...I'm trying to envision what is actually happening inside the engine....

Here's what's happening....

As already mentioned, there are no mechanical changes to the piston or crank's movement. The piston is still moving up and down through its stroke just like it did before. The valves are still opening and closing at the same time too.

What's different is the where in the stroke the piston is at when the spark plug decides to fire. This is called ignition timing. It's measured in degrees of crank rotation Before Top Dead Center (BTDC). In nearly all gasoline engines, the spark plug fires while the piston is still traveling upward on the compression stroke.

At 2000rpm and below, the stock 400EX ignition advance is 8 degrees BTDC. So, if you put the piston right at TDC and then rotated the crank backwards 8 degrees (we're talking rotational degrees here; 1 full revolution = 360 degrees), that piston location is were the spark plug normally fires.

The plug never fires at or after TDC--always before TDC. The reason for this is because it takes a certain amount of time for the air/fuel mixture in the cylinder to begin burning near the plug and then spread outward through the combustion chamber. The plug needs to fire while the piston is still on the way up so there is enough time to burn all the air/fuel mixture and create peak pressure a little bit After Top Dead Center (ATDC). Generally speaking, we want combustion pressures to peak somewhere around 15deg ATDC. This allows the engine to make best use of the pressure created by the expanding gases produced by combustion.

As the engine rpm climbs, the ignition advance also increases. The CDI box does this through electronic circuitry. On a 400EX, between 2000rpm and about 4100rpm the ignition advance gradually changes from 8deg BTDC to 27deg BTDC. Before the days of electronic ignitions, ignition timing was advanced as rpm increased through mechanical means.

The reason for increasing the ignition advance at higher rpm is because as the rpm increases there is less time for the air/fuel mixture to complete its burn. So, when the piston is moving faster (higher rpm), the burn has to start sooner to achieve peak combustion chamber pressure at the appropriate time ATDC.

The CDI gets it's information about where in the stroke the piston is by a little magnet on the flywheel. The magnet sweeps past a fixed sensor called an ignition pulse generator. Every time the flywheel magnet passes the pulse generator a signal gets sent to the CDI. That's how the CDI knows what the crank/piston is doing.

The aftermarket flywheel keys reposition the flywheel on the crank by some amount (typically 6 degrees for a 400EX). That makes the timing of the ignition spark happen 6 degrees of crank rotation sooner than stock. So instead of having 8-27deg of advance (depending on rpm) you'd have 14-33deg of ign adv.

Clear as mud?

[BEAVER.989]

With all of this being said, I guess what I want to know is this...
Can this thing effect the reliability of the motor?

[Eviltanker]

I'm running one in my 06' no issues a little warmer thats all

[alping45]

Does anyone know how long these keys for the 400ex have been around? I'de like to hear from someone that has had one in for a couple years to see if they've had any reliability issues.

I've had mine in for quite some time and its still running great. Advanced timing i shuge in engine building and tuning. People have been doing in in cars messing with it for more performance for longer than i've been alive thats for sure.

[jcs003]

it can have reliability issues. GPracer gave you a drawn out explanation.

what the 'key' it is doing is envoking preignition. preignition will allow a more complete burn of combustion gasses, creating higher combustion chamber pressure. this in effect will produce more heat and can create detontion in certain conditions.

to explain more about what happens in a system with changes in heat, pressure and types of gasses see: bernoulli's principle.

[buster024]

I gotta tell ya', when I initially wrote this thread, I hoped for GPracer to come back from the dead and answer it in the way only he can. Nice to see I got my wish. Great stuff GP.....EXACTLY what I was looking for.:D

[Dangerous400EX]

Yeah excellent answer, thanks.

[F-16Guy]

Okay, to add a little to this:
GP -- There is a lot of controversy about whether the key advances the timing too much for a higher compression ratio engine. What is your take on how close the key is to too much advance? Is it likely to cause detonation on a quad with mods like mine (since I have probably the most common 400ex mods)? Have you seen or ridden any quads with similar mods and an advance key? How about a CDI box with stock base timing and a more aggressive curve as opposed to a key?

Edit -- I'm running (and would like to continue to run), 93 octane pump fuel. I'm guessing that you could get away with more advance with a higher octane fuel, but I like the cheap stuff.

Originally posted by F-16Guy
How about a CDI box with stock base timing and a more aggressive curve as opposed to a key?

Edit -- I'm running (and would like to continue to run), 93 octane pump fuel. I'm guessing that you could get away with more advance with a higher octane fuel, but I like the cheap stuff.

The timing needs to be advanced more as the piston speed increases. Thats why most CDI boxes with extra RPM have a small advance so it will keep pulling. The thing you wont get is a torquey feeling bottom end. I was reading that advanced timing to a certain point and higher compression pistons are the best for a motor. They were saying that higher compression pistons do not change the flame front speed but the advanced timing does. Now asked whats better: advanced just before you get preignition? higher compression just before you get preignition? or piston/timing combo and what would perform best. The answer was the combo. They went into full detail, if i knew where the article was i would show everyone. I run 93 otane and the key no problems, piston wise most i care to go is 10:1. It depends on the cam and riding to where you start getting preignition. I know some were running 11:1 and a 6* advance on 93, then another running 10.8:1 and 6* advance would have a heavy knock with 93 and required a higher octane. Is 10.5:1 a safe limit for all? Dont know, a little cure that most havent thought a lot about is spark plug choice. Heat is the enemy, if you have more than bolt ons run a colder plug. Run a higher weight oil and preferably synthetic. Maybe try a larger oil cooler and/or a fan. Last resort is higher octane, take th key out or set less of an advance. With the sparks key you can set it anywhere from stock to 6* advance.