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Discussion Starter #1
4 degree timing mod from www.sa750.com

Hey folks,

I know I've asked a similar question before but I really want to be sure I have my head wrapped around the concept before I do this mod. Since I am not an engine mechanic but can do simple bolt-on mods...

So... when I do this mod I am advancing the timing 4 degrees. This means that the spark fires earlier? Assuming I am correct, this would mean less compression, right? (since the piston had not traveled all the way to the top of the cylinder) and would not less compression equal less power? Obviously there is a flaw in my understanding somewhere, else this would not be something anyone would want to do.

thanks,

Scott
 

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Someone can correct me if I'm wrong, but I think the idea behind advancing the timing, which does cause the plug to fire earlier as you said, is to allow the flame front in the combustion chamber to be well developed just as the piston is starting past top dead center. This will create higher cylinder pressure and more down force on the piston. This can also cause the engine to "ping" especially under load at low speed, so a higher octane fuel may be required. I also plan to try this mod on my bike this spring. Hope this helps.
 

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Compression ratio is simply how much volume in the cylinder is compressed. It is only affected by adding or subtracting actual space. (different pistons, thicker head gaskets, etc.) Spark timing is measured in degrees of crankshaft rotation. The spark has to fire a little before the piston reaches top dead center to ensure all the fuel burns. If it fires too early, you get detonation, which is really bad. However, right before you get to the "really bad" you make the most power. Honda sets the spark timing to give good power and be on the safe side. It is also optimized for emission purposes. 4 degrees should make more power, it could require 89 octane fuel. If it runs fine on 87 with the timing advanced, you should be fine.
 

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Is this only for the 750 or will it work on the 1100's too?

Where can I get more int on this?
 

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Re: 4 degree timing mod from www.sa750.com

silvershadow55 said:
Hey folks,

I know I've asked a similar question before but I really want to be sure I have my head wrapped around the concept before I do this mod. Since I am not an engine mechanic but can do simple bolt-on mods...

So... when I do this mod I am advancing the timing 4 degrees. This means that the spark fires earlier? Assuming I am correct, this would mean less compression, right? (since the piston had not traveled all the way to the top of the cylinder) and would not less compression equal less power? Obviously there is a flaw in my understanding somewhere, else this would not be something anyone would want to do.

thanks,

Scott
Let me try to put this in a clear way, since there is a lot of people that
have similar misconceptions.

As stated before, the compression doesn't change.

As the piston goes up in the cylinder on the "compression" stroke,
the fuel/air mixture is compressed.
When the piston is at 0 degrees that is perfectly straight up on the crank.
Any backwards for forwards movement from 0 degrees will cause
the piston to fall.
Advancing the ignition timing is causing the plug to fire earlier in the up stroke of the piston. Retarding the timing, is just the opposite.

As the piston is on the up stroke, the plug will actually fire long before the piston reaches 0 degrees (top dead center).
This gives the fuel/air mixture time to ignite. As the fuel ignites,
the gases expand. The expansion rate is a given and is conservatively
setup so that as the piston goes over top dead center and starts back
down in the hole, the expanding gases push the piston down in the hole.
Advancing the timing, gives the gases a little more time to expand.
If the piston is trying to compress the gases as they are trying to expand,
then suddenly the piston goes over center, energy is released to the top
of the piston and transmitted to the crank.
If you wait, and fire the plug later in the stroke and the gases don't
have time to expand while in a compressed state, they expand in to open
space and don't exert as much energy on the top of the piston... hence,
not as much power transmitted to the crank rotation.

Advancing the timing a little allows the gases to get a head start on
expanding, a little more before the piston hits TDC (top dead center).
If you go too far advanced, the plug will ignite the gas and super heat
the cylinder. This will cause the lower half of the cylinder gases to
detonate (ignite from heat/pressure, instead of spark).
When this happens, the two flame fronts collide and cause what you hear
as pinging. This collision releases a tremendous amount of energy in
the form of heat and can do severe damage.

I have done the timing advance on my VLX and it really woke the bottom
end up. Mid-range and high RPM are about the same.

And as stated above, be prepared to possibly have to change to mid-grade gasoline.
Mine runs file on '87 in the winter, when the ambient air is 70 degrees or under.
In the summer, I have to go up to 89 octane or it'll ping. Runs great on 89.
 

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Discussion Starter #6
thanks Litnin! that was exactly the explanation I was looking for.
 

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I fully understand how this mod can help, I would like to know what method you used to advance the timing, I can think of a couple of ways but will always listen to those with the experiance, Thanks, Don.
 

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Chainsaw said:
I fully understand how this mod can help, I would like to know what method you used to advance the timing, I can think of a couple of ways but will always listen to those with the experiance, Thanks, Don.
You have to modify the mounting bracket that the pulse generators (crank angle sensor) is mounted too.
Then, by using the bolt head as a reference, figure out and mark
degree marks and rotate the mounting hole X number of degrees.

Since a bolt head is 360 degrees, one flat or one point is 60 degrees.
Then just sub-divide between the flats or points.
 

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Sniper Bob said:
Is this only for the 750 or will it work on the 1100's too?

Where can I get more int on this?
I have done this to my 1100 Spirit, but have yet to drive it. I do know that you dont need to choke much if any after advancing the timing.

I can tell you this: The position of the magnetic pickup is different, and the wheel it senses is larger than in the 750. I am not sure how far I have advanced the timing, but I moved the pickup about twice as far as what Blacktop's mod shows. (Because the part that it is sensing is alot larger than the one in the 750s, it should get me in the ballpark)

The other thing is that the lower frame on the RH side must be removed to get the side cover off. That isnt too hard to do, so it sounds more intimidating than it actually is.

You will want a Honda or Clymer manual for your bike and a torque wrench or two to make sure you get the bolts back on to the correct tightness. Other things to have handy are some permatex 2A, zip strip (for old gasket removal), brake cleaner (For cleaning the surface after gasket removal)

PM me if you are really interested in doing this yourself. I am sure there are things I have missed.

Oh, and the shifter guts can fall out of place inside the engine if you are not careful. I was able to get mine to line back up on the first go...

While your in there, check your clutch plates to see if they are blued. If they are, you might want to change them out. I put in the Barnett replacement on mine. (Beware, the springs on the Barnett replacement are a fair bit stronger than stock.)

Hope this helps some. Been a few years since I even had a bike to work on. This time I actually have a heated garage (albiet small) to work on them in.

Mark
 

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I ride my ss750 everyday. It is my main mode of transportation. I have pipes and the bike have been professionally re-jetted/tuned. Will this 4 degree timing mod affect the life of my engine. 95% of my driving is in town on 87 octane. I don't ride like a maniac but it is nice to twist the throttle back occasionally....well ... regularly... :cool:
 

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I ride my ss750 everyday. It is my main mode of transportation. I have pipes and the bike have been professionally re-jetted/tuned. Will this 4 degree timing mod affect the life of my engine. 95% of my driving is in town on 87 octane. I don't ride like a maniac but it is nice to twist the throttle back occasionally....well ... regularly... :cool:
Advancing your timing 4 won’t wear out your engine; in fact it may even lengthen its life.
But the amount you rotate your throttle will definitively have an effect on engine life… :wink:... :-D:lol::mrgreen:... LOL..
 
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