Increased compression - retard timing

[erik_w]

Hello!

Just to get this straight, if I increase the compression I migt have to retard the timing to avoid knocking?

Erik

[LowRyter]

perhaps so, yes and try premium gas

[erik_w]

ok! when you say premium, what octane do you mean?

my engine has started knocking slightly after new heads and I am suspecting the increase in compression that comes from good valves has caused it?

[Moto]

ok! when you say premium, what octane do you mean?

my engine has started knocking slightly after new heads and I am suspecting the increase in compression that comes from good valves has caused it?

The octane you need depends on the elevation above sea level, and on the rating system used for octane where you are. I think the best answer is to buy the highest available octane in your area, barring special racing or aviation fuels. Gas stations sell different octanes as premium in different parts of the U.S., mostly as a reflection of elevation.

The explanation you offer for the knocking makes sense. But there are other possibilities too, like timing or even pre-ignition from a sharp edge in your newly machined head.

Guzzis like high octane! Mine have taught me to bite the bullet and buy it.

Moto

[redrider]

Higher octane burns more slowly and with timing at say, 20 deg BTDC, this allows the piston to reach TDC just as a burn cycle is completed. "Modern" heads have much less burn space and require timing of only a few degrees BTDC and lower octane burns more quickly to extract maximum power from the mixture. Knock is when your burn is completed too soon and it tries to stop the piston from completing the compression stroke. Pre-ignition is caused by the mixture self igniting due to hot spots with no input from the ignition system. Both will destroy an engine.

[erik_w]

Hello!

you mean a sharp edge in the chamber that gets superheated and starts igniting the mixture? I am using 95 octane now but there is 98 avaliable, I can try that .

as for knock, how can I tell between knock and pre ignition?

[oldbike54]

 What bike is this ?

  Dusty

[Rough Edge racing]

Higher octane burns more slowly and with timing at say, 20 deg BTDC, this allows the piston to reach TDC just as a burn cycle is completed. "Modern" heads have much less burn space and require timing of only a few degrees BTDC and lower octane burns more quickly to extract maximum power from the mixture. Knock is when your burn is completed too soon and it tries to stop the piston from completing the compression stroke. Pre-ignition is caused by the mixture self igniting due to hot spots with no input from the ignition system. Both will destroy an engine.

 This isn't exactly true.....higher octane fuel prevents the spontaneous combustion of end gasses, what we all call detonation. As the mixture burns some gases around the edges  ignite explosively  causing the detonation. The actual rate of burn of high octane is a touch slower or may be the same or even faster in the case of  racing fuels...

[Aaron D.]

If you get detonation when you increase compression you may have to retard timing to stop it, but it may also be much less efficient and cause other trouble. It will depend on the combustion chamber, plug placement etc.

Generally the "better" your combustion chamber the less advance you need.

[JoeW]

You could install an EGR system! Just kidding! How about dual plugging the heads? I don't know what bike we're talking about but, not too difficult on a round head, a little more complicated on a square. You can also deck the cylinder so the piston protrudes slightly over the top. There is a quench area around the perimeter of the piston head caused by the thickness of the head gasket. The quench area is where detonation usually begins because the flame front can't reach it. Or, try making a steel shim gasket. Since I've brought this up, does anyone know how much a Guzzi rod grows when the engine reaches full operating temperature?

[Peter from Sch'dy]

You could install an EGR system! Just kidding! How about dual plugging the heads? I don't know what bike we're talking about but, not too difficult on a round head, a little more complicated on a square. You can also deck the cylinder so the piston protrudes slightly over the top. There is a quench area around the perimeter of the piston head caused by the thickness of the head gasket. The quench area is where detonation usually begins because the flame front can't reach it. Or, try making a steel shim gasket. Since I've brought this up, does anyone know how much a Guzzi rod grows when the engine reaches full operating temperature?

One inch of steel expands 0.00000645 inches per degree Fahrenheit increase in temperature.. .0000126" for aluminum
 
Best,
Peter

[JoeW]

One inch of steel expands 0.00000645 inches per degree Fahrenheit increase in temperature.. .0000126" for aluminum
 
Best,
Peter

WOW! Thanks for that, I love this forum.

[oldbike54]

One inch of steel expands 0.00000645 inches per degree Fahrenheit increase in temperature.. .0000126" for aluminum
 
Best,
Peter

 What is that in Whitworth 

  Dusty

[JoeW]

What is that in Whitworth 

  Dusty

I don't know Dusty but, if the rod increases in temp 200 degrees it grows .00516" That's enough to tag a valve when decking a cylinder.

[Peter from Sch'dy]

I don't know Dusty but, if the rod increases in temp 200 degrees it grows .00516" That's enough to tag a valve when decking a cylinder.

Don't forget to include the cylinder/piston increase in length..

Best,
Peter

[dl.allen]

Dumb question: So if I want to retard the timing on a spec 20 DBTDC by say 2 degrees, I would set it at 18DBTDC?  Correct?

[rodekyll]

Yes.   

[erik_w]

It is regarding my Ural 650.

Since I installed new heads it has started knocking when reaching full temperature..

The difference is that the new heads have tighter valves and are a bit ported for better flow.

I was thinking that would require me to retard the timing below spec (dot on flywheel).