Tunning for Spark advance

[whittlebeast]

Impossible. It's a Big Twin and those in the know say BT riders never exceed 2500 RPM. North of 2500 is reserved for sportster riders. 

Here is a friend of mine running his bagger on a typical his Sunday ride.



Beast

[Steve Cole]

I really been lugging the engine for the spark and o2 tunning. Normally when I ride RPMs are north of 2500.

Impossible. It's a Big Twin and those in the know say BT riders never exceed 2500 RPM. North of 2500 is reserved for sportster riders. 

You know those sportster riders............. they tell a new tale every few hours! 

[Tsani]

Impossible. It's a Big Twin and those in the know say BT riders never exceed 2500 RPM. North of 2500 is reserved for sportster riders. 

Here is a friend of mine running his bagger on a typical his Sunday ride.



Beast

I gotta walk away. This is too damn easy.

[60wt]

Just went through a tune myself on a 2007 RK and somewere i saw where some said max spark advance shouldnt be more than 40 degrees max.... I see that in the Master Tune program under table selection theres front and rear spark advance the chart shows up to 49 degrees... was wondering if one should  bring thoese numbers down to 40 degrees???

[Doc 1]

Just went through a tune myself on a 2007 RK and somewere i saw where some said max spark advance shouldnt be more than 40 degrees max.... I see that in the Master Tune program under table selection theres front and rear spark advance the chart shows up to 49 degrees... was wondering if one should  bring thoese numbers down to 40 degrees???

It all depends on the build what timing is needed, higher compression, larger displacments, require less timing than a bike with stock cams and stock heads. I've seen stock timing tables over 50 degrees in areas.
Doc

[whittlebeast]

Doc

What is making these motors so slow burn that makes them require 45 degrees in the midrange?

Beast

[Doc 1]

Doc

What is making these motors so slow burn that makes them require 45 degrees in the midrange?

Beast

Low compression and restricted air flow is the main items that allows the timing to be at 45* or more.

[build it]

Doc,
Could you be a little more specific regarding timing advance and engine configuration? Meaning, in general terms, is it displacement that necessitates less spark advance, or is spark advance related more to bore or stroke values? Also, does rod length factor into this?

On a slightly different note, is it possible to put together an engine with a configuration that requires less spark advance compared to the more traditional means to accomplish a given displacement? What are the positive and negative consequences for this within the context of spark advance?     

[whittlebeast]

Doc

So would you expect a small bore motor with 230 lbs of cold cranking compression to hammer with 40 degrees in the midrange?  What about with 89 or 91 octane fuel would you expect it to hammer?

Beast

[mayor]

On a slightly different note, is it possible to put together an engine with a configuration that requires less spark advance compared to more traditional means to accomplish the same displacement? What are the positive and negative consequences for this within the context of spark advance?   

the more efficient the combustion process, the less advance is needed to achieve MBT.  If you can keep the flame front from happening in the edges of the combustion chamber, you can improve the efficiency.  That's were the thinner head gaskets come in.  The thinner head gaskets push the flame front back into the chamber rather than out in the squish band.  It also increases compression, which is another method of improving the combustion process.  There are points of diminished returns for both of these though.   Another method to improve the combustion process is adding the squish grooves to direct the combustion process back into the chamber, but there are folks that swear by them and folks that swear at them. 

So would you expect a small bore motor with 230 lbs of cold cranking compression to hammer with 40 degrees in the midrange?  What about with 89 or 91 octane fuel would you expect it to hammer?

why would anyone be stupid enough to try to run mid grade gas in an air cooled engine with 230# of ccp, and why are you asking the question when you have the ability to see knock retard events for yourself? 

[whittlebeast]

why would anyone be stupid enough to try to run mid grade gas in an air cooled engine with 230# of ccp, and why are you asking the question when you have the ability to see knock retard events for yourself?

Mayor

Would you expect even worse results if the intake air temps are in the 160 degree range?

Beast

[mayor]

Mayor

Would you expect even worse results if the intake air temps are in the 160 degree range?

do you believe that that air temp is real?  The intake air temps in a car efi system is likely more believable since the intake tract can be longer, and routed under a hood near a giant heat source.  I seriously doubt that the actual air temp going through a short motorcycle intake port is as warm as the sensor is showing.  The sensor on a bike is likely showing the transfer of heat through the metal intake manifold, and not the actual air temp.  Air acts as an insulater and is not a good heat conductor, so I highly doubt that the air gets that warm for the few milliseconds it's traveling through the manifold. 

[rbabos]

Mayor

Would you expect even worse results if the intake air temps are in the 160 degree range?

do you believe that that air temp is real?  The intake air temps in a car efi system is likely more believable since the intake tract can be longer, and routed under a hood near a giant heat source.  I seriously doubt that the actual air temp going through a short motorcycle intake port is as warm as the sensor is showing.  The sensor on a bike is likely showing the transfer of heat through the metal intake manifold, and not the actual air temp.  Air acts as an insulater and is not a good heat conductor, so I highly doubt that the air gets that warm for the few milliseconds it's traveling through the manifold.

Real or not, that's what it's telling the ecm.
Ron

[wolf_59]

Did anyone notice the IAT on Thumper Buttercups post here? http://harleytechtalk.com/htt/index.php/topic,41897.150.html
Could it be the air cleaner set up? still would be interesting to see how that effects the tune and fuel milage, may have to try one out

[Doc 1]

Doc,
Could you be a little more specific regarding timing advance and engine configuration? Meaning, in general terms, is it displacement that necessitates less spark advance, or is spark advance related more to bore or stroke values? Also, does rod length factor into this?

On a slightly different note, is it possible to put together an engine with a configuration that requires less spark advance compared to the more traditional means to accomplish a given displacement? What are the positive and negative consequences for this within the context of spark advance?   

There is too much involved in what the advance should be for optimum power and I know this is going to turn into a long thread once we get going on it. First off lets talk about when the max cylinder pressure should occur in a Twin Cam motor (Iron Head Sportsters, Shovel Heads, Pan Heads, etc are much different so lets keep this to Twinkies). Max Cylinder Pressure, BMEP, (Break Mean Effected Pressure) in a Twinkie should happen at 12 to 17 degrees ATDC (After Top Dead Center). To achieve this pressure at 12* to 17* combustion has to start some where between 5* and 50* BTDC (Before Top Dead Center) depending on the RPM, the rate the mixture burns. There are several factors that determine the optimum ignition advance, some internal and some external. The internal factors are; combustion chamber design, piston shape, turbulence, compression ratio, cam timing, bore, rod/stroke relationship, number of plugs, AFR, fuel quality, and RPM. Some of the external factors are; bike weight, gear ratios, air temperature, altitude, and humidity. As you can see this gets very complicated.
The reason we want the max pressure at the 12 to 17* is this is where you will fine the most torque....a few degrees before 12* will drop power because the advance was way too early and the cylinder pressure is pushing down on the piston as it tries to reach TDC causing the flywheels to slow down. A few degrees after 17* you will have the piston out running the flame front....ie...the piston travels the fastest around 45 degrees ATDC and as the piston travels down the cylinder the combustion pressure drops very quickly so the pressure being too late will not push on the piston and again the flywheels slow down.
Also when more of the cylinder is exposed to the flame front, from retarded timing, the engine will run much hotter.
We want the timing as far advanced as we can get it without detonation happening. On a dyno I can find the sweet spot 12* to 17* ATDC buy watching the power increase or decrease with 2* timing changes....however without a dyno advancing the timing to detonation then backing it off a few degrees seems to be the norm. (being able to read the timing line on the spark plug works very well when you don't have a test track or a dyno.....another topic for another time).
Lets say the timing is set at 20* at idle or 1000 rpms, the time or interval we have to burn the mixture is about .0025 of a second, at 2000 rpm that time is cut in half, and at 4000 rpm it is cut in half again, so the higher the rpm the less time we have to burn the fuel so more advance is needed to get a jump on the less time we have. Now in reality after 3000 rpm the timing advance levels off because the rate of burn is offset by increased combustion turbulence and heat so less timing advance is needed on top end.
The higher the compression the less advance you need because the higher compression generates more heat as the mixture is compressed, the molecules are finer (a mist) and the activity is greater which is the rate of burn......if the mixture burns faster the less advance is needed to reach the sweet spot. Lets put it this way...a leaner mixture, less turbulence, lower combustion chamber temperature, lower density of air, and residual combustion gases all lengthen combustion time which needs more advance. Shortening combustion time to have less advance your conditions will need to have a richer mixture, more turbulence, higher temperature, higher pressure, and higher air density.
As mayor pointed out there are squish settings that promote better fuel rate of burn.....your squish is most effective between .025 and .040. Any thing below .025 you are very close to detonation with ever stroke and as carbon starts to build this gap closes even further, anything above .040 the squish is now ineffective and no longer promotes turbulence.....I like to see .030~.035.
I hope this gets your mind rolling and helps you to understand all involved in finding what the advance should be....each engine will be different however for a basic street bike a few degrees here and there chang is all you guys should be seeing for max power from the TTS timing tables. I see a lot of maps from people sending me their tunes from all over this world and I must say there are a few so called tuners out there that are killing these timing tables......you can't expect to remove 15 to 20 degrees of timing and expect the bike to run crisp with good throttle response....the bike will be sluggish to say the least.
Be careful just changing values without knowing the consequences.....
There is a lot more to all this, however, I'll leave it open to questions and discussion from here.

Andy if you are trying to jerk me around like you do Steve with those off the wall questions there wil be no answers.
Doc   

[wolf_59]

Doc, Thanks that is the best explaination of timing that I have seen on this forum as of yet

[Jeffd]

Doc refresh my memeory is the timing line on the plug on the threads or on the strap?

[Doc 1]

Doc refresh my memeory is the timing line on the plug on the threads or on the strap?

Ahhh...I knew someone would have to get to that question....lol
The timing line is on the negative electrobe strap......your timing looks very good however those plugs need to be relaced......

[Jeffd]

I just did about a week ago lol they had about 16000 miles on them.  thanks jeff

[N-gin]

N-gin

Have you gone thru the V-Tune process per the directions?  Is this bike anything out of the ordinary?  Is the motor camed for the under 3500 RPM that you normally ride?  Is there any chance you have an exhaust leak?

Beast

Engine is as follows
crank is trued, welded, pro plugged, lightened,  balanced, and has oversize rod bearings.
pistons are cast SE flat tops 103, has woods 555cam and SE tappered push rods.
Heads have been bead blasted to remove the powdercoat from the ports and chamber. The have been cut. 035 to get 80.5 cc. The valve springs were checked and have 135 psi of seat presure. Head gaskets are custom made,  they have a 3.875 bore and are. .027 thick. Stock throttle body, injectors, lifters, valve springs, stock gearing. ThunderHeader exhaust.

[Tsani]

Damn Doc, That is a mindful.

[pauly]

Cool Doc,
I've got a few questions too but am on a tablet keyboard - soon 
Pauly

[N-gin]

I have new info in the media section. I did three tunes this morning. Some side streets and mostly highway.
This is the tune that was loaded.

[attachment removed after 60 days by system]

[Thumper Buttercup]

Did anyone notice the IAT on Thumper Buttercups post here? http://harleytechtalk.com/htt/index.php/topic,41897.150.html
Could it be the air cleaner set up? still would be interesting to see how that effects the tune and fuel milage, may have to try one out

Wolf,

    We are adding the HPI backplate and 51mm throttle body to the mix while
we are in here fixing things.  We'll report back when we get to ride in the spring.



Doc,

    Thanks for taking all the time to explain all of this.

Mark & Kim

[build it]

Doc,
I've got one last question for you, and my purposes here will come to a close. It is another factor, maybe; I'd like to know if case vacuum has an affect on the advance curve? Lets say you built a motor with a short stroke, long rod, big bore configuration, and hooked up a vacuum pump to a relay that kicked in at a certain RPM only, would THAT affect the advance curve? Am I talking about pumping losses here? I know this is in far left field fwiw.   

Thank you.