Lightweight pistons, Ignition timing and skirt length

Here’s a photo of a lightweight piston with a perfect wear pattern. The oil varnish near the crown shows that there is no rocking going on. It only shows light burnishing/wear in the middle of the skirt and some polishing near the sides.

A quote from the owner of the pistons.
"Those pistons have done 60 races or qualifying/practice sessions over 11 meetings. Each race or session lasted about 15 to 20 minutes each."



Another lightweight piston (below) shows wear at the top (near the oil ring) and at the bottom of the skirt. This piston suffered from detonation (lean mix etc). The owner could hear it on the dyno. When there is detonation the skirt slams and scrapes against they cylinder wall and this probably the reason for the wear at the top and bottom of the skirt.

See photo below of piston that suffered detonation and wear on the skirt.



A few people think “the more ignition advance the better”. But this is not what is necessary. Nortons with higher compressions (smaller relative combustion chambers) need less advance because the smaller combustion chamber provides more efficient combustion – less time and less advance is needed for the burn. Nortons with more combustion chamber volume, medium compression and short rods may run well with 31 deg advance but longer rod high compression Norts do better with less advance. Some run best in the 24 to 26 degree ignition range. Note that the longer rods give more dwell time at TDC for more fuel burn efficiency and this could be a reason for less ignition advance. Too much advance will only cause detonation and wear.

Below is an interesting photo showing the skirt length of a MAP Triumph piston on the right compared to the JS piston on left. You can clearly see that the JS skirt is longer.

Good pic comparing the JS and MAP pistons Jim. Interestingly, I have run pistons in Triumphs with even shorter skirts, with no noticeable ill effect, and with Norton cranks.

I am not totally sure as I didn't measure it accurately at the time. But when my ign timing on the Commando was out, it was probably at around 22 degrees. This was approx 10 BHP down on the previous Dyno run when it was set at 32 degrees.

This is with JS rods and pistons, small combustion chamber volume due to the 10.5:1 CR, a tight squish band clearance, ceramic coated piston crowns and combustion chambers. In other words, many of the things that should allow more retard.

I will do a little bit more systematic testing of ign timing when I get it back on the Dyno after having my head has been re-worked by Lord Comstock (can't wait for that!).

Different configurations will probably have different findings. Just thought I'd share mine.

[quote="jseng1.......Below is an interesting photo showing the skirt length of a MAP Triumph piston on the right compared to the JS piston on left. You can clearly see that the JS skirt is longer. [/quote] That is certainly interesting to see Jim, thx for the post. My brother had some work done to his Harley and it was interesting to see that the aftermarket pistons that his builder/tuner used, looked to be much shorter than stock, following the same school of thought it seems

Fast Eddie said:

Good pic comparing the JS and MAP pistons Jim. Interestingly, I have run pistons in Triumphs with even shorter skirts, with no noticeable ill effect, and with Norton cranks.

I am not totally sure as I didn't measure it accurately at the time. But when my ign timing on the Commando was out, it was probably at around 22 degrees. This was approx 10 BHP down on the previous Dyno run when it was set at 32 degrees.

This is with JS rods and pistons, small combustion chamber volume due to the 10.5:1 CR, a tight squish band clearance, ceramic coated piston crowns and combustion chambers. In other words, many of the things that should allow more retard.

I will do a little bit more systematic testing of ign timing when I get it back on the Dyno after having my head has been re-worked by Lord Comstock (can't wait for that!).

Different configurations will probably have different findings. Just thought I'd share mine.

Click to expand...

I suggest that three things work together - ignition advance, comp. ratio and fuel mixture. For any given comp.ratio the ignition advance and the fuel mixture are interdependent in achieving the optimum combustion temperature. So if your ignition timing had retarded by ten degrees, you might have regained most of your power loss by leaning off the mixture. That is not to say that there are not limits as to how much the ignition advance may be out. I suggest for max. performance, the trick is to find the optimum to suit the comp. ratio and the fuel you are using. I use methanol fuel so my jetting changes are half as critical as yours' would be on petrol. What some of us have found when racing Triumphs etc. on methanol is that at 10 to one compression it is better to advance the timing by about 4 degrees. At 12 to one compression, to use the standard petrol timing. At 14 to one, retard by 4 degrees. What you then find is that at 12 to one compression and standard petrol timing, the jets are huge compared to what is needed at 10 to one comp.
So what I am saying is that if you retard your ignition on a petrol motor, you can expect to have to jet down to get decent power. This might be useful in endurance racing.

It might be interesting to run some dyno tests keeping everything on the motor constant except the static ignition advance and mixture, then finding out how much top end power is actually lost by using various combinations of mixture and advance. I suspect it would not be very much at all. However the torque characteristics might change.

acotrel said:

Fast Eddie said:

Good pic comparing the JS and MAP pistons Jim. Interestingly, I have run pistons in Triumphs with even shorter skirts, with no noticeable ill effect, and with Norton cranks.

I am not totally sure as I didn't measure it accurately at the time. But when my ign timing on the Commando was out, it was probably at around 22 degrees. This was approx 10 BHP down on the previous Dyno run when it was set at 32 degrees.

This is with JS rods and pistons, small combustion chamber volume due to the 10.5:1 CR, a tight squish band clearance, ceramic coated piston crowns and combustion chambers. In other words, many of the things that should allow more retard.

I will do a little bit more systematic testing of ign timing when I get it back on the Dyno after having my head has been re-worked by Lord Comstock (can't wait for that!).

Different configurations will probably have different findings. Just thought I'd share mine.

Click to expand...

I suggest that three things work together - ignition advance, comp. ratio and fuel mixture. For any given comp.ratio the ignition advance and the fuel mixture are interdependent in achieving the optimum combustion temperature. So if your ignition timing had retarded by ten degrees, you might have regained most of your power loss by leaning off the mixture. That is not to say that there are not limits as to how much the ignition advance may be out. I suggest for max. performance, the trick is to find the optimum to suit the comp. ratio and the fuel you are using. I use methanol fuel so my jetting changes are half as critical as yours' would be on petrol. What some of us have found when racing Triumphs etc. on methanol is that at 10 to one compression it is better to advance the timing by about 4 degrees. At 12 to one compression, to use the standard petrol timing. At 14 to one, retard by 4 degrees. What you then find is that at 12 to one compression and standard petrol timing, the jets are huge compared to what is needed at 10 to one comp.
So what I am saying is that if you retard your ignition on a petrol motor, you can expect to have to jet down to get decent power. This might be useful in endurance racing.

Click to expand...

That is dangerous advice.

I was on a dyno with and air/fuel ratio display and can tell you with certainty that there was no such clear relationship. In fact the fuel curve was virtually identical prior to the ignition timing error and after.

If someone were to experiment by retarding their ign by 10 degrees AND leaning it out, I think they would be buying some new engine parts pretty shortly afterwards...

acotrel said:

It might be interesting to run some dyno tests keeping everything on the motor constant except the static ignition advance and mixture, then finding out how much top end power is actually lost by using various combinations of mixture and advance. I suspect it would not be very much at all. However the torque characteristics might change.

Click to expand...

That's kinda what I hope to do next time. I also don't expect it to make huge differences to power. I don't expect much effect on torque either. But we shall (hopefully) see.

Fast Eddie said:

Good pic comparing the JS and MAP pistons Jim. Interestingly, I have run pistons in Triumphs with even shorter skirts, with no noticeable ill effect, and with Norton cranks.

I am not totally sure as I didn't measure it accurately at the time. But when my ign timing on the Commando was out, it was probably at around 22 degrees. This was approx 10 BHP down on the previous Dyno run when it was set at 32 degrees.

This is with JS rods and pistons, small combustion chamber volume due to the 10.5:1 CR, a tight squish band clearance, ceramic coated piston crowns and combustion chambers. In other words, many of the things that should allow more retard.

I will do a little bit more systematic testing of ign timing when I get it back on the Dyno after having my head has been re-worked by Lord Comstock (can't wait for that!).

Different configurations will probably have different findings. Just thought I'd share mine.

Click to expand...

Yours is a good one to keep track of. As you say - there will always be different findings (because of variables). I have pistons in 3 different heights and compressions including domed. Some get up near 12 to one CR on gas. So the chambers are all different sizes. We should see some different timings depending on the setup. Still, I expect to see less advance in some Hi CR race motors sustaining continued WOT.

Herb Becker just built a short stroke for Doug MacRae with small combustion chambers that result in 11.8 CR with the domed pistons and long rods (2 to 1 rod stroke ratio). Here's a quote from Herb. "we adjusted the timing from starting 30 deg. to 25 degrees with no loss of power."

Hey Steve A
What about your long rod short stroke? What timing do you use?

Acotrel
When I ran a Methanol mix I remember retarding a few degrees. This was at methanol related compression.

Hello.

Interesting. Thanks. What ignition timing would you surguest for long rods/std. comp?

I´m running a Pazon Smart fire ignition on my Commando. The instruction sayes to run 31 degree instead of the 28 degree standard. The same ignition is used for Triumph and BSA´s and the instruction for these sayes to use standard settings.

Thanks, Dan.

dantriumph said:

Hello.

Interesting. Thanks. What ignition timing would you surguest for long rods/std. comp?

I´m running a Pazon Smart fire ignition on my Commando. The instruction sayes to run 31 degree instead of the 28 degree standard. The same ignition is used for Triumph and BSA´s and the instruction for these sayes to use standard settings.

Thanks, Dan.

Click to expand...

Regular compression allows more advance than HI compression. I would go for the recommended advance provided my the EI manufacturer if you have stock pistons & rods. If you have the longer JS rods in your Commando then I would use the 28 deg standard. For the longer rods and higher compression pistons (10 or more to one), 26 deg is being used by some. The smaller combustion chambers are more efficient and need less advance. I'm trying to collect more dyno results to verify this.

It is not easy to change compression ratio, so the optimisation usually has to be by adjusting ignition advance and jetting. If higher compression ratios require bigger main jets at a fixed ignition advance when the plug readings are right, then the extra fuel must go into making power ?
My brother has two motors on over 16 to one comp. using methanol. One is a 600cc Jawa two valve, the other is a 1000cc V-twin JAP - they are both excellent. My own commando engine is only on 9 to one comp. uses 4 degree advance over the usual 28 degrees and extremely small jets, it makes very good power, however I know the next strep must be Jim's long rods and 12 to one comp. pistons.

" What some of us have found when racing Triumphs etc. on methanol is that at 10 to one compression it is better to advance the timing by about 4 degrees. "

That's about what I use on my 500 Norton twin when going to methanol.

10.25 :1 compression.

28 Deg advance on 98 octane petrol.

32.5 deg on methanol.

Has proven very reliable on both settings.

Fast Eddie said:

acotrel said:

Fast Eddie said:

Good pic comparing the JS and MAP pistons Jim. Interestingly, I have run pistons in Triumphs with even shorter skirts, with no noticeable ill effect, and with Norton cranks.

I am not totally sure as I didn't measure it accurately at the time. But when my ign timing on the Commando was out, it was probably at around 22 degrees. This was approx 10 BHP down on the previous Dyno run when it was set at 32 degrees.

This is with JS rods and pistons, small combustion chamber volume due to the 10.5:1 CR, a tight squish band clearance, ceramic coated piston crowns and combustion chambers. In other words, many of the things that should allow more retard.

I will do a little bit more systematic testing of ign timing when I get it back on the Dyno after having my head has been re-worked by Lord Comstock (can't wait for that!).

Different configurations will probably have different findings. Just thought I'd share mine.

Click to expand...

I suggest that three things work together - ignition advance, comp. ratio and fuel mixture. For any given comp.ratio the ignition advance and the fuel mixture are interdependent in achieving the optimum combustion temperature. So if your ignition timing had retarded by ten degrees, you might have regained most of your power loss by leaning off the mixture. That is not to say that there are not limits as to how much the ignition advance may be out. I suggest for max. performance, the trick is to find the optimum to suit the comp. ratio and the fuel you are using. I use methanol fuel so my jetting changes are half as critical as yours' would be on petrol. What some of us have found when racing Triumphs etc. on methanol is that at 10 to one compression it is better to advance the timing by about 4 degrees. At 12 to one compression, to use the standard petrol timing. At 14 to one, retard by 4 degrees. What you then find is that at 12 to one compression and standard petrol timing, the jets are huge compared to what is needed at 10 to one comp.
So what I am saying is that if you retard your ignition on a petrol motor, you can expect to have to jet down to get decent power. This might be useful in endurance racing.

Click to expand...

That is dangerous advice.

I was on a dyno with and air/fuel ratio display and can tell you with certainty that there was no such clear relationship. In fact the fuel curve was virtually identical prior to the ignition timing error and after.

If someone were to experiment by retarding their ign by 10 degrees AND leaning it out, I think they would be buying some new engine parts pretty shortly afterwards...

Click to expand...

If you wish to investigate this area I recommend reading the chapter on combustion in Gordon Blair's book Four Stroke engines. (A weighty tomb but full of useful information - but you had better have your first year university math ready!!) He presents data from experimental engines in the lab at Belfast university. They tested a number of engines, fuel, ignition timing, CR to determine fuel burn rate and maximum pressure. From memory the perfect result is to have fuel burn maximised about 15 deg ATDC. This would depend a bit on rod ratio I guess. Again from memory the methanol burn is clearly slower and this formed part of the background of my initial experiments for converting to methanol on my engine.

There are Blair articles at:
http://www.profblairandassociates.com/RET_Articles.html
Hope some are related to this thread. I started with 'Back to Basics'.
Ta.

Hmmm, some good stuff in this thread!

I have never heard that 15 degrees ATDC as the best burn point figure before.

I am trying to visualise this, and the effect of long rods on what goes on....

JS states "The increase in power is a result of more dwell time at the top of the compression stroke, allowing more time for the fuel charge to burn completely and exert maximum force on the piston throughout the power stroke, This translates to more power for the amount fuel you burn".

Question: Is it correct to translate this into saying that, if all else is equal between two engines, the pistons will effectively be moving more slowly in the longer rod engine in the moments before and after TDC?

" I have never heard that 15 degrees ATDC as the best burn point figure before. "

Please note . This is from memory of about 6 years ago. I am about 20,000 km from my copy of his book right now so it would be great if someone could check.

However I am certain it was several degrees after TDC.

Fast Eddie said:

Hmmm, some good stuff in this thread!

Question: Is it correct to translate this into saying that, if all else is equal between two engines, the pistons will effectively be moving more slowly in the longer rod engine in the moments before and after TDC?

Click to expand...

Have a read of this.

http://victorylibrary.com/mopar/rod-tech-c.htm

Fast Eddie said:

Hmmm, some good stuff in this thread!

I have never heard that 15 degrees ATDC as the best burn point figure before.

I am trying to visualise this, and the effect of long rods on what goes on....

JS states "The increase in power is a result of more dwell time at the top of the compression stroke, allowing more time for the fuel charge to burn completely and exert maximum force on the piston throughout the power stroke, This translates to more power for the amount fuel you burn".

Question: Is it correct to translate this into saying that, if all else is equal between two engines, the pistons will effectively be moving more slowly in the longer rod engine in the moments before and after TDC?

Click to expand...

This is from my web page
http://www.jsmotorsport.com/technical_rodspistons.asp

"For example: When a Norton crank is at 30 BTDC the distance from piston crown to TDC with our longer rod is 6.8mm compared to 9.2 mm with standard rods. So both sets of pistons are going from 6.8 and 9.2 mm BTDC to TDC in the same amount time. You can see that the piston with the longer rod spends more time closer to TDC (more efficient burning & power) and is moving slower when it reverses direction (less vibration). "

So in principle, if the piston is moving slower, and fuel still burns at the same speed, and IF the point of max burn stays the same (15 degrees ATDC), then longer rods should require less advance.

Shouldn't they?

Fast Eddie said:

So in principle, if the piston is moving slower, and fuel still burns at the same speed, and IF the point of max burn stays the same (15 degrees ATDC), then longer rods should require less advance.

Shouldn't they?

Click to expand...

Assuming the increased dwell brought upon by the longer rod (or better R/S ratio) improves the chamber's efficiency, then yes, you should require less advance. That's assuming the increased dwell causes the flame-front speed to increase. A lot of assumptions...

Nathan