Source: Cycle World
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)Thank you, Moparnut72. Thank you! You’ve provided an excuse for us to exchange our best ignition timing stories.First, the story about Don Garlits and the arrival of the 426 Hemi Chrysler (to replace the well-developed 392 Hemi that preceded it). Supposedly the 426 was a disappointment when “suited up” with the supercharger, nitro fuel, and tuning techniques that had worked well on the 392. But later it was discovered that the 426 “came alive” when a bunch more ignition advance was cranked in.I’m sure I read some of the same things you did in regard to this, but what stood out to me was one writer’s remark that the 426′s ports were greatly enlarged as compared with those of the 392. As I recall, it was said that the 392′s intake ports flowed something like 225 cubic feet per minute (on whatever pressure) versus 300 CFM for the 426. The 426′s displacement was an 8 percent increase over the 392, but the airflow difference was 33 percent. Sounds like the 426 had really big ports.That triggered a memory. Back in 1961 Formula 1 went to a 1.5-liter formula, so engine manufacturer Coventry-Climax decided to build a V-8. They were encouraged by the strong performance of a 750cc four they had built, but when scaled up as a 1500 it produced disappointing power. Investigating, they found the 1500′s intake ports to be bigger than those of the 750. When the ports in the heads were bored and sleeved back to the original (smaller) dimension, power came back.Two reasons. First, the smaller the intake ports, the higher the velocity, and it is the turbulence resulting from high intake speed as it enters the cylinder that after ignition accelerates flame speed. Bigger ports would slow flame speed, requiring more ignition advance. And second, it is the high velocity of the inlet flow that keeps it coasting into the cylinder long after bottom dead center, thereby increasing torque. The big ports, by reducing inlet velocity, allowed the rising piston to stop intake flow early and even to back-pump some of what had just been taken in.The 426 that Garlits tested had the usual big blower on it, so back-pumping surely wasn’t the problem. More likely that its reduced intake velocity, relative to that of the 392, agitated the fuel-air charge in the cylinder less, resulting in lower flame speed and a need for more ignition advance than had been found best on the 392.Now for a happier story. Back when Don Tilley and Dick O’Brien were developing their Harley-based Battle of the Twins bike “Lucifer’s Hammer,” they had just completed an engine build and had made the first few pulls on the dyno. Big disappointment! Where did the power go?Never mind; they set about advancing the distributor, expecting to see the power of the previous build return. Nope. Instead, power went down. Being practical men, they reversed direction and began to retard the ignition. More power. Another retardation, and here came more power yet. Really good power.O’B said, “We had that distributor pulled around so far it was about to hit the cylinder. Finally we ended up making best power at under 30 degrees BTDC!” And it was really good, strong power. Next time they saw “the Champion man” Bobby Strahlman, they asked him about this.“You boys lucked out. Every once in a while you’ll get one like this, that makes really strong power on very little ignition timing. Maybe the winds that blow in the cylinder are coming from just the right direction to make the light-up extra quick.”Moparnut72 also mentioned cross-flow two-stroke outboard motors that needed timing “set clear into next week to make them really work.” If you’ve seen the piston crown on such an engine, you’ll understand why they needed so much advance: There is a big fencelike deflector cast into the piston top, to turn the flow from those cross-flow transfer ports to go straight up, across the underside of the cylinder head, then down the far cylinder wall, rather than straight across and out the exhaust port. If you try to build a reasonably high compression ratio into such an engine, the combustion space at TDC is so thin and complicated that mixture turbulence gets damped out, slowing flame travel and making the engine need very early ignition for best torque.And then there’s what I think of as “the linguistic problem.” Would you rather be thought of as advanced? Or retarded?I have seen so many people at Daytona, trying to make their poor engines accept another quarter degree of ignition advance, and not really getting anywhere. What we want and need is quick combustion, so hot flaming gases are held against the piston crown and cylinder head for as short a time as possible. Heat from the intensely hot gas is rushing into the cold metal that surrounds it, taking with it valuable energy that should be acting on the piston crown.Thus, the less ignition timing your engine needs, the more efficient its combustion.The two-stroke racebike engines of 1965 to 1984 had simple combustion chambers that you could machine on a lathe. No valves to get in the way. One spark plug in the exact center. And a completely circular squish band. As the piston rose to TDC, mixture in that band was “squished” rapidly toward the center of the open combustion chamber, resulting in superstrong turbulence and very rapid flame propagation. It was typical for GP two-strokes to give best torque at an ignition timing of 15 degrees BTDC.So instead of trying to make their engines seize with another quarter degree of advance, they should have been congratulating themselves on how little timing their engines needed.At Daytona in 1977, rider Rich Schlachter and I, with a new-but-modified Yamaha TZ250D, started out with the stock timing of 20 BTDC but when we pulled it back to 15 degrees we picked up an immediate extra 300 revs down the back straight. That was in the bad old days of fixed ignition timing, which was always a compromise—too little for the slow combustion below 9,500, too much for on-the-pipe running at 11,000.It had worked that same way for engine designer Keith Duckworth too. By discovering how to achieve strong in-cylinder turbulence that lasted all the way to TDC, he was able to get his DFV Formula 1 V-8 to make best torque on ignition at 27 degrees BTDC, and won a record number of Grands Prix.Conclusion: Pay attention to your ignition timing. It may be trying to tell you something.
Full Text:
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)
Thank you, Moparnut72. Thank you! You’ve provided an excuse for us to exchange our best ignition timing stories.
First, the story about Don Garlits and the arrival of the 426 Hemi Chrysler (to replace the well-developed 392 Hemi that preceded it). Supposedly the 426 was a disappointment when “suited up” with the supercharger, nitro fuel, and tuning techniques that had worked well on the 392. But later it was discovered that the 426 “came alive” when a bunch more ignition advance was cranked in.
I’m sure I read some of the same things you did in regard to this, but what stood out to me was one writer’s remark that the 426′s ports were greatly enlarged as compared with those of the 392. As I recall, it was said that the 392′s intake ports flowed something like 225 cubic feet per minute (on whatever pressure) versus 300 CFM for the 426. The 426′s displacement was an 8 percent increase over the 392, but the airflow difference was 33 percent. Sounds like the 426 had really big ports.
That triggered a memory. Back in 1961 Formula 1 went to a 1.5-liter formula, so engine manufacturer Coventry-Climax decided to build a V-8. They were encouraged by the strong performance of a 750cc four they had built, but when scaled up as a 1500 it produced disappointing power. Investigating, they found the 1500′s intake ports to be bigger than those of the 750. When the ports in the heads were bored and sleeved back to the original (smaller) dimension, power came back.
Two reasons. First, the smaller the intake ports, the higher the velocity, and it is the turbulence resulting from high intake speed as it enters the cylinder that after ignition accelerates flame speed. Bigger ports would slow flame speed, requiring more ignition advance. And second, it is the high velocity of the inlet flow that keeps it coasting into the cylinder long after bottom dead center, thereby increasing torque. The big ports, by reducing inlet velocity, allowed the rising piston to stop intake flow early and even to back-pump some of what had just been taken in.
The 426 that Garlits tested had the usual big blower on it, so back-pumping surely wasn’t the problem. More likely that its reduced intake velocity, relative to that of the 392, agitated the fuel-air charge in the cylinder less, resulting in lower flame speed and a need for more ignition advance than had been found best on the 392.
Now for a happier story. Back when Don Tilley and Dick O’Brien were developing their Harley-based Battle of the Twins bike “Lucifer’s Hammer,” they had just completed an engine build and had made the first few pulls on the dyno. Big disappointment! Where did the power go?
Never mind; they set about advancing the distributor, expecting to see the power of the previous build return. Nope. Instead, power went down. Being practical men, they reversed direction and began to retard the ignition. More power. Another retardation, and here came more power yet. Really good power.
O’B said, “We had that distributor pulled around so far it was about to hit the cylinder. Finally we ended up making best power at under 30 degrees BTDC!” And it was really good, strong power. Next time they saw “the Champion man” Bobby Strahlman, they asked him about this.
“You boys lucked out. Every once in a while you’ll get one like this, that makes really strong power on very little ignition timing. Maybe the winds that blow in the cylinder are coming from just the right direction to make the light-up extra quick.”
Moparnut72 also mentioned cross-flow two-stroke outboard motors that needed timing “set clear into next week to make them really work.” If you’ve seen the piston crown on such an engine, you’ll understand why they needed so much advance: There is a big fencelike deflector cast into the piston top, to turn the flow from those cross-flow transfer ports to go straight up, across the underside of the cylinder head, then down the far cylinder wall, rather than straight across and out the exhaust port. If you try to build a reasonably high compression ratio into such an engine, the combustion space at TDC is so thin and complicated that mixture turbulence gets damped out, slowing flame travel and making the engine need very early ignition for best torque.
And then there’s what I think of as “the linguistic problem.” Would you rather be thought of as advanced? Or retarded?
I have seen so many people at Daytona, trying to make their poor engines accept another quarter degree of ignition advance, and not really getting anywhere. What we want and need is quick combustion, so hot flaming gases are held against the piston crown and cylinder head for as short a time as possible. Heat from the intensely hot gas is rushing into the cold metal that surrounds it, taking with it valuable energy that should be acting on the piston crown.
Thus, the less ignition timing your engine needs, the more efficient its combustion.
The two-stroke racebike engines of 1965 to 1984 had simple combustion chambers that you could machine on a lathe. No valves to get in the way. One spark plug in the exact center. And a completely circular squish band. As the piston rose to TDC, mixture in that band was “squished” rapidly toward the center of the open combustion chamber, resulting in superstrong turbulence and very rapid flame propagation. It was typical for GP two-strokes to give best torque at an ignition timing of 15 degrees BTDC.
So instead of trying to make their engines seize with another quarter degree of advance, they should have been congratulating themselves on how little timing their engines needed.
At Daytona in 1977, rider Rich Schlachter and I, with a new-but-modified Yamaha TZ250D, started out with the stock timing of 20 BTDC but when we pulled it back to 15 degrees we picked up an immediate extra 300 revs down the back straight. That was in the bad old days of fixed ignition timing, which was always a compromise—too little for the slow combustion below 9,500, too much for on-the-pipe running at 11,000.
It had worked that same way for engine designer Keith Duckworth too. By discovering how to achieve strong in-cylinder turbulence that lasted all the way to TDC, he was able to get his DFV Formula 1 V-8 to make best torque on ignition at 27 degrees BTDC, and won a record number of Grands Prix.
Conclusion: Pay attention to your ignition timing. It may be trying to tell you something.
