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Altering timing

Old 08-15-2007, 11:16 PM
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Default Altering timing

Could I ask you guys for some links on engine timing if you wouldn't mind sharing a few? I have moded a few engine components and would like to take it a step further with learning to alter timing profiles. Any info would be just super. thanks a ton in advance.
Old 08-16-2007, 01:18 AM
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Default RE: Altering timing

Ok This is going to be and interesting thread.
I'll share info:
Please post your findings on altered timing on engines youve done. I am interested as this is also what I like about nitro.

Here goes:

The reciprocating motion (up and down) of an engine's piston allows it to act as an air pump. Initially, the air/fuel mixture is moved into the crankcase below the piston, and then it's forced into the cylinder (above the piston) where it's compressed and united. As the gases burn, temperature-and pressure-soars. This pressure forces the piston toward the bottom of its stroke to where exhaust gases are finally purged. It sounds simple, but very accurate port design-shape, size and location-and timing are vital if you want sensational engine performance.
INDUCTION PORT. RC nitro engines use the crankshaft rotary-valve induction system. Here's how it works: a port machined through the crankshaft journal's surface is aligned with the engine block's air-induction hole (under the carburetor) once per revolution. The air/fuel mixture flows through the open port in the crankshaft journal and then through a hole bored in the center of the crankshaft and, finally, into the crankcase.
TRANSFER PORTS. These are holes machined through the cylinder wall and that are alternately covered and uncovered by the piston. The air/fuel mixture in the crankcase (below the piston) is moved through bypass channels between the engine block and the outside of the cylinder to the transfer ports.
Nitro 2-stroke engines use a variety of transfer-port combinations. There can be as few as two and as many as 10 or 11 transfer ports in all shapes and sizes-- plus an exhaust port or ports (yes, there can even be several exhaust ports).
Dozens of transfer- and exhaust-port configurations have been used in 2-stroke engines, but RC engines use a basic configuration that's known as Schnuerle porting, so we'll discuss only this type.
In a Schnuerle system, two transfer ports are angled upward and away from a single exhaust port that's between them. The fresh fuel mixture is intended to be directed at a point that's farthest from the exhaust port. At this point, the fresh mixture loops toward the cylinder head and forces spent gases out through the exhaust port.
The boost port is an important improvement on the basic Schnuerle port arrangement. It's opposite the exhaust port and is easily distinguished by its sharp upward angle relative to the other cylinder ports. The boost port not only creates another passage through which air/fuel mixture can be transferred into the cylinder, but it also does so at an angle that forces the mixture toward the glow plug, which is at the top of the cylinder. This promotes better "packing" of the cylinder and improves exhaust scavenging.
More significant than the number of ports are port timing (when the ports open and close), duration (how long they stay open) and area (the ports' sizes), so don't be impressed by how many ports are advertised for a given engine. A properly configured 3-port engine can be more powerful than a less well-- designed 7-port engine. Don't base your purchasing decision on port count alone.
In engine terminology, "scavenging" means to clear a volume-in other words, to clear exhaust gases out of the cylinder and to move fresh air/fuel mixture from the crankcase and into the cylinder. To an engine designer, clearing exhaust gases from the cylinder is only half of the problem; simultaneously replacing these gases with fresh, cool, air/fuel mixture is the other. When a nitro engine operates, some of the fresh gases transferred to the cylinder mix with scavenging exhaust gases and reduce the engine's efficiency and power. Over the years, many porting systems have been tried to minimize this mixing and contamination; designs have improved, but the condition continues to impact 2-stroke performance. The size, location and direction of these ports determine how successful scavenging is and how well your engine performs.
Transfer and exhaust ports permit pressurized gas to escape from above and below the piston during the engine's cycle. Having adequate time (port duration) to do this is only half the story; having a large enough port (port area) is the other half. Stated differently: the time required to move a quantity of gas through a port depends on the port's area.
An analogy will be helpful: 50 people have 30 seconds to exit a room after the fire warning has sounded. If the exit door opens completely, they all file out smoothly in just under the time limit. If the exit door malfunctions and opens only partway, people can still get out, but there's a jam that ultimately allows only 35 to exit within the time limit. Arithmetic shows that the partly opened door allowed only 70 percent of the people to exit within the allotted time. A similar situation exists for gases attempting to pass through transfer and exhaust ports. If flow is too restricted, the port can be widened to increase its area, or it can be made higher to increase both its area and its duration. Each remedy has a different effect; deciding which is better is the subject of years of study and experience.
The goal of most racing engine modifiers is to increase horsepower. The easiest way to do this is to make the engine run faster. When top-end speed is increased, however, the ports stay open for a shorter time. Based on experience with a particular engine, the modifier either widens or raises the port-or a combination of both. The practice is known as "porting."
The shapes, sizes and positions of ports are all very critical to engine performance, and you can't make one change without it affecting engine performance elsewhere. There's always a trade-off.
Dave Gierke's book, "2-Stroke Glow Engines for RIC Aircraft" is available from Air Age Publishing. Go to
I recently spoke with noted 2-stroke engine modification expert Dennis Richie of Texas. Dennis modifies hundreds of engines for his boat and car customers each year. He graciously gave his time to discuss ports, port timing and porting.
Dennis sees a noticeable difference in port-- timing design philosophy between high-dollar.12 and .15 RC nitro engines and .21s. According to Dennis, the smaller engines are more conservatively ported. Here's a typical example:
* INDUCTION-open 40 degrees ABDC (above bottom dead center); close 48 degrees ATDC (at top dead center); duration 188 degrees.
* EXHAUST-open 78 degrees BBDC (below bottom dead center); close 78 degrees ABDC; duration 156 degrees.
* TRANSFERS-open 60 degrees BBDC; close 60 degrees ABDC; duration 120 degrees.
He says, "Although the duration of the exhaust and transfers is somewhat low, the greatest boost in high-speed performance is gained by increasing the induction-port duration: According to my calculations, if the port opening remains the same and the closing is advanced to about 65 degrees ATDC, the duration of the induction port would be extended to 205 degrees-a 9 percent increase. The best .21 engines already have very advanced port timing.
Here are some typical port duration numbers for these more advanced .21 engines:
-induction 210 degrees;
-exhaust 180 degrees;
-transfer 126 degrees.
Dennis says that these engines are "happy" using 30 percent nitromethane fuel and that the peak brake horsepower occurs between 33,000 and 34,000rpm after modifications.
When asked about porting tools, Dennis laughed and said, "You'll be surprised to learn that my main tool is a Dremel [grinder] fitted with a special cutter." He suggested that he learned most by modifying parts and making mistakes. Although it's expensive, he says, "There is no substitute for getting in there and doing it yourself."
Old 08-16-2007, 09:01 AM
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Default RE: Altering timing

Thanks for sharing Jerseyboy. I also got this link but I dont feel there is enough to clearly explain all timing tricks.
Old 08-16-2007, 11:48 AM
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Default RE: Altering timing

Ill post some my work when I finish studying this in depth.
Old 08-16-2007, 11:22 PM
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Default RE: Altering timing

Very good information, Jersyboy. And thankyou for posting what you know.

The 210 degree induction timing IS definitely a badge of a high revving perfromance engine, but as for typical prifile in releation to the crank angle, does not such a longer induction timing open moe like 30 to 35degrees ABDC?
Also, STS engines have an induction timing that exceed 220degrees and open 25degrees ABDC (I think, I cannot remember, I lost my notes on the measurements I took on my D28M and D30M engines).

For all those who want to change the timing profile of a nitro engine, a long induction timing with the timing profiles of the sleeve to match as well as other engine designs all add to perfromance, the 210 induction, 180 exhaust, and 125 laterals alone will contribute the most to increase performace outputs. I would probably be more conservative as for the exhaust, maybe a 175degree duration; I would like to preserve the trapped compression ratio to help maintain a higher torque output. But I would definitely accept the other durations of the timing profile to better performance over the stock timing profile.
Old 12-09-2023, 02:27 PM
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Default Running a tuned pipe yes or no

Performance from port timing modifications does help all engines, but you can have higher port timing with an engine running a tuned pipe. I mostly fly free flight and we are not allowed to run a pipe. We still use the 180 degrees of exhaust timing as a max for our flying.
Old 12-10-2023, 06:10 AM
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Itís always interestingly funny when these old threads get resurrected because oftentimes thereís only certain parts mentioned. With buggy and truck engines, the liner port timing is a minute fraction of the equation. Quite honestly, more can be gained by properly setting the squish and boring & smoothing the crankshaft bore. Many times, just opening the bore of the crank and smoothing the port window will add more than enough extra. However, this is not a task to be taken lightly. Quality engines have extremely hard crankshafts.

The moral of the story is: engine modifications are a package deal. If one thing is modified, everything else has to be modified as well. Otherwise you can very easily cause more harm than good.

Aircraft engines do not benefit nearly as greatly as truck engines do because of the different circumstances the engines are operated.

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