Rotor/Crank Timing

[Steve Anderson]

.

 

[Kim J]

The largest induction timing on a model engine I know of, had a 360* duration.

It was a ***** to start, spitted a lot of fuel at low RPMs but would otherwise run decent.

Kim J

 

[Andy Brown]

The largest induction timing on a model engine I know of, had a 360* duration.

It was a ***** to start, spitted a lot of fuel at low RPMs but would otherwise run decent.

Kim J

Yep! I've seen that too! After several failed launch attempts and some head scratching the guy took his boat back to his pit area only to find the Zimmerman disc laying on the table where he had just reassembled his engine. :unsure:

 

[Kim J]

Yep! I've seen that too! After several failed launch attempts and some head scratching the guy took his boat back to his pit area only to find the Zimmerman disc laying on the table where he had just reassembled his engine. :unsure:

Andy

This was on a Rossi91 DF engine if my memory is correct. He had problems with the drum causing the drive pin to break.

He started making the drum in plastic - problem solved - and also once tried without the drum.

 

[Tim_Duggan]

Good engine simulation programs can answer a lot of these questions. They need actual dyno tests to confirm the findings, but have been pretty accurate. They take a lot of practice and a good understanding of engine concepts to learn.

Good point Lohring. Some big gains have been made thanks to utilising software. If you consider the cost of trial and error when cutting up parts to test, the software si cheap. Knowing how to use it is the hard part.

I've been fortunate to benefit from some of it first hand via a friend with MOTA. It's great for simulating some of these theories prior to cutting up parts, to decide if the hype is worth it. It is really easy to backwards PERFORMANCE WISE on these engines!

 

[Andy Brown]

Good engine simulation programs can answer a lot of these questions. They need actual dyno tests to confirm the findings, but have been pretty accurate. They take a lot of practice and a good understanding of engine concepts to learn.

Good point Lohring. Some big gains have been made thanks to utilising software. If you consider the cost of trial and error when cutting up parts to test, the software si cheap. Knowing how to use it is the hard part.

I've been fortunate to benefit from some of it first hand via a friend with MOTA. It's great for simulating some of these theories prior to cutting up parts, to decide if the hype is worth it. It is really easy to backwards PERFORMANCE WISE on these engines!

Engine software is great up to the point of known quantitative effects. Those "known quantitative effects" are, in most cases more than what 99.9% of the engine builders out there know. With that said Engine software is of great benefit to many. However, the engine software engineers have not thought of everything, so there will always be room for "testing hunches" and "cut & try".

I designed a thing called a MAC 45. It has an exhaust port about 2/3's the normal size. The guru's and software engineers say that power is directly related to exhaust port size and smaller means less power. In that case their software would have proven the MAC 45 to be an engine of moderate power.

In actuality the MAC 45 has proven to be one of the most powerful 45's ever produced.

 

[seaducerpete]

Good engine simulation programs can answer a lot of these questions. They need actual dyno tests to confirm the findings, but have been pretty accurate. They take a lot of practice and a good understanding of engine concepts to learn.

Good point Lohring. Some big gains have been made thanks to utilising software. If you consider the cost of trial and error when cutting up parts to test, the software si cheap. Knowing how to use it is the hard part.

I've been fortunate to benefit from some of it first hand via a friend with MOTA. It's great for simulating some of these theories prior to cutting up parts, to decide if the hype is worth it. It is really easy to backwards PERFORMANCE WISE on these engines!

Engine software is great up to the point of known quantitative effects. Those "known quantitative effects" are, in most cases more than what 99.9% of the engine builders out there know. With that said Engine software is of great benefit to many. However, the engine software engineers have not thought of everything, so there will always be room for "testing hunches" and "cut & try".

I designed a thing called a MAC 45. It has an exhaust port about 2/3's the normal size. The guru's and software engineers say that power is directly related to exhaust port size and smaller means less power. In that case their software would have proven the MAC 45 to be an engine of moderate power.

In actuality the MAC 45 has proven to be one of the most powerful 45's ever produced.

it shure was Andy Dick jones won d mono straightaways at the nats 4 or 5 years in a row

 

[Tom Foley]

Good engine simulation programs can answer a lot of these questions. They need actual dyno tests to confirm the findings, but have been pretty accurate. They take a lot of practice and a good understanding of engine concepts to learn.

Good point Lohring. Some big gains have been made thanks to utilising software. If you consider the cost of trial and error when cutting up parts to test, the software si cheap. Knowing how to use it is the hard part.

I've been fortunate to benefit from some of it first hand via a friend with MOTA. It's great for simulating some of these theories prior to cutting up parts, to decide if the hype is worth it. It is really easy to backwards PERFORMANCE WISE on these engines!

Engine software is great up to the point of known quantitative effects. Those "known quantitative effects" are, in most cases more than what 99.9% of the engine builders out there know. With that said Engine software is of great benefit to many. However, the engine software engineers have not thought of everything, so there will always be room for "testing hunches" and "cut & try".

I designed a thing called a MAC 45. It has an exhaust port about 2/3's the normal size. The guru's and software engineers say that power is directly related to exhaust port size and smaller means less power. In that case their software would have proven the MAC 45 to be an engine of moderate power.

In actuality the MAC 45 has proven to be one of the most powerful 45's ever produced.

The ,45 Mac I had was stupid fast . please replicate at your earliest opportunity !!

 

[Tim_Duggan]

Engine software is great up to the point of known quantitative effects. Those "known quantitative effects" are, in most cases more than what 99.9% of the engine builders out there know. With that said Engine software is of great benefit to many. However, the engine software engineers have not thought of everything, so there will always be room for "testing hunches" and "cut & try".

I designed a thing called a MAC 45. It has an exhaust port about 2/3's the normal size. The guru's and software engineers say that power is directly related to exhaust port size and smaller means less power. In that case their software would have proven the MAC 45 to be an engine of moderate power.

In actuality the MAC 45 has proven to be one of the most powerful 45's ever produced.

Andy,

I always found it ineresting where some guys would quote Jennings or Blair calculations, yet ignore software (which has the same equasions embedded in it, plus a bunch more)

The best performing MAC 45 I ever saw (it was a beast) had a full exhaust port and a billet piston with a longer skirt. Couldn't keep crankshafts alive in it though.

Very easy to pick a stock exhaust port MAC45 powered boat - they have that distict sound.

 

[Andy Brown]

Engine software is great up to the point of known quantitative effects. Those "known quantitative effects" are, in most cases more than what 99.9% of the engine builders out there know. With that said Engine software is of great benefit to many. However, the engine software engineers have not thought of everything, so there will always be room for "testing hunches" and "cut & try".

I designed a thing called a MAC 45. It has an exhaust port about 2/3's the normal size. The guru's and software engineers say that power is directly related to exhaust port size and smaller means less power. In that case their software would have proven the MAC 45 to be an engine of moderate power.

In actuality the MAC 45 has proven to be one of the most powerful 45's ever produced.

Andy,

I always found it ineresting where some guys would quote Jennings or Blair calculations, yet ignore software (which has the same equasions embedded in it, plus a bunch more)

The best performing MAC 45 I ever saw (it was a beast) had a full exhaust port and a billet piston with a longer skirt. Couldn't keep crankshafts alive in it though.

Very easy to pick a stock exhaust port MAC45 powered boat - they have that distict sound.

Tim,

We built the first batch of MAC 45's with a full piston skirt just in case the short exhaust port did not prove to work well. That way we could just mod the port and be good to go.

Anyway after some test time with both port layouts we found the short port to be the fastest.

The full port could run a shorter pipe (equal to what normally works on a Picco 45), but still did not produce the same top end as the short (stock) MAC exhaust port. The MAC 45 requires a longer length pipe because the port does not allow as much raw mixture to dump into the header & pipe. This causes the pipe to run hotter and therefore the longer length requirement for a given rpm range.

 

[Ian Inverarity]

Tim,

We built the first batch of MAC 45's with a full piston skirt just in case the short exhaust port did not prove to work well. That way we could just mod the port and be good to go.

Anyway after some test time with both port layouts we found the short port to be the fastest.

The full port could run a shorter pipe (equal to what normally works on a Picco 45), but still did not produce the same top end as the short (stock) MAC exhaust port. The MAC 45 requires a longer length pipe because the port does not allow as much raw mixture to dump into the header & pipe. This causes the pipe to run hotter and therefore the longer length requirement for a given rpm range.

Andy,

That's interesting, you had a demonstrable improvement from the MAC45 exhaust port design. Was the benefit specific to high nitro marine 45's or would it work on other engines? Was that style of exhaust tried on other engine sizes or was it considered for other MAC engines?

 

[Andy Brown]

Tim,

We built the first batch of MAC 45's with a full piston skirt just in case the short exhaust port did not prove to work well. That way we could just mod the port and be good to go.

Anyway after some test time with both port layouts we found the short port to be the fastest.

The full port could run a shorter pipe (equal to what normally works on a Picco 45), but still did not produce the same top end as the short (stock) MAC exhaust port. The MAC 45 requires a longer length pipe because the port does not allow as much raw mixture to dump into the header & pipe. This causes the pipe to run hotter and therefore the longer length requirement for a given rpm range.

Andy,

That's interesting, you had a demonstrable improvement from the MAC45 exhaust port design. Was the benefit specific to high nitro marine 45's or would it work on other engines? Was that style of exhaust tried on other engine sizes or was it considered for other MAC engines?

I wanted to build the MAC 21 with the same port design, but CMB would not do it. I built a MAC 67 P/L with the short exhaust port and made it a 5 port similar to the MAC 45, reshaped the 67 case port to match.

It ran 89 mph the first day out in our SG race boat which was 3-4 mph faster than the current MAC 67 at that time.

 

[lohring]

Programs are only as good as the assumptions they're based on. None of the low cost programs can simulate combustion or scavenging from an engines physical description. They mainly simulate the pulsating flow between the various parts of the engine. That's what makes reeds and tuned pipes work and is the one area the programs are very useful. Other than cut and try, there is no other way to design pipes. All the formulas out there on the web are based on empirical observations of what has been working.

There are big discussions on how to layout ports and ducts for best scavenging. Some computed fluid flow programs can simulate this, but the run time is very long. Combustion simulations have similar problems. Port angles, shapes, and duct shapes can all be investigated with mechanical simulations that can be incorporated into the simple programs. There is still a lot of room for experiment and investigation.

I used MOTA to design the 35cc SAW pipe for the CMB. The power predictions were off, but the best pipes in MOTA were the best pipes on the dyno. EngineMod2T is an even better simple program. One of the best things about any of these programs is that you can "test" hundreds of changes in the time it takes to build one pipe. By looking at the pressure waves throughout the engine you can get an idea of what is going on. The physical equipment to do this is way beyond my means.

Lohring Miller

 

[tnrcboatracer]

I wanted to build the MAC 21 with the same port design, but CMB would not do it. I built a MAC 67 P/L with the short exhaust port and made it a 5 port similar to the MAC 45, reshaped the 67 case port to match.

It ran 89 mph the first day out in our SG race boat which was 3-4 mph faster than the current MAC 67 at that time.

Wouldn't happen to have any of those old 67 short port sleeves laying around, would ya?

 

[Ian Inverarity]

I wanted to build the MAC 21 with the same port design, but CMB would not do it. I built a MAC 67 P/L with the short exhaust port and made it a 5 port similar to the MAC 45, reshaped the 67 case port to match.

It ran 89 mph the first day out in our SG race boat which was 3-4 mph faster than the current MAC 67 at that time.

Andy, interesting info indeed, thanks.

 

[Ian Inverarity]

Programs are only as good as the assumptions they're based on. None of the low cost programs can simulate combustion or scavenging from an engines physical description. They mainly simulate the pulsating flow between the various parts of the engine. That's what makes reeds and tuned pipes work and is the one area the programs are very useful. Other than cut and try, there is no other way to design pipes. All the formulas out there on the web are based on empirical observations of what has been working.

There are big discussions on how to layout ports and ducts for best scavenging. Some computed fluid flow programs can simulate this, but the run time is very long. Combustion simulations have similar problems. Port angles, shapes, and duct shapes can all be investigated with mechanical simulations that can be incorporated into the simple programs. There is still a lot of room for experiment and investigation.

I used MOTA to design the 35cc SAW pipe for the CMB. The power predictions were off, but the best pipes in MOTA were the best pipes on the dyno. EngineMod2T is an even better simple program. One of the best things about any of these programs is that you can "test" hundreds of changes in the time it takes to build one pipe. By looking at the pressure waves throughout the engine you can get an idea of what is going on. The physical equipment to do this is way beyond my means.

Lohring Miller

Lohring,

How have you found these simple programs for working on inlet, transfer and exhaust timings and port areas? You suggest EngMod2T is better than Mota, better in what ways, more representative results?

Ian.

 

[carlnoki]

Now, how would the intake timing being disgust here work on a 21 valvola?

Anyone?

thanks

Carl

 

[lohring]

EngineMod2T has the same core assumptions as MOTA. However, it is still being actively developed with input from some hard core two stroke tuners. it gives very accurate results for kart and motorcycle engines. It looks close to my dyno tests on 25cc gas engines. I understand it is being used for glow engines (it has inputs for shaft rotary valves), but I have no experience in that area. The key is accurate modeling of scavenging, combustion, and friction, as well as accurate engine dimension inputs. Small engines have relatively more friction than larger ones. All of these factors need a good guess of what is actually happening. I've fudged a lot of factors to try to match the programs to dyno results. EngineMod 2T needs less fudging to get close. It also has a time area measurement section, a crankshaft balance section, and the ability to model lots more variations than MOTA. Other valuable sections give head volume and squish velocity from head dimensions; crankcase compression ratio from easily measured volumes; port height measurements from timing complete with pictures; and a great but empirical pipe design program to use as a starting point.

Again, the biggest value of these programs is in pipe design, especially as related to port timing. You can see what various changes do to the pressures in the engine as well as the power. It's very educational. However the learning curve is steep. Expect to spend a lot of time to get up to speed. You can get an idea of what's involved for free at http://www.lesoft.co.uk/index1.html Look at the downloads section for the free program. It has a tutorial to get you started. It models both two and four stroke engines, but is limited and not particularly accurate for our size engines.

Lohring Miller

 

[Ian Inverarity]

EngineMod2T has the same core assumptions as MOTA. However, it is still being actively developed with input from some hard core two stroke tuners. it gives very accurate results for kart and motorcycle engines. It looks close to my dyno tests on 25cc gas engines. I understand it is being used for glow engines (it has inputs for shaft rotary valves), but I have no experience in that area. The key is accurate modeling of scavenging, combustion, and friction, as well as accurate engine dimension inputs. Small engines have relatively more friction than larger ones. All of these factors need a good guess of what is actually happening. I've fudged a lot of factors to try to match the programs to dyno results. EngineMod 2T needs less fudging to get close. It also has a time area measurement section, a crankshaft balance section, and the ability to model lots more variations than MOTA. Other valuable sections give head volume and squish velocity from head dimensions; crankcase compression ratio from easily measured volumes; port height measurements from timing complete with pictures; and a great but empirical pipe design program to use as a starting point.

Again, the biggest value of these programs is in pipe design, especially as related to port timing. You can see what various changes do to the pressures in the engine as well as the power. It's very educational. However the learning curve is steep. Expect to spend a lot of time to get up to speed. You can get an idea of what's involved for free at http://www.lesoft.co.uk/index1.html Look at the downloads section for the free program. It has a tutorial to get you started. It models both two and four stroke engines, but is limited and not particularly accurate for our size engines.

Lohring Miller

Lohring,

EngMod2t sounds like it will be worth trying, thanks for your opinions!

Ian.