Squish Band Width

[Ian Inverarity]

" It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation."

Marty,

For years I have been hearing you say that the small engines do not have any excees horsepower. I just wonder why you say that? Does any racing engine have "excess horsepower"? As you know, a good .21 will put out over 2.5 HP. That's nearly 12 HP per cubic inch. Does a CMB 101 put out 12 HP? I don't think so. Maybe what you are really saying is that our .21 boats are SLUGS....too heavy for the 2.5 HP engine. Lets see. A 4 pound .21. That's 1.6 pounds/HP.

Extrapolating from Jim's 6.5 HP .70 on 65% nitro, That's 9.3 HP/ CI. A single 90/101 boat would come in about 10 pounds. That's about 1.1 pounds/HP.

A 16 pound twin 101 comes in at a skinny .86 pounds per HP. Yet the 101 engine is putting out only 9.2 HP/CI, about 20% less HP/ci than the .21

So I guess that is what you are really saying is that the .21 boats are too heavy, because certainly the .21 engine are among the most powerful engines on earth. I have even heard of some 3 HP .21'S. Maybe you have too? That would ba a whopping 14HP/CI. If it was a 500CI Top Fuel engine that would be 7000 HP. Hmmmm sounds like a number I've heard from the Top Fuel guys.

Well looking at all of that, while I don't think any race engine has "excees horsepower", I do think the .21's produce the "most Excessive horsepower" for their size.

Guess we need to get to work on those 101'S. Those 18 horsepower twins should be 28 Horsepower twins! Yeah Baby!

Even with a 3 HP .21 engine the boat could only weigh 1.7 pounds to accelerate like that 28 HP TWIN BADBOY!!!

Interesting points there Andy, but some of the power to weight is simply related to volumes not scaling linearly. A 101 is 4.8 times the cubic capacity of a 21, but isnt even twice as tall. I weigh a nova 21 as 300g and a 101 at 1020g, both with collet, so the 101 is 4.8 times the capacity but only 3.4 times the weight. Altho if the HP is 2.5hp for the 21 and 7.5hp for the 101 the 21 still has the better power per pound, even with the greater weight per cubic capacity. But many of the other items in the boats are not scaled either, for example the radio receivers, switches, antenna tubes, 3rd channel needle assemblies, mixture and throttle servos and so on are the same size for the 21 as the 101!

I hadnt considered the power to weight disadvantage of 21 boats before tho, good point. B)

 

[Andy Brown]

What is causing this apparent drop in the HP/CI as the size of the engine keeps getting larger. Is it friction losses, fluid dynamic losses, poor induction design, poor carburetor design, poor tuned pipe design, poor transfer design, poor metallurgy, etc., etc. If a .90 size engine made the same HP/CI as a .21 size engine, you would think that 9.5 HP would be easily obtainable.

None of the .90 size engines I built ever exceeded 7.5 HP. To achieve this 80% nitro was required & the engine had to turn 30,000 RPM. A good .45 engine can make 4.5 HP which means there is something really wrong with the design of larger size engines, mine included.

Jim Allen

"None of the .90 size engines I built ever exceeded 7.5 HP. To achieve this 80% nitro was required & the engine had to turn 30,000 RPM. A good .45 engine can make 4.5 HP which means there is something really wrong with the design of larger size engines, mine included."

It could be a blessing that the progrees of getting more HP from the 90/100 engines has been so slow. A 30 HP twin could be down right dangerous (as if they already are not), especially with the radio equiptment we had availble 20-30 years ago.

 

[Andy Brown]

" It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation."

Marty,

For years I have been hearing you say that the small engines do not have any excees horsepower. I just wonder why you say that? Does any racing engine have "excess horsepower"? As you know, a good .21 will put out over 2.5 HP. That's nearly 12 HP per cubic inch. Does a CMB 101 put out 12 HP? I don't think so. Maybe what you are really saying is that our .21 boats are SLUGS....too heavy for the 2.5 HP engine. Lets see. A 4 pound .21. That's 1.6 pounds/HP.

Extrapolating from Jim's 6.5 HP .70 on 65% nitro, That's 9.3 HP/ CI. A single 90/101 boat would come in about 10 pounds. That's about 1.1 pounds/HP.

A 16 pound twin 101 comes in at a skinny .86 pounds per HP. Yet the 101 engine is putting out only 9.2 HP/CI, about 20% less HP/ci than the .21

So I guess that is what you are really saying is that the .21 boats are too heavy, because certainly the .21 engine are among the most powerful engines on earth. I have even heard of some 3 HP .21'S. Maybe you have too? That would ba a whopping 14HP/CI. If it was a 500CI Top Fuel engine that would be 7000 HP. Hmmmm sounds like a number I've heard from the Top Fuel guys.

Well looking at all of that, while I don't think any race engine has "excees horsepower", I do think the .21's produce the "most Excessive horsepower" for their size.

Guess we need to get to work on those 101'S. Those 18 horsepower twins should be 28 Horsepower twins! Yeah Baby!

Even with a 3 HP .21 engine the boat could only weigh 1.7 pounds to accelerate like that 28 HP TWIN BADBOY!!!

Interesting points there Andy, but some of the power to weight is simply related to volumes not scaling linearly. A 101 is 4.8 times the cubic capacity of a 21, but isnt even twice as tall. I weigh a nova 21 as 300g and a 101 at 1020g, both with collet, so the 101 is 4.8 times the capacity but only 3.4 times the weight. Altho if the HP is 2.5hp for the 21 and 7.5hp for the 101 the 21 still has the better power per pound, even with the greater weight per cubic capacity. But many of the other items in the boats are not scaled either, for example the radio receivers, switches, antenna tubes, 3rd channel needle assemblies, mixture and throttle servos and so on are the same size for the 21 as the 101!

I hadnt considered the power to weight disadvantage of 21 boats before tho, good point. B)

Yes Ian, those things like recievers and mixture valves and such are the same weight. The crank pin in a 21 is much more beefy than the pin in a 90 as are many other parts if you scale them accordingly. If in fact we were getting 14 HP out of a 101 they would need to be much heavier to survive. Otherwise we would be replaceing things like cranckshafts every 10 runs like the TOP FUEL guys do....at $4000 a pop!

 

[Marty Davis]

I am not sure about this at all (the assumption that nitro content makes little difference)..... I am curently testing 3 different head buttons with the exact same volume, just different percentage squish band percentage. I am POSITIVE that squish velocity is the controlling factor in excess detonation. It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation. The controls of squish velocity are in 2 classifications: The first are those items that affect SV tremendously with only a very small change and those that change the SV only a little bit with changes.

The first group is Squish Width, Squish Angle, Peak RPM, Head Clearance.

Those that fit into the second group are Compression Ratio, Exhaust Duration

It is really a balancing act with SV. If you get it too high, the engine will detonate, too low and the engine will not have good low end power.

" It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation."

Marty,

For years I have been hearing you say that the small engines do not have any excees horsepower. I just wonder why you say that? Does any racing engine have "excess horsepower"? As you know, a good .21 will put out over 2.5 HP. That's nearly 12 HP per cubic inch. Does a CMB 101 put out 12 HP? I don't think so. Maybe what you are really saying is that our .21 boats are SLUGS....too heavy for the 2.5 HP engine. Lets see. A 4 pound .21. That's 1.6 pounds/HP.

Extrapolating from Jim's 6.5 HP .70 on 65% nitro, That's 9.3 HP/ CI. A single 90/101 boat would come in about 10 pounds. That's about 1.1 pounds/HP.

A 16 pound twin 101 comes in at a skinny .86 pounds per HP. Yet the 101 engine is putting out only 9.2 HP/CI, about 20% less HP/ci than the .21

So I guess that is what you are really saying is that the .21 boats are too heavy, because certainly the .21 engine are among the most powerful engines on earth. I have even heard of some 3 HP .21'S. Maybe you have too? That would ba a whopping 14HP/CI. If it was a 500CI Top Fuel engine that would be 7000 HP. Hmmmm sounds like a number I've heard from the Top Fuel guys.

Well looking at all of that, while I don't think any race engine has "excees horsepower", I do think the .21's produce the "most Excessive horsepower" for their size.

Guess we need to get to work on those 101'S. Those 18 horsepower twins should be 28 Horsepower twins! Yeah Baby!

Even with a 3 HP .21 engine the boat could only weigh 1.7 pounds to accelerate like that 28 HP TWIN BADBOY!!!

Andy:

Your way of looking at this is exactly correct. My only problem is and has always been the HP to weight ratio on the .21 sized boats. Guess we need some different building methods and materials. I was able to get to 3.25lbs on my last boat and it was really great at acceleration, etc. My term "excess hp" is probably not the best way of saying what is happening, but it is my way of explaining that the .21 engines are working harder than any of the other engines because of the HP to weight ratio. On a .67 sized boat, the engine does not have to work very hard and it launches, accelerates, etc easily. On a .21 sized boat, the needle setting, prop, etc must be really well optimized - because of the HP to weight ratio. So, I guess that the statement that a .21 sized boat does not have any excess HP is really true it is working harder than any size that we run.

 

[Andy Brown]

I am not sure about this at all (the assumption that nitro content makes little difference)..... I am curently testing 3 different head buttons with the exact same volume, just different percentage squish band percentage. I am POSITIVE that squish velocity is the controlling factor in excess detonation. It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation. The controls of squish velocity are in 2 classifications: The first are those items that affect SV tremendously with only a very small change and those that change the SV only a little bit with changes.

The first group is Squish Width, Squish Angle, Peak RPM, Head Clearance.

Those that fit into the second group are Compression Ratio, Exhaust Duration

It is really a balancing act with SV. If you get it too high, the engine will detonate, too low and the engine will not have good low end power.

" It is especially bad in the small engines with no excess horsepower. You have to ask them to put out more than they want and the result is detonation."

Marty,

For years I have been hearing you say that the small engines do not have any excees horsepower. I just wonder why you say that? Does any racing engine have "excess horsepower"? As you know, a good .21 will put out over 2.5 HP. That's nearly 12 HP per cubic inch. Does a CMB 101 put out 12 HP? I don't think so. Maybe what you are really saying is that our .21 boats are SLUGS....too heavy for the 2.5 HP engine. Lets see. A 4 pound .21. That's 1.6 pounds/HP.

Extrapolating from Jim's 6.5 HP .70 on 65% nitro, That's 9.3 HP/ CI. A single 90/101 boat would come in about 10 pounds. That's about 1.1 pounds/HP.

A 16 pound twin 101 comes in at a skinny .86 pounds per HP. Yet the 101 engine is putting out only 9.2 HP/CI, about 20% less HP/ci than the .21

So I guess that is what you are really saying is that the .21 boats are too heavy, because certainly the .21 engine are among the most powerful engines on earth. I have even heard of some 3 HP .21'S. Maybe you have too? That would ba a whopping 14HP/CI. If it was a 500CI Top Fuel engine that would be 7000 HP. Hmmmm sounds like a number I've heard from the Top Fuel guys.

Well looking at all of that, while I don't think any race engine has "excees horsepower", I do think the .21's produce the "most Excessive horsepower" for their size.

Guess we need to get to work on those 101'S. Those 18 horsepower twins should be 28 Horsepower twins! Yeah Baby!

Even with a 3 HP .21 engine the boat could only weigh 1.7 pounds to accelerate like that 28 HP TWIN BADBOY!!!

Andy:

Your way of looking at this is exactly correct. My only problem is and has always been the HP to weight ratio on the .21 sized boats. Guess we need some different building methods and materials. I was able to get to 3.25lbs on my last boat and it was really great at acceleration, etc. My term "excess hp" is probably not the best way of saying what is happening, but it is my way of explaining that the .21 engines are working harder than any of the other engines because of the HP to weight ratio. On a .67 sized boat, the engine does not have to work very hard and it launches, accelerates, etc easily. On a .21 sized boat, the needle setting, prop, etc must be really well optimized - because of the HP to weight ratio. So, I guess that the statement that a .21 sized boat does not have any excess HP is really true it is working harder than any size that we run.

Yes Marty! Or said another way, Our .21 boats are pigs that need to go on a diet! LOL

 

[Ian Inverarity]

Yes Ian, those things like recievers and mixture valves and such are the same weight. The crank pin in a 21 is much more beefy than the pin in a 90 as are many other parts if you scale them accordingly. If in fact we were getting 14 HP out of a 101 they would need to be much heavier to survive. Otherwise we would be replaceing things like cranckshafts every 10 runs like the TOP FUEL guys do....at $4000 a pop!

Andy, maybe we need multi cylinder engines to get the power up in larger capacity engines, altho the weight, cost, size, internal losses would also rise.

 

[Andy Brown]

Yes Ian the costs would go up. There could be better power from a multi but we don't need to go there yet. Plus to keep the weight down the materials would need to be more exotic. A six cylinder 1.0" would cost 5K or it would have to be a slug. There is more to be had from the big single cylinder. Just a matter of doing the right things with it....

 

[lohring]

A very simple scaling analysis explanes why small engines produce more power per cubic inch. If you assume that power is limited by breathing, the available cylinder wall area available for ports increases as the square of an engines dimensions. The cylinder volume to be fed by the ports increases as the cube of the dimensions. A 3.5 cc engine is a 1.6 scale version of a 15 cc engine, that is the 15 cc engine will have a bore and stroke that are about 1.6 times as big as the 3.5 cc engine. The displacement is 4.3 times as much so you would expect that the power would be 4.3 times as much at the same rpm. However, the ports of a 15 cc engine are only 2.6 times the area of the 3.5 cc engine. Since the 15 cc engine has more than 2.6 times the power, the little engine is suffering from some of the other losses that small scale brings. A high cylinder surface area to volume may help the port area, but it also means greater friction and heat loss for the displacement.

Lohring Miller

PS The figures are rounded to one decimal place for readability. They will seem to be somewhat off because squares and cubes are involved.

 

[Ian Inverarity]

Good analysis Lohring, that is pretty much why I was thinking about multi cylinder engines.

 

[Luca Gray]

delete

 

[aaron alberico]

Andy

This is why 21 rigger is my best class!!! nothing beats rocking up to a race and my 21 rigger

is radared as quick as the 90/101 twins, no better feeling B)

Aaron

 

[Marty Davis]

A little more information on the Toroidal head button shape. I indicated the information in the thread about this type head to Brian Callahan and he did some investigation. Finally wrote back and this is his reply:

"How has it been going with the toroidal combustion chambers? I finally saw an answer from a reputable source about the best edge condition of the bowl lip. Sharp. Jan Thiel on the Aprilia RSA125 did a series of tests trying radiuses everywhere from sharp to 5 mm. The answer was crystal clear: sharp is best.

They also run zero squish angle. The idea is to not minimize the amount of any fresh charge push its way into the squish area. So, when it does detonate, it doesn't have a lot of energy in it. That effectively lets you raise the compression ratio that much more. Remember you will need to choose a shorter pipe as the compression ratio goes up to soften the "brick wall" at high rpm.

BC"

 

[Jack ODonnell]

A little more information on the Toroidal head button shape. I indicated the information in the thread about this type head to Brian Callahan and he did some investigation. Finally wrote back and this is his reply:

"How has it been going with the toroidal combustion chambers? I finally saw an answer from a reputable source about the best edge condition of the bowl lip. Sharp. Jan Thiel on the Aprilia RSA125 did a series of tests trying radiuses everywhere from sharp to 5 mm. The answer was crystal clear: sharp is best.

They also run zero squish angle. The idea is to not minimize the amount of any fresh charge push its way into the squish area. So, when it does detonate, it doesn't have a lot of energy in it. That effectively lets you raise the compression ratio that much more. Remember you will need to choose a shorter pipe as the compression ratio goes up to soften the "brick wall" at high rpm.

BC Hi marty, Can you explain more on ( the need to choose a shorter pipe as com. ratio goes up ) J.O'Donnell

 

[Marty Davis]

A little more information on the Toroidal head button shape. I indicated the information in the thread about this type head to Brian Callahan and he did some investigation. Finally wrote back and this is his reply:

"How has it been going with the toroidal combustion chambers? I finally saw an answer from a reputable source about the best edge condition of the bowl lip. Sharp. Jan Thiel on the Aprilia RSA125 did a series of tests trying radiuses everywhere from sharp to 5 mm. The answer was crystal clear: sharp is best.

They also run zero squish angle. The idea is to not minimize the amount of any fresh charge push its way into the squish area. So, when it does detonate, it doesn't have a lot of energy in it. That effectively lets you raise the compression ratio that much more. Remember you will need to choose a shorter pipe as the compression ratio goes up to soften the "brick wall" at high rpm.

BC Hi marty, Can you explain more on ( the need to choose a shorter pipe as com. ratio goes up ) J.O'Donnell

Jack:

Here is what Brian said in this regard:

"Higher compression ratio also means higher expansion ratio. Peak gas temperature in the cylinder doesn't change much because that is more a function of combustion instead of compression. A higher expansion ratio means the gas has expanded more and will be colder by the time it is released into the exhaust pipe, lowering EGT. That makes the pipe act longer and will chop off the power at high rpm. So, increasing compression ratio must be accompanied by a shorter pipe."

Marty

 

[Jim Allen]

Marty,

This information is dead on! Jan Thiel, Frits Overmars, & others have had many in depth discussions about how to set the compression ratio, deck clearances, squish area ratio, plug cooling, coolant head temperature, exhaust gas temperature, squish velocity, how the flame front reaches the squish curtain, tuned pipe design, port geometry, spark plug modifications, etc., etc.

So far the already tested theories work both in gas & nitro engines!

Jim Allen