head buttons

[steve wood]

i've been rereading the 2 stroke tuners handbook. again. have been studying the part about squish and looking at the piston with an angle that matches the angle on the head. i see the new nova has this type of set up and wondering if anyone here has tryed to do this to one of the older novas. seems to me this would be the best of both worlds, tight squish for power and the angle to aim the fuel at the plug a little. this could be done on current engines if you lower the liner to compensate for the angle.

 

[DaveMarles]

i used to us a flat squish band.. had TONS of detitnation.. i have since switched to a tapered with better results.. what i cannot figure out is some mfg switch back and forth between bowl shapes... and at what point is the squishband essentially none effective.
chris

Chris , when you changed the head did you keep to the same squish clearance at the narrowest point? Did you measure the squish at the narrowest point and at the edge of the bowl?

Dave

 

[Jerry Wyss]

The data in the Jennings book deals mostly with gas two stroke's where excessive heat in the piston

crown area is a real problem. That squish area is designed (as you know) to trap a layer of unburned

air/fuel/lube to keep it cool. Using the fuel that we do, with such low volumes (in relation) that problem

is not as apparent.

I was taught (Tim) that detonation was not such a bad thing if reached only briefly. It is an indicator

that your really close to the engines full potential. Does the angled squish keep detonation at bay?,

it appears so,, so will just merely making it richer, increasing the bubble volume and also increasing

headspace.

I've cut an angled squish on the Mac O/B button (.18cc stock) to .16cc without problem. It was easier

on the plugs, engine ran well in hot humid weather and no detonation.

I think once the engine has detonated, a small volume adjustment, partial squish, .01cc inrease,

.002" more headshim or just cutting the water a little more while richening could put you on the spot.

I think many engine manufacturers use the angle squish in they're motors for the use of lower nitro,

they all seem to come with more than enough headshim in them.

Totally doing away with the flat squish just seems a little extreme to me,, but I could be wrong too

Jerry

 

[steve wood]

i used to us a flat squish band.. had TONS of detitnation.. i have since switched to a tapered with better results.. what i cannot figure out is some mfg switch back and forth between bowl shapes... and at what point is the squishband essentially none effective.
chris

Chris , when you changed the head did you keep to the same squish clearance at the narrowest point? Did you measure the squish at the narrowest point and at the edge of the bowl?

Dave

we set the squish at the narrowest point. i actually seen a head that some one had "modded" with the angle running the wrong way, like chris has mispoken about. the guy couldn't figure why his motor wouldn't run right!

 

[Marty Davis]

Chris:

The charge that is under the squish band really doesn't burn completely. The flame front starts at the plug and burns outward in all directions from that point. The squish band is primarily used to dissipate heat and create a "squish velocity" moving the charge from under the squish band toward the center of the chamber. The closer the clearance of the squish band and piston, the more it can transfer heat from the piston to the squish band (head button) and then to the water cooling in the head. The raw charge transfers and cools the squish band and the piston and directs the heat to the water cooling. The closer the clearance the better (limited by rod, crank, piston pin slop). The detonation that you get, is as a result of the clearance being too tight and the compression ratio to high and is measured by the excess squish velocity (above about 70 m/sec). The squish velocity calculations will be in the new EAP and will help you determine the range that you will want to be in. I have found that you will get considerable detonation at 70 m/s but also great squish performance. If you get much below about 55-60 m/s you will not have good performance from your head button. About 65 m/s seems to be about optimum.

The squish clearance is VERY important to be kept close to help with cooling.

Hope this helps some....

We still do not know the penalty that you pay (if any) by adding an angled squish band (AND keeping the same compression ratio). Also, we do not know the optimum squish angle. Also, we do no know if the angled squish band takes away too much performance from a flat squish band. Hopefully, we will know some of these answers soon with the tests on the dyno that are ready to go.

I would be interested in the findings of Dave Marles in this area, if he is willing to share.

Marty Davis

 

[DaveMarles]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high.

I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

 

[Marty Davis]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave:

Send me your e-mail address and I will send you a Beta of the Squish Velocity portion of the New EAP for your testing.

Marty Davis

 

[Marty Davis]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave:

I am in EXACTLY the same position as you are with not knowing the best configuration of the combustion chamber and the squish band. With the ability of Brian Callahan to use his company's CNC machinery, I was able to design a series of heads with the exact same volume and different configurations. I have wanted to know what is the optimum for MANY years and finally I should be able to find out. I will not know the plug height questions at this point, but maybe sometime int he future for that. I have no idea whether that is important for our use. Some of the control line competitors say that it is important to them for re-starting reasons. Not really affects us.

I finally took an entry level AutoCAD class which allows me to design head shapes with exactly the same volume, as well as many other uses for the drawing program. I would encourage everyone who is really into model boat racing to do the same. Find the course at your local university, it will be some of the best time that you will spend on your hobby!

Marty Davis

 

[Jack ODonnell]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

 

[Jerry Wyss]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Hi Dave, I just wonder how close the two stroke gas engine data is going to be useful for our nitro motors??

There has to be a significant difference between the events of the two different fuels and mode of ignition

that could make the data far off from what "we" need,, anyone, what'cha thinkin'?

JW

 

[DaveMarles]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Hi Dave, I just wonder how close the two stroke gas engine data is going to be useful for our nitro motors??

There has to be a significant difference between the events of the two different fuels and mode of ignition

that could make the data far off from what "we" need,, anyone, what'cha thinkin'?

JW

Jerry , What other data do we have to go on? The basics are the same, compression, ignition, powerstroke. The 3 main differences are fuel composition, ignition and engine size. I would think that ignition has to be the biggest difference because the ignition point on a nitro motor is variable with load on the motor, needle setting, pipe length and temperature.

Dave

 

[DaveMarles]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

 

[moparbarn]

dave, could it be because, with less squish %, you had less trapped (unburnt) fuel? burning more of the fuel in the cylinder could account for the difference.

 

[Jack ODonnell]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

Dave; I have found the same thing as you on head volume. The bigger i can make the head volume and still get the engine on the pipe, the more top rpm`s/overrev the engine has. And i have read studies on tuned pipes that say a properly tuned pipe will most often require a bigger head volume. I also found that the ABC piston/sleeve fit has a big influence on head volume. Jack.

 

[Chris Wood]

Dave and Jack

As i tend to agree with both of you on the head volume.. we made some heads several yrs ago for our 80 motors.. with bigger volumes both in depth and width and kept the same or relatively close compression ratio.. but the motors spooled up MUCH better with the smaller volume?? what would cause that..

also, we made all our bowl a HEMI shape on the 80 motors.. we made a BATHTUB shape like what used to come in the 67 motors and it didn't like it at all.. any thoughts or comments.

very good topic.. wish more where like this.

 

[Marty Davis]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

Dave; I have found the same thing as you on head volume. The bigger i can make the head volume and still get the engine on the pipe, the more top rpm`s/overrev the engine has. And i have read studies on tuned pipes that say a properly tuned pipe will most often require a bigger head volume. I also found that the ABC piston/sleeve fit has a big influence on head volume. Jack.

Jack and Dave:

Think of it this way: The higher the compression ratio the more difficult that the engine will have to squish the trapped charge as the piston comes up. That is kind of like putting on the brakes for the rotating assembly. The less compression ratio, the less difficulty the piston and the rest of the rotating components have in squishing the trapped charge. The same thing with the squish band. The more area, the harder it is to squish the trapped charge and the more of a brake is put on the rotating mass. The trade off is that you will not have nearly as much torque across the band which will make you have to use rpm with smaller props. Both ways will work for sure. As you said Dave, with 8.5:1 you get great rpm but suffer in torque. You have found that you want the rpm and can live without the torque. For Jack, as long as his teather car can have enough torque to get on pipe, the less squish band area and the less compression ratio, the greater the potential rpm. For Jack's use, and going over 200 mph, I would think that the head would be the most important part of the puzzle, other than the pipe design.

For SAW, the lower the CR and the less squish area you can get away with the better if you are looking for HIGH RPM. Exactly the same as for Jack's Teather Cars.

Marty Davis

 

[Charles Perdue]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

Dave; I have found the same thing as you on head volume. The bigger i can make the head volume and still get the engine on the pipe, the more top rpm`s/overrev the engine has. And i have read studies on tuned pipes that say a properly tuned pipe will most often require a bigger head volume. I also found that the ABC piston/sleeve fit has a big influence on head volume. Jack.

Jack and Dave:

Think of it this way: The higher the compression ratio the more difficult that the engine will have to squish the trapped charge as the piston comes up. That is kind of like putting on the brakes for the rotating assembly. The less compression ratio, the less difficulty the piston and the rest of the rotating components have in squishing the trapped charge. The same thing with the squish band. The more area, the harder it is to squish the trapped charge and the more of a brake is put on the rotating mass. The trade off is that you will not have nearly as much torque across the band which will make you have to use rpm with smaller props. Both ways will work for sure. As you said Dave, with 8.5:1 you get great rpm but suffer in torque. You have found that you want the rpm and can live without the torque. For Jack, as long as his teather car can have enough torque to get on pipe, the less squish band area and the less compression ratio, the greater the potential rpm. For Jack's use, and going over 200 mph, I would think that the head would be the most important part of the puzzle, other than the pipe design.

For SAW, the lower the CR and the less squish area you can get away with the better if you are looking for HIGH RPM. Exactly the same as for Jack's Teather Cars.

Marty Davis

One other thing that you have to consider is that we are running approximately 6 to 1 air to fuel ratio with a compression ratio at approximately 10 to 1, the fuel/air mixture in the combustion chamber at top dead center, except for the fact that we are not filling the cylinder to 100%, is almost compressed back to a liquid or am I completely off base?

 

[Marty Davis]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

Dave; I have found the same thing as you on head volume. The bigger i can make the head volume and still get the engine on the pipe, the more top rpm`s/overrev the engine has. And i have read studies on tuned pipes that say a properly tuned pipe will most often require a bigger head volume. I also found that the ABC piston/sleeve fit has a big influence on head volume. Jack.

Jack and Dave:

Think of it this way: The higher the compression ratio the more difficult that the engine will have to squish the trapped charge as the piston comes up. That is kind of like putting on the brakes for the rotating assembly. The less compression ratio, the less difficulty the piston and the rest of the rotating components have in squishing the trapped charge. The same thing with the squish band. The more area, the harder it is to squish the trapped charge and the more of a brake is put on the rotating mass. The trade off is that you will not have nearly as much torque across the band which will make you have to use rpm with smaller props. Both ways will work for sure. As you said Dave, with 8.5:1 you get great rpm but suffer in torque. You have found that you want the rpm and can live without the torque. For Jack, as long as his teather car can have enough torque to get on pipe, the less squish band area and the less compression ratio, the greater the potential rpm. For Jack's use, and going over 200 mph, I would think that the head would be the most important part of the puzzle, other than the pipe design.

For SAW, the lower the CR and the less squish area you can get away with the better if you are looking for HIGH RPM. Exactly the same as for Jack's Teather Cars.

Marty Davis

One other thing that you have to consider is that we are running approximately 6 to 1 air to fuel ratio with a compression ratio at approximately 10 to 1, the fuel/air mixture in the combustion chamber at top dead center, except for the fact that we are not filling the cylinder to 100%, is almost compressed back to a liquid or am I completely off base?

Charles:

You might be about right on that. I know that the mixture is VERY rich in volume with high nitro fuels.

AND, that makes even harder to compress.

Marty Davis

 

[dave roach]

Has any one tryed two plugs in a 21 and what would the out come be.I beleave this would help the fuel to burn more even.Just thinking out loud.

Dave Roach

 

[Jack ODonnell]

Marty, I wish I knew the answers on head design. I'm running with a very small angle squish with a 1mm flat on on the outside edge so that I know when I'm measuring the squish clearance correctly Your MSV at 60 to 70 metres per sec seems incredibly high. You sure thats correct? My program caculates MSV on my 3.5 at 34,000 rpm with 0.3mm squish and just under .19 chamber at just over 27 m/s which I thought was a bit high. I always stick with the same squish clearance on all of my nitro engines which is as close to 0.35mm as possible but have played a lot with squish band width which is very important for allowing the motor to rev out or not as the case may be.

Here is a link to an interesting article written by Neils Vannik a well respected tuner and two stroke software designer. Heads

Dave

Dave; Could you explain with a little more detail on the squish band width,which is very inportant for allowing the motor to rev out or not as the case may be. Jack

Jack, I just extracted some testing info from my files but firstly as I said I don't know the answers on head design and I can say nothing definitive except what works for me. I found in general that when I had a session of serious testing on the 3.5 to extract more performance, I could increase the compression ratio by various means and get improved throttling, pulling away from the shore and a shade bigger on the prop but actual lap times did not fall because top speed was down which I put down to less revving past peak hp. ie overrev. My files show my motor running with 8.5:1 trapped CR, squish 0.4mm and squish of 60%. This was my good race motor which was fast and easy to use but I'm always looking for better. I reduced the squish to 0.2mm which increased the compression ratio to 10:1 and the motor was instantly better out of the corners and better acceleration but top rpm on the water was down by close to 800 rpm. After a lot of playing around I found I could run the same 10:1 with .2mm squish and maintain rpm if I used a head with reduced squish area of around 45%. I don't know the reason for that, only the result.

Dave

Dave; I have found the same thing as you on head volume. The bigger i can make the head volume and still get the engine on the pipe, the more top rpm`s/overrev the engine has. And i have read studies on tuned pipes that say a properly tuned pipe will most often require a bigger head volume. I also found that the ABC piston/sleeve fit has a big influence on head volume. Jack.

Jack and Dave:

Think of it this way: The higher the compression ratio the more difficult that the engine will have to squish the trapped charge as the piston comes up. That is kind of like putting on the brakes for the rotating assembly. The less compression ratio, the less difficulty the piston and the rest of the rotating components have in squishing the trapped charge. The same thing with the squish band. The more area, the harder it is to squish the trapped charge and the more of a brake is put on the rotating mass. The trade off is that you will not have nearly as much torque across the band which will make you have to use rpm with smaller props. Both ways will work for sure. As you said Dave, with 8.5:1 you get great rpm but suffer in torque. You have found that you want the rpm and can live without the torque. For Jack, as long as his teather car can have enough torque to get on pipe, the less squish band area and the less compression ratio, the greater the potential rpm. For Jack's use, and going over 200 mph, I would think that the head would be the most important part of the puzzle, other than the pipe design.

For SAW, the lower the CR and the less squish area you can get away with the better if you are looking for HIGH RPM. Exactly the same as for Jack's Teather Cars.

Marty Davis

Marty; I have to think about your idea of putting the brakes on the rotating assembly, you might have something there. I have made a lot of different heads,squish band and non squish, plugs in different locations. For me it seems to be the total volume that makes the differance. Every thing i have read,say the squish aera is to be as tight as possible. I can understand this on a fixed spark ignition. But on a glow plug there is no way of positively knowing the exact time of the ignition. For me its piston fit,pipe,head volume in that order. Boats or cars. Jack