1989 Blaster Engine Rebuild - Part 10, Two-Stroke Software Review

1989 Blaster Engine Rebuild - Part 10

Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10 - Part 11 - Part 12 - Part 13 - Part 14

Refinement

Two-Stroke Software Review

Part 1

Refinement
It became clear to me after running some numbers through the two-stroke software I'm evaluating for the other part of this article that I could obtain the target HP value I was shooting for by using a combination of a 1 mm thick aluminum base plate spacer and its required additional base gasket. This raises the barrel a total of 1.5 mm. Not only would it let the transfer ports become fully uncovered and increasing their time area, it would allow the piston edge to be completely below the exhaust port when it was at BDC. I never like it when a piston is always exposed to the hot exhaust gasses - never having a chance to get out of the hot exhaust stream to cool off like it should. It seems that even a small adjustment here could mean the difference between detonating and not. Designing the engine this way requires milling the deck of the barrel to compensate for the base plate spacer setup. Decking was already going to be done since I previously discovered the squish thickness was far too thick for good throttle response but now I can calculate exactly how much to remove because I know what the final port timing specs are going to be. With the cylinder raised to its final position the exhaust port now opens at 88.5° ATDC for a duration of 183°. The transfer ports now open at 116.5° ATDC for a duration of 127°.
Head & Barrel Work

Since there is a small difference in the lower deck height of all engines it is ideal to have the lower end that accompanies the correct top end while performing these types of modifications - in other words the whole engine. This particular engine also had its lower deck lapped - a slight removal of material, so calculating the deck height figures from this assembled engine can be taken right to the absolute precise limit. There are those who may only be able to cut a specific "safe" amount of material from the surface of a cylinder or head because the correct block was not used to measure it from. In cases like this it is common to find a wide margin of safety (as much as .010" to .020" or more) to prevent a condition which produces too much compression (detonation) and possibly disaster for its owner. It could be off in the other direction as well - though the engine may not melt down because of the loose specifications it probably won't add much in the power department.

Originally the engine had a deck height of -.63 mm (.63 mm below the deck), add to this the amount the cylinder was raised when the base plate spacer and additional gasket was added, and the piston edge is even further from the deck at -2.13 mm (.63 + 1.0 + .5 = 2.13). On top of this the copper head gasket that will be used is .82 mm thick. The head will be cut flush with the edge of the squish band - there will be no step up or down. Knowing this I can deck the cylinder so the piston will be right at the correct height for the squish band thickness I'm shooting for - in this case the target thickness is 1.1 mm. To give myself a small amount of room to work with and to leave some extra material that I'll remove during the lapping process I cut 1.75 mm from the top of the cylinder surface. This changed the -2.13 mm deck height to -.38 mm. When I add the .82 (.80 installed) head gasket thickness to the deck height (.80 + .38 = 1.18 mm) the result is a little on the thick side. This is just fine - that .08 mm amounts to .002", I'll remove most of that during lapping.

This combustion chamber was designed using MSV (maximum squish velocity) technology - it is a good way to design a combustion chamber for a two-stroke engine. It's velocity is measured in meters per second (m/s). When using a combustion chamber shape like the one pictured above or the actual one below, the software can be a wonderful tool. The software allows entering different dimensions into it so I can see what the result will be before I make a cut. It is necessary to enter the bore and stroke, connecting rod center to center length, exhaust port timing and the targeted peak horse power RPM. From these parameters a head can be designed. For this engine I've selected a central dome - hemisphere design - that is to say the combustion chamber will be centered directly over the piston crown with no offset to the front or back.

The squish area ratio is defined as the amount of the bore that's being squished - as a percentage of its area. A ratio of .5 would be 50% of the bore area. I designed this head using .55 as the ratio (55%). The squish area is cut at 11° for a width of 10.75 mm. That's about one degree more than the piston crown - actually the piston crown has a radius on its top, but for this project I averaged it out at 9.5 to 10°. I wanted to put an off-the-shelf piston in this engine so I made no fancy cuts to its top or anywhere else for that matter. I could have radiuses the squish area instead of cutting an angle there but this will be fine - the angle opens up slightly toward the center of the piston crown to help prevent detonation. This design leaves an area in the center that's 45 mm wide to hold most of the mixture. A radius of 24.3 mm is used on the inside of the combustion chamber dome. A radius of 4.5 mm blends the outside squish area to the inner bowl.

This is the way it looked after coming off the lathe. Nothing fancy here - just simple machine work.

A closer look reveals its smoothness and some small pits that were found once the aluminum was cut.

You'll have to go back to Part 7 to compare this shot to the picture of the head the way it started. It used to hold much more fluid than it does now. It's volume has been reduced considerably.

Recently I used the following statement to describe the Blasters personality when exchanging email with an online friend.

The power delivery of the stock Blaster could be described as being "impaired." Add an after market tuned pipe, and its rating moves up to "fatigued." It's power band is so flat it could be considered "flush." Yamaha describes the power delivery as being "tractable", what they seem to mean by it is "obedient."

That may be a little harsh but I like these engines - I want to improve it. The head received a lot of consideration during this project. I wanted to get rid of the lazy-revving feel the stock engine exhibited. Properly designing a head is a great way to accomplish this. I designed it toward the maximum safe squish velocity to help the engine pull the heavy-ish Blaster around (29.2 m/s - meters per second). A (s)lower squish velocity would have worked for other power delivery characteristics - such as tuning for power higher up the RPM range.

When compared to the second photo on the page of the Part 7 page it's easy to see how much material was removed to be able to achieve this kind of design. Several millimeters of material had to be skimmed from the surface in order to be able to make the shape I wanted. In fact the previously raised surface is now recessed. I had to trim my flat plastic plate to fit within the recessed area in order to CC it when it was done. I was not able to lap the head surface after coming off the lathe. I would have had to cut material across the complete head surface in order to be able to use it against my lapping slab or glass. I felt the head would be better off keeping the material for strength.

Top Volume
I relocated TDC using a piston stop and sealed the piston with a small amount of grease - I wouldn't want any fluid to leak past the piston when I checked the volume. I put the head on the engine with the head gasket in place and proceeded to pour in fluid from the graduated burette - its volume was now 17 cc. The small amount of grease used to seal the piston during this procedure amounts to a few tenths of a cc - this reduces the compression ratio slightly but I won't consider it here since it is such a small reduction and its in the direction of reducing the compression ratio, it won't matter. I continued on to calculate the new compression ratio (198 + 17 = 215 cc, 215 / 17 = 12.65:1). This new full stroke (uncorrected) compression ratio along with new port timing gives the engine a trapped (corrected) compression ratio of 7.5:1 (up from 6.11:1). It's estimated static compression is predicted to be 160 PSI at sea level. The octane required to run this engine is still within the range of what I can buy at the pump - 92.

1989 Blaster Engine Rebuild - Part 10

Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10 - Part 11 - Part 12 - Part 13 - Part 14

Refinement

Two-Stroke Software Review

Part 1

Refinement It became clear to me after running some numbers through the two-stroke software I'm evaluating for the other part of this article that I could obtain the target HP value I was shooting for by using a combination of a 1 mm thick aluminum base plate spacer and its required additional base gasket. This raises the barrel a total of 1.5 mm. Not only would it let the transfer ports become fully uncovered and increasing their time area, it would allow the piston edge to be completely below the exhaust port when it was at BDC. I never like it when a piston is always exposed to the hot exhaust gasses - never having a chance to get out of the hot exhaust stream to cool off like it should. It seems that even a small adjustment here could mean the difference between detonating and not. Designing the engine this way requires milling the deck of the barrel to compensate for the base plate spacer setup. Decking was already going to be done since I previously discovered the squish thickness was far too thick for good throttle response but now I can calculate exactly how much to remove because I know what the final port timing specs are going to be. With the cylinder raised to its final position the exhaust port now opens at 88.5° ATDC for a duration of 183°. The transfer ports now open at 116.5° ATDC for a duration of 127°. Head & Barrel Work Since there is a small difference in the lower deck height of all engines it is ideal to have the lower end that accompanies the correct top end while performing these types of modifications - in other words the whole engine. This particular engine also had its lower deck lapped - a slight removal of material, so calculating the deck height figures from this assembled engine can be taken right to the absolute precise limit. There are those who may only be able to cut a specific "safe" amount of material from the surface of a cylinder or head because the correct block was not used to measure it from. In cases like this it is common to find a wide margin of safety (as much as .010" to .020" or more) to prevent a condition which produces too much compression (detonation) and possibly disaster for its owner. It could be off in the other direction as well - though the engine may not melt down because of the loose specifications it probably won't add much in the power department. Originally the engine had a deck height of -.63 mm (.63 mm below the deck), add to this the amount the cylinder was raised when the base plate spacer and additional gasket was added, and the piston edge is even further from the deck at -2.13 mm (.63 + 1.0 + .5 = 2.13). On top of this the copper head gasket that will be used is .82 mm thick. The head will be cut flush with the edge of the squish band - there will be no step up or down. Knowing this I can deck the cylinder so the piston will be right at the correct height for the squish band thickness I'm shooting for - in this case the target thickness is 1.1 mm. To give myself a small amount of room to work with and to leave some extra material that I'll remove during the lapping process I cut 1.75 mm from the top of the cylinder surface. This changed the -2.13 mm deck height to -.38 mm. When I add the .82 (.80 installed) head gasket thickness to the deck height (.80 + .38 = 1.18 mm) the result is a little on the thick side. This is just fine - that .08 mm amounts to .002", I'll remove most of that during lapping. This combustion chamber was designed using MSV (maximum squish velocity) technology - it is a good way to design a combustion chamber for a two-stroke engine. It's velocity is measured in meters per second (m/s). When using a combustion chamber shape like the one pictured above or the actual one below, the software can be a wonderful tool. The software allows entering different dimensions into it so I can see what the result will be before I make a cut. It is necessary to enter the bore and stroke, connecting rod center to center length, exhaust port timing and the targeted peak horse power RPM. From these parameters a head can be designed. For this engine I've selected a central dome - hemisphere design - that is to say the combustion chamber will be centered directly over the piston crown with no offset to the front or back. The squish area ratio is defined as the amount of the bore that's being squished - as a percentage of its area. A ratio of .5 would be 50% of the bore area. I designed this head using .55 as the ratio (55%). The squish area is cut at 11° for a width of 10.75 mm. That's about one degree more than the piston crown - actually the piston crown has a radius on its top, but for this project I averaged it out at 9.5 to 10°. I wanted to put an off-the-shelf piston in this engine so I made no fancy cuts to its top or anywhere else for that matter. I could have radiuses the squish area instead of cutting an angle there but this will be fine - the angle opens up slightly toward the center of the piston crown to help prevent detonation. This design leaves an area in the center that's 45 mm wide to hold most of the mixture. A radius of 24.3 mm is used on the inside of the combustion chamber dome. A radius of 4.5 mm blends the outside squish area to the inner bowl.
This is the way it looked after coming off the lathe. Nothing fancy here - just simple machine work.	A closer look reveals its smoothness and some small pits that were found once the aluminum was cut.
You'll have to go back to Part 7 to compare this shot to the picture of the head the way it started. It used to hold much more fluid than it does now. It's volume has been reduced considerably.	Recently I used the following statement to describe the Blasters personality when exchanging email with an online friend. The power delivery of the stock Blaster could be described as being "impaired." Add an after market tuned pipe, and its rating moves up to "fatigued." It's power band is so flat it could be considered "flush." Yamaha describes the power delivery as being "tractable", what they seem to mean by it is "obedient." That may be a little harsh but I like these engines - I want to improve it. The head received a lot of consideration during this project. I wanted to get rid of the lazy-revving feel the stock engine exhibited. Properly designing a head is a great way to accomplish this. I designed it toward the maximum safe squish velocity to help the engine pull the heavy-ish Blaster around (29.2 m/s - meters per second). A (s)lower squish velocity would have worked for other power delivery characteristics - such as tuning for power higher up the RPM range. When compared to the second photo on the page of the Part 7 page it's easy to see how much material was removed to be able to achieve this kind of design. Several millimeters of material had to be skimmed from the surface in order to be able to make the shape I wanted. In fact the previously raised surface is now recessed. I had to trim my flat plastic plate to fit within the recessed area in order to CC it when it was done. I was not able to lap the head surface after coming off the lathe. I would have had to cut material across the complete head surface in order to be able to use it against my lapping slab or glass. I felt the head would be better off keeping the material for strength.
Top Volume I relocated TDC using a piston stop and sealed the piston with a small amount of grease - I wouldn't want any fluid to leak past the piston when I checked the volume. I put the head on the engine with the head gasket in place and proceeded to pour in fluid from the graduated burette - its volume was now 17 cc. The small amount of grease used to seal the piston during this procedure amounts to a few tenths of a cc - this reduces the compression ratio slightly but I won't consider it here since it is such a small reduction and its in the direction of reducing the compression ratio, it won't matter. I continued on to calculate the new compression ratio (198 + 17 = 215 cc, 215 / 17 = 12.65:1). This new full stroke (uncorrected) compression ratio along with new port timing gives the engine a trapped (corrected) compression ratio of 7.5:1 (up from 6.11:1). It's estimated static compression is predicted to be 160 PSI at sea level. The octane required to run this engine is still within the range of what I can buy at the pump - 92.