Hey guys,
I've done a search on this and only found a few, half-assed, non-dedicated threads that don't discuss it thoroughly enough.
I'm doing research for my 7M-GE engine build up, with desire to increase compression in the quest for more power.
from my calculations, decreasing the gasket thickness by approximately 1mm (39 thousandths of an inch) would provide a standard 7mge with a compression ratio of 10.5:1...which seems like the safe limit for 91 octane fuel. such an increase will theoretically net a 7hp gain at the flywheel. Since the head must be machined and I usually use premium gas anyways, it seems like a worthwhile thought.
obviously the ideal way to increase this compression is by higher compression/higher dome pistons, but that may not be the most efficient way to do it if your bottom end is still in good shape.
Now, it's not possible to purchase a .37mm thick headgasket (stock is 1.37mm) so material must be removed from either the block or head, to reduce the effective deck height. To avoid changes in the combustion chamber, this material would ideally be removed from the block. upon visual inspection, the pistons already seem dangerously close to the gasket surface, and thus contacting the head. Head squish is great, but I'd rather not reduce that safety margin.
This means our sights need to be set on the head. "fine" you're thinking, just shave some off of there. Well, That's what I'm thinking too, so long as any drawbacks are considered:
Piston to Valve Clearance
if keeping the motor non-interference is important (and it should be) this must be considered, including the use of aftermarket camshafts if planned that have extra lift.
I had a conversation with ED of toymods.net fame over the phone and he suggested there was " *ahem* loads of space", even after removing a total of 40 thou from the block and head combined, he measured 1.2mm of clearance still remaining.
Timing Belt Slack
This is often an argument against shaving the head, why? has anybody measured how much belt the timing belt tensioner can compensate for? I can't imagine Toyota's safety factor for such a thing to be so small that the belt's tension is compromised when only 1mm worth of material is removed. The tensioner would effectively have to take up 2mm worth of slack, since the belt completes a loop and this height change effects both the left and right side of the belt.
Valve Timing
This is a valid downside...but not a certain one. I measured the cam gears diameter to be 120mm...this means the circumference is 377mm. dividing this by 360 degrees means each mm moved on the outside diameter means 1.05 degrees of rotation. Applying the 2mm of movement to this degree change means 2.1 degrees of movement will be present at each of the gears.
The 7m has one tensioner, located on the exhaust side....so this 2 degrees of movement will rotate both cam gears counterclockwise (retarded).
To correct this, adjustable cam gears can be fitted, the bolts loosened and the cams advanced 2 degrees to set them back to stock.
I've heard "it won't run right even with adjustable gears, it screws up the timing." I fail to see how this is possible, the cam gears diameter does not change, nor does the crank pulley...since the gear ratios remain the same I fail to see how the timing can be wrong or changed at all if the cam gears are adjusted properly.
Combustion Chamber Shape
another valid drawback, but upon visual inspection, it doesn't like like it will change that much....how I'd quantify this, I don't know...but with material removed, the combustion chamber would also have a decrease in volume....I don't know what side effects this may have as I haven't taken internal combustion engines yet in school (next year). However, the bore side of the chamber will have a smaller diameter, increasing head squish area, which is definitely a good thing.
so the question remains, is this a draw back or a benefit? I'm not educated enough to tell...
Ignition Timing
this has been another argument against removing material. in my mind, everything I stated about valve timing applies to this too. The gear on the cams and the gear on the distributor both stay the same, turning at the same speed with respect to one another. Again, if the cam gears are set up with the same TDC relationship, I fail to see how this can be negatively effected.
Fuel type
to avoid detonation, premium fuel must be used
obviously, these key areas would need to be measured and inspected to give definite answers. I'd love to hear some other's thoughts on this subject, and if anybody has measured any of these key parameters. Please feel free to correct my thoughts or errors in calculations.
Steve
I've done a search on this and only found a few, half-assed, non-dedicated threads that don't discuss it thoroughly enough.
I'm doing research for my 7M-GE engine build up, with desire to increase compression in the quest for more power.
from my calculations, decreasing the gasket thickness by approximately 1mm (39 thousandths of an inch) would provide a standard 7mge with a compression ratio of 10.5:1...which seems like the safe limit for 91 octane fuel. such an increase will theoretically net a 7hp gain at the flywheel. Since the head must be machined and I usually use premium gas anyways, it seems like a worthwhile thought.
obviously the ideal way to increase this compression is by higher compression/higher dome pistons, but that may not be the most efficient way to do it if your bottom end is still in good shape.
Now, it's not possible to purchase a .37mm thick headgasket (stock is 1.37mm) so material must be removed from either the block or head, to reduce the effective deck height. To avoid changes in the combustion chamber, this material would ideally be removed from the block. upon visual inspection, the pistons already seem dangerously close to the gasket surface, and thus contacting the head. Head squish is great, but I'd rather not reduce that safety margin.
This means our sights need to be set on the head. "fine" you're thinking, just shave some off of there. Well, That's what I'm thinking too, so long as any drawbacks are considered:
Piston to Valve Clearance
if keeping the motor non-interference is important (and it should be) this must be considered, including the use of aftermarket camshafts if planned that have extra lift.
I had a conversation with ED of toymods.net fame over the phone and he suggested there was " *ahem* loads of space", even after removing a total of 40 thou from the block and head combined, he measured 1.2mm of clearance still remaining.
Timing Belt Slack
This is often an argument against shaving the head, why? has anybody measured how much belt the timing belt tensioner can compensate for? I can't imagine Toyota's safety factor for such a thing to be so small that the belt's tension is compromised when only 1mm worth of material is removed. The tensioner would effectively have to take up 2mm worth of slack, since the belt completes a loop and this height change effects both the left and right side of the belt.
Valve Timing
This is a valid downside...but not a certain one. I measured the cam gears diameter to be 120mm...this means the circumference is 377mm. dividing this by 360 degrees means each mm moved on the outside diameter means 1.05 degrees of rotation. Applying the 2mm of movement to this degree change means 2.1 degrees of movement will be present at each of the gears.
The 7m has one tensioner, located on the exhaust side....so this 2 degrees of movement will rotate both cam gears counterclockwise (retarded).
To correct this, adjustable cam gears can be fitted, the bolts loosened and the cams advanced 2 degrees to set them back to stock.
I've heard "it won't run right even with adjustable gears, it screws up the timing." I fail to see how this is possible, the cam gears diameter does not change, nor does the crank pulley...since the gear ratios remain the same I fail to see how the timing can be wrong or changed at all if the cam gears are adjusted properly.
Combustion Chamber Shape
another valid drawback, but upon visual inspection, it doesn't like like it will change that much....how I'd quantify this, I don't know...but with material removed, the combustion chamber would also have a decrease in volume....I don't know what side effects this may have as I haven't taken internal combustion engines yet in school (next year). However, the bore side of the chamber will have a smaller diameter, increasing head squish area, which is definitely a good thing.
so the question remains, is this a draw back or a benefit? I'm not educated enough to tell...
Ignition Timing
this has been another argument against removing material. in my mind, everything I stated about valve timing applies to this too. The gear on the cams and the gear on the distributor both stay the same, turning at the same speed with respect to one another. Again, if the cam gears are set up with the same TDC relationship, I fail to see how this can be negatively effected.
Fuel type
to avoid detonation, premium fuel must be used
obviously, these key areas would need to be measured and inspected to give definite answers. I'd love to hear some other's thoughts on this subject, and if anybody has measured any of these key parameters. Please feel free to correct my thoughts or errors in calculations.
Steve
