Comp ratio and head milling

[R1000]

Anyone know how much the static compression rate will go up with 0.01 and 0.02 head surface milling? The head that is under porting needs a surface clean up, and it would be fine to increase the comp ratio to about 12:1 since a slightly hotter cam will be installed. And yes, we do have high-octane fuel here, which in combination with polished combustion chambers will prevent detonation

[Para045]

Anyone know how much the static compression rate will go up with 0.01 and 0.02 head surface milling? The head that is under porting needs a surface clean up, and it would be fine to increase the comp ratio to about 12:1 since a slightly hotter cam will be installed. And yes, we do have high-octane fuel here, which in combination with polished combustion chambers will prevent detonation

Can't the guy who is machining your head CC the combustion chambers and work that out for you :icon_think: , I recently rebuilt a V8 motor in my F100 and took the LPG head off the stuffed 5.0L and put it onto the 5.8L and the head reconditioner worked out what the comp ratio was going to be as the 5L has smaller chambers than the 5.8L so pushed up the comp ratio to ~11.5-1 great for LPG but I do get slight pinging with petrol under load using 95 RON Good thing I basically only use petrol for startup the switch to LPG :icon_think:

[R1000]

Anyone know how much the static compression rate will go up with 0.01 and 0.02 head surface milling? The head that is under porting needs a surface clean up, and it would be fine to increase the comp ratio to about 12:1 since a slightly hotter cam will be installed. And yes, we do have high-octane fuel here, which in combination with polished combustion chambers will prevent detonation

Can't the guy who is machining your head CC the combustion chambers and work that out for you :icon_think: , I recently rebuilt a V8 motor in my F100 and took the LPG head off the stuffed 5.0L and put it onto the 5.8L and the head reconditioner worked out what the comp ratio was going to be as the 5L has smaller chambers than the 5.8L so pushed up the comp ratio to ~11.5-1 great for LPG but I do get slight pinging with petrol under load using 95 RON Good thing I basically only use petrol for startup the switch to LPG :icon_think:

Sure he can and me to, but if the figures are already well known there is no big reason to CC my head. I will get a small safety margin anyway since the chambers are a little larger than stock due to unshrouded valves.

[tomek]

You can go 13-1 comp ration on modern 4-5 valve on pump gas with no problemo.

You factor in squish area of, let`s say 15-20 % of bore area,if you know stock CR and stroke you can calculate new CR pretty accurate..

Milling the head is not the same as milling block/thinner head gasket.I know XX has a 1 piece block and upper crankcase design,so it is lots of work to mill the block,but if stock head gasket is a 3 piece design I would measure squish on stock motor and see if 1 layer could be pulled out.

You could safely close squish to 0.75 mm on XX I guess.To measure squish on 4 valve engine place 4 pieces of solder, symetrically on the piston between valve pockets,torgue cylider head,turn motor couple of times,pull cylider head ,measure the smacked solder,average the 4 numbers.Armed with all that you can figure out how thinner head gacket needs to be.

In case of XX with stock CR 11.0 and bore of 59, 0.02 thou mill ( 0.5 mm) CR would be about 11.7

Here is math

stock stroke is 59 mm and CR =11.0

X is a tickness of cumbustion chamber if it was a slice of cylinder ( I know it is not but it does not matter for this calculation)

59 +X

------------ = 11

X

X =5.9 mm

If you mill 0.5 mm of you head you removing only about 80 % of the slice ( you are milling squish area also),so it is like only milling 0.4 mm

so X1 =5.5 mm

59+5.5

--------=11.7,that is your new CR with 0.02 mill

5.5

If valves are unmasked,CR most likely would be in 11.5 range.

Removing 1 layer of gasket (0.2-0.23mm I guess) and milling head 3 thou (0.75mm) would give 12.5 CR,but I would check piston to valve clearance before doing that,there might be problem here.

[R1000]

You can go 13-1 comp ration on modern 4-5 valve on pump gas with no problemo.

You factor in squish area of, let`s say 15-20 % of bore area,if you know stock CR and stroke you can calculate new CR pretty accurate..

Milling the head is not the same as milling block/thinner head gasket.I know XX has a 1 piece block and upper crankcase design,so it is lots of work to mill the block,but if stock head gasket is a 3 piece design I would measure squish on stock motor and see if 1 layer could be pulled out.

You could safely close squish to 0.75 mm on XX I guess.To measure squish on 4 valve engine place 4 pieces of solder, symetrically on the piston between valve pockets,torgue cylider head,turn motor couple of times,pull cylider head ,measure the smacked solder,average the 4 numbers.Armed with all that you can figure out how thinner head gacket needs to be.

In case of XX with stock CR 11.0 and bore of 59, 0.02 thou mill ( 0.5 mm) CR would be about 11.7

Here is math

stock stroke is 59 mm and CR =11.0

X is a tickness of cumbustion chamber if it was a slice of cylinder ( I know it is not but it does not matter for this calculation)

59 +X

------------ = 11

X

X =5.9 mm

If you mill 0.5 mm of you head you removing only about 80 % of the slice ( you are milling squish area also),so it is like only milling 0.4 mm

so X1 =5.5 mm

59+5.5

--------=11.7,that is your new CR with 0.02 mill

5.5

If valves are unmasked,CR most likely would be in 11.5 range.

Removing 1 layer of gasket (0.2-0.23mm I guess) and milling head 3 thou (0.75mm) would give 12.5 CR,but I would check piston to valve clearance before doing that,there might be problem here.

Thanks a lot for a professional comment!