Getting My Head Shaved.. But How Much?

[5ksev]

hey,

 

well hopefully today I'm going to take my head into town to get shaved for more compression

 

iv got a 5k block and head..

 

but what I'm stuck on is, how do i work out what compression ratio am i running.. and what should i get it shaved to?

[altezzaclub]

You measure the volume of the combustion chamber with a sheet of plastic over it and a burette or pipette full of turps.

 

The 4K is about 31cc. Then you add up the combustion chamber+head gasket(5cc or so)+piston dish volume+cylinder volume. That is your total volume when the inlet valve is open.

 

Under compression the volume is that minus the cylinder volume, and the compression ratio is the ratio of the two.

 

You need a target compression ratio you want, say 10:1, and then measure the area of the combustion chamber with a piece of graph paper. That will let you work out how much to shave off to get the CR you want.

 

Pictures here-

 

http://www.rollaclub.com/board/topic/42407-the-girls-ke70/

[5ksev]

You measure the volume of the combustion chamber with a sheet of plastic over it and a burette or pipette full of turps.

 

The 4K is about 31cc. Then you add up the combustion chamber+head gasket(5cc or so)+piston dish volume+cylinder volume. That is your total volume when the inlet valve is open.

 

Under compression the volume is that minus the cylinder volume, and the compression ratio is the ratio of the two.

 

You need a target compression ratio you want, say 10:1, and then measure the area of the combustion chamber with a piece of graph paper. That will let you work out how much to shave off to get the CR you want.

 

Pictures here-

 

http://www.rollaclub.com/board/topic/42407-the-girls-ke70/

i do understand that.. but i really doubt ill end up with an acurate answer..

 

I'm going to a head place today.. they should be able to work it out for me hey?

[Spencer[RL]]

You would hope the could do it :D

Don't forget tO take a piston in with you or atleast know the cc's of it.

[ae824ate]

You measure the volume of the combustion chamber with a sheet of plastic over it and a burette or pipette full of turps.

 

The 4K is about 31cc. Then you add up the combustion chamber+head gasket(5cc or so)+piston dish volume+cylinder volume. That is your total volume when the inlet valve is open.

 

Under compression the volume is that minus the cylinder volume, and the compression ratio is the ratio of the two.

 

You need a target compression ratio you want, say 10:1, and then measure the area of the combustion chamber with a piece of graph paper. That will let you work out how much to shave off to get the CR you want.

 

Pictures here-

 

http://www.rollaclub.com/board/topic/42407-the-girls-ke70/

Altezzaclub is onto it! Do it this way, it is a very accurate way of doing it. We used a thick oil (gbox/diff oil) and a syringe. This is calculating your static CR though, not dynamic CR. If its possible, find out how many deg abdc the intake valve seats, and work off that volume of the cylinder.

For example, the tighe 150A cam that i had, the intake valve didnt seat untill 71*abdc. Cylinder volume for a 4k-325cc.

 

325cc/360*=.9027

So 1 degree of revolution is .9027cc.

.9027ccx71*=64cc.

325cc-64cc=261cc.

So that is the dynamic volume of one cylinder, then figure out the volume of your chamber and add a few cc for the head gasket. Without knowing the specs of the stock cam, the simplest way is just use the static volume.

[altezzaclub]

Give it a go, then have the head shop check your answer. Otherwise you are at the mercy of some local engineer forever.

 

What do you plan to run the car on?? 98octane all the time? and what do you want to do with it?? road car? Occasional hill-climb or track day? As many track days as you can?? That will determine the cam you use, and the cam determines what compression you can run. If you want to race it enthusiastically, do what ae82 said as dynamic compression is important for fat cams.

[5ksev]

yeah 98, going to run a 20-60 cam with 10:1 compression..

 

how can i go by working out my compression now, without having to take the engine out.. due to it being on a lean?