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If you go from 86mm pistons to 89mm pistons do you lose compression? Also to what extent do you have to reshape the head to match the diameter of the block?
Depends on what kind of pistons you get. As far as the head, I am unsure. I think w/ 3mm, nothing is probably necesarry.
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My real name is Kyle
2002 Maxima SE (6MT) -Drop-in K&N and a brand new longblock VQ35
1994 G20 - SR20VE, JWT pop, SSAC 2.5", 2.5" VRS catback
If you go from 86mm pistons to 89mm pistons do you lose compression? Also to what extent do you have to reshape the head to match the diameter of the block?
O
compression has nothing to do w/ the bore. Changing bore means new pistons, which have to have a certain deck height, dish or dome to get a desired CR. The head doesn't have to be reshaped unless the piston goes higher than the deck at tdc (not likely). However, more displacement with the same port size means there'll be greater flow demand at higher revs, so it'll benefit a little bit more from a porting job up top than it would've at a smaller displacement.
All other things being equal, a larger bore will yield a higher CR. More swept volume compressed into the same size combustion chamber equals higher CR.
However, a larger bore indeed means new pistons so all things may not be equal. You can change the dome or dish shape and volume to change the CR.
You don't necessarily have to reshape the combustion chamber when going to a larger bore, but it may be beneficial.
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George Roffe
91 SE-R (well modded)
84 944 SCCA ITS race car under construction
http://www.nissport.com
If the pistons have the same dome or top you will raise your compresion. Remember the Compression ratio on a simplified formula is: Volume of each cylinder/the sum of Combustion chamber volume plus Head gasket volume plus Volume for piston top land( It is the volume of the piston between the top and the block surface at Top dead Center) plus the Volume for valves recess. The total fo the operation Plus 1 is the Compresion ratio.
Any incresase in the Volume of each cylinder means an increase in the C:ratio. But: any decrease on the divisor will decrease the C.R. or any increase in the divisor will reduce C.R.
You will always gain or increase the Volume of the cylinder increasing the bore or increasing the stroke.
Volume of the cylinder= the radio of the piston cirunference ( diamenter/2) squared ( ratio times ratio) X 3.1416 X stroke.
If you will use 89mm pistons it means:4.45X4.45X8.6X3.1416=535.02 per cylinder and this is an engine with total displacement of 2140.08 cc.
What Kind of pistons do you intend to use?
Regards.
well no one makes ve pistons but Arias will make some for $145 each and I would like to just have them make sr20de pistons just at a higher compression. I just didn't know if the shear size of the pistons would make a difference in compression. I would like to go 12:1 but i want to know what all i have to factor in in asking for a 89mm piston that will give me 12:1 compression on a motor that hardly anyone has heard of.
well no one makes ve pistons but Arias will make some for $145 each and I would like to just have them make sr20de pistons just at a higher compression. I just didn't know if the shear size of the pistons would make a difference in compression. I would like to go 12:1 but i want to know what all i have to factor in in asking for a 89mm piston that will give me 12:1 compression on a motor that hardly anyone has heard of.
O
a very important consideration is the valve reliefs - I'm guessing you're going to run at least C3's, probably C6's so you'll want to make sure you have some leeway for advancing/retarding the cams w/o mulching the valves. Of course, bigger reliefs mean a bigger dome to maintain the same CR. I would suggest you tear apart an engine and start playing around w/ clay to check some clearances and dome volume
Measure the volume of you cumbustion chamber with the valves closed and sealing properly and the spark plug you intend to use screwed in. Call JE and tell them what you want in terms of bore and final CR. As far as I know the VE and DE piston's top designs are interchangeable. They take into account the range of cam lifts on the market and build the reliefs into the pistons to suit, they then compensate the piston's compression height to suit the final CR. If you are not sure send a piston to JE.
They do a good job and you can get them for just over $700 including rings.
I know a wako who has put the RB25 pistons with appropriately sized pin bushings, into the VE block to get an ultra high compression, it blew up but no valve/piston problems. Once the "dome" is not larger on the diameter than the standard 86mm pistons dome you should be ok.
