Compression Ratio question..... Cosworth
01-06-2010, 09:23 PM
#1
PassionFord Post Whore!!
Thread Starter
Compression Ratio question..... Cosworth
One for the folks who know how to do the relevant calculations.....
Just been chucking ideas about today and talking about stuff we have worked on in the past and one car cropped up that had 3.3mm skimmed from the top of the piston to lower the comp and was still boosting over 30 psi and 8k rpm with no problems for years.
That led to the question of doing the same to a standard Cossie piston, what would it drop the comp to??
Cheers
Just been chucking ideas about today and talking about stuff we have worked on in the past and one car cropped up that had 3.3mm skimmed from the top of the piston to lower the comp and was still boosting over 30 psi and 8k rpm with no problems for years.
That led to the question of doing the same to a standard Cossie piston, what would it drop the comp to??
Cheers
02-06-2010, 09:38 AM
#10
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Because the bottom end is like this now, It has run 36psi for the last 2 years and is still in good condition but now i have a T4 to go on and was sitting wondering what sort of cr it was..
Y waste money on new pistons if it does not need them......was told when i bought the bottom end that the cr was around 6/9-1....
Y waste money on new pistons if it does not need them......was told when i bought the bottom end that the cr was around 6/9-1....
02-06-2010, 09:56 AM
#11
before having guesses at it,. you need to know how much the std piston sticks out of the top of the block
if the block has been skimmed alot, then taking 3mm off will not effect it as greatly as you may think
it needs to be measured properly so the mapper knows what he is dealing with, a guess isnt good enough imho
if the block has been skimmed alot, then taking 3mm off will not effect it as greatly as you may think
it needs to be measured properly so the mapper knows what he is dealing with, a guess isnt good enough imho
02-06-2010, 09:59 AM
#12
PassionFord Post Whore!!
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Good point Jimbo. I know my block has had a skim and the pistons used to stick out by a fannys hair where as my spare block the pistons used to sit dead flush.
02-06-2010, 09:59 AM
#13
You dont want to end up with the outside edges of the piston being too far from the head at the top of the cycle, this squish region is important.
I always aim to have them outer edges of the piston protruding into the headgasket slightly (.1 -.2 mm typically)
So ideally you want to take material out of the centre to lower the CR not from the edges, but as per the excellent picture above, there is only a finite amount you can take off of course!
I always aim to have them outer edges of the piston protruding into the headgasket slightly (.1 -.2 mm typically)
So ideally you want to take material out of the centre to lower the CR not from the edges, but as per the excellent picture above, there is only a finite amount you can take off of course!
02-06-2010, 11:05 AM
#22
Because you will make less power like that.
a proper squish band massively increases the amount of turbulance that the mixture sees, which makes it burn a lot better, especially at high rpm, so without it working properly you will lose power.
The prescense of much gas in the squish region can also increase the risk of detonation, which means you have to run less timing to avoid det, which means less power as well.
It will basically make your engine more inefficient and sluggish if you lose this part of the piston.
a proper squish band massively increases the amount of turbulance that the mixture sees, which makes it burn a lot better, especially at high rpm, so without it working properly you will lose power.
The prescense of much gas in the squish region can also increase the risk of detonation, which means you have to run less timing to avoid det, which means less power as well.
It will basically make your engine more inefficient and sluggish if you lose this part of the piston.
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