993 connecting rod length
10-15-2002, 10:14 PM
#1
Racer
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993 connecting rod length
I'm building a 993 race motor using stock mahle pistons (11.3:1 CR) and would like to increase the CR by going to a longer rod. Is there a mathematical formula for calculating the increased CR based on increased length ? I'm looking for approximately 12:1 CR with RS valves.I'm aware there could be clearance problems especially with RS cams.
Thanks in advance!
Thanks in advance!
10-16-2002, 10:55 AM
#2
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Howard,
Surely it would be cheaper to deck the head and chain housings rather than have longer rods made?
Anyway, you can work it out from (CC+SV)/SV (Combustion chamber volume, swept volume) but the best bet is to dummy up the piston/barrel/head assembly, fill the cylinder through a drilled out spark plug with the volume you require then measure the height of the piston in the barrel and compare it with standard. It's fiddly, but ultimately it is more accurate than measuring individual components and adding up the volumes. As long as you do it the same each time and get all the air out, the comparitive measurements will be accurate.
You should also aim to get the squish at .030 to .040", no more, (maybe less if you are brave).
Surely it would be cheaper to deck the head and chain housings rather than have longer rods made?
Anyway, you can work it out from (CC+SV)/SV (Combustion chamber volume, swept volume) but the best bet is to dummy up the piston/barrel/head assembly, fill the cylinder through a drilled out spark plug with the volume you require then measure the height of the piston in the barrel and compare it with standard. It's fiddly, but ultimately it is more accurate than measuring individual components and adding up the volumes. As long as you do it the same each time and get all the air out, the comparitive measurements will be accurate.
You should also aim to get the squish at .030 to .040", no more, (maybe less if you are brave).
10-16-2002, 12:38 PM
#3
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Colin,
Thanks for the info. I'm ordering new rods from Pauter so specifying rods 2 or 3mm longer is no problem. I just thoguht there would be a rule of thumb, so to speak. I would think that if I reduce the combustion chamber volume by 10 % the CR would increase the same amount.
If the head surface is machined and 1 mm is removed, how much would this increase the CR ? Then I would need to machine the chain housing mounting surface as well, and the timing chains may need to be shortened.
Thanks again!
Thanks for the info. I'm ordering new rods from Pauter so specifying rods 2 or 3mm longer is no problem. I just thoguht there would be a rule of thumb, so to speak. I would think that if I reduce the combustion chamber volume by 10 % the CR would increase the same amount.
If the head surface is machined and 1 mm is removed, how much would this increase the CR ? Then I would need to machine the chain housing mounting surface as well, and the timing chains may need to be shortened.
Thanks again!
10-16-2002, 07:21 PM
#4
Race Director
What about the quench/squish zones? Those are typically as close as you can get at about 1mm. Longer rods will have the pistons contact the heads. Best to CC the head and pistons, compute the desired compression-ratio change and see how much you have to mill off the head.
Another way is to just get custom pistons made.
Another way is to just get custom pistons made.
