lower compression?
08-01-2006, 02:16 AM
#16
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It has a sealing ring.. Not a gasket that you can stack.. The oring sits inside a groove that is cut in the liner.
Another option is to install 3.6 turbo pistons from a 965
Another option is to install 3.6 turbo pistons from a 965
08-01-2006, 10:12 AM
#17
Three Wheelin'
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Swaybar,
Sorry not to have been clearer. Those two sentences are meant to be read together.
The technique of extending the inlet closing time to reduce dynamic compression is well known. The 993 cam already is using the principle.
Unless a custom cam grind was done - very expensive- the cams available to extend the inlet closing point have more overlap , in other words just bigger duration on inlet and exhaust.
With pressure fed motors this will lead to increased low speed boost loss which could spoil the conversion.
A square law compressor makes boost from mid to high rpm if geared correctly . Low boost at mid rpm and high boost at high rpm. This charateristic better matches the detonation sensitivity of the motor and makes compression reduction less necessary .
I hope that makes sense !! Just the ramblings of an old motor guy.
Geoff
Sorry not to have been clearer. Those two sentences are meant to be read together.
The technique of extending the inlet closing time to reduce dynamic compression is well known. The 993 cam already is using the principle.
Unless a custom cam grind was done - very expensive- the cams available to extend the inlet closing point have more overlap , in other words just bigger duration on inlet and exhaust.
With pressure fed motors this will lead to increased low speed boost loss which could spoil the conversion.
A square law compressor makes boost from mid to high rpm if geared correctly . Low boost at mid rpm and high boost at high rpm. This charateristic better matches the detonation sensitivity of the motor and makes compression reduction less necessary .
I hope that makes sense !! Just the ramblings of an old motor guy.
Geoff
08-01-2006, 10:51 AM
#18
Nordschleife Master
Martin, The 993tt pistons will not work with the 964 heads since the pistons extend above the cylinder and would hit the 993 heads unless they were machined to accept the 993tt pistons. For 6psi of boost, the 3.6T pistons would likely be too low a compression ratio since they are advertised by Porsche at 7.5:1 and are more like 7.2:1. Again, the simpliest and easiest way would be to take the 993 piston and machine it down.
FWIW, custom grind camshafts cost less than Porsche camshafts and are built off of new chill cast cores, not reground. They are not expensive. I tend to run large overlap camshafts in boosted engines and don't agree with the large overlap issue. My turbo camshafts had over 54 degrees of overlap.
FWIW, custom grind camshafts cost less than Porsche camshafts and are built off of new chill cast cores, not reground. They are not expensive. I tend to run large overlap camshafts in boosted engines and don't agree with the large overlap issue. My turbo camshafts had over 54 degrees of overlap.
08-01-2006, 12:01 PM
#19
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Geoffrey,
FWIW I have done quite a lot on supercharged motors and with the motors concerned have found the overlap situation to be more critical than with a turbo.
I put that down to the supercharger having a fixed maximum air delivery at any motor speed whereas the boost control on a turbo can vary the air available too compensate ?
Just one of those things that probably comes down to personal experience on particular motor set ups.
Interesting observation on custom cam availability .Maybe the late inlet closing is a viable option . Any idea on prices for one off cams ?Just a rough guide.
Geoff
FWIW I have done quite a lot on supercharged motors and with the motors concerned have found the overlap situation to be more critical than with a turbo.
I put that down to the supercharger having a fixed maximum air delivery at any motor speed whereas the boost control on a turbo can vary the air available too compensate ?
Just one of those things that probably comes down to personal experience on particular motor set ups.
Interesting observation on custom cam availability .Maybe the late inlet closing is a viable option . Any idea on prices for one off cams ?Just a rough guide.
Geoff
08-01-2006, 02:02 PM
#20
Nordschleife Master
Retail pricing for custom NEW billet camshafts are in the $800 range. This is to use airflow and velocity data from a flow bench, P/V clearance, compression ratio and desired powerband and run it through a computerized model and build a set of templates and grind the cams from new chill cast cores.
