99 Pete wrote:
Cam timing and duration affect dynamic compression ratio because although the compression stroke starts as the piston passes BDC, actual compression doesn't start until the inlet valve is closed which will be sometime after BDC. How much after depends on the duration and timing of the cam. One of the reasons why highly tuned N/A engines with long duration cams have very high static compression ratios.......to make up for the otherwise very low dynamic compression ratio that would otherwise result.
Hope that helps to explain, otherwise you'll find plenty of info and probably better explanations if you Google it!
OF COURSE JUST TO ADD TO THIS,THAT THE 99 TURBO HAS A VERY 'MILD' DESIGN OF CAMSHAFT,AND THAT SPECULATION BACK IN THE DAY STATED THAT THE 'EMS' CAM WITH SLIGHTLY HIGHER LIFT,WOULD GIVE 'AMAZING' PERFORMANCE INCREASE'S......IF FITTED?
IN 'LAYMAN'S TERMS. THE ADVANTADGES OF A 'HIGH' LIFT CAM,IN N/A ENGINE.... ARE EXARCERATED IN A 99 TURBO,BY THE TURBO'S THIRST FOR AIR,(OR BOOST )WHICH SIMPLY FORCES THE COMPRESSION,STRAIGHT BACK OUT THE CYLINDER ,IF A 'LONG' DURATION/HIGH LIFT CAMSHAFT,IS UTILISED......
TO ADD TO 'PETE 99' THE N/A A-SERIES 1380 MINI STAGE RALLY CAR, WE RAN WITH FORGED STEEL CRANK,AND OVER 11 TO 1 COMPRESSION RATIO,HAD A SLIGHTLY MILDER CAMSHAFT FITTED(THAN FULL RACE SPEC)TO AID TORQUE AND LOW DOWN TRACTABILITY OUT OF MUUDY FOREST CORNERS. IF WE HAD USED IT FOR TARMAC RACING,AN 'ULTIMATE RACE SPEC' HIGHER LIFT CAM AGAIN,WOULD HAVE BEEN SELECTED,WHICH WOULD MOVE THE POWER BAND HIGHER UP THE REV RANGE...WHICH INCIDENTALLY, IT WOULD REV TO 9000 RPM.
BUT THIS OF COURSE,IS TO ILLUSTRATE,THE 'DIFFERENCE' BETWEEN A FULL ON N/A 11.1 TO 1 C/R RACE ENGINE,THE 9.0 TO 1 B202 16V TURBO APC ENGINE,OR INDEED THE 7.2 TO 1 CR B20 99 TURBO ENGINE.
SOMETIMES IT IS RATHER DIFFICULT TO FIND THE INFORMATION YOU NEED ?
AS REGARDS 99 TURBO DEVELOPMENT,THIS IS PROBABLY 'GOLD DUST'.....

IT ANSWERS A LOT OF QUESTIONS,MAYBE EXPLODES A FEW MYTHS....?
viewtopic.php?f=12&t=77024&hilit=99+TURBO+RALLY+MODS
viewtopic.php?f=12&t=66005&hilit=MORE+POWER+FOR+YOUR+99+TURBO
I TOOK THE LIBERTY OF ASKING A RATHER ENDEARING 'GENTLEMAN' ENGINEER,WHOM HAS ONE OF THE EARLIEST&FASTEST 3 DOOR 99 TURBO'S(PICTURED) IN THE UK,IF HE HAD ANY FIGURES IN THE EXTENSIVE&METICULOUS SERVICE PORTFOLIO,THAT HE KEEPS FOR HIS 99 TURBO?
THIS CAR IS PROBABALY THE LONGEST 'OWNED' 99 TURBO,BY ANY INDIVIDUAL OWNER CROSS THE GLOBE,NEVER MIND THE UK,SO WE CAN SAFELY SAY THAT HIS FIGURES ARE 'CONCRETE',OR 'SET IN STONE' AS IT WERE.....
THE COMPRESSIONS WERE TESTED BY HIMSELF IN 2018 WITH THE MILEAGE AT 106K,ON THE ORIGINAL ENGINE.
1.155 PSI
2.153 PSI
3.155 PSI
4.155 PSI
ALL DONE COLD&DRY.