na svt
03-12-2012, 12:09 PM
The intake valve closing point is the most important cam event as is sets the RPM at which peak power occurs and also how much mid range power and tq are made. The second most important cam event is the intake valve opening point. The Marauder intake valves (@.050") close around 33 deg ABDC (which is okay for a 10:1 engine) and the intake valve opening point is 23 deg ATDC.
The intake valve opening point of 23deg ATDC means the intake valve doesn't reach .050" open until the crank is well past top dead center. This is a problem as cylinder filling begins before TDC and the first half (TDC to 90 deg ATDC) of the intake stroke is the best time to fill the cylinder. To open the valve sooner the intake cam would need to be advanced a lower lobe center (LC), but it's not that easy.
Here's where the problem lies, advancing the Marauder intake LC to say, 108, will increase cylinder filling and as a result add a good amount of mid range hp and tq, however this will also lower the RPM at which peak power occurs. Peak power in the Marauder occurs low enough and advancing the intake cams would place peak around 5000rpm and power will drop off quickly thereafter.
Some people think that longer duration cams will kill tq and mid range power, even the Cobra cams. Well that is incorrect and here's why, their longer duration allows for a more efficient intake stroke while keeping the intake valve closing at a point that allows for peak power to occur anywhere from 5800-6300rpm (depending on intake LC). When installed at a lobe center of 108, the .050" intake valve opening point is 6 deg ATDC and the intake valve closing point is 30 deg ABDC. The intake valve opens 17 degrees earlier allowing far more air/fuel into the cylinder. The intake valve closing point is earlier which raises the dynamic compression ratio resulting in higher tq and mid range power.
Mid-range hp and tq will decrease and the powerband will shift to the right as the intake lobe center is retarded from 108. The RPM at which peak power occurs will also shift to the right and if using a stock intake manifold it will go no higher than 6300 on an n/a combo. All cars with a 6200 or less shift point should set the intakes at 108, once again n/a. So, the cobra cams with longer duration will increase tq, mid range power and peak hp. Centri cars can shift higher as teh pressurized intake charge forces the intake manifold to make power beyond it's n/a limit. PD blown combos have no runners therefore there is no intake runner imposed barrier to RPMs.
The next topic, how cobra cams can fix (sorta) the low compression, no torque producing, Cobra shortblock. The static compression of this engine is too low and as such should never be installed in a heavy, automatic transmission vehicle. At 12psi this shortblock is down 50rwhp/rwtq when compared to a 10:1 combo. You can get a lot of the hp/tq back by installing cobra intake cams at a lobe center no higher than 108, me, well I think I'd go with 106 for combos shifted at 6000rpm. Advancing the stock cams increases tq as much as 30ft lbs, cobra intake cams add as much as 50 ft lbs at 4000rpm. The intake cams can be run at lower than stock lobe centers with FI also, I'd set them at 108 for centri combo and no lower than 110 for a PD blower.
The factory lobe centers for the Cobra intake cams is 119/110 giving them an intake valve closing point of 41 deg ABDC and the opening point of 17 deg ATDC. Installing these cams at at their factory lobe centers will reduce tq and mid range power.
Maintaining tq and mid range hp with even longer duration cams is possible if the LC is no higher than 106 and the intake valve closing point is no later than 36 degrees ATDC. With this in mind, good cams for an n/a combo would look something like 220 deg @.050" with a 106 lobe center.
The intake valve opening point of 23deg ATDC means the intake valve doesn't reach .050" open until the crank is well past top dead center. This is a problem as cylinder filling begins before TDC and the first half (TDC to 90 deg ATDC) of the intake stroke is the best time to fill the cylinder. To open the valve sooner the intake cam would need to be advanced a lower lobe center (LC), but it's not that easy.
Here's where the problem lies, advancing the Marauder intake LC to say, 108, will increase cylinder filling and as a result add a good amount of mid range hp and tq, however this will also lower the RPM at which peak power occurs. Peak power in the Marauder occurs low enough and advancing the intake cams would place peak around 5000rpm and power will drop off quickly thereafter.
Some people think that longer duration cams will kill tq and mid range power, even the Cobra cams. Well that is incorrect and here's why, their longer duration allows for a more efficient intake stroke while keeping the intake valve closing at a point that allows for peak power to occur anywhere from 5800-6300rpm (depending on intake LC). When installed at a lobe center of 108, the .050" intake valve opening point is 6 deg ATDC and the intake valve closing point is 30 deg ABDC. The intake valve opens 17 degrees earlier allowing far more air/fuel into the cylinder. The intake valve closing point is earlier which raises the dynamic compression ratio resulting in higher tq and mid range power.
Mid-range hp and tq will decrease and the powerband will shift to the right as the intake lobe center is retarded from 108. The RPM at which peak power occurs will also shift to the right and if using a stock intake manifold it will go no higher than 6300 on an n/a combo. All cars with a 6200 or less shift point should set the intakes at 108, once again n/a. So, the cobra cams with longer duration will increase tq, mid range power and peak hp. Centri cars can shift higher as teh pressurized intake charge forces the intake manifold to make power beyond it's n/a limit. PD blown combos have no runners therefore there is no intake runner imposed barrier to RPMs.
The next topic, how cobra cams can fix (sorta) the low compression, no torque producing, Cobra shortblock. The static compression of this engine is too low and as such should never be installed in a heavy, automatic transmission vehicle. At 12psi this shortblock is down 50rwhp/rwtq when compared to a 10:1 combo. You can get a lot of the hp/tq back by installing cobra intake cams at a lobe center no higher than 108, me, well I think I'd go with 106 for combos shifted at 6000rpm. Advancing the stock cams increases tq as much as 30ft lbs, cobra intake cams add as much as 50 ft lbs at 4000rpm. The intake cams can be run at lower than stock lobe centers with FI also, I'd set them at 108 for centri combo and no lower than 110 for a PD blower.
The factory lobe centers for the Cobra intake cams is 119/110 giving them an intake valve closing point of 41 deg ABDC and the opening point of 17 deg ATDC. Installing these cams at at their factory lobe centers will reduce tq and mid range power.
Maintaining tq and mid range hp with even longer duration cams is possible if the LC is no higher than 106 and the intake valve closing point is no later than 36 degrees ATDC. With this in mind, good cams for an n/a combo would look something like 220 deg @.050" with a 106 lobe center.