[Stan Weiss]

Originally Posted by Mark G

In general with a nitrous application I open the exhaust valve sooner to give the exhaust port more time to evacuate the cylinder. Since an oxidizer is being introduced to the cylinder in an attempt to burn more fuel and make more power, more spent exhaust gas is created as the result. To sustain power throughout the operating range with this added exhaust gas(especially at the high end of the operating range) and expel excess heat generated by the faster burn of the oxidizer being introduced, the valve needs to open sooner. Since nitrous burns so quickly, it creates a sharp spike in cylinder pressure early after TDC on the power stroke. Because of this more work is being put to the crank in a shorter period of crank degrees. The exhaust valve can be opened sooner without losing a lot of torque(if any in the usable operating range) which carries more power upstairs.

If the exhaust valve is not opened sooner in a nitrous application versus a cam optimized for the same engine but for motor usage, power will not carry as far into the operating range as it did on motor. Peak power will be made at a lower RPM.

This is caused by spent exhaust gas not being evacuated from the cylinder. Spent exhaust gas displaces fresh air/fuel charge and it also heats it up. Added power cannot be made by attempting to burn spent exhaust gas, let alone sustain at the level it was.

I also like to close the exhaust valve later in an attempt to expel the last bit of remaining exhaust gas present in the clearance volume. You don't want to force the piston to do the majority of the work in pushing the spent exhaust gas out on the exhaust stroke as that would incur a large pumping loss. The size of the motor, the piston speed it generates and the amount of nitrous used determines where the exhaust valve should open among other things. Opening the exhaust valve at the right point on the power stroke allows the pressure differential between in cylinder pressure and atmospheric pressure in the exhaust port to do the majority of evacuation for us. Opening the valve at the right point on the power stroke ensures that we incur the smallest pumping loss possible on the exhaust stroke by removing the most spent exhaust gas it can before starting the exhaust stroke. Opening it too early will expel cylinder pressure that could of been used to do more work on the piston driving it down and producing more torque. Thus slowing the car especially off the line and on the gear change. A car with a 2 speed trans may become an absolute turd if this happens. A 3 speed auto may not be affected as much nor would a 4 or 5 speed.

That said, not all of the spent gas will be forced out during blow down or on the exhaust stroke. Leaving the exhaust valve open a little bit longer and closing it later will allow the negative reflected wave that arrives during overlap to help pull that last bit of remaining higher pressure spent gas present in the clearance volume out the cylinder.

Yes you lose some charge doing this. The benefit to this is removing more spent gas from the cylinder means we can now cram more fresh charge into the cylinder than we could have had we not closed the exhaust valve later. This almost always shows a power increase versus closing the exhaust valve earlier in a nitrous application that operates at higher RPM's. A lower RPM nitrous engine may not need this as much as a higher RPM nitrous engine.

I like to close the intake valve later in a nitrous application as well. Nitrous engines that are utilizing large amounts of nitrous are timing sensitive more than likely. A later intake valve close event helps to bleed some cylinder pressure off by pushing some air/fuel mixture out of the cylinder from the late closing event. This also helps to soften the hit on a smaller tire car or one that needs some help in the traction department.

Intake valve open event I like to delay a bit to ensure that reversion doesn't occur at lower RPM's(coming off the starting line) and cause a nitrous backfire. I may be totally wrong in doing this, but I feel it helps. I normally only do this on heavy nitrous engines. ( Engine Builder words not mine) Please add your thoughts ..

Can you ask him about some actual numbers. If we look at the turn force on the crank. Let say a 4.5" stroke with a 6.8" rod. This geometry will have the turning force raise quickly and peak at about 25 ATDC. It will then descend at a somewhat slower rate.

Edit. - Let me also add. What is the difference is CR between the N/A engine and the Nitrous engine? If I make the same peak cylinder pressure at the same degrees ATDC the lower the CR the higher the cylinder pressure will be later in the power stroke.

Stan