When finishing the combustion chamber Tom worked on eliminating sharp edges that could form a hot edge point that could be a source of pre ignition. Finally Tom polished the port bowls and combustion chamber with 220 grit cartridge rolls for a smooth and consistent finish.
After this work was done, Tom cc’ed our combustion chambers to determine their volume, the unshrouding work increases the chamber volume and reduces the compression ratio. After cc’ing, Tom milled our head’s deck surface slightly to bring the chamber volume down to our desired 43.5cc.
|A bunch of flow gained without much work or cost! The cool thing is that since the port diameter and volume was not changed, it is likely that power gains are going to be across the board since port velocity will probably be improved, even at low rpm. Headwork usually gains at high rpm and loses some at low rpm due to stagnation in port flow due to an increase in port volume when the ports are enlarged.|
As a last step to prove his point to us, Tom put our head on his Superflow flow bench to show us just how much better our head flowed. Our intake port picked up about 15-20 cfm across the board all the way up to 0.500″ of lift. The exhaust side showed gains as high as 26 cfm, especially above 0.250″ of lift.
The SR20VE has some of the best flowing ports of any Nissan 4 cylinder so you can expect to see even larger gains on some other Nissan heads. A large percentage of flow increase found in extensive headwork is found in the valve job and pocket of the port. Complete full porting would include narrowing down the port splitter and raising the roof of the ports but this is expensive and time consuming. Now we are worried that our head may make too much power. If so we will electronically limit it and tune for a wide powerband via cam timing and electronics.
So our head is fixed and the most difficult part of fixing my motor is behind us. Stay tuned, we will be building an econo engine while we are in there fixing things.