Over the past 3 years we have built over 15 EJ motors, a dozen RB configurations and ALOT of B-series engines.
through trial and knowledge we have found what has worked best for us when it comes to ring gap with specific engine set up. And we are still learning. This write up is a hands on, experience POV that we hope helps you in you engine building process.
What is the proper ring gap you should use when building your engine? There is no correct answer to that question... There are multiple configurations depending on boost level, nitrous, meth, etc. How you set up the ring gap will play a large role in your engine's performance and longevity. But the short answer is there is not a one size fits all ring gap.
Top Ring (compression ring)
In a perfect world you would want the most compression with the smallest ring gap, but due to thermodynamics and heat expansion there are several factors to take into account when gapping your compression rings. As heat and pressure increases, you need more gapping to prevent the expansion of the rings, reducing the gap and frying a whole in your piston like Brian O'Conner. On the flip side, over gapping a compression ring will make a big difference in performance as it will reduce the compression of the engine. Thus, with boosted applications, you want to run a larger ring gap. When we say larger, it is actually fractions, but fractions make a big difference, as you will see here in a bit.
CP Piston provides a good "rule of thumb" you can you as a guide if you are new to gapping, but when you are building engines more frequently and can see the impact that it play's, you will get more comfortable.
Image below is courtesy of CP Pistons:
Oil Ring - The oil ring (second ring) plays a large part in oil lubrication. It acts as a scraper and removed excess oil from the cylinder bore. It is very important to set the ring gap properly. If you set the gap to tight, you can scrap too much oil from the walls and cause a lubrication issue. If you set them too loose, you will have blow by issues and excess oil making its way into the combustion chamber.
We recently experienced this issue in the black R32 we built. We got way to aggressive with out oil ring gap and made it about .004-.005" too large, causing a blow by issue. Although this wouldn't be an issue with race engines, or track cars, we have not tracked this car, and want to scale it back.
Example on a RB26 engine, running CP pistons, and Borg Warner 7163 Turbos. Max boost is estimated to be 30psi, so we scaled down out ring gap .004".
At the end of the day, it is really important that you take the time to understand the changes in the engine operation when you set things like ring gap.
We will update this posts with more information as we continue reassembling the RB26.