Not shifting at high RPM is mostly due to clutch moment area of inertia being too high for the synchros. Simple solution - go to a smaller clutch disk since this is proportional to R^2 of the disk. A single 7.25" clutch disk can probably just hold a 2GR's torque reliably. A twin 7.25" disk would easily hold it and still drive pretty nicely with a typical ~10-12 lbm flywheel.I myself see two separate questions to address.
1. Would there be an advantage to higher output above the current peak at/near 7K rpm or so? Answer yes, definitely. It should be obvioius that if you are revving in that range then more output makes the car faster, even with a 8K rev limit.
2. What the heck is the deal with the 8K limit on the e153. Lots of contradictory info on this one. Some builds report shifting without problem at 9k. Others start having issues as low as 6.5K. It has been suggested that this could a clutching issue rather than a internal trans issue. I myself have no problem shifting at 8K. Just lucky, I guess? If I did not have some reservations about taking my hydraulic lash adjusters aka lifters above 8K, then I would try upping the rev limit to see what happens. With solid lifters I would not have this fear of spitting. Solid lifters along with oversize valves are on the goodies wish list.
Sorry, brainfart there. Was listening to a meeting while typing. Basically, changing valves can quickly spiral to a lot of additional work to fit them. Some valve seats can't even take +1 mm sized valves well (the valve ends up sitting too far out for the seat to really support it well with high spring rates and high RPM usage).I'm not sure if I am getting the meaning of this: "it can quickly spiral into extra capacity if you can get the stock valve seat to work."
I see a lot of online debate about larger valves, do they help, do they hurt (by reducing port velocity), are they just plain useless.
Re the cams there is a new billet cam offering from Piper. "Fast" cam with 270 duration, "Ultimate" cam with 285 duration. I don't know how these are measured. Compare to 256/266/272 for MWR Stage 1/2/3. Again, I don't know how these are measured, i.e. at what clearance. It could be that Piper and MWR are measured at the same clearance - or not.
Another possibility for a budget build is to adapt a high-duration cam from a 2gr-fks. The simulated Atkinson cycle requires much higher duration cams, but I don't have their spec. . Someone is taking this approach with the 2AR. PS. I have just been informed this can be scratched off the list, it is a no-go, because of different camshaft construction.
I looked into the 2GR-FKS cam specs, and they actually weren't much longer than 2GR-FE cam duration. 8 degrees or so according to the engine data FSM? I was surprised they were barely longer duration.