Quote:

is that with performance springs or 1200gx springs?

double springs allows mine to rev to 8500 easily...next time i take to the track Im expecting 9200

Also, if you fit larger valves, too stiff a valvespring can cause the head to crack in the reduced area between the valves.

I use the Performance Springs valvesprings. They weren't very expensive. Got them from Kelford Camtech. I shimmed them to give 90lbs on the seat and they don't valvebounce before my 8000rpm redline. Higher seat pressures not only wear the valvetrain more quickly, it robs horsepower.

The springs I have are shimmed up to handle about 8000 rpms. With another 40 thou of shimming, it could probably rev higher. (on a missed shift once it reved god only knows how far past 8grand) Of coarse, having springs that hard don't mean your engine will make power all the way up to 8. Plus, stiffer springs wear out your cam faster, and recsess your valve seats quicker too. If you want to rev past 8000, a balanced bottom end is mandatory. Otherwise the flywheel bolts work loose/snap.

my car only revs to 6500 usually before the valves bounce which i wasa little dissapointed with cos my old L's series used to make more around 7000. what sort of rev increase could i expect?

Not really a performance improver in themselves. They allow you to rev much higher without valvebounce + they do snap the valves shut more quickly.

Quote:

any rough estimates on the price? has anyone here actually used these and knows what sort of improvment is made by purely the double valve springs and to what extent???

A14/15 valves are ever so fractionally longer than A12 ones. It shouldnt matter though.
Performance springs make a replacement double spring.
Be sure to measure your installed heights, and shim them up accordingly.

I'd stay away from "pre loved " valve springs, they do have a working life if you want to run big revs. Given time they will lose their ability to handle high Rpm.

1. L-series valves can be used, the stem dia. is right but they are too long. You will have to cut them down and cut new collet grooves. I think it is L16 or L18 inlets that are about right head dia.??? from memory. There doesn't seem to be much around for aftermarket performance A-series valves.
2. Unsure but I think they are same for all A-series? The valvesprings supplied by the cam grinder who did mine said A-series datsun AND K type toyotas. Thought that was a coincidence!