valve float symptom

 What is valve float? u can ask mr. google.
    Briefly   it can lead your engine to miss fire normally happen caused of numbers of reason examples:
    increasing rev limit, changing to bigger or faster spool turbo, inadequate valve spring press, higher cam shaft lift and a lot more.
 how to identify the valve float?
    I not the expert of identifying the symptom but i just share on what happen to me.
          -on dyno graph (blue line) u will see the oscillation line indicate miss fire. when this is happen mostly people will start to look at the ignition problems by changing and testing the spark plug, plug gapping, coil,  leads, wiring, and exciter box ect.. the thing that i notice was what ever you change or test the oscillation line/ missfire will occur at the same rmp. in my case @ 5300. the power from here suppose to be increasing but it couldn't.
           - after testing ignition components then go for testing stock camshaft together with stock valve spring. Most of the cases on 4G93 the valve still float but only after 2 or 3 dyno run. in another word, the valve float symptom happen during 'hot' engine temp (hot means normal operating temperature).
           - Another one more to confirm the valve float is, when removing head gasket. If you have correct tools you can change the spring without removing head gasket, but i you do remove then inspect the combustion area. there is a possibility of having a 'carbon wash out' near the valve to cylinder wall area. carbon wash out is a clean surface that wiping out some black carbon  in combustion area. base on Mr. google it should happen on intake valve but in my case it happen on the exhaust valve area, it shows that the valves were running hotter.

On 4g93,How to over come this problem?
        there are few option examples like changing valve spring to higher rated, adding some shim underneath, using beehive spring, dual valve spring ect.. it is difficult to find for 4g93 dual spring and beehive because of different size of valve stem lock, evo stem and 4g93 stem/retainer are not the same even though the springs can be interchangeable. if you want to do so then look for 'COMP cams' as it provide stem dimension that suite your engine.
       Below is the valve spring comparison  between 4g93t stock, JUN 4g93, BC evo, stock 4g67

 

surprisingly I have to change my JUN  valve spring. it is because of inadequate spring pressure base on JUN website take alook at evo spring press.

                evo spring press  =  26mmkg = 53lb@ set load

              4g93spring press   =  not mentioned,but according at 4g93 manual, stock= 58lb@set load

so stock 4g93 spring is stiffer than evo JUN spring? how about JUN 4g93 spring?

 from above picture we can see that stock 4g93 has more coil than JUN 4g93. but both have similar spring rate pressure.

 above are JUN 4g93(left) vs BC evo

 Stock 4g93 vs BC evo. why i choose BC evo spring?
         coz of difficulty in finding tomei 4g93 spring alone and expensive i choose BC coz of price and srping pressure stiffer than other brand. Also the dimension is just a bit bigger by 0.5 mm still can suite stock retainer. base on rough calculation BC evo on 4g93 set load can be around 94- 104 lb load. equivalent to dual valve spring pressure .

 0.006 inches is the limit for checking coil bind to prevent any binding that can cause serious engine damage. but after checking the BC clearance i got way2 more space.