Performance and racing.
Nearly all agree that dual valve springs are needed for engines revving over 7000 rpm. We recommend these for all track cars and any M10 using high lift cams like the 304, 306, 316, etc. For installation, you'll need to either use aftermarket spring retainers (such as our titanium retainers or our chromoly steel units), or perform a very-slight modification to the factory steel retainers due to the smaller I.D. of the inner valve spring. Sold as a complete set, 8 inner and 8 outer.
Note: You MUST use late-style M10 valve guides and seals with these springs (the old style stem seals interfere with the inner spring).