12G295 Large Collet valves
Turbo Mini Forums

I learned the hard way that the triple collet valve keepers are designed to rotate the valves some and are not a good choice at all for performance use. I'm about to build a 998 turbo (gt15) and want to use a 12G295 head instead of the 12G940 heads we've been using. My current valve selections are slim.

Are the large collet grove valves/caps up to the task of about 7200 peak rpm, 3000-6500 sustained for 8 hours nonstop? Should the little spring clips be used (despite Vizard saying in one of his books to toss them)?

I have loads of new valves in the correct diameter with large collet in the 3.44 length but the only longer and larger valves in the small single grove keeper style.

Are the large collet grove valves/caps up to the task of about 7200 peak rpm, 3000-6500 sustained for 8 hours nonestop?

YES!

If running NA (skimmed head) with a standard bore, you might have trouble with OS exhaust valves. FYI.

BENROSS, should I use the spring clips as well or just keep them off?

Vegard-- can you explain the trouble I might experience with the 12G295 valves in a standard bore? I'm planning on boosting this engine so it will not be skimmed to the point of the valves being able to contact the block, if that's what you are referring to.

if you use a standard unskimmed head then exhaust valves size up to 29.5 can be used but dont go over a cam and rockers 1.3 with lift over it total of 0.318" ................8.08mm

that would be a N/A mg cam with 1.3 rockers the saftey margin would be a tad under 1mm before the ex valve hits the block without pockets been machined in there

as vegard says thoug 27mm is the best ex size to use without de-shrouding the chamber around that larger valve of 29.5mm

OH! nearly forgot i personally would use the clip for the collets if you have them and they are in good condition?

EDIT:- i did a 295 head for a 998 for bennys friend on here with valve sizes of 33mm inlets and 29.5mm exhausts and it fast motor none turbo so benny quotes

Edited by BENROSS on 4th Oct, 2011.