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Stock valves for A12 engine are 35 mm intake, 29 mm exhaust. You can fit the A14-size 37mm inlet/30mm exhaust. And valves up to 42mm have been fitted for ultra high-RPM use.
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Overview
The stock A12 valves (35/29) are good for small-bore (1200cc) use below for up to 6000 RPM. For larger bore engines -- or higher RPM -- the 37/30 valves from an A14 are better.
For racing at high RPMs, more power can be made with larger valves, but care must be taken for proper valve flow.
The 40s may require relief/flow notch in the side of the cylinder wall?
POST COUP1200: My A14 makes 140hp & its not EFI or turbo. I have twin 40mm webers, good extractors the head flowes 141hp. All your power gain is in the head big valves extencive amounts of porting good spark ... I dont run roller rockers & have not had a problem.you dont need to go to big with the cam mine about 74deg ... Mine makes power to 7500rpm.
feral: COOPS 1200 SEDAN AFTER RUNNING A 2:11 AT PHILLIP ISLAND CIRCUIT ... The car sounded awesome and handled very well. In typical 1200 fashion showed alot of more exotic machinery including a Porsche 924 turbo and a lotus elite a clean pair of tailights :-)
Feral
Feral used 40mm steel Inlet valves, tuliped and swirl polished, with 35mm Exhausts steel polished.
And don't forget to dial in the camshaft timing
41mm
GX head with 41 mm intake valves & 32 mm exhaust valves
42mm Valves
42 mm intakes are perhaps the limit for A-series cylinder heads.
Dual & Triple Valve Springs
Large valves work best at high RPM. When revving over 6000 RPM, dual valve springs are called for. The stock A12GX springs are good for 6600 RPM (and some report higher RPMs).
Tomei Valves
40 mm inlet 33 mm exhaust 7 mm stem (fits stock guides) thin heads (not cut-down valves)
Tomei valves Fitted to A12 GX head. Tomei valves with 7mm stems. Intake valves 40mm, exhaust valves 33mm. The stems have a thinner design for maximizing air flow rate within the port with the reduced air flow resistance. The cylinders need extra notching at the top of the bore to accommodate these large valves.
kale
"POST this is what you want for an a15 - 42mm inlets and 33mm exhaust valves this head allows 81kw at the wheels"
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