Lotus Elan

Odd oversize exhaust valves

PostPost by: benymazz » Sat Jun 22, 2019 6:16 pm

When I took apart my father's old racing TwinCam a few months ago I noticed that the intake valves were the stock size, but the exhaust valves had been changed to oversize at one point. I measured the head diameter to be 1.364" probably +/- .004", the valve was not extremely clean) and I measured the overall length to be 3.943" on one valve and 3.945" on another.

My knowledge isn't the best in this department, all I really have to go on is that I know the stock valve had a head dia of 1.32"/3.85" OA and the D production exhaust valve was 1.375"/4.04".

I have never seen a stock TC exhaust valve before so I do not know if they have the concave impression in them similar to the intake valves, but these valves did not have that. The face was flat.

Soooo, has anyone seen these valves used in a TC before? And what might the rationale be, if any, behind using high lift long duration cams, slight intake porting, stock size intake valves (engine was built to race spec in 1969 - before the "Big Valve" TC was invented), and oversize exhaust valves?
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PostPost by: rgh0 » Sun Jun 23, 2019 12:33 am

The twincams exhaust valves and ports are really too big unless you get to the very top end of the development range, so there is no real need for larger exhaust valves in an engine with standard inlet valves and mild porting. Valves come in both flat faced and dish shaped designs depending on the supplier and hard to tell what was originally fitted.

It sounds like the engine was rebuilt in the traditional racing style of the day but they needed new exhaust valves and found something they could fit

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