Lotus Elan

+2 Twin Cam

PostPost by: Rob_LaMoreaux » Wed May 12, 2004 2:07 pm

one thing to check is your head gasket. The proper Elan head gasket has smaller holes for the coolant near cylinders 2 and 3 to make sure the coolant does not bypass the cylinder head and go straight to the thermostat. The wrong head gasket will make cylinders 1 and 4 have hotter valves and thus be more likely to burn.

Unless you drive on the highway an awfull lot it is extremely unlikely that you have valve seat recession even if you did not run an additive to your unleaded fuel. Just ask all the folks over here in the USA who have run their car with stock valvetrains for 20+ years on unleaded. The seat inserts in the aluminum head are hard enough for normal use.

Your valve could easily have burned due to it being too thin on the edge after being reground to fix the first problem. A thinner head valve will run hotter and thus burn easier.

Now I just need to check Holbay about that 288degree 0.389 lift CAM. It sounds just perfect for what I am looking for for my Stromber engine then later for my weber engine when I get time and money to rebuild it.

Rob
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PostPost by: david.g.chapman » Wed May 12, 2004 7:21 pm

Thanks - My old head gasket had the small holes.

My pet theory is now overground valves that are just too thin.
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PostPost by: steveww » Thu May 13, 2004 11:02 am

Rob,

I would be interested to hear a report if you go for the Holbay cam.
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PostPost by: steveww » Thu May 13, 2004 4:46 pm

General principals say that a valve flows most efficiently when lift is 0.25 its diameter, any more lift than this does not produce significantly more flow.

Some valve sizes:

Std 1.528 / 4 = 0.382
Sprint (Big Valve) 1.563 / 4 = 0.39
Race 1.7 / 4 = 0.425

As a general rule of thumb the output of any engine is governed by the size of the inlet valves. Lets do some math....

Starting with standard valves:
Total area of all inlet valves = 4732 mm2
Divide by 30 (magic constant) = 158bhp

Big Valve: 4951 mm2 / 30 = 165bhp
Race: 5856 mm2 / 30 = 195bhp

Thus big lift cams with standard valves do not really give you much more. The Big Valve engine is only worth another 7bhp at most. If you are going for big valves go really big.

BTW: plotted out the Holbay cam and the overlap (Stromberg) happens at the same place as the 26/66 Lotus cam, the valve will just have more lift at this point. The duration of the overlap is slightly longer. 92 Lotus vs 108 Holbay 17% increase. MOP is the same for both.
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PostPost by: sotul86 » Mon May 17, 2004 11:40 am

Hi

I?ve had heard that it?s bigger chance to burn valves especially No 4 with standard exhaust manifold ....so You should have a aftermarket manifold if You don?t already have one.

Good luck

Johan
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