Lotus Elan

Head port size

PostPost by: benymazz » Tue Oct 22, 2019 2:32 pm


I have a Weber TC head in my possession that has intake "throats" (as measured at the end of the runner where it mates to the carb) with a diameter of 45mm. It is my understanding that 1) this dimension on a standard Weber head is 40mm and 2) on a Weber 40DCOE the throat diameter is 40mm. The question is, if 40DCOEs are put on this head, will the abrupt change in diameter cause turbulence or a pressure drop and adversely affect performance? If so, what is the best course of action to correct this situation? Buying 45's? or making custom spacers to go between the carbs and the head that provides a smooth "aerodynamic" transition?

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PostPost by: rgh0 » Wed Oct 23, 2019 9:51 am

A step up in diameter after the Carb is not a great issue unless your looking for the last few HP and if thats the case then you probably want 45's anyhow. In general the ports on a standard head cannot be ported to give a good transition for 45's as you run out of metal and you will find they close up quickly back to the 40 mm diameter taper of the original ports. To get the best out of 45's you need the new larger inlet heads or the McCoy conversion

Steps up and down have some strange effects in both exhaust and inlets and can be negative or positive depending on how done due to the pulsating waves in the inlet and exhaust

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