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Su Dashpot Oil Choice

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#31 Ethel


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Posted 04 August 2011 - 11:30 AM

Just because i'm a sucker for detail...sorry...! Maximum torque is produced when the engine is at it's most efficient at filling the cylinders, I.e. peak V.E. (volumetric efficiency) which coincides with inlet port peak velocity. Maximum power is produced when the engine is pumping the greatest volume of air which is when the carb sees the greatest airflow.

You're right. However, peak VE is when the cylinder needs its largest dose of fuel (per cycle). Above peak VE the fuel dosage would decrease as the air density decreases. A carb is a fuel metering device and the SU does that by the needle taper.

As the revs go higher the increasing airspeed will still create more depression across the wide open venturi to suck fuel through the jet at a faster (but directly proportional) rate. That's why some carb needles are shorter than others yet still manage to meter the fuel.

#32 ado15


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Posted 04 August 2011 - 03:06 PM

Just to throw yet another spanner in the works, I've always favoured ATF in the SU's! I haven't run an SU equipped A series for some years, but have used it extensively in Rover V8's with success.

#33 1.3Cooper


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Posted 04 August 2011 - 06:10 PM

I always assumed they specified 20/50 for the carb as it was the same as went in the engine and so was easy and readily available and did the job ok.

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