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MG MGA - Thinning the Throttle Shaft

Has anyone out there tried thinning the throttle shaft within the carb body and cutting the butterfly bolts to improve airflow past the shaft? I read about that technique recently, and I'm wondering how effective the technique is in practice.

Regards,
M.D.
'57 Coupe
M. D.

I understand thinning the throttle shafts but by cutting the butterfly bolts, are you referring to filing them flat once they are mounted on the shafts.Therefore reducing the amount of turbulance of the air flow over the heads of the screws.
gord
Gordon Harrison

Gordon:

Yes, I meant filing the butterfly bolts flat to reduce turbulance.

Regards,
M.D.
M. D.

I think this kind of thing helps to sell books but whether it has any benefit is another matter!
I think even the most simple mind would be able to understand that any modification in that area would only affect the performance at full throttle, at all other times the SU damper will be having an enormous affect upon flow through the carb as would the angle of the throttle disk.
So how often do you have extended full throttle operation to gain some if any benefit MD?
Now if you were to race your car around the banking at Indy with full throttle for a few miles perhaps a horsepower may be a benefit.
Bob (robert) I am turning? yes I once owned an MGWasp!!

The difference is so small as to be not worth the risk of going too thin and having the torque of the return spring on one end and the cable on the other twist the shaft - permanently. I saw one guy that managed to do that.

If you think that the piston on an SU is completely out of the way of the venturi even at full throttle, I suggest that you stare into the bore while having a friend floor it in neutral......:-)
Bill Spohn

Thanks all; and excellent point about the piston, Bill.

Regards,
M.D.
'57 Coupe
M. D.

There have been dyno results posted with the amount of air flow that can be gained from cutting away half the throttle shaft, knife edging the throttle plate, and using smaller "button head" screws. I've done the modification to my SUs, as described in detail in David Vizard's book. Its good for an air flow increase of 20 cfm per carb. Its not exactly necessary unless you've exhausted all other routes to gain power and you actually need the extra airflow!

Tapering the lead edge of the piston by 15 degrees DOES increase air flow at lower speeds and does give better throttle response, while not providing any noteable horsepower. Using a thinner dashpot oil would probably be another means to the same end.
Jeff Schlemmer

Jeff I don't understand increased airflow at lower speeds. If the airflow increases then the car goes faster or if you close the throttle the car runs slower, how is that affected by thinning the throttle disk. Surely it just means the throttle disk would need to be a slightly different angle when cruising, wouldn't it? No benefit would be had would it?
Bob (robert) I am turning? yes I once owned an MGWasp!!

This thread was discussed between 12/09/2006 and 16/09/2006

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