Which Holley throttle body should I use?

[51geezer]

I see advertised the vacuum-secondary style, and the mechanical-secondary style. Which is correct for a 425 mixer?

[storm]

The best one to use is the mechanical secondary for the squarebore carb.
You can use a vacuum secondary but you will loose 2 barrels if you do because the secondaries won't open effectively.

[TroyT]

Vacuum secondary can be modified to work but a standard mechanical secondary is best.

One note pay attention to the mounting holes, some base plates have a hole right in the middle that a 425 doesn't use. That needs plugged, drill and tap for an 1/8" pipe plug is the easiest, install plug in the top to prevent any risk of it falling out in the top the engine.

[51geezer]

Thanks for the replies. I have seen the vacuum secondary throttle body advertised for sale as a good mate for the Impco 425 but it was apparent there would be a problem controlling the secondaries. Thanks for validating my concerns. So, mechanical linkage it will be, but another question - does throttle bore size matter, ie 1 9/16" or 1 11/16"? My motor won't need the larger size but maybe the Impco carb would respond better to one or the other (?).

[storm]

I can't comment on the size of the throttle body based on experience. I'd love to test different TBs on a 425 to see if different ones make the 425 flow less than advertised or not. I think you can assume a TB that fits a Holley 600 cfm carb will be suitable and not restrict flow.

[Frank]

My guess is that the mixer has a bigger effect on flow than the throttle valves. I would go with the smaller bore size for better part-throttle control and make sure that the secondaries are set up for progressive opening.