Which Holley throttle body should I use?
Which Holley throttle body should I use?
I see advertised the vacuum-secondary style, and the mechanical-secondary style. Which is correct for a 425 mixer?
Re: Which Holley throttle body should I use?
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.
You can use a vacuum secondary but you will loose 2 barrels if you do because the secondaries won't open effectively.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Re: Which Holley throttle body should I use?
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.
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.
Re: Which Holley throttle body should I use?
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 (?).
Re: Which Holley throttle body should I use?
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.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456
Re: Which Holley throttle body should I use?
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.