Engine issues - advice needed

[Chris Sale]

Thanks Mike. Made the mistake myself when I had my '66 with the L79 327. Replaced the stock 585 CFM vacuum-secondary Holley with a 750 CFM double-pumper. Took me a long time to accept/realise that it never ran as well with this. Went back to stock after seeing the chart and normal performance was restored.

Chris Sale
'64 Coupe

 

[Roscobbc]

An interesting chart - especially where it shows the relationship between RPM and WOT and how stall speed of a torque converter plays an important part. In the case of oversized carb usage a vacuum secondary Holley will only ever open the secondary throttles if the engine vacuum allows it. A kinda 'failsafe' for incorrect carb selection. Quadrajet has a similar working - if the engine vacuum isn't sufficient it won't open the secondaries.
Double pumpers have no such restrictions and 'mashing' the throttle to the floor on a lower powered, high geared auto box with stock converter can over-fuel the engine and create major bore wash in extreme conditions.

 

[Chuffer]

Thanks Mike. Made the mistake myself when I had my '66 with the L79 327. Replaced the stock 585 CFM vacuum-secondary Holley with a 750 CFM double-pumper. Took me a long time to accept/realise that it never ran as well with this. Went back to stock after seeing the chart and normal performance was restored.

Chris Sale
'64 Coupe

I had similar issues . When I bought the C3 it had a 750 CFM Holley on the 305 cu inch that is from a from a 1985 Camaro IROC -Z . Totally over carbed and never seemed to run well and ultimately got worse and worse . Swapped it out for a brand new Edelbrock 600 cfm Performer and once properly set up and the timing correct it was amazing the difference in performance .

 

[Forrest Gump]

An interesting chart - especially where it shows the relationship between RPM and WOT and how stall speed of a torque converter plays an important part. In the case of oversized carb usage a vacuum secondary Holley will only ever open the secondary throttles if the engine vacuum allows it. A kinda 'failsafe' for incorrect carb selection. Quadrajet has a similar working - if the engine vacuum isn't sufficient it won't open the secondaries.
Double pumpers have no such restrictions and 'mashing' the throttle to the floor on a lower powered, high geared auto box with stock converter can over-fuel the engine and create major bore wash in extreme conditions.

Exactly. Quadrajets are rated at 750 so theoretically a bit big for the thousands of [low performance] small block cars GM fitted them to….but the vacuum secondaries cater for that.