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.

 

[62 C1]

https://documents.holley.com/techlibrary_selecting_a_carburetor.pdf

This is the whole article and only two pages nut very useful - some pointers if you have manual transmissions and why double pumpers work well

 

[Chuffer]

https://documents.holley.com/techlibrary_selecting_a_carburetor.pdf

This is the whole article and only two pages nut very useful - some pointers if you have manual transmissions and why double pumpers work well

Interesting data and information that continues to explode the myth that bigger is better . It certainly validates my down size to an Edelbrock `Performer ` 600 cfm that has mechanical secondaries instead of the gnarly old 750cfm Holley that I inherited when I bought the car . It always tended to bog down when hitting the gas but now performs a treat . As I have mentioned elsewhere , I run an ex Camaro IROC - Z 305 cu inch so the graphs seem to line up spot on for me .

 

[Forrest Gump]

For my engine and using the first chart:
383 cu in, max revs 6000 equates to about 700 cfm on the chart. Then multiplied by the efficiency factor of 85% (for a performance engine) to get required carb size: 85% x 700 = 595cfm.
The 600cfm Holley double pumper that I have is the correct size

 

[Roscobbc]

Interesting graph - offered my 489 cu in and max rpm of 6200 rpm against it and it shows perhaps 925 cfm on chart - 925 divided by 85% = 786 cfm. My 950 cfm Holley DP is certainly too large according to those calc's (I agree and knew that when I bought the carb) - It's a generic branded race spec' Holley and in fairness it took a fair bit of tweaking to set-up.
Reality is that all these measurements are with carb at WOT with all four barrels fully open and flowing max cfm. Whereas 95% of our driving will be done on the two primary barrels where perhaps carb tuning/jetting/accelerator pump settings are of more relevance?

 

[CaptainK]

Looking at that chart, with my little 327 @ 5000rpm max gives around a 500cfm carb. I have a 600cfm Edelbrock 1406 on it and it seems ok.