Holley carb factory rating & related questions?

[Warren F.]

For several years now, different publications have stated CFM ratings for the 4150 model as 780 or 800. Which is correct? I'm referring to my application as to the '71 LS6, would be interested on the LT1's as well.

Second, there are several different metering block stamping #'s used on different hp/cid configurations, both primary & secondary, do these have anything to do with the cfm ratings?

As an example, model 4150, List 4555 for '70 LT1 shows primary metering block #6333, however for 4150, List 4803 for '71 LS6, it shows primary metering block #6808. Obviously different #'s must indicate a change of something?

Can someone explain this?

[Clem Z.]

Re: Holley carb factory rating & related questions

the difference in the metering body could be the size of the power valve restriction,the size and number of emulsion bleeds and if the carb has ported or manifold vacuum distributor advance

[Clem Z.]

PS

the CFM rating difference could be a difference in the type of booster nozzles even if the venturi and throttle bore are the same size

[Warren F.]

Re: Holley carb factory rating & related questions

Clem:

Thank you for the explanation. I've been looking at Colvin's Corvette by the numbers book, on the Holley Carburetor Specification Chart page. The '71 LT1 and LS6 w/ST use same primary & secondary metering blocks. The '70 LT1 has a different primary metering # and the CA version of the '70 has yet another metering # for the primary. I was curious and wanting to understand why the need for different metering block #'s for the same model carburetor.

I had heard from many sources that the 780 cfm rating was GM's and that the 800 cfm was Holleys, however in GM publications it has been cited as both.

[bruce11495]

Re: Holley carb factory rating & related questions

Warren.......I've had an LT1 for over 20 years. I have lots of technical info on the Holley from GM. I've never seen a Corvette engine rated with a Holley (single carb) at 800CFM. I've heard the Chevelles were rated higher, I have an article that states" the largest carb installed on a GM product was 800 CFM on the Buick Riviera GS (72)", but it was a Quadrajet. I had one of these cars.

I agree with Clem. A lot of the metering blocks could be interchanged and you would probably not notice any performance change. But you notice the regular carb and the CA carb, because of the strict emissions in CA there would be a difference for sure. Probably easier to check now with the sophisticated emission equipment on the market.
Hope this helps....

[Mark #28455]

Re: Holley carb factory rating & related questions

The L88 carbs 3418, 4054 (both vac secondary) and 4296 (double pumper) were all rated at 850 CFM by GM and Holley.

[bruce11495]

Re: Holley carb factory rating & related questions

I STAND corrected!!!! Doesn't take long here to get to the truth. NOW, we all want to know what the differences were in the carbs, besides numbers on the metering blocks.........
Thanks...Bruce

[Mark #28455]

Re: Holley carb factory rating & related questions

See Clem's first note to explain differences in the carbs. Every time even one thing was changed, Holley assigned a new List number or a revised list number to the carb.(I think the original List 3310 is now a 3310-3) For instance, the List 3418 carb is a vacuum secondary 850 CFM for the 1967 L88. The List 4054 is a vacuum secondary 850 CFM carb for the 1968 and early 1969 L88. The only difference is that the base plate on the 4054 is tapped for a PCV vacuum source! (all internal passages, jets, orifices, power valves, etc. are identical) The List 4296 is for the late 1969 L88 and only differs in that it is a double pumper and the vacuum port for the PCV comes out the rear of the base plate.
The metering blocks for the 3418 and 4054 are 5247 and 5253. The blocks for the 4296 are 5895 and 5900. I last looked at the original Holley spec sheets several years ago but at that time, I noted that all the internal restrictions, power valves, etc. were the same, the only difference was the holes for the secondary "pumper". Yet Holley put a different number on the primary metering block for the 4296 anyway. The only way to know the exact differences is to look up each carb in the original Holley spec sheets.