454 - 425 hp, 800 CFM really 780 CFM

[Bruce N.]

I'm trying to find out if the 71 LS6 454/425 came with a 800 CFM or was it really a 780 CFM? Thanks.

[Roy V.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

the technical information manual and judging guide for 1970-1972 states that in 1971, the 425 (LS6) with a 4 speed used Holley list#4803 and the automatic equipped LS6 used Holley list #4802 both were 780 cfm carburetors.There is also a note " A carburetor revision apparently took place with the LS-6 manual-transmission equipped cars during 1971 model year production. Holley model 4150 (List#4803) original production carbs used a primary metering block with stamped numbers 6333 and a secondary metering block with stamped numbers 4519. These appear on LS6 cars from beginning of production. Sometime in March 1971, Holley model 4150 (list #4803-1) came into use, with the primary metering block having the stamped numbers 6808, while the secondary metering block remained 4519. Both carburetor versions have been seen on LS6 manual-transmission cars built during the April time frame. The second version is more prevalent during the remaining months of production."
Hoped this helped some.
Regards
Roy

[John H.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

I'm trying to find out if the 71 LS6 454/425 came with a 800 CFM or was it really a 780 CFM? Thanks.

Bruce -

Holley lists them as 780 CFM.

http://www.holley.com/data/TechServi...%20Listing.pdf

[Bruce N.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

Roy, John

Thank you very much. This is exactly what I needed to know.

[Joe L.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

I'm trying to find out if the 71 LS6 454/425 came with a 800 CFM or was it really a 780 CFM? Thanks.

Bruce------


Very few Holley carburetors were ever rated at 800 CFM and I don't think that any 1972 or earlier original carburetors were so-rated.

[Warren F.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

I'm trying to find out if the 71 LS6 454/425 came with a 800 CFM or was it really a 780 CFM? Thanks.

GM rated the LS6 carburetor at 800 cfm while Holley stated 780 cfm.

[Bruce N.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

Warren,

GM must have had a lot of problems with typo's in those days..lol. They also claimed this motor had 9 to 1 compression. Thanks.

[Duke W.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

The '71 LS6 was advertised at 9:1, and like all other '71 and later engines was designed to operate on 91 RON (same as today's 87 PON) unleaded fuel per GM management mandate.

Do you have measurements that show otherwise?

Duke

[Bruce N.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

Key word - Advertised. My motor was 8.5 :1.

[Duke W.]

Re: 454 - 425 hp, 800 CFM really 780 CFM

That's typical for the C1-C3 era. If you pull the engines apart and measure everything required to compute the CR, you find that the actual is about 0.5 point below the advertised value.

It was mostely due to the decks being high - up to .015" above nominal, and I don't think I've ever heard of a OE machined block being below nominal.

Duke