1969 Tri-Power Carb CFM Ratings?

[David D.]

I am trying to find the individual CFM ratings for my 1969 Tri-power carbs #3659A and 4055-1A. Any help?

[John H.]

Re: 1969 Tri-Power Carb CFM Ratings?

I am trying to find the individual CFM ratings for my 1969 Tri-power carbs #3659A and 4055-1A. Any help?

David -

The 4055-1A is 350 CFM, and the 3659A's are 466 CFM each, per the Holley website "Carburetor Numerical Listing".

[Wayne M.]

Re: 1969 Tri-Power Carb CFM Ratings?

I am trying to find the individual CFM ratings for my 1969 Tri-power carbs #3659A and 4055-1A. Any help?

A table in "Peterson's Basic Carburetion and Fuel Systems" lists the R-4055-1 at 250 CFM, and the R-3659's at 350 each.

Interesting to note that for '67, they show the R-3660 center carb at 245 CFM.

[Clem Z.]

Re: 1969 Tri-Power Carb CFM Ratings?

don't forget 2 barrel carbs are rated differently than 4 barrel carbs. 2 barrel CFM rating X .71 = 4 barrel CFM rating

[Rich P.]

Re: 1969 Tri-Power Carb CFM Ratings?

Don't remember individual ratings But do remember reading back in the day the
total was 1050 CFM don't know if thats correct and would be interested to see.

Rich

[Clem Z.]

Re: 1969 Tri-Power Carb CFM Ratings?

David -

The 4055-1A is 350 CFM, and the 3659A's are 466 CFM each, per the Holley website "Carburetor Numerical Listing".

correct so the total CFM is 1282 X .71= 910 CFM the way 4 barrel carbs are rated.

[Bill M.]

Re: 1969 Tri-Power Carb CFM Ratings?

Don't remember individual ratings But do remember reading back in the day the
total was 1050 CFM don't know if thats correct and would be interested to see.

Rich

Corvette News Vol. 10, no. 1 "3 X 2 = 6" has "up to 1000 CFM". I remember seeing 1050 also...somewhere.

[Clem Z.]

Re: 1969 Tri-Power Carb CFM Ratings?

go thru a holley catalog and find a 4 barrel carb with the same venturi size and throttle bore size as the 2 barrel and divide the flow rate by 2 and you will have the true CFM of the 2 barrel carb. 4 barrels are measured at 1.5" Hg pressure drop and 2 barrels are measured at 3" Hg of pressure drop across the carb because at WOT with a 2 barrel there will still be a readable vacuum in the intake manifold at WOT as the 2 barrel is a restriction to the needed air flow. when there are more than one 2 barrel carb it acts like a 4 barrel carb so you have to read the air flow differently

[Duke W.]

Re: 1969 Tri-Power Carb CFM Ratings?

don't forget 2 barrel carbs are rated differently than 4 barrel carbs. 2 barrel CFM rating X .71 = 4 barrel CFM rating

I remember when this discussion came up early in the days of the TDB, circa 2000, and I'm sure you can find it by searching the archives, which has all the math. It was a long thread.

What most guys don't realize is that 2-bbl. carb flow is speced at 3" Hg depression, but 4-bbl. carbs are flow rated at 1.5"; 3" depression for 2-bbls and 1.5" for 4-bbls are the defacto industry standards for flow testing.

So the 2-bbl test depression is double four-barrel test depression, but since flow varies with the square of depression, converting from one to the other is simple. To convert 2-bbl flow to "equivalent 4-bbl flow" you divide 2-bbl flow by the square root of 2, which is 1.414, which is the same as mulitiplying by 0.71, as Clem did. In other words, if you double the test depression, flow only increases by the square root of two - 1.414. Likewise, if you half depression flow will be 71 percent of the original depression.

When you add up the 3" depression flow of trips, which is somewhere in the range of 1100 CFM and convert it to equivalent 4-bbl flow, it comes out to about 780 CFM, which is about the same as the single Holley used on L-72. Thus, trips don't really "flow more".

Beyond this, IMO the trips manifold design is compromised compared to the 4-bbl manifold, and I would bet that the same SHP long block with a properly tuned L-72 induction system will make marginally MORE power than with a properly tuned L-71 induction system.

Trips were primarily a marketing gimmick.

Duke

[William F.]

Re: 1969 Tri-Power Carb CFM Ratings?

Duke,
I don't follow why you say trip manifold is compromised compared to 4 bbl. Would seem to me that trip setup would give more even filling of end cylinders. Am I wrong? On the hydraulic lifter big blocks, not talking about highest end solid lifter engines, wouldn't the trip setup have an advantage?

[Ken A.]

Re: 1969 Tri-Power Carb CFM Ratings?

I remember when this discussion came up early in the days of the TDB, circa 2000, and I'm sure you can find it by searching the archives, which has all the math. It was a long thread.

What most guys don't realize is that 2-bbl. carb flow is speced at 3" Hg depression, but 4-bbl. carbs are flow rated at 1.5"; 3" depression for 2-bbls and 1.5" for 4-bbls are the defacto industry standards for flow testing.

So the 2-bbl test depression is double four-barrel test depression, but since flow varies with the square of depression, converting from one to the other is simple. To convert 2-bbl flow to "equivalent 4-bbl flow" you divide 2-bbl flow by the square root of 2, which is 1.414, which is the same as mulitiplying by 0.71, as Clem did. In other words, if you double the test depression, flow only increases by the square root of two - 1.414. Likewise, if you half depression flow will be 71 percent of the original depression.

When you add up the 3" depression flow of trips, which is somewhere in the range of 1100 CFM and convert it to equivalent 4-bbl flow, it comes out to about 780 CFM, which is about the same as the single Holley used on L-72. Thus, trips don't really "flow more".

Beyond this, IMO the trips manifold design is compromised compared to the 4-bbl manifold, and I would bet that the same SHP long block with a properly tuned L-72 induction system will make marginally MORE power than with a properly tuned L-71 induction system.

Trips were primarily a marketing gimmick.

Duke

Duke,
I respectfully disagree re: marketing gimmick. A well set up 427 of 67-435 configuaration always dynoed more HP & torque than any 4 bbl setup until we got up to dominator size (1050). With a little ignition work & carb rework & cam springs replaced along with those dumb pushrods replaced, we saw 550 HP with repeatability. Bumping up the compression ratio got you nearly 600 HP. However, in stock "smog' tune, it was like a street Hemi-easilt out run by a well setup 350 small block.The only limit was the tires, which would go up in smoke at the touch of the throttle
Ken

[Duke W.]

Re: 1969 Tri-Power Carb CFM Ratings?

Do you have any actual data to share?

I'm talking about starting with a "pure stock" L-72. Then replace the L-72 induction system with a stock L-71 induction system.

NO OTHER CHANGES, so it's an apples to apples comparison with the induction systems being the ONLY change.

Duke

[Duke W.]

Re: 1969 Tri-Power Carb CFM Ratings?

Duke,
I don't follow why you say trip manifold is compromised compared to 4 bbl. Would seem to me that trip setup would give more even filling of end cylinders. Am I wrong? On the hydraulic lifter big blocks, not talking about highest end solid lifter engines, wouldn't the trip setup have an advantage?

Most V-8 manifold designs are compromised to one degree or another. The ideal set up would be identical inlet runner geometry on each cylinder (although Can-Am engines used two different length inlet trumpets, which helped smooth out the torque curve, but other than trumpet length, the runners were pretty much identical.)

A typical dual plane manifold has four short and four long runners, and overall runner geometry (think curvature and height) is also different because of the way the runners are stacked on top of each other and the different lengths from the plenum to the head port boundary.

Most tri-power manifolds have even more runner geometry variation, because of the longer plenum, which further compromises the design from the ideal, which is why I doubt that the L-71 system produced anymore power than the L-72 set up, and I might even be willing to make an even odds bet that it was marginally less.

And remember, at 1.5" Hg depression, trips flowed about the same as the L-72 four-barrel, so any difference in power would come down to runner flow variation and fuel distribution issues in the manifolds.

Duke

[Dick W.]

Re: 1969 Tri-Power Carb CFM Ratings?

Seen more than one 425 vs 435 side by side race, never saw a 435 win. There is a definite "seat of the pants" difference between the two, the 425 feels stonger. Just my 1/2 cent worth

[William F.]

Re: 1969 Tri-Power Carb CFM Ratings?

Duke,
You may have answered my question. It would seem that the advantage to the trip setup would be in more even fuel distribition since end carbs are closer to end cylinders.I don't know of any direct comparison between single 4 barrel and trip setup on CORVETTE big blocks, but if you search, you can find SOME Pontiac apples to apples comparisons saying the trip setup turned significantly better quarter mile times. I say SOME because some articles (but not performance comparisons) in '67 when Pontiac ditched the trip setup for quadrajet said horsepower was equal-but here again they were talking about a quadrajet, not a Holley.Go figure.