Dissimilar carb CFM comparison discussion

[My427stang]

This is ABSOLUTELY not a "which carb do I need" question, it's more of a discussion on: "How can you compare advertised flow numbers on dissimilar intake manifolds and carburetors?"

As a background, I am potentially building a 575-600 HP 464-ish FE hyd roller Cobrajet stroker for the street, this owner wants to use a set of already-owned fancy 450 Holleys on a Tunnel Wedge (big plenum short runner single plane factory dual quad intake) with Ford progressive linkage. With a 6000-ish RPM peak HP rpm, I'd likely put dual 600s or even 715s on this build with the same linkage, but he isn't really chasing a specific horsepower number and has a real nice set of 450s he'd like to use. Basically we are looking at the trade off of "good enough" after tuning in terms of his view of cost/benefit

Also as an input, I recently finished a 461 inch FE Cobrajet, hyd roller motor, it made 568 HP @ 5500, and we used a ported dual plane stock Ford PI intake, no carb spacers, and a vacuum secondary Quick Fuel 780 vac secondary with a choke installed and open. Needless to say it had very good heads and the intake matched well.

On the new motor, I will of course match the rest of the build to the use of the car, so it's not a 1:1 engine comparison, but the peak numbers are close enough to show what I am asking about

So the question is, if a single 780 made that kind of peak power, could I match or exceed airflow with two 450s on a common plenum? I imagine that it will be more fussy on HSAB tuning with the small venturi, but I really am asking, does 900 cfm in 8 small Holley barrels flow equal to, less, or more than a single 780 QFT 4 barrel?

I can lower the peak RPM on the engine, but as a reference, the 780 build needed to stay at 15 inches of vacuum, this one runs more gear and no power brakes, so it has some room to be a little more lumpy, so initial plan is to let it spin a bit higher, but it's not an absolute requirement. Customer likes a 575 HP number, but it was just bench racing

Thoughts?

[Joe-71]

I just recently did a back to back dyno comparison on a 600 hp engine with 650 dp, 660 dp, 750 dp, and 860 dp carbs, and there was a 40 hp swing in the power ratings. All carbs were QF, or Pro Systems, or highly modified for tunnel ram. We also tried a set of 600 vs, and 750 vs 4160 style carbs, and the engine ran much cleaner with the double pumper carbs. I believe your friend is asking too much with those little 450 cfm carbs. I have them on a 289 8V, and that is what they were designed for. Joe-71

[PackardV8]

As Joe's tests validate, on the street today, the low-rise 2x4s are mostly for looks. The better 1x4s will match or out-perform, get better fuel economy, more stable return to idle and be easier to tune.

This from the guy who's running 2x4s on his Packard V8 just for looks.

[My427stang]

As Joe's tests validate, on the street today, the low-rise 2x4s are mostly for looks. The better 1x4s will match or out-perform, get better fuel economy, more stable return to idle and be easier to tune.

This from the guy who's running 2x4s on his Packard V8 just for looks.

I appreciate your feedback and would generally agree completely, but this intake is likely not comparable to a common low rise 2x4 of the 60s. (pics below) The Tunnel Wedge medium riser from Ford flows 370-390 cfm per port and is a large plenum single plane (Joe correct me on flow if incorrect) and although it may be softer on the bottom, it'll crank up top. Much different than a FE Low Riser 2x4, although parts of the curve may be happier with a single 4, this should feed the 335 cfm heads without breaking a sweat.

The fuel economy and low end, I agree completely on your assessment on a dual plane. In fact, the engine is likely a better match for a medium riser 2x4, but it is what it is as an ex racer. What I really am trying to do is see how two 450s would stack up for total airflow when I would normally use 2 x 600+ on this intake with this type of build

In the end, I may just dyno with both, before and after, then see if he wants to go bigger. I also absolutely love the Packard as you have it, I'd run it too!

[F-BIRD'88]

Find ,borrow a set of Edelbrock carbs to dyno test compare against the 450's. can be 2x750's.

Then try each on the street.

[1980RS]

In the early 80's I ran a Weiand Hi-Ram with 2 450 Holley's on a mild 327 street engine. It was the best combination for performance at that time, it always got great fuel mileage and the it looked cool also. Like other have said, today there are carb and intake setups that would blow most of the old stuff away.

[My427stang]

In the early 80's I ran a Weiand Hi-Ram with 2 450 Holley's on a mild 327 street engine. It was the best combination for performance at that time, it always got great fuel mileage and the it looked cool also. Like other have said, today there are carb and intake setups that would blow most of the old stuff away.

Thanks, I have run them on mild 428s and a 454 inch 427 FE street stroker, almost 20 years ago, and we used a set synchronized on a 410 FE with an Edelbrock cross ram. There is lots of good stuff out there, both of my FE strokers are port injected EFI, but this is a classic racer with a pedigree, not much, absolutely NO, option for the new stuff and the intake is what it is. I do agree though, if it was a max effort thing, the Tunnel Wedge would still be a competitor, but I wouldn't be running a hyd roller or contemplating the 450s

Find ,borrow a set of Edelbrock carbs to dyno test compare against the 450's. can be 2x750's.

Then try each on the street.

Thanks, but I will really only dyno and deliver in this case, not local to me and not my car to experiment later. I do think a pair of Edelbrocks would run OK, but not really a classic Ford retro thing.

At this point, I think that my additional detail with the other engine caused confusion. Let me simplify ......

Does anyone have an opinion (or experience) on whether two 450 Holleys flow more or less air than a single 780?

[PRH]

If you’re talking about an old school type 780, like a 3310, that still has the choke blade installed, etc...... then I’d expect to 2x450’s to flow more.

I’ve never flowed one of those 450’s, but the testing I have done on the old classic style Holley carbs has shown that they usually flow pretty close to what they were rated at.

The flow BS seemed to start when the HP950 came on the scene.

[sjre]

Several years back a friend threw together a 406 sbc with a street tunnel ram and 2 450s.Broke it in with single plane and 700 dp holley was a 10.5 speed pro hyper pistons,swap meet brownfield heads, 260/268 roller.Made 515 with 1 carb 520 with 2 450s.I thought they would be crap but the were surprisingly ok. Only changes were 50 cc pumps and some jetting.

[BLSTIC]

Didn't you say the 780 was on a dual plane manifold with no spacer, and the dual 450s were on a shared plenum?

Dual plane manifolds only allow each cylinder to see half the carb. It's not quite half the airflow because you've only got one intake stroke at a time vs 1.5, but typically a dual plane manifold will reduce airflow vs the same carb on a single plane.

I'd say you have more airflow by some large margin than the single 780

[My427stang]

Didn't you say the 780 was on a dual plane manifold with no spacer, and the dual 450s were on a shared plenum?

Dual plane manifolds only allow each cylinder to see half the carb. It's not quite half the airflow because you've only got one intake stroke at a time vs 1.5, but typically a dual plane manifold will reduce airflow vs the same carb on a single plane.

I'd say you have more airflow by some large margin than the single 780

I thought I answered yesterday, but somehow it isn't here

I thought the same BLSTC, BTW the carbs actually turned out to be 465 vac sec not 450 mechanical as I thought. Same dimensions as a 450 but are reproduction Ford carbs with correct linkage etc, not required for a stock progressive linkage setup but a nice clean install

At this point though, I think I am going to build a pair of old core generic 1850s and put them on the dyno after the 465s and see what they do. If they are significantly more powerful, the customer will likely buy a set. He isn't cheap, but he has these and has dyno'd them on a 490 inch motor with good results. We will see, won't be until after the 1st of the year

[6.50camaro]

I like your idea of back to back testing the 465's vs 600's . I would make sure to data log manifold vac. readings . That will help determine if the 465's are creating a restriction or not. There is a formula that uses actual manifold vac reading vs. 1.5"hg to calc. the real cfm that is flowing thru the carbs . Dan