500 cfm 3bbl

[Grey owl]

As I continue to plan the 347 been trying some different options on the stock 302. This 500 cfm is a nice upgrade from the 2100. It's a manual choke version and has the same tuning options as a Standsrd

 

[DirtDonk]

Was that intended to say “2bbl“ instead?
Nice polished version it looks like. We tried both the 500 and the 350 CFM versions on a friend’s 72 way back in the day.
The 500 felt like it had a little more at the top end, but the fuel mileage sucked and the 350 ran better all around. But we didn’t know much about tuning Holley’s and didn’t spend any time trying to tweak them.
So both could’ve probably been better.

 

[DirtDonk]

I forgot to ask. Do you run a PCV system at all?
I see the return air port is plugged on the air cleaner housing and just a breather on the driver’s valve cover.
Is there anything at all on the passenger valve cover?

 

[Broncobowsher]

There was a 3-barrel carb from Holley. The secondary was a big oval. But it still had 4 venturis.

People often don't know that 2-barrel carbs are rated on a different scale from a 4-barrel. The 2-barrel is rated at 3" of vacuum while a 4-barrel is rated at 1.5" of vacuum. If you compare the 2-barrel to the 4-barrel scale, the 500 CFM rating is like a 375 CFM on a 4-barrel scale.

 

[Grey owl]

I forgot to ask. Do you run a PCV system at all?
I see the return air port is plugged on the air cleaner housing and just a breather on the driver’s valve cover.
Is there anything at all on the passenger valve cover?

Yes there is a spacer under the carb for the PCV system

 

[ared77]

This is why I love this site, I learn something new all the time! I can honestly say I'd never heard this before -

"People often don't know that 2-barrel carbs are rated on a different scale from a 4-barrel. The 2-barrel is rated at 3" of vacuum while a 4-barrel is rated at 1.5" of vacuum. If you compare the 2-barrel to the 4-barrel scale, the 500 CFM rating is like a 375 CFM on a 4-barrel scale."

 

[Grey owl]

This is why I love this site, I learn something new all the time! I can honestly say I'd never heard this before -

"People often don't know that 2-barrel carbs are rated on a different scale from a 4-barrel. The 2eyeybarrel is rated at 3" of vacuum while a 4-barrel is rated at 1.5" of vacuum. If you compare the 2-barrel to the 4-barrel scale, the 500 CFM rating is like a 375 CFM on a 4-barrel scale."s

Thats interesting but the fact remains this carb is a significant upgrade to the 2100. As for how they determine the cfm flow I could careless , Holley also has 2bbl they rate at 500 cfm. I am pretty sure they have justification and are not trying to mislead consumers.

 

[Broncobowsher]

Thats interesting but the fact remains this carb is a significant upgrade to the 2100. As for how they determine the cfm flow I could careless , Holley also has 2bbl they rate at 500 cfm. I am pretty sure they have justification and are not trying to mislead consumers.

I'll toss in a couple links I quickly found https://grassrootsmotorsports.com/forum/grm/2bbl-vs-4-bbl/82339/page1/ and https://www.jalopyjournal.com/forum/threads/diff-between-2-barrel-vs-4-barrel-cfm-rating.527079/

While yes they do measure 500 CFM and they advertise it as so, it is a different scale from what the 4-barrel carbs are rated at. More confusing is they say the same number. 500 CFM isn't a brick wall. It is a point on a curve. The more vacuum in the manifold, the less torque and power you are making. That's the idea of a throttle. Hold back the airflow, engine makes vacuum trying to pull air in that the throttle doesn't allow. You get a low pressure in the manifold and very little cylinder filling. Engine power (at a fixed RPM) is based on manifold pressure filling the cylinder. Weather it be atmospheric or boost. Pilots don't care what the manifold vacuum is, the care about what the mainfold pressure is. This way they can base power output at altitude.

So the 500 CFM 2-barrel only pulls 375 CFM on a 4-barrel scale. That means a 390 CFM 4-barrel will outflow a 500 CFM 2-barrel.
Put that 500 CFM on a big power engine that can make a lot of power (circle track class that has a 2-barrel rule) and it might be making 6" of vacuum. That doesn't mean it has hit a wall and is only flowing 500 CFM. The greater the pressure drop the higher the flow for an opening. That 500 CFM carb might actually be pulling 600-700 CFM. It can flow it, but the engine can't make the power since the manifold pressure is down (or how most of us read it, you have manifold vacuum).

Don't go thinking that holley claims a number, it's right. Well it is. But only if you are comparing it to the rest of the crate of oranges. When you start looking at that crate of apples, they are different. Still truthful in what they are. But not the same.

There is a limitation, choked flow. Once the flow reaches the speed of sound the pressure on the low side can change and not change the flow through a hole. Only changing the high pressure side will change the flow. It is outside that of a carburetor, but here is a description of that max speed through a hole https://www.revo-rebreathers.me/wp-content/uploads/2015/04/Understanding_Constant_Mass_Flow.pdf And if you want some fun if you try and force beyond that, Randell will keep you entertained.

or "What if you funneled Niagara Falls through a straw?" pre-video version https://what-if.xkcd.com/147/

 

[Grey owl]

That's great info. What I can tell you from a practical perspective when I put it on my 302 there was a noticeable difference in throttle response and smoother idle. But I guess that should be expected taking off a 56 year old carb and putting on something far more modern will do that.

 

[ntsqd]

I'll toss in a couple links I quickly found https://grassrootsmotorsports.com/forum/grm/2bbl-vs-4-bbl/82339/page1/ and https://www.jalopyjournal.com/forum/threads/diff-between-2-barrel-vs-4-barrel-cfm-rating.527079/

While yes they do measure 500 CFM and they advertise it as so, it is a different scale from what the 4-barrel carbs are rated at. More confusing is they say the same number. 500 CFM isn't a brick wall. It is a point on a curve. The more vacuum in the manifold, the less torque and power you are making. That's the idea of a throttle. Hold back the airflow, engine makes vacuum trying to pull air in that the throttle doesn't allow. You get a low pressure in the manifold and very little cylinder filling. Engine power (at a fixed RPM) is based on manifold pressure filling the cylinder. Weather it be atmospheric or boost. Pilots don't care what the manifold vacuum is, the care about what the mainfold pressure is. This way they can base power output at altitude.

So the 500 CFM 2-barrel only pulls 375 CFM on a 4-barrel scale. That means a 390 CFM 4-barrel will outflow a 500 CFM 2-barrel.
Put that 500 CFM on a big power engine that can make a lot of power (circle track class that has a 2-barrel rule) and it might be making 6" of vacuum. That doesn't mean it has hit a wall and is only flowing 500 CFM. The greater the pressure drop the higher the flow for an opening. That 500 CFM carb might actually be pulling 600-700 CFM. It can flow it, but the engine can't make the power since the manifold pressure is down (or how most of us read it, you have manifold vacuum).

Don't go thinking that holley claims a number, it's right. Well it is. But only if you are comparing it to the rest of the crate of oranges. When you start looking at that crate of apples, they are different. Still truthful in what they are. But not the same.

There is a limitation, choked flow. Once the flow reaches the speed of sound the pressure on the low side can change and not change the flow through a hole. Only changing the high pressure side will change the flow. It is outside that of a carburetor, but here is a description of that max speed through a hole https://www.revo-rebreathers.me/wp-content/uploads/2015/04/Understanding_Constant_Mass_Flow.pdf And if you want some fun if you try and force beyond that, Randell will keep you entertained.

or "What if you funneled Niagara Falls through a straw?" pre-video version https://what-if.xkcd.com/147/

Then there are the "re-dye" 4412 "500 CFM" carbs that have been modified to flow even more air. They're quite illegal in the 2 bbl class, but they're out there winning races.

On the mild 302 in the Ranchero I tried a 4412 500 CFM 2 bbl, a similar Holley 2 bbl with smaller venturi's and throttle plates (it was an OEM application carb and I could find no CFM rating for it), a borrowed 390 CFM 4bbl and the ubiquitous 600 CFM Holley 4 bbl. The carb that ran the best, and had the best compromises of power and all-around drivability was the Carter AFB (625 CFM rating). I was in the process of getting a Q-Jet qued up to try it when I sold the car.

There is no way that I would put an AFB on a vehicle going off pavement. I have no such concerns about going off pavement with a Q-Jet. Q-Jets do want a lower fuel pressure than Holley's do. While a Holley might tolerate up to 8psi, what I recall is that 6 psi is the upper limit for a Q-Jet.

 

[DirtDonk]

I remember the old Cagle fuel pressure regulators with the vacuum attachment for the Quadrajet had a much lower PSI rating. Something in the neighborhood of 2.5 to 4. I might have the numbers wrong, but it was notably lower than all the other carburetors.

 

[ntsqd]

I was remembering 4 psi as well, but since I know that is what Webers want I wasn't sure if I was recalling correctly that Q-Jets also want that. I do recall MISF exploring what the Q-Jet on his Pontiac 455 powered '69 Gran Prix would work with or at least tolerate and 6 psi was the max.

 

[bmc69]

Then there are the "re-dye" 4412 "500 CFM" carbs that have been modified to flow even more air. They're quite illegal in the 2 bbl class, but they're out there winning races.

Way back when...I had a carb shop in Indiana that I operated as a side-line business to my day job..automotive machinist. We build lots of cheating engines for the "stock" class racers and I built the carbs that went on top of them. Good times...

 

[ntsqd]

My old next door neighbor had three race engines and two 500 CFM carbs. One was a stock carb, the other a re-dye. One engine was a 'to the rules' engine, one had stock CR, but was a (much) larger displacement, the third engine had stock displacement but more CR. Since the protest rules only allowed you to protest one specific rules infraction he was always playing a guessing game of what was going to get protested next. I don't think that they ever figured out his game. He's gone now, but if you've ever seen a "Gunsmoke" episode you've likely seen him. Gene Evans was his name.

 

[gnpenning]

Gene was in a bunch of old Westerns. Interesting that he raced. Do you recall what type of racing?

 

[ntsqd]

There's not a lot of Westerns from the late 50's into the 70's that he wasn't in. As you might guess, Gene was a fantastic story teller. He once told me why he owned a Maserati (Mistral). Word had gotten out around Hollywood's casting agents that he raced Sprint Car at Ascot, and his race car tow truck got raced occasionally at Lion's Drag-strip (it had a 392 Hemi in it). His agent told him that he was having trouble getting him work because they were afraid that he'd go racing in the middle of a shoot and get kilt dead or otherwise unable to continue with the shoot. So he sold all of the race car stuff and bought the Maserati, which was far more acceptable to the casting agents. High performance sports cars were acceptable, but a race car wasn't. James Dean taught them nothing.
When Gene retired from acting he bought a place in TN that had it's own ~3 acre lake on it. Not long afterwards he started racing again, as the team owner, in TN. A dirt oval as I understand it. It was thru him that I met Jack Elam and was introduced to Slim Pickens. He and Jack played poker weekly.
Fantastic guy to have as your next door neighbor while growing up.

 

[Speedrdr]

I remember with great fondness spending time at Lion’s Raceway watching motorcycles race. It’s been a long time past and it’s my understanding that it closed down.

 

[gnpenning]

Ntsqd@ great information. You knew some highly prolific actors. All 3 kept pretty busy. You would be hard pressed to find a western with out at least one of them. The best part was they probably didn't come with egos. Met Ben Johnson in the late 80s early 90s. Like the others a great role player. He and slim as you know, were stuntmen.

Thanks for sharing.b

 

[ntsqd]

Lions closed in 1972. (I had to look it up.)

At the time they were just friends of my neighbor. It wasn't until much later that I realized who they really were. In a weird, 7 degrees of Kevin Bacon moment, it was the father of a high school friend of mine who was the Pilot in Command during all of Dr. Strangelove's aerial B-52 footage. Never met him, he passed in Walla Walla, WA when the ILS beacon was mis-calibrated.

Reminds me, I've got some odd Holley's that I should sell off. 650 CFM Double-pumpers, but due to some weirdness in Pro Stock rules at one time they were limited to those venturi's but not to those throttle plates. So this pair of carbs has 850 D-P's throttle plate assemblies. They are supposedly a tunnel ram matched set. At one time I had an Edelbrock STR-10 cross-ram that I had thought to put them on. The STR-10 went on Cammie Edlebrock's DZ302 from what I was told. No idea if it's still there or if that ever happened, but it was Edelbrock's guy who worked on their private car collection who got it from me for them.

 

[Speedrdr]

Dang! I done got old! Didn’t realize just how long ago I was in Cali.