Hot Rods 327 Chevy how much CFM for carbs is needed.

Helping a club member trying to get his 327 Chevy powered Three window Deuce coupe ready for the season.
The engine has a 6 carb McGurk intake the carb base pattern for the carbs is early Ford 94 type three bolt base. The carbs being used are original Stromberg 97's.

Since its impossible to tune it with all six carbs due to there being no easy way to get to the idle mixture screws on the center and rear carbs for now were trying to tune the engine using just the center two carbs. The other four carb locations two in front and two at the rear are just blocked off so theres no carbs for now.

We have found it impossible to get the engine to run correctly on just the two Stromberg's in the center even when trying more main jet and modifying the imolstion tubes nothing helped.
I pointed out the fact I have never seen any two deuce intakes for a Chevy 327 engine or for any small block for that matter. So I'm thinking there's not enough carb CFM to make this tune up possible. What do you guys think?
Ronnieroadster

 

I'd be more likely to believe that the intake manifold design is preventing it from running correctly with just two carbs.

What's the plenum like?

 

@SamIyam ran the Roach Rod on two carbs. I would suspect fuel distribution issues with that manifold

 

Ronnie, do you think maybe the front and rear cylinders are lean and the center ones are ok or fat? Must be a pretty hot set up if client looking to run 6 97s. And maybe starving with only 2
Please keep us updated

 

Maximum intended RPM, a**** other thing's, usually determines CFM needed. Tell us more about the combination for a better answer. A 327 will run reliably with a single 2 barrel, as long as that 2 barrel doesn't have a plenum the size of a watermelon to fill. It's gonna take alot of carb to fill up those plenum's to make it run right, even at an idle.

 

Absolutely correct. You can't have a serious discussion about "what CFM?" without considering intended use, maximum operation RPM, and matching carb/s with manifold ........ and cam....... and compression....... etc.
It really is a package deal where the sum of the parts is greater than what each separate part offers.

 

Go to the FLAPS and acquire some small vacuum hose.

Cut the hose in several pieces each about 4 inches long.

Paint a small line parallel with the tube at each end of each tube.

Turn the idle mixtures screws in on the center and rear carburetors until they are "thumb tight".

Fit one of the tubes over each of the idle mixture screws on the center and rear carburetors with the painted line in the 12:00 o'clock position (or whatever clock position meets your fancy). The line will now help you know the position of each screw.

Re-install the carbs, and tune the idle mixture screws by rotating the tubes. Once you are satisfied with the tune, the tubes may be removed so as not to interfer with the eye-candy.

I am not familiar with the manifold you are using. As some have previously mentioned; the design of some multi-carb intakes REQUIRE that all carburetors be present and functional. Only an inspection of the internal design of the manifold can determine if some of the carburetors may be removed.

Jon

 

Interesting information. This intake is being used on the street the car is a driver only not being used in any type of compe***ion. The intake is an open plenum design with a small cross over runner at each end of the runners to help balance. So it may be this will never work with only two carbs. The McGurk intake with six carbs is great eye candy but the car is a driver. Carbking great idea on getting to the idle screws Thanks for that.
The stock original Stromberg 97 has 170 CFM so running two is 340 CFM. If there was a stock 327 that ran on only a two barrel would anyone know the CFM of that carb?
Ronnieroadster

 

They were not 340 for sure.

I for one cannot see how this running one carb on the log of of 3 on each would not work just fine especially if the two logs have balance tubes on each end. If it will run with them blocked off it will run with them used and the throttles closed. Yes they will naturally bleed in some air and maybe not evenly because of shaft wear. Each idle mixture screws may need to be just off the seats as Carb King states and definitely not all even for what air is leaking past the shafts and throttle plates. Once it idles and is repeatable .. then work on the progressive linkage.

From experience I could not get my sons 3-2’s on his 302 Jimmy 6 to repeat idle. They were on a Howard log style manifold. I found it was shaft leakage and bought br*** bushing kits and nylon strips to fit the shaft that went between the end of the bushing and the throttle plates. A pain to get all to fit right but the engine idles perfect now has a repeatable idle and can take all 3 open when he wants as long as the engine is warmed up.

 

The small GM 2 jet is less than 300 cfm if I recall. No reason it shouldn't run on 2 carbs. Maybe you have a vacuum leak somewhere?

 

I don't really like 6x2 on a V-8; basically just twice the problems of a 3x2 on a V-8; HOWEVER:

The OP asks about CFM. CFM is basically a liar's game as ratings are generally supplied by marketing, nor engineering. For decades I have believed that the individual with the greatest understanding of CFM was Mark Twain. Mr. Twain stated that "figures don't lie, but liars figure"!

Forget CFM for a moment, and use the "old-school" method of comparing carburetors.

If you calculate the venturi area of the EE-1 (so-called 97) it is 1.47 square inches.
Now calculate the venturi area of the primary side of a Carter 3721s AFB (factory on the 327). This calculates to 2.45 square inches.

So 2 of the EE-1 (center carbs only) would have 2.94 square inches of venturi area, which is 16 percent greater than the primary side of the 3721s.

Enough difference so that idle characteristics would be much better by idling on all 6 carburetors; using the center carbs for up to about 70 percent throttle. The venturi air velocity is lower than it should be on the 2 center carbs, thus the idle is too lean. If this were a single carb application, and the idle mixture was too lean, most would simply increase the size of the idle jets. With the extra carbs, no need to do so, just add idle mixture from the other carbs.

Now calculate the area of the secondary side of the 3721s, and the answer is 3.83 square inches. So adding the primary and secondary totals about 6.3 square inches for the 3721s.

Now multiply 6 times 1.47 (6 EE-1 carbs) and the total is 8.82 square inches; significantly more than the 3721s.

BUT: using the linkage, utilize the center carbs up to about 70 percent throttle and start opening the other 4, restricting the opening of the other 4 to 60 percent capacity. Now the total of 2 centers at WOT and 4 ends at 60 percent add up to about 6.4 square inches at WOT, compared to 6.3 square inches for the 3721s.

Throttle restriction is not a new concept; virtually every Y-block Ford factory dual quad I have seen had throttle restrictors on the secondary sides of both carbs.

So you won't have too much carburetion, it will just be in the wrong places on top of the engine!

Jon

 

L78 327 had a 585/600 cfm Holley. Medium duty truck 327s probably had ~200 cfm Rochester carbs.

Cfm numbers on Holley Four barrels are measured at a different pressure drop than two barrels, from what I remember.

I wouldn’t call it a liars game, but you do have to have some knowledge of engineering and fluid mechanics to understand such things, I suppose.

 

The L-79 327s also were using factory installed 585 cfm Holley carbs..

 

You need 1.67 cfm per Hp at wide open throttle.

So a 300 hp 327 needs 501 cfm and it should hit peak volumetric efficiency at 5284 rpm

Here's the Hp/Tq for a 1961 327 Corvette [with a Carter WCFB of approx 525 cfm, but some claim only approx 400 cfm]

 

Not your exact setup but here's what I had years ago. In my 29 Roadster I had .A 1955 Pontiac 287 with a Edelbrock X2 6x2 intake with just the center 2 working on a pair of Holly 94s and the other ones just blocked off ,the thing idled and ran great and got 20 miles of the gallon but it was just a worn out 287. I ended up getting a 347 and bored it to 354 with 10:00 to 1 Pistons ,it still ran good on the center 2 once I put the intake back on but was lean on the top end so I ended up running the outside for carburetors and blocked off the center too and it ran great, never did get to making all six work as I wanted to try a different manifold. Just my experiences and if you've got something similar I don't see any reason the two'97's shouldn't work

 

I think it was Hot Rod Power Your this past year, there was a YouTuber who had experimented with a lawnmower carb on his SBF, he actually swapped from a 4 barrel to the tiny lawnmower carb during the tour and ran the rest of it like that. He used some magic devices to keep it from being lean, power was down but his mileage was incredible.

I mention this to say that CFM isn't all that critical but for all-out max RPM performance. As has been well covered, most multi-carb intakes should run fine on the center carb(s) with a progressive linkage, so you most likely have a tuning problem.

 

Overall I'm thinking this combination is not going to work the way its set up presently. Time to go to plan B. Thanks for all the impute it has been very informative I certainly know lots more now and to me this is the best part when I can learn more.
Thanks Everyone
Ronnieroadster