Inliners International Forums General Discussion Engines to triple or double-offenhauser intake choice

Hey guys as you all know I am working on a later model 261 punched 60 out with porting/polishing, cam-3/4 or so, 267 duration and .455 at intake-.475 at exhaust, headers, HEI, etc. I was recently givin an opportunity to grab a offy double intake for this motor for FREE and I'm sitting on an offy triple at the moment, but I only have 2 carter/webbers at the moment. So, would 2 or these carbs be adequate to properly feed an engine of this size and build orientation or does it really need a third carb on this triple carb intake? Would it be easier for a break in to just use the double set up? Thanks in advance, Tye.

i have two Holley Webers on a 320cid GMC that made 362 ft-lbs of torque at 3050 rpm and 265 horsepower at 4000 rpm with an Ellis manifold. If you want more power at higher rpm you may need that third carburetor. Otherwise those two will likely do.

God's Peace to you.

d
Inliner #1450

wow thats a lot of power your GMC makes lol I wish mine would make those kinds of numbers. Does anyone know what these carter webers run CFM wise-is it like 200 cfm. Arent the holley/webbers bigger than carter webbers-Anyone know what the CFM rating on the holley/webbers is by chance? Thanks in advance peoples.

Info on Internet says the Holley Weber is about 350 cfm rated as a 2bbl.Use .7 to compare to a 4 bbl,or 245cfm.
10 years ago I wrote an article for Inliners on my many induction experiments on a 261 Chevy.The engine was mild,block and deck milling for about 9-1 compression,Fenton headers,stock head with a quality valve job,cam .428 lift,206 degrees at .050 lift.
Original set up was two stock Rochester off mid 50's 235 car engines Then two Holley Webers,various four barrels on a Clifford intake.Carbs were tuned properly for the engine.All intakes were water heated.
The testing was the subjective "feel" and the time it took between two objects on a back road using full throttle in second gear of a 3 speed from 2000 to about 4200-4400 rpm.This happened over about 2 years time.Just a hillbilly dyno .
Big surprise,the two leaky Rochesters with stock jetting and power valve "jet" reduced about 15 percent were quite good.The Holley Webers had a slight top end advantage as did the four barrels.Going larger than 400 cfm rated four barrels gave no added power in my tests with a rpm limit of 4400.
Best power was from a 500 Holley 2305 staged two barrel,a rare carb..........I also think multiple carbs function a bit better that a single carb of any size.

In round numbers, 269" at 90% VE turning 4,500 RPM needs about 347 CFM, if rated at 1.5" Hg (like 4 bbl.).
Using 1 or 2 bbls. about 491 total at 3.0" Hg, for 2 carbs on a common manifold divide by 2.
The .7 correction is the square root of the difference between the 2 rating pressure drops.

The power figures are very interesting, but they can't take into account relative manifold efficiency (how restrictive, plenum volume) or mixture distribution imbalance (3-4 port is richer than 1-2 or 5-6) - those are pretty much decided by the manifold manufacturer unless you want to do some surgery.

I bought my triple Holley/Webers 5200 from Tom Langdon years ago and they make my warmed over 292 run like a demon. He said they only flow about 150-175 CFM. Seems to be some difference somewhere. I adjusted my carbs to open up only when the pedal is on the floor. Otherwise, it ran way too rich! I changed the primary jets to 48's and the secondaries to 52.

Rap

so the holley webber 5200's only flow like 150-175 cfm is that in 4 barrel at 1.5 hg rating or is that at the 2 barrel 3.0 hg rating value because that seems like its super low for a 2 barrel werent some people saying like 350 cfm or so? Im wondering if I should sell these two carters and just get holleys and just have 2 instead of 3 carters if its needed lol, IDK at this point but I just want to get it running great and just not worry about it anymore lol.

The CFM ratings on the those carbs vary greatly depending on who is talking about them.Generally speaking most guys run two of them to support about 180 hp.
Like the CFM guide mentioned above is only a static guide .The actual demands of the engine needs to figured out from real world experience on your particular engine.
And then there's subjective opinion
Some guys bolt a 600 cfm carb on a very mild 250 cube inline and say it runs like bear.Another guy might drive it and say it's a bogging pig and half the carb would be too much.

I have 2 Carter Webers on my 292 and I love them. They are actually Ford Escort 1.6 carbs.

pretty cool so I left a post with tom langdon and he told me that although there are no cfm info on these carter webbers, that two of them are like 340 CFM in 4 barrel rating, so (340 cfm)/(.71)= 493 total CFM at 3 hg in 2 barrel form so each carb should be half of that so around 246 cfm each at 3hg, does that sound about right to anyone lol? (The .71 is the conversion factor back to 1.5 hg from 3.0 hg) He also said it would work up to 5000 rpm but not 6000 rpm or so (dont know why I would ever have it that high lol but ok). He actually said he takes his 250 with two these carbs to 5500 rpm no problem so thats good so a 261 shouldnt be too different. So I think that two these are gonna be easier to deal with/tune/dial in than three of these are give me some clearance for a P/S pump

On the Carter Webers, I have had two different dual carbs setups using the same intake on the same engine. The Carter Webers, and two matching Rochester 1bbls. The Rochesters had noticably more nuts but the C/W carbs manners were so much better at idle and did not leak. Just my two cents.

That sounds right - never compared them, but a progressive is generally going to have better manners, part-throttle smoothness and easy transitions.

had a .060 over 261 with the complete 'vette system and with those primative YH Carters (roughly the size of three rochesters), I had no troubles, they did not stumble at low speeds at all even with throttle positioned metering rods rather than the more refined vacumm controlled power jet rochesters. Should be an equal in size to the rochesters and were not a progressive setup either. Two YHs were stock on some Nashs, as well as some Kaisers, (maybe the sportier models the Healey and Darrin), the turbo corvairs, and a bunch of inboard/outboard boat conversions, all using various carbs per engine.

It seems with the more primative setups the rule is to err on the rich side, and you can cover up the rough spots...but you cant hide the MPGs.

Have lost my good Carter book, but do remember that the YHs were a side draft version of their down draft YF series, and a single bbl version of their WDG series 2 bbls (maybe a stretch there). I wish I could tell what cfm they were, probably about 210 cfm(2-bbl talk) or 147 cfm(4-bbl talk).

Important thing to remember is that they were a FACTORY setup used on a PowerGlide 235 engine, and stock the car could flirt with 20 mpg figures. So I would not classify it as being overly carburated--even on the 235. I have driven a six cylinder Vette and they were wll behaved (sure wish they had a standard trans).Is Ol'Hank around--it had a 3.55 rear axle ratio too!

I've seen mention that the YH's are around 128-135 CFM, about the same as the Stromberg 2 bbls. How would (3) of the YH's work on the 194-292's with a similar intake design like the Blue Flame engines?

Should be fine, but the extra vacuum (250/292 = bigger and more efficient) may need the jetting adjusted down a bit.

I'm not sure how the Corvette manifold functions. As said, it came with Powerglide - not a very forgiving arrangement with low stall speed, and wide RPM drop on the shift.
If the YH is about 128 (I got that from somewhere?) and the Corvair 180 hp variant a bit more with its larger venturi (1-3/8" vs. 1-5/16"), they would be a bit much if used together in a log-type manifold (big open common plenum) with each cylinder drawing from all 3 carbs = 384 CFM. The drive test suggests that even though the 3 runners have connecting passages they may be pretty small and only balance idle vacuum.

Anybody ever measure the connection ID?

i used to use holley model 1920 1 barrels on my dual and tripple carb setups. they run good, have manuel chokes and they look killer. i think i even have 4-6 of them still in my stash. wish i could post pic,s. !!!!

never did, but would guess from 1/4 " to 3/8" tops allowing for a some wall thickness.--as you say just for balancing at idle. The manifold shape was a slight horizontal "s" shape to match the head port size. The intake roar thru some small paper air cleaners closly imitated the exhaust pulse (probably more so than a triple setup on a log manifold- offy), they were louder than stock mufflers!

Take a brake cyclinder hone and open up the venturi and blend it down to the throttle bores and try for the late model diameter. bigger can be better!

due to cam lope, I had to safety wire air cleaners with some guitar type strings to prevent loss when engine shook them off! Just drilled a slight depression in air horn lip to allow air cleaner set screw to have a good "bite", an bolted other end of guitar wire to float bowl screw- had sort of old Airplane look to it.

These carbs will have one last accellerator pump discharge went engine is shut off, and air cleaners can receive a dose of raw gasoline if not parked level-- could be 3 Molitov coctail effect. When used on a Nash-Healey they had a metal drip tube ('bout 12 to18" long) threaded into the bottom of the carb to collect this raw gas and escort it safely down past the hot exhaust manifold. The chevy carb just had a threaded plug at this location. Intake manifold vacumm kept the accellerator pump in the lifted position until a lack of vacumm allowed it to overcome a spring sending a flush of gas, instead of hard linkage doing the job.

Besides CFM considerations, shouldn't a more even fuel distribution from a third carb enter into the equation?

With what manifold?

Tye's Offy triple.

having that third carb probably would make for better fuel distribution but would it also not probably lower the draw signal by having a third primary sharing the same vacuum signal decreasing the air velocity through each carb which might make atomization and draw lower at each location? Nonetheless I am gonna try and run two of them on this double, and if it ends up not being quite what I am looking for then I will just put the triple with another carb and go from there.

The distribution on a 3 × 1 like the Offy is largely based on which linkage type is used: progressive (center carburetor is the primary) or simultaneous (idles on all 3).
Just based on visual, if it's progressive I would guess the center (#3-4) cylinder pair will run rich based on proximity to the center carburetor. Various manufacturers have modified their manifolds over the years, or published tips on curing distribution problems (Chrysler), but they all involve minor internal surgery such as ditches or obstructions ("popsicle sticks") across a port runner entrance.
Needless to say, the mixture should be better with all 3 idling, but the response will be much worse since (as said) the vacuum signal will be split between the 3.

yeah since the carbs I have are progressive 2 bbls I am just gonna run both of the carbs synchronously so that they both idle fuel, if I had three I would run them the same way in that I would rather they idle on all three primaries of each carb that way the 3-4 cylinder ports wouldn't be closest to, and hogging the only carb that idles. Happy Thanksgiving from California everybody.

Dbane261,
If you are really serious and am not adverse to costs talk to Clifford's. They have an intake with Weber's already setup plus a great header setup. That with a decent cam, valve train and a HEI will help you wave at the small block V-8's as you pull away. I have one and it is a great runner.
Normbc9

I'm not impressed with the design of many triple manifolds. What's surprising is that (unlike today) the speed equipment manufacturers frequently did a worse job than Chevrolet. Many manifolds are designed like plumbing with right-angle joints.
The McGurk at least has a curved runner from the plenum to the head. Harper looks excellent but it's IR with no plenum, and must idle on all 3.