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265 Carburetor CFM Needs?


Matt Wilson

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I'm building a 265 engine, and I plan to install a dual-carb setup, but I am wondering what CFM carbs I should be looking to install to get the most out of this engine that I can.

After doing a pretty extensive search on this site, I see that a number of people are using the Carter-Webers that Langdon offers, but many of those folks are driving 201's, 218's or 230's.  I wonder if two of those carbs would be enough for this engine?  I don't plan to race it, but just cruise between 1200 - 3000 rpm.  I want to have good low-end torque as well.  Langdon's Carter-Webers are for a 1.6L Ford Escort engine, which is just under 100 cubic inches, so even two of them seem to be short of what my engine might want.

Any comments and guidance would be appreciated.

Thanks,

Matt

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Langdons site shows the Weber 32/36's as the ones he sells. A quick check shows each is rated around 300 to 380 CFM. You would be looking at 600-760 CFM, which would be plenty for what you are running. We put 2 of these on an OT 235, and it runs great!

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I have a pair of these Langdon carburetors that a friend sent me several years ago. I have never installed them except on the log manifold pictured below that is sitting in front of me as I write this. I have a good friend who installed them on a highly modified 230 CI engine with good results. I have no idea of the CFM so the choice is yours.

air1.jpg

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3 hours ago, Matt Wilson said:

I'm building a 265 engine, and I plan to install a dual-carb setup, but I am wondering what CFM carbs I should be looking to install to get the most out of this engine that I can.

After doing a pretty extensive search on this site, I see that a number of people are using the Carter-Webers that Langdon offers, but many of those folks are driving 201's, 218's or 230's.  I wonder if two of those carbs would be enough for this engine?  I don't plan to race it, but just cruise between 1200 - 3000 rpm.  I want to have good low-end torque as well.  Langdon's Carter-Webers are for a 1.6L Ford Escort engine, which is just under 100 cubic inches, so even two of them seem to be short of what my engine might want.

Any comments and guidance would be appreciated.

Thanks,

Matt

You can do the math , I don't know the max rpm or I would do it for you . So here is the formula .rpm x ci / 3456 =cfm. Hope this helps .

 

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4 hours ago, Matt Wilson said:

I'm building a 265 engine, and I plan to install a dual-carb setup, but I am wondering what CFM carbs I should be looking to install to get the most out of this engine that I can.

After doing a pretty extensive search on this site, I see that a number of people are using the Carter-Webers that Langdon offers, but many of those folks are driving 201's, 218's or 230's.  I wonder if two of those carbs would be enough for this engine?  I don't plan to race it, but just cruise between 1200 - 3000 rpm.  I want to have good low-end torque as well.  Langdon's Carter-Webers are for a 1.6L Ford Escort engine, which is just under 100 cubic inches, so even two of them seem to be short of what my engine might want.

Any comments and guidance would be appreciated.

Thanks,

Matt

No one has ever mistaken me for Don Gartlits,but it seems to me that cubic inch displacement isn't as important a factor as you may think because even though your 265 is a bigger engine,it doesn't turn up the RPM's that a OHC 4 cylinder does. Regardless of the engine or the size of the carb,flow and volume are the primary considerations. The 4 cylinder Pinto was putting out as much power as your 265 at twice the RPM. One indisputable fact it is takes X amount of gasoline and X amount of air to produce X amount of horsepower at X amount of RPM's.

If one of those carbs can provide enough air and gas flow to produce 100 hp in a 4 OHC 122 cubic inch 4 cylinder Pinto engine,2 of them would seem to be plenty to me to feed a 265 flathead 6. Yeah,you can tweek your engine to put out more than 200 hp if you want to spend the bucks to do so,but it really doesn't seem to be worth the effort. If you really wanted more power you could put a 318 or 360 in there cheaper and have more power to boot.

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Not so much total flow you need to worry about but fuel air mix in the correct range through the working rpm range.  While adding a second carb would seem to add more air and therefore more fuel, you need to factor into the equation that the air flow through each carb or bbl is now halved.  Less air flow, less fuel through the jet.  But hopefully better atomized and balance a long the length of the manifold.

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2 hours ago, greg g said:

Not so much total flow you need to worry about but fuel air mix in the correct range through the working rpm range.  While adding a second carb would seem to add more air and therefore more fuel, you need to factor into the equation that the air flow through each carb or bbl is now halved.  Less air flow, less fuel through the jet.  But hopefully better atomized and balance a long the length of the manifold.

Greg,doesn't that amount to more flow as well as more effeciency? You only have 3 cylinders being fed by one carb that is a much more efficient design than the 1940's or 50's carb.

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Dodge trucks use the 265 with dual carter carbs.  I'd use that info as a start.

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Knuckle if the engine is pumping the same amount of air and that air is now flowing through two openings, there might be a slight gain in volume due to less resistance but it would be negligible.  To change the amount being sucked in you need the valves open sooner,longer, through cam mods.

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6 hours ago, greg g said:

Knuckle if the engine is pumping the same amount of air and that air is now flowing through two openings, there might be a slight gain in volume due to less resistance but it would be negligible.  To change the amount being sucked in you need the valves open sooner,longer, through cam mods.

Maybe/probably I am not doing a good job of explaining it,but a dual carb setup on a inline engine is much more efficient than a single carb setup,regardless of the single carb design or flow rate because of the intake itself.  Add carbs with a more efficient design to  the mix,and you even end up with better fuel mileage as well as more power.

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There is a standard formula used to calculate the cfm requirements for a given engine, which uses displacement, estimated volumetric efficiency and anticipated maximum rpm. I did a calculation for a mildly warmed over 440 I built years ago. I assumed 85 % VE, max rpm of 6000 and the cfm requirement came out to be somewhere between 650 and 700 cfm. 

CFM = (CID X rpm X VE)/ 3456

440 example - (440 X 6000 X .85)/3456 = 649 cfm

So most folks over carb their motors for normal street use without knowing it. For racing over sized larger carbs are usually used.

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Which was my point. You don't need to get tied up in worrying about total cfm's, of and by it self the dual set will work better inspite  of itself with stock type carbs.  Maybe not so much with two Briggs and Stratton carbs, but with just about any pair of single bbl carbs from the era would be fine.

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That calculation is all well and good for high peromance application that run at Wide Open Throttle. Before you warmed over the engine did it like most of them out there originally have a dinky little 2 bbl on it?  And even if it was a 4 bbl engine were not the primary throttles about an inch in diameter?  Peak rpm in a road motor I'd 3200 at 70 % efficiency in real life.  What does the calculation look like with those numbers. Works out to just about 290 for the 440.  The 265 @2800 with 65% vme needs around 130 cfm.  

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Correct, that's how the calculation works. You choose the max rpm you intend to run.

Nope - all 440's used in cars that I know about, have a 4 bbl of about 650-750 cfm or larger on them. Thermoquads used on later 440's are about 850 cfm, six packs I think were about 1050 cfm ? 440's always were a high output motor to my knowledge. Depending on the application the 4 bbl primary butterflies are about 1 1/2 inches on TQs and for AVS or AFB carbs the primaries are 1 7/16, or 1 1/2 " IIRC. The real flow determination is the Venturi size anyway.

My point was to show there is a method of calculating the engines demand for air flow. 

I believe putting a dual carb set up on a flat six is an excellent idea. Mainly for fuel distribution problems the stock set up has. Theoretically the center two cylinders are going to run richer than the two end cylinders because of where the stock manifold places the carb. Dual carb manifolds equalize the fuel distribution by the carb placements.

 

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9 minutes ago, Dartgame said:

...440's always were a high output motor to my knowledge.

 

LOTS of 413's and 440's were used in motor homes and big trucks. I have no idea what carbs they used,but they did use small valve heads and cams ground for torque instead of HP. They also had lower compression ratios. Because of this I suspect the carbs they used flowed a lot less than 750-850 cfm.

GREAT motor home engines. They just seem to last forever.

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What I guess I was remembering was an uncle who had a 61 Imperial maybe a 413.  It had what looked like the same carb that was on a 2bbl 318. Don't know how efficient it was, but it ran nice and did everything a car should do.  What do you really need for an occasional use cruising vehicle.  Day to day reliability or peak power?

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On ‎5‎/‎9‎/‎2017 at 11:00 AM, Matt Wilson said:

I'm building a 265 engine, and I plan to install a dual-carb setup, but I am wondering what CFM carbs I should be looking to install to get the most out of this engine that I can.

After doing a pretty extensive search on this site, I see that a number of people are using the Carter-Webers that Langdon offers, but many of those folks are driving 201's, 218's or 230's.  I wonder if two of those carbs would be enough for this engine?  I don't plan to race it, but just cruise between 1200 - 3000 rpm.  I want to have good low-end torque as well.  Langdon's Carter-Webers are for a 1.6L Ford Escort engine, which is just under 100 cubic inches, so even two of them seem to be short of what my engine might want.

Any comments and guidance would be appreciated.

Thanks,

Matt

For a 265 C.I. engine running up to 4,000 RPM, Carter suggests 295 CFM total carburetion.

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21 hours ago, nonstop said:

Langdons site shows the Weber 32/36's as the ones he sells. A quick check shows each is rated around 300 to 380 CFM. You would be looking at 600-760 CFM, which would be plenty for what you are running. We put 2 of these on an OT 235, and it runs great!

Wow, you're using two 32/36's?  It seems like that would be way too much carb for your engine, and mine.  Are you sure they're not the Carter-Weber 32's?  Those are also on Langdon's website, and much smaller than the 32/26's.  Assuming you are using the 32/36 carbs, how much adjustment was there to get them working well throughout your rpm and power range?  Reason I ask is because I've read that these carbs are highly adjustable, and need a fair amount of mixture adjustment when first installed, to get them dialed in, as opposed to the Carter-Weber 32's, which usually don't need much.  I'd much rather not deal with carbs that need a lot of mixture adjustment to get them working well.  Getting them synchronized will be enough "fun" for me, I think.

 

21 hours ago, justold said:

You can do the math , I don't know the max rpm or I would do it for you . So here is the formula .rpm x ci / 3456 =cfm. Hope this helps .

 

Thanks, I did the math, and included an assumed volumetric efficiency factor of 0.75 (I don't know how close that is to reality), and came up with 215 cfm for cruise operation at 2800 rpm.  It might be good to have a little cushion beyond that, for when I want to go a bit faster or pass someone (probably somewhat rare).  One thing I found after my original post is that the carbs on the market are rated for cfm's in an inconsistent manner.  Apparently some have cfm ratings associated with a certain amount of pressure drop (across the throttle plate, presumably?), while others have cfm ratings for a different amount of pressure drop, and I think there were other variables, so comparing one carb to another by cfm rating is often not an apples-to-apples exercise.  The information I found seemed to indicate that it is almost pointless  to use the calculated cfm needs of the engine when deciding what carburetor(s) to use, as the carb cfm figures can vary wildly depending on how they were measured.  So my original question may be somewhat pointless, as it turns out.

 

19 hours ago, knuckleharley said:

No one has ever mistaken me for Don Gartlits,but it seems to me that cubic inch displacement isn't as important a factor as you may think because even though your 265 is a bigger engine,it doesn't turn up the RPM's that a OHC 4 cylinder does. Regardless of the engine or the size of the carb,flow and volume are the primary considerations. The 4 cylinder Pinto was putting out as much power as your 265 at twice the RPM. One indisputable fact it is takes X amount of gasoline and X amount of air to produce X amount of horsepower at X amount of RPM's.

If one of those carbs can provide enough air and gas flow to produce 100 hp in a 4 OHC 122 cubic inch 4 cylinder Pinto engine,2 of them would seem to be plenty to me to feed a 265 flathead 6. Yeah,you can tweek your engine to put out more than 200 hp if you want to spend the bucks to do so,but it really doesn't seem to be worth the effort. If you really wanted more power you could put a 318 or 360 in there cheaper and have more power to boot.

Yes, this crossed my mind as well, and I think you have a valid point.  As best I can tell, the Ford Escort engine that used these carbs was a 65-hp engine, and that probably occurred at 4000 or 5000 rpm (couldn't find that info), whereas my engine will be used to cruise long distances at about 2800 - 3000 rpm, with occasional bursts to 3500 rpm.  So two of these carbs on my engine might be ok, but it doesn't seem like a slam-dunk necessarily.  Tom Langdon recommended these carbs for my application, but that's partly because they are the carbs he has available.  If there are other carbs that will do a better job, then I might rather get those.  I'm sure his recommended carbs will make the engine run fine, but will they get as much out of the engine as they could?

 

15 hours ago, Dodgeb4ya said:

Dodge trucks use the 265 with dual carter carbs.  I'd use that info as a start.

Yep, good point, and I did that.  I looked up a "cheat sheet" that I believe Tim Kingsbury put out a while back, and it lists a couple of that were used on the 265 2.5-ton truck dual-carb setups.  The cheat sheet doesn't provide cfm ratings, though.  And of course, as I mentioned above, the ratings from one type of carb to another cannot often be compared directly, so I'm not sure that it helps.  I suppose if I could find a pair of those exact carbs, then I would know that I had the right ones and they would be about as good as it gets.  Maybe I should do a search for those, but I'm just not optimistic about finding them.  They are E9K1 or 39Y1.

With all of the above in mind, the best is to get empirical data from folks who have used these carbs, and preferably used some other carbs, to know which ones worked better.

Anyway, I appreciate all the responses.  Keep 'em coming!

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52 minutes ago, jeffsunzeri said:

For a 265 C.I. engine running up to 4,000 RPM, Carter suggests 295 CFM total carburetion.

Hmmm, good info.  As I mentioned in my post a couple of minutes ago, it's often not valid to compare cfm ratings from one carb type to another, and even varies with one manufacturer's selection of carburetors, depending on how they set up their tests, and depending on if you're talking 1-bbl, 2-bbl, 4-bbl and other variables, if I understood last night's research correctly.  Where did you get this information?  Is it on a website somewhere that I can access, or is it in a manual that you have?  Thanks.

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50 minutes ago, Matt Wilson said:

Hmmm, good info.  As I mentioned in my post a couple of minutes ago, it's often not valid to compare cfm ratings from one carb type to another, and even varies with one manufacturer's selection of carburetors, depending on how they set up their tests, and depending on if you're talking 1-bbl, 2-bbl, 4-bbl and other variables, if I understood last night's research correctly.  Where did you get this information?  Is it on a website somewhere that I can access, or is it in a manual that you have?  Thanks.

This is from the Carter AFB Selection and Tuning Guidelines publication #1601. My only copy is in .pdf format from which I can't cut/paste from my computer at work. I'll bet you can google it and find the publication. Although the table is intended for the 4bbl carb, the overall numbers are very cross compatible. All engines are air pumps to a certain degree, and the difference I believe you'll find from 1 4bbl, to 2 1bbl's, to 2 bbl.'s, won't be very great. It sounds like you've got enough technical sense to extrapolate from the factory suggestion. 

The Carter calculation of CFM is qualified as the air quantity passing through the carb at full throttle, with a specified vacuum below the throttle plate. In this chart, they are assuming 1.5" of vacuum.

 

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2 hours ago, Matt Wilson said:

>>Tom Langdon recommended these carbs for my application, but that's partly because they are the carbs he has available.  If there are other carbs that will do a better job, then I might rather get those.  I'm sure his recommended carbs will make the engine run fine, but will they get as much out of the engine as they could?<<


I wouldn't know Tom Langdon if I ran into him,but have you considered that he is in this business to make money,and to make money in this business he has to produce products that work well for the intended purposes? There must be a dozen or more carbs he could be using that are are cheaper than the ones he uses,and maybe even more than that if you are talking about larger carbs. If they worked better for inline flat 6's don't you think those would be the ones he would be recommending and selling?

Not trying to be snide here,but if his setups didn't work we would have heard complaints about them long ago,and he would no longer be selling them. Granted,they are probably not the carbs you want to run if you built your car to make passes at Bonneville,but I'd be willing to bet they work pretty damn good for flat 6's driver cars.

If your concern is they aren't big enough to feed a 265,ask him. If he tells you they work just fine with 265's,chances are they work just fine with 265's in driver cars. If you plan on putting a hot cam,high compression head and pistons,and bigger valves,tell him what you are building and ask for his recommendations.

 

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Fair point, Knuckle.  I imagine you are correct.  I just like to do my research beforehand, as much as I can, because I really dislike getting to a point where I am unhappy with a situation and feeling like I didn't do my homework.  It almost always pays to do your homework.  Without going into specifics, there have been disagreements over which carbs and manifolds are adequate, coming from people who specialize in selling these setups.

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I have several factory dual carb dodge trucks.

My trucks are 2-3/4 to 4 tonners. They perform just right at all speeds and loads.

Factory installed equipment has lots of good engineering designed into it.

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On ‎09‎/‎05‎/‎2017 at 3:15 PM, nonstop said:

Langdons site shows the Weber 32/36's as the ones he sells. A quick check shows each is rated around 300 to 380 CFM. You would be looking at 600-760 CFM, which would be plenty for what you are running. We put 2 of these on an OT 235, and it runs great!

Can you tell me where you got the CFM  rating of 300-380 cfm ?    When they were released for the 225 ci slant six the carter webers were listed as 185 cfm and that is what I was told by Tom Langdon that they were equivalent to,

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