Two Barrel Carburettors.

[64beam]

Hi,

I have put on hold the four barrel conversion on the Tiger. I am considering a Holley 350 two barrel carby as a replacement for the standard Autolite carby. What is the largest two barrel carby that could be put onto a stock 260ci engine without flooding it?

Thank you, Robin.

 

[Bill Blue]

Well, guys use the Holly 500 cfm on highly tuned Pinto 2.0's, so it should be fine on the 260. Got to remember that the two bbl carbs are rated at a higher vacuum than the four barrel, so direct comparison to flow rates is impossible. To compare a two barrel to a four, divide the two barrel flow by 1.17. My math gives the carb a four barrel flow rating of 427 cfm.

Bill

 

[husky drvr]

With Holley carbs rated at 3.0 inchHG for two barrels and at 1.5 inchHg for four barrels, I think that the conversion factor is 1.414.


500 / 1.414 = 353.6 cfm ( two barrel large enough to produce the same restriction to flow as a four barrel rating )


I am thinking that Weber rates carbs at 2.0 inchHg and would use the 1.17 conversion factor to compare flow at the four barrel restriction level.

 

[64beam]

Hi,

Thanks for the replies. As I have been researching carby information over the internet, I stumbled across another formula for choosing a carburettor. The formula uses the engine displacement multiplied by the maximum RPM, divided by 3456 to get your required cfm. It does note that for standard engines, the engines volumetric efficiency is approximately 85%. So for my engine the formula works out at 376 cfm and taking into account the volumetric efficiency, is 320 cfm. So I suppose I have answered my own question.

Thanks again, Robin.

 

[saltykracker]

Before this rebuild, I had too much cam, can look it up but what I remember it was a 3/4 crane race cam. A few notches hotter than a marine cam. With it I was running a 500 cfm holley 2 bbl, be it the cam or carb there was a hesitation I could not tune out. When this motor got on the cam it was a beautiful thing. I now changed the cam and this is the reason I'm looking for a new 4bbl carb.

 

[Bill Blue]

Hi,

Thanks for the replies. As I have been researching carby information over the internet, I stumbled across another formula for choosing a carburettor. The formula uses the engine displacement multiplied by the maximum RPM, divided by 3456 to get your required cfm. It does note that for standard engines, the engines volumetric efficiency is approximately 85%. So for my engine the formula works out at 376 cfm and taking into account the volumetric efficiency, is 320 cfm. So I suppose I have answered my own question.

Thanks again, Robin.

Robin, isn't 320 at zero depression? If so, the carb must be larger in order to flow 320 cfm at a realistic manifold pressure.

Bill

 

[64beam]

Robin, isn't 320 at zero depression? If so, the carb must be larger in order to flow 320 cfm at a realistic manifold pressure.

Bill

Hi Bill,

I'm not sure what you mean by zero depression . By the formula I found, the 260 ci requires 376 cfm obviously depending on the specs of the motor. I got the 320 cfm based on the 85% rule for the volumetric efficiency. In reality, I would install a 350 cfm carby (I'm not sure if that answers your question).

Regards, Robin.

 

[Bill Blue]

Robin, how much vacuum do you have to pull to get your 350 to flow 320 cfm? As I understand the numbers game, the four barrel carbs are rated at a vacuum you normally see on an engine operating at wide open throttle. The Holly two barrel carbs are rated at a vacuum that is much greater. A comparable four barrel would flow 247 cfm. The "four barrel" rating of the Holly 500 is 353 cfm, so it would be a better fit to your engine, if you want it to pump 320 cfm. To put it another way, the 500 will allow 320 cfm to flow to your engine, the 350 won't.

If I've totally confused you, ask Don. He explains things a lot better than I.

Bill

 

[64beam]

Hi Bill,

I am a bit confused. A few carby specialists that I have spoken to, suggested to stick with the 350 carby as the 500 would be too much for the stock 260 engine. When Holley or Edelbrock for example, specify the carby cfm, is'nt that what it is rated for? So if your engine requires 376 in the Tigers case, you choose a carby closest to that figure.

Regards, Robin.

 

[V_Mad]

Hi Robin
I think I know where you are going with your enquiries. Like most guys, you are looking for an improvement in 'performance'? Forgive me if I am teaching my grandmother etc. and for the long post but here's my two pennyworth.

Stock carbs are chosen by manufacturers as a compromose between power, fuel efficiency, and driveability. As classic car owners, we may want to change that balance towards performance

To get more power we need to ensure that more mixture (of the 'correct' fuel/air ratio) gets into the combustion chambers. To help with that we can open up our venturi sizes in the carb, but we must also ensure that there is still enough depression (vacuum if you like) to draw the correct amount of fuel in with the air.

The formula you mention is a general rule of thumb guide to a size of carb that should be capable of being set up for reasonable performance.

But as you increase the venturi size, your depression (for a given engine speed) reduces, and so driveability/economy can suffer. It is difficult to meter the fuel correctly at low depressions.

The advantage of going to 4 venturis is that at low speed/revs the carburettor can more accurately meter the fuel/air in the correct ratio (it acts just like your old 2V carb), and at the same time transform itself into a much better flowing carb at high speeds, when the additional two venturis open. Carbs with vacuum secondaries ensure that they only open when they are needed.

So, if you want more go with good driveability and, without draining your tank too much every time you boot it, I think your best bet is to go 4 barrell carb rather than a larger 2-barrel. Admittedly its more work/expense, but I think its worth it.

 

[64beam]

Hi V_Mad,

Thanks for the reply. If I was looking at performance, then I would go for the four barrel setup. My reason for the upgrade is better reliability out of the carby and I think the 350 Holley would suit the engine better without compromising reliability and economy. I find the stock 260 to have enough power for my liking. Who knows, with the electronic conversion kit and a distributor service booked in for this Thursday may be the answer.

Thanks again, Robin.

 

[V6 JOSE]

Hi V_Mad,

Thanks for the reply. If I was looking at performance, then I would go for the four barrel setup. My reason for the upgrade is better reliability out of the carby and I think the 350 Holley would suit the engine better without compromising reliability and economy. I find the stock 260 to have enough power for my liking. Who knows, with the electronic conversion kit and a distributor service booked in for this Thursday may be the answer.

Thanks again, Robin.

Why would you go to a bigger carburetor, just to get more reliability?? If you are not looking to improve performance (horse power), why change? A larger carguretor will sacrifice a bit on the bottom, because of what Chris just explained. You will gain on top, but if your aim is reliability, the stock two barrel carb that came with the 260 is the best.

I, personally, would go with the four barrel carburetor, because, usually, the primaries are smaller than the two barrel, making it carburete better, plus with the smaller bores, will give better mileage too, when cruising. The two barrel carb needs to be bigger in order to give more power when the throttle is opened all the way, so must be a compromise between the two.

The four barrel carb will outperform the two barrel carb in every aspect, be it economy, or power. Of course, if the pedal is to the metal with the four barrel, it will eat more gas, but driven normally, it won't.

Jose

 

[64beam]

Why would you go to a bigger carburetor, just to get more reliability?? If you are not looking to improve performance (horse power), why change? A larger carguretor will sacrifice a bit on the bottom, because of what Chris just explained. You will gain on top, but if your aim is reliability, the stock two barrel carb that came with the 260 is the best.

I, personally, would go with the four barrel carburetor, because, usually, the primaries are smaller than the two barrel, making it carburete better, plus with the smaller bores, will give better mileage too, when cruising. The two barrel carb needs to be bigger in order to give more power when the throttle is opened all the way, so must be a compromise between the two.

The four barrel carb will outperform the two barrel carb in every aspect, be it economy, or power. Of course, if the pedal is to the metal with the four barrel, it will eat more gas, but driven normally, it won't.

Jose

Hi Jose,

The Tiger I have retains the original Autolite carby on the two barrel manifold. Initially after its restoration it ran beautifully for my Dad, but over time deteriorated by flooding easily causing hard starting. The carby has been reconditioned earlier this year, but still likes to flood. As I wrote previously, I am taking the Tiger to get the ignition side checked out which I hope is the problem. I wanted the Holley 350 as I have heard that they are a good reliable carby and I wanted to retain the two barrel more for insurance reasons than anything.

Regards, Robin.

 

[V_Mad]

Hi Jose,

The Tiger I have retains the original Autolite carby on the two barrel manifold. Initially after its restoration it ran beautifully for my Dad, but over time deteriorated by flooding easily causing hard starting. The carby has been reconditioned earlier this year, but still likes to flood. As I wrote previously, I am taking the Tiger to get the ignition side checked out which I hope is the problem. I wanted the Holley 350 as I have heard that they are a good reliable carby and I wanted to retain the two barrel more for insurance reasons than anything.

Regards, Robin.

64Beam
When you started this thread, and other threads on the carb, you were asking about bigger carbs, etc, and several people have given their time to share information and opinions with you on that subject. Then you tell us that you dont really want more performance, only reliability (which has nothing to do with size). You then change the reason again to insurance. What is the relevance of that? It is easy to get cover for even radical modifications. Many owners have fitted highly tuned 302 engines. Your little carb change is not even significant provided you tell them about it.

As if thats not enough, now, all of a sudden you mention a problem with flooding (which nothing to do with carb size etc) which you hope will be solved by looking at the ignition! Wheres the logic in that?

I am sorry, but I cant help feeling foolish for spending my time trying to help solve your problems when I dont know what the question really is or was?

Or have I completely lost the plot here?

 

[Bill Blue]

Robin, there is little mechanical difference between the 350 and 500 and they have the same reliability. The 500 is simply larger.

If the original carb is flooding with no apparent cause, I would wager the replacement carb will do the same. Carbs don't malfunction because they are old, they malfunction because something is wrong. If there is nothing wrong in the carb, there is something wrong elsewhere. Like crud in the fuel system causing the needle valve to stick open or a fuel pump that is putting out too much pressure and overpowering the float.

Bill

 

[64beam]

To all who have replied,

Thank you all for your input into my questions as it is much appreciated. As I wrote in a previous post, I am new to the Tiger scene and due to that there will be questions that may seem basic to some (not necessarily to me). I have been asking about two and four barrels so that I can decide what is best for my needs at present. If I change my mind it's not to annoy or waste the time of the forum users (only my time if anyone's). I thought we are here to help each other out, not disparage or humiliate people trying to get as much information they can before making a decision.

Kind Regards, Robin.

 

[pdq67]

On just about any forum, you will find folks hiding in the dark corners waiting to ambush you. It's probably an inferiority complex. These are the same folks who would not pass the foxhole test. Don't worry about it. For every wiseass out there, there are ten great people who will jump in to help. The forum is the ultimate proof that the only stupid question is the one you don't ask. Enjoy your car and the Forum.