I just performed a very crude experiment. Have a Z head, stipe 280 cam, Metz (larger than T) intake, modified NH carb (inside passage enlarged and smoothed). Has been running very well but I began to get a desire to be more stock and installed a very nice Ford script stock T intake. The venturie area of the carb is still the smallest passage the air/fuel mix must traverse before it enters the cylinders even with the stock intake manifold. I noticed (sorry, no real measurements) no drop in performance/perceived power. Some areas seem improved. Still exploring the top end, but so far seems to still be there. I now have better response starting from stops that I attribute to the smaller manifold size giving faster maintained air/fuel velocity around that first bend, resulting in little or no puddling as can happen in the larger intake. That pool or rain-out fuel that collects in the first elbow then causes some stumbling at take off is now gone.
It may be my imagination, but the engine feels more peppy with less throttle input. Why, I am not sure.
I am doing a good size tour tomorrow so will get more testing time in.
Anyone have any thoughts on this matter?
Further, I can see a larger intake manifold may be of greater advantage if using with a non-T carb with larger internal air passage, I assume.
Air velocity plays a big part and the stock manifold with it's smaller passage will lend it's self to a higher velocity....It's always a balance between the volume and velocity. A given amount of cubic inches can only handle a given amount of fuel at a given RPM. You may be more efficient with the stock manifold...
Of course the cam, intake passages, and valves can be made to improve the efficiency as well. This is especially true with this engine...
I tried the bigger intake manifold and also noted no difference in performance on my very modified engine. I was surprised.
I don't think too much of a difference would be noted until you push a T engine past 2500 RPM and on to 5000. After that you need much more volume than can be had from stock. I think one of the reasons the OF Stromberg works so well is that they stuck to the small venturi and worked from there....
You bet. Are you going to have old blue out there? Will see you there.
At low speeds, especially starting speeds, the smaller manifold will carry the gas vapor to the cylinders faster with less chance of fuel droplets falling out of the stream. Ford decreased the size of the manifold from the very early cars IMHO to make them start better.
Every carburetor is a compromise between low and high speed performance. The Stormberg OF was about the first to exploit the idea of a separate circuit for low and high speeds and the use of an air injection circuit into the high speed circuit (which they called an economizer) to lean the mixture at idle speed and low power requirement speeds. The fuel leaves the supply orifice as droplets of un-vaporized fuel. It is necessary for the fuel to evaporate into a gaseous mixture before it will ignite in the cylinder. Air flow speed through the Venturi is a major factor in how large those droplets are when they leave the orifice and how rapidly they vaporize. It's a whole science into and of itself.
With side curtains on!
Bugga the science!! my 16 has a reamed out (by an 1/8") NH straight through and a bigger manifold, gives me all the power I need to stay in top gear on hills and keep up with the Z headers if I need too. Just picked up 2 more s/thru NH's today so my other T's will get the same mods.
So why have a Venturi at all? Why not just ream it out to the size of the valve or cylinder and have all the power you can use if you can ever get it to run.
These cars, including their carburetion and intake system were designed over 100 years ago when gasoline ranged in octane from the 40's to a high of around 65. By the 1920's in the US a lot of the fuel sold was as much as half Kerosene. Also by the 20's cars were being driven in temperatures down to 30 below zero F. Trucks were expected to start and work on the coldest, snowiest days of winter and the hottest, driest or wettest days of summer. EVERY carburetor and intake system was and is a compromise to get top efficiency at every temperature, fuel and load possible.
Starting an engine on a hand crank in cold weather with poor fuel is nearly impossible and especially so with a carburetor with a large venturi and intake manifold. Ford knew that it was necessary to compromise some other advantages of carburetor design to be able to start those cars on a hand crank, have good low speed pulling power and still have reasonable high speed performance -- which was probably the least important of the three in the day when they were designing and building these systems.
If you study carburetor design, (Science) nearly all of the better carb designs of the day revolve around some type of variable venturi size. There were dozens of designs, most of which were either controlled by air, like the Bootie, or mechanical; related to throttle position, like the H & N. A small venturi opening which causes high air flow speeds at low cranking speeds is best for starting, a larger Venturi is better for mid to high range operation.
As John said: Air velocity plays a big part and the stock manifold with it's smaller passage will lend it's self to a higher velocity....It's always a balance between the volume and velocity. A given amount of cubic inches can only handle a given amount of fuel at a given RPM. You may be more efficient with the stock manifold..
As I said: Every carburetor is a compromise between low and high speed performance.
As every carburetor designer or physics teacher teaching about carburetors that ever lived said: We need a smaller venturi for increased air flow at cranking and idling speeds and a larger venturi for mid and higher speeds.
Well, I'm off to my carburetor shop, trying to get ready for Chickasha.
I have seen at least one carb from the T era that had a mechanical variable venturi actuated by a lever. Wish I could recall the name of it.
Keith, real men don't need no stinking side curtains........(I say it because I haven't any)
Sorry to drift my thread.
Probably a Harrington. They were pretty popular.
Scroll down for a photo of one.
There were well over 100 accessory carburetors made for the Model T. There was a reason.
I hope you have a spare carburetor. Once you ream the venturi, you cannot go back. It would be a good experiment to see what effect reaming would give you, but you need a good stock carburetor to compare it with and to use in case the modification fails.
Keep us informed on the result.
After reading Stan's last post it is easy to figure out why the four-barrel carbs were so popular and why almost 100% of 4 cylinder cars and small trucks had two barrel carbs that ran on one small venture until the engine speed was fast enough to have high velocity going through the second venture.
Computer controlled fuel injection keeps the mixture right for all loads and speeds as well as altitude compensation and cold weather starting.
Having the right fuel mixture plays a big part in why engines the size of a model T are now developing over 200 HP stock right from the factory.
When I bought the Toquet carburetor for my '27 Tudor I was shocked to see the size of the venturi.......it narrows WAY down and I wondered how in creation enough air could get through it to run the engine.
It works like a dream other than the choke, having fixed jets has no adjustments and does show a noticeable improvement in performance.
I run it with no stove and it never ices which, to me, is a miracle.
Norm, I have lots of carbs and have done several stock, several smoothed, and several slightly opened. So far have not over done one yet but can see how it would be very possible to do that. The standard swayback NH is my favorite even over the straight through NH. Have worked up several of those and have found they just don't work out as well for all around drive-ability, let alone the top end. I can do better with a slightly modified swayback.
There is a reason Stromberg sold over half a million OF's in the day when they cost three or four days' wages for the average Model T owner.
They sold thousands of Rayfield F's, Wheeler Schebler and other accessory carbs. Ford couldn't get an NH to be anything but adequate and cheap.
A few years ago, someone in the MTFCI ran flow bench tests on a bunch (6 or more) stock T intakes and published an article on it in the Model T Ford Times. I don't remember which issue it was in, but I do remember the best flowing intake was the 1914 cast iron manifold. It was a manifold ast from the same or a similar mold as the 1913 cast aluminum version. I have, and used one of the cast iron manifolds for 20 years and I swear it provides a healthy increase. I'm busy getting parts ready for Chickasha, but if I have time tomorrow, I'll mike the inside diameters of it and a stock manifold and post them here.