MTFCA Forum

Thinking Nothing complicated, but believe it will improve performance. No other changes and have dist ignition.

What carb are you replacing, and what makes you believe a down-draft will improve performance (Other than the new fuel pump it will require)?

It is impossible to offer a suggestion for improvement when the peanut gallery has no idea what is being improved.

FWIW, the Montana 500 rules limit the cars to an NH and they are routinely exceeding 50 MPH.

I don't think a downdraft would help except possible easier to flood the engine because excess fuel would go right down into the engine. If you are modifying the engine by using an overhead valve head, then a downdraft might improve performance. Henry wanted to keep it simple so he used gravity flow with the carburetor lower than the gas tank and it works very well except for the cars made before 1926 had a hard time going up a steep hill unless the tank was full. With down draft, as mentioned above, you would need a fuel pump which is just another non stock part to have trouble with and also if you are running on magneto, you would need a source of electricity if it were an electric pump. Some of the older cars had a vacuum tank on the firewall which drew in gas which then flowed from there to the carburetor, but those were still updraft carbs. You need a pressure system to get the gas up to the carburetor. Anyway, just something else to cause problems and non stock so you should carry spare parts along if you go on tours, because most of the other members would not have any along.
Norm

With a stock T engine in good condition, the limiting factor on performance is probably the very low stock compression ratio. Adding lift and duration on the cam will move the power range up the rpm range, which might call for more carburetion, but without spending a good deal of money, you don't have very far to go in that direction. Raising compression, if you haven't already, is probably the best way to get more kick out of the engine at useable speeds. The two speed transmission and stock gearing needs an engine with good low end torque. More carburetion and a hotter cam may actually lower low end torque, and make the car a real dog at anything but higher road speeds, unless you go to a lower axle gearset, which will get you moving but limit higher speeds.

At one time I ran a VW carb. Worked very well.(accelerator pump) On a 27. Did get rid of it because of going thru fuel pumps every year or 2. Our gas ate them up.

Thank you all for your comments. I am just brainstorming the idea and your feedback is helpful, Sounds like I should leave well enough alone. I have had mid 1930's Buicks with updraft and found when converted to downdraft all around performance was greatly improved.

Don

I would imagine that the performance improvement was in the carb and not the fact that it was just a downdraft. As far as T carburetion goes, there are many aftermarket side draft carbs and some updrafts that will make a T go MUCH better than with a stock carb, and without all of the fuel pump, oil dilution, and cylinder flooding problems (and frankly the increased fire hazard) associated with a down draft on a T.

I own Ts for their simplicity, reliability and ease of maintenance; my enjoyment of the hobby is getting into one at a moment's notice and driving them. Other people own them to add lots of stuff on them and personalize them and to that end, their hobby is spent in the garage. Because I drive them and am not looking for more complications, small performance improvements on my car don't include any downdrafts.

A good cam, an advanced timing gear and a mild compression improvement along with an NH will make the car GO and result in no loss in reliability or easy maintenance.

I am presenting this not as a lecture but as a perspective as to why my opinion is what it is. Everyone is free to do what they wish and you will eventually settle on a solution that works for you. Good luck and have fun.

SPEEDYTINC- What size V W Carb?

TeveS-Nor Cal wrote: ↑

Mon Jul 26, 2021 12:42 pm

SPEEDYTINC- What size V W Carb?

28 & 30. seemed to be no noticable difference. I preferred a manual choke. The higher velocity of a smaller throat-ed carb helps with low speed operation but not effect high speed for a t motor that runs under 4K rpm.

Hi Don,
My first T was my speedster. I set it up with a home made header & a WF Carter down draft on a manifold made from a model T intake. For a old worn out T it ran pretty well. Some feel that the size of the T carb. and exhaust govern the speed and this was designed into the car for reliability. A couple of things to consider are if you use the stock exhaust a down draft will have to go around it. If the carb. leaks it will drip on the hot exhaust and maybe
the sparking generator brushes. I run a Shebler up draft now its larger than a NH and is still adjustable from the drivers seat ( some thing that very few down drafts do ). Unfortunately most up draft manifolds are centered and do not leave enough room for the generator.
Good Luck, Craig.

Most carbureted aircraft use updraft carburetors. Guess why?

In my opinion (and experience), modifications should be considered a “package”, if you want to get a decent driving car. Attached is a picture of a modified T engine I built in 1978!

This set up works real good. I can pull hills right along with Model A's. As stated you will need a fuel pump and I run a regulator. I'm currently running a Solex 34 PICT-3 carb while my Winfield is on the bench.

I ultimately moved the engine into this car and changed to a Stromberg 97 (flathead V8 carb). Car is capable of 100 mph!

The carb is a Carter WO, as used in Jeeps, and other stuff. Has an accelerator pump. Rebuld kits have been easy to get from the vintage military vehicle suppliers. Peppy!

How many of you guys are running downdrafts on purely stock engines with no hot cam, large valves or high compression head or a combination thereof?

My guess is "none".

It's my impression that the OP simply wants to add a down draft and expects great things from a stock engine thus fitted. It's not where I would start, nor encourage anyone else to do so, either.

I think Les is giving good advice re: "package"

But again, that is not what is apparently presently planned, so my advice stands.

I think Scott has summed it up quite well!!

Les Schubert wrote: ↑

Tue Jul 27, 2021 10:31 am

I think Scott has summed it up quite well!!

First thing. Is the motor @20 hp? Good shape? To do only 1 thing. The "1" thing to add for more performance is a higher compression head. 5-6 to 1
Just a bigger carb might, maybe make a noticeable difference, but not as much as the head change. And with a down draft one needs a fuel pump & regulator. I stay away from electric fuel pumps, because of their failure rate.

Does anyone have a vacuum gauge tapped into a stock T manifold just above the carburetor? Does it show any vacuum when the engine is turning 2000 rpm at wide open throttle in high gear? (Stock engine, NH carb, full-fendered car)

It will only show no vacuum when it is not running. The amount of vacuum will change through the RPM range, though once the throttle is opened beyond idle, there is a surprisingly small change...it is not particularly dramatic, though if you have timing issues, valve or valve guide issues, etc., they become immediately obvious if you know what you're looking at. They used to be a staple of mechanics everywhere. Now, not so much.

What you didn't ask, and very few people realize, is that the level of vacuum and the steadiness of the reading is influenced by the position of the spark lever as well as the throttle opening particularly if you are running a cam with a good deal of overlap on intake/exhaust.

The firing order in a T is not really ideal and takes a good deal of flow to even out the pulsations created by flow alternating back and forth through only two intake ports.You might want to look up the Winfield two-up/two-down race engine which did some really ground-breaking stuff: http://www.mtfca.com/discus/messages/70 ... 1490791225

There was also the reverse-flow engine created which swapped intake/exhaust ports and is remarkable in it's own right: http://www.mtfca.com/discus/messages/41 ... 1407698295

Just for interest speaking of different carbs, this is a SU I'm fitting up on my Speedster. The motor is pretty much stock but for Chevy valves, aluminum pistons, stock cam grind with a advanced gear and a low head plus a distributor. Ran pretty good with a Simmons carb. Interested to see what the SU does for it.