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OT - How hard is it to machine a throttle body?

G

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
For reference, a typical throttle body is shown below....just a simple butterfly on a shaft in a bore.

Over the years, I've heard people say there is more to these than meets the eye. The precision, the fits, etc.

So...if I could bore a piece of aluminum within .001" and make or buy a plate, drill and ream the shaft holes etc. would I wind up with a decent throttle or a piece of junk?



throttle-blocks-single-40mm-55mm-Bores.jpg
 
Seems to me a qualified machinist could do it.

Sure, you probably have close tolerances buuuuut........
 
Just some numbers from old production and prototyping. I may be forgetting something. The throttle shaft bore is offset from the throttle body bore .030". That way it won't overcenter and lock open when slammed shut(high vacuum can do funny things to a throttle body/intake manifold). Bore tolerance .0012". Maybe about 15 degrees angle on throttle blade when throttle is closed. The throttle blade OD is actually machined on that angle, to give a good seal in the bore when closed.
 
Thanks, good info on the offset. It seems the current trend is to tighten up the 12-15 deg. angle to 6-7 degrees. The theory is that it provides a smoother off-throttle response as the 'amount of air flow per degree' is much smaller. In other words, if you crack a 15 degree plate just a little, you get an abrupt change in airflow. On a 6 degree plate, the differential is much smaller so the throttle tip-in is more even.
 
Is it even a straight bore? I would expect it to be a fancy curve for a venturi effect. Folks do all sorts of things to improve airflow like polish the intake paths cast into the head. Old carburators wanted a somewhat turbulent flow to mix the fuel with the air not a pure laminar flow streams of clean air and. a spearate stram with fuel mixed in.
Bill D
 
Bill D said:
Is it even a straight bore? I would expect it to be a fancy curve for a venturi effect. Folks do all sorts of things to improve airflow like polish the intake paths cast into the head. Old carburators wanted a somewhat turbulent flow to mix the fuel with the air not a pure laminar flow streams of clean air and. a spearate stram with fuel mixed in.
Bill D
Click to expand...

likey they go through extensive testing that costs car compnies a ton of money that makes them .0005% better , but that times about 250 million cars makes it very important....
The screws could fall into the intake and puke you engine...
One screw coud come lose and waste some fuei...
The flapper could stick open and blow an engine or cause an accident..

Still to buy one used would cost so much less than building one...
 
Pretty much what the others said. Not hard if you have a good lathe and a good boring head. You can mount the oversize plate on a piece or round stock with one end milled at the angle you need then just turn it. I've made several for prototype drone engines. The first time I saw the print for the plate I thought how the hell am I going to make that angle on the OD. They showed me how the other guy did it with the fixture and it was "AHA!"
 
I know an easy way of making the butterfly a perfect fit in a round hole - at almost any angle you desire.

Dang Mike 74 beat me to it / I didn't read his post properly.
 
GreSY.
You are on the right track. The throttle plate is not a true circle. A well made plate creates an amazingly good seal in a well made carb/throttle body.

A guy I used to work with in Oakland CA, by the name of Buzz (Buzz's Carburators) used to make his own for carbs he couldn't buy parts for. He made a special split mandrel for the lathe with the split at an angle to the long axis of the mandrel to match the angle of the closed throttle plate to the long axis bore in the throttle body.

There are other tricks for compensating the opening rate such as cam shaped cable pulleys on the throttle shaft and intermediate linkages.

I must confess that he never actually showed me how to make one though he promised he would one day... especially if I bought out his business. He was getting on in years and wanted to retire. He upped and died of a heart attack before I ever learned the trick.

-DU-
 
Thanks all.

Well, to give myself some credit I 'knew' right away how to angle a throttle plate....the lathe idea for some reason was easy to dream up. But I'm glad I'm hearing it from others cuz it helps prove I'm not wrong.

I should add...making the plates cut at an angle will also make them no longer perfectly round, right? A slight oval...?

Why make my own? Because what I want, I can't necessarily buy. Those IMPCO TB's are so unattractive I couldn't use them if I wanted, lol. I haven't identified anyone selling what I want...but I also know if I did find it, the price would be high. Most people seem to want $200 per hole and I need 8 so it gets appealing to make my own.

As for wanting turbulence in the air path, I think that has some validity still but it's far less valid with EFI systems that are mostly flowing just air without fuel suspended in it. Most throttle bodies are round where the plate is but otherwise pretty quickly become shaped otherwise.

I have a couple feelers out on some existing parts I can use with good results, but if they don't come through I'll try making my own.
 
Bill D said:
Is it even a straight bore? I would expect it to be a fancy curve for a venturi effect. Folks do all sorts of things to improve airflow like polish the intake paths cast into the head. Old carburators wanted a somewhat turbulent flow to mix the fuel with the air not a pure laminar flow streams of clean air and. a spearate stram with fuel mixed in.
Bill D
Click to expand...

A venturi is required for a carburetor but is not needed for fuel injection.

The purpose of the venturi is to create a low pressure area (suction) that draws fuel from the bowl through a passageway. In a fuel injection system the throttle body just regulates air intake.
 
If the throttle is not exactly perpendicular to the body when closed it would need to slightly elliptical, i believe.
 
Langley claimed to have made a flyable airplane before the Wright brothers. He did take off downhill and crash at a lower elevation into the river a few times before the wright brothers took off from level and went up. Well after the Wright brothers, 1915, he added a modern ignition system and carburetor, more efficient modern propellers and an experienced pilot was able to make it go up 2-3 feet. Wonder where the pilot got his experience?
I believe the original carburetor was a handful of sawdust soaked in gasoline. Kind of like a modern air filter.
Because Langley ran the Smithsonian institution his "plane" is still displayed there. He did not want the Wright brothers plane but the museum was forced to take it 1943 after he had died. He was trying to claim patent rights for flying machines.
Bil lD.
 
One other point regarding throttle response upon initial opening with fuel injected engines is the idle air valve. These seam to be quite common on fuel injected engines. I have a 2003 40hp outboard with one. As you begin to increase the throttle the idle air valve allows air to bypass the throttle plate, the computer can regulate how much it opens.
 
There is always a way, if you have the gumption. Parts options were much more scarce in the early 90's. A machinist buddy wanted to remove the significant constriction in the center of the MAF in his 5.0L Mustang. The MAF was an aluminum casting, with a thin wall and external support ribs around the constriction. A Coors can was the perfect diameter for a uniform cross-section. He bored through the walls of the MAF and well into the support ribs. The can was inserted, epoxied, and painted.

Looking at it, you would not believe it had been modified. The only problem, whenever someone was told it was a coors can, there was an irresistable urge to poke it with your finger, which was a huge No. It held up fine until he upgraded many years later. Also, he had connections who did the essential ECU calibration for the new config.

maf90mustang-228x228.jpg
 
kb1cjp said:
One other point regarding throttle response upon initial opening with fuel injected engines is the idle air valve. These seam to be quite common on fuel injected engines. I have a 2003 40hp outboard with one. As you begin to increase the throttle the idle air valve allows air to bypass the throttle plate, the computer can regulate how much it opens.
Click to expand...

The IAC regulates air when it idle. The computer opens it more or less as needed to maintain the targeted idle speed. But most EFI systems lose total interest in the IAC at anything above 1% throttle opening. The IAC does its work when your foot is off the gas.
 
Fisher Controls makes an excellent gas control valve known as the v-ball, I always thought a mini version would make an excellent TB.

Sent from my SM-G930R4 using Tapatalk
 
I made some throttle butterflies a while back. I milled a 10 degree angle on the end of a piece of bar, milled another short bit at 10 degrees and drilled the pivot screw holes and tapped the bar end. I sandwiched the future butterfly between the bar and short bit then turned the required diameter. Next remove the screws and there you have it, a slightly oval butterfly that is a perfect fit, edges parallel to the bore when closed.
 
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