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This is in addition to my previous question about throttle bodies. I know it may sound a bit subjective, but I think it has a clear technical aspect, and I have done a considerable amount of homework.

To elaborate; I'm looking for a throttle body to replace my SU carburetors. The carburetors, have a bore of 45mm. It's obvious that you don't want a TB (throttle body) that's too small because that would restrict the airflow. You wouldn't want to go too big either, because the response would probably be really weird. If 20% throttle would give you full power already, that would be annoying. I suppose manufacturers choose a TB that is, with a little margin, just big enough to be not restricting at WOT at max rpm.

I was wondering if it would be smart to go for a bigger TB that is not restrictive anymore at WOT at max rpm, when it's only opened for say, 50 degrees. I'd set the pedal travel accordingly; pedal to the metal means throttle opened for 50 degrees from shut. Because the TB is much bigger, but opened only partially, you still have the swirl/turbulence that the throttleplate creates, while also keeping maximum manifold pressure or volumetric efficiency.

I have flow tested it in SolidWorks and you can clearly see the difference. It's a 2 litre 4 cylinder twin carb/TB. So at 6000rpm you'd get 50l/sec per TB. The ambient pressure is 1013hPa. The colour of the arrows represent the air speed in m/s.

At WOT, there's virtually no swirl, and the MAP barely differs from ambient pressure.

At 50degrees opening, there's much swirl but the MAP drops significantly.
At 50 degrees, but with 5/3rd of the volume airflow(30L/sec), you still get the swirl, while also keeping a MAP that's almost as high as ambient.

SolidWorks FlowSimulation image

The same would be true for 50L/sec but with a bigger TB. So in my case i'd choose a 75mm TB over a 45mm one; 5/3rd bigger than normal. More swirl means better atomization, better atomization means more complete combustion, more efficiency, and more power.

I'm inclined to think there's a reason manufacturers don't do this, but I can't see a downside in this concept. Obviously one would need to make an smooth conical-like adapter to properly fit the TB to the manifold.
What do you guys think?

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    I think we need more questions like this is what I think!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 23, 2017 at 12:39
  • @Pᴀᴜʟsᴛᴇʀ2 I hope you also have an opinion about the idea? :)
    – Bart
    Commented Jan 23, 2017 at 15:07
  • I think I've seen some figures on doing this, but not sure. I'll take a look later today after I get home from work to see if I can find something. I'd bet Zaid may have some input in this area. The thing about sizing a TB is the engine will only use as much air as it can use. Oversize is much better than undersize, but if the engine cannot use it, you may be wasting your money. It's like putting a huge cam in a car ... if you don't have the supporting mods to go with it (ie: exhaust & intake mods), you're doing your engine a disservice and will most likely have created a dog in the end.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 23, 2017 at 15:12
  • @Pᴀᴜʟsᴛᴇʀ2 Yeah indeed, so the maximum airflow would be achieved already, when the valve is not yet fully open. The valve being partially closed won't be a restriction then, but it does create more swirl. In the pictures of the simulation the partially closed valve barely causes more restriction than the fully open one, but it has way more swirl. the only thing is that you have to set up the accelerator to open the valve only partially while the pedal fully depressed. A bit more tweaking may be needed to get the engine's response to the accelerator right.
    – Bart
    Commented Jan 23, 2017 at 16:27
  • Would having two throttles, like a small one for low power and a large one, which starts opening when the small one is nearly fully open, be an option?
    – Andrew Morton
    Commented Nov 9, 2019 at 11:44

2 Answers 2

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One of the issues is air control at very small openings. Smaller throttle bodies work better at or near idle (in general).

As for maximum performance, a key input is V.E. (volumetric efficiency). To achieve high numbers, smooth airflow is important. Smooth flow promotes high velocity flow which in turn creates momentum. So when the piston hits bottom, and is no longer drawing air, the air continues to flow into the cylinder because of the momentum on the intake track (similar to scavaging by the exhaust with piston nears TDC on exhaust stroke). So smallest opening that doesn't restrict is typically the best. Turbulence is the enemy.

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Wow! Very technical. If you are turbocharged then put a bigger turbo volume to give more air or maybe just increase your boost on yours to make up for your big throttle body, then see how it goes and you can also increase your injector spray time? I think your good with the bigger throttle body and just adjust the rest if it if you have too?

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    will the bigger TB give more flow or will all the other components be the limiting factor? What about the waste gate?
    – Solar Mike
    Commented Jul 27, 2019 at 5:19
  • Thanks for your positivity, but the larger TB would not be to get more air into the engine. It is assumed that the current TB is not the limiting factor in that. Rather, a larger TB will give the same airflow at WOT while not even fully opened yet. Because of that, it double functions as a swirl valve, improving atomisation and homogenisation.
    – Bart
    Commented Aug 4, 2019 at 19:01

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