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Anyone with experience on the Diameter of individual throttle bodies

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I have a theory that its important to keep the diameter smalle of a ITB so as to maintain the air velocity going into the engine in that way there is a positive pressure onto the combustion chamber just before valve closing.

I have a Research paper on NA F1 engines and they were getting better cylinder fill at 13,000 rpm then at 7,000 rpm and from the pressure transducers it shows the effect of velocity as opposed to harmonics. If anyone what a copy or the link to it let me know.


I would be interested in a link to the research paper. I am effectively using smaller throttle bodies at a higher RPM and have been impressed with the cylinder fill I've been able to achieve at 8000+ RPM.

Hi David are you rpepared to share some number eg size for a cylinder volume of X. The paper is really interesting because in short at low rpm harmonics are a big factor but at high rpm its the inertia of the air velocity that makes the difference and rams the air in.

Link here,;jsessionid=7023F4994CF7FA78F40DC35F008676AE?doi=

Graph at 13,000 rpm shows how you can get more pressure into the engine.

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Sure -- I am using 42mm Ram Pipe/Extension tubes with AT Power Throttles direct-to-head throttle bodies (these have larger oval-shaped throttle plates). The engine is a 2L Mazda MZR (aka Ford Duratec HE), with 499.4cc cylinder volumes and a 10.8:1 compression ratio with two 35mm intake valves. My VE table shows 122% VE at 8000 RPM.

Hi David,

Are yours like these

Yes, except I use the non-injected versions which don't have an extra injector mount.

The stock head is setup for the injectors to mount just above the valve split. Other than that they are identical. I expect to be receiving some different length extension tubes and ram pipe lengths to test in the next few weeks.

BTW -- these are really nice pieces and I am a US-based distributor for AT Power for those in the states.

Interesting research paper, thanks for sharing.

I am agree that it is important to not oversize on diameters on the inlet tract to maintain air velocity and have

good cylinder filling but given the Mazda example with 35mm inlet valves and a 42mm throttles bodies keep in mind

that the two valve throats (valve seats) belonging to the two 35mm inlet valves should have aproximately 30.5 mm of diameter each.

Generelly valve seats are the main restriction to the flow path of air on the inlet tract on the engines .

My point is that the square area of one 42mm throttle bodie is : 1385.44mm (without substracting the shaft area because is a shaftless setup)

And the two inlet valve throats (valve seats) squares areas added are : 1461.23 mm , so in this case the throttle bodie is smaller in area than the two

inlet valve throats . With all that I am not saying that the 42mm throttle bodies are small for that engine maybe the valves are a little big for the engine size and the RPM range that the engine is on use , it could be the reason why the 42 mm throttle is not restricting air, if it is not doing it.

It could be interesting to test a 45 mm or 48 mm throttle bodies set up and look the behaivor of the torque and HP curve and ofcourse focus on

the usage that the engine is going to see , road racing, rally , drag racing , each setup is going to have and advantage with each racing activity.

I have drag race engine is a Toyota 3sge engine 2l , 11.8 c.r, race cams , 50mm throttle bodies and it have 187 HP @ 8050 rpm and 190 Nm @6550 rpm all that to the wheels and with a very basic tune I think that with a little more cam timing setup and messing more with the fuel and ignition I could achive maybe 5 more HP but I doubt that this engine have more than 105 or 110 ve % I dont think that ecu VE% tables relates to exactly how much VE% of air the engine is really ingesting .

Hi Peru,

Keep this number in mind every NA engine produces between 100 and 120 Nm per litre, at one point around 2004 a NASACAR engine produced 120 nm per lire and a F1 NA engine 122 NA this is of course engine torque number. Now the trick is to get it to produce that number at or around the red line so you get maximum hp.

What I am doing is I have a flat 6 cylinder dropped back to two cylinder and following on from what David has tried I purchased 2 of each of the following sizes. 36, 38, 40, 42 & 45mm ITB so I can experiment with what works best at what rev on a give capacity per cylinder motor. I have also purchased 4 sets of Kistler pressure transducers which I plan to mount in the combustion chamber and get pressure reading to study. They are the same type uses in the research paper. From that data I want to be able to work out a air velocity that work best so we can revers calc what size for a given motor and rpm. Hope that makes sense. I also have my own engine dyno that I plan to use with the testing. I run Link G4+ ECU's and part of my aim is to get a NA engine to develop full power at 10,000 rpm.

I will be happy to share the data I just need to find a suitable forum to post the results.