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Throttle body diameter and inlet length tuning/calculating on turbo engines?.

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Hi two questions really, figuring the inlet length and throttle body diameter is fairly simple on a n/a in order to align to either the first or second harmonic for either addition torque or addition hp gains for your setup.

But how is this best done in turbo applications taking into account the inlet length to the throttle body then on to intake length that goes right back through the intercooler to the turbo? Also with this how is best to calculate the throttle body diameter itself.

On throttle body size - it's very difficult to find that information. The only source I found it in was Maximum boost book. Basically it talks about 300 ft/sec of air velocity as a reference point that shouldn't be exceeded...

Here is picture of that article in the book.

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I've always used the calculation formula:

Engine displacement/number of cylinders x rev limit..

then do that to ^0.5 then × 0.8 ..

This will get you about right for itbs. But wonder for on a turbo how you would do this same equation. Then on to second part of my question which is on a turbo how do you go about setting the inlet length to match harmonic? As unlike that of simply calculating trumpet length to match first or second harmonic 🤔

(For the calculation above the constants Being the 0.8 is based on VE avarage at peak for most N/A engines, and the 0.5 being of VE avarage at idle.)

E.g. 1.6÷4=0.4 x7500 = 3000

3000^0.5=54.7722 (55)

55x0.8= 43.817 (44)

So 44mm throttle body be about your ball park to consider.

So call it 45mm to be fitted just to be on the safe side of no restriction on peak power if building it for a race engine.

Or 43mm if was for day to day road use and just want good low end response.

Just to confirm, this is a single throttle body on a plenum that feeds individual runners to the cylinders? My thoughts as discussion points - don't take them as gospel.

In theory, you should be able to use one of (or several and take the mean value(s)) the available formulae for the overall runner length, as you would for N/A, and modify it for the change of the pressure wave velocity under the higher pressure. I looked for a few minutes and there doesn't seem to be easily found data on the affect of pressure, but i seem to recall an engineer saying the difference was negligible, but may be mis-remembering it.

The runner diameters should be the same, as it's moving the same volume of air, or so I would expect.

In this application, I wouldn't concerm myself with anything up-stream of the TB, as what pressure waves aren't dampened, absorbed and dissipated within the plenum shouldn't be able to make much affect as upstream there is considerably larger volumes of air, flexible hoses and fittings, and a charge cooler that will act like a massive pressure dampener.

If it's something you are really interesed in following up, you could fabricate a manifold with two main parts - the planum and slip tubes, and the head flange with matching tubes that allow the two to slide like a trombone. Heck, maybe incorporate into the vehicle with a suitable controller.

For the TB diameter, as has been said, around 45mm shouldn't restrict it and still allow a degree of throttle control at small openings - some sort of progressive linkage should help a lot there.

OK so I get what your saying when it comes to pressure not changing the harmonic bit like blowing on a trumpet harder your blow still gets the same note. And yes the idea of using just upto the TB makes sense being as you say the intercooler would act as a dampier to the harmonic infact so would a plenum so just upto there.

Two questions thou So in running a plemun and single tb setup the runner diameter would be figured like ITB's would "but" the calculation above then wouldn't work straight off for that of your peak VE would be higher in boost depending on amount of boost etc etc..

so only way i can see to figure accurate runner diameter would have to make a ruff ball guess on diameter then map the car see what your getting max VE in boost then adjust diameter of the pipe proply based on that to then make the calculation work for you and be more accurate.

Am taking that again the plenum is going to work as a dampner so be making the runner length calculation work from plenum to back of valve match harmonic. Rather than end of trumpet?

Still as say how do you then calculate throttle body diameter accurately for single tb turbo. (Itb's is case of just match the runner diameter you have calculated e.g. 45mm as above) but for single TB I see alot based on throttle body size figured off rpm and power output so (e.g. 63mm working for 200bhp engine 7500rpm peak.) Even thou 63mm gives 223bhp on the graph they say to go 10% bigger than to that what the engines making to play safe to no restriction. So would this be to take again a ruff ball educated guess to what it's going to make, set it up, Map it see what your get power wise then correct from there to refine? Was hoping for a more mathematical calculation to figure single TB from the start rather than this of needing to wastefully be making inlets, getting TB's to test and home it in that way 🤔

Also the new question possed here is to how to calculate plenum size if focusing on just upto the TB.?

That is EXACTLY why I loathe the use of VE as compensation for forced induction engines - it's a false measurement. IMO, VE should ONLY be used for the relative filling of the cylinder in relation to the air source. Something like "mass efficiency" would be MUCH better term!

As such, use the VE estimate for NA engines in the equation(s).

The plenum is basically the equivalent of a cold air box on NA engines, so the total runner length would be from the valve seat to the opening of the runner/trumpet in the plenum/air box. Can't recall the recommended volume range for the plenum/air box, but should be easy emough to find on-line.

You don't need to worry about testing with variable trumpet lengths unless you really want to get as much out of the engine as possible - like some race series where they have multiple variations they're testing - and have the time and money to invest on diminishing returns, or are interested in experimenting for fun and/or curiosity.

Right so have found a calculator for accurately calculating the runner lengths from valve to the plenum to be on target to the 2nd or 3rd harmonic to of what i wanted.


As for the plenum size and design general rule of thumb I formed was that a plenum volume be of approximately double the engine capacity is a good place to be and if end fed from a throttle body to have a ruff taper across the plenum to maintain the air speed towards the furthest cylinder away from the throttle body and to have them all be feed equal, plus have all the runners be trumpet shaped of the plenum into the runners to the Chambers.(So just like a set of individual intake trumpets but inside the plenums base plate)

To the the final question still to get of a most accurate calculation on a throttle body size rather than just vaguely set it on rpm peak to bhp (which you dont know till its built). What I can find to be accurate on setting this or least not cause restriction or cause torque control reduction will be to run individual itb's on the plenum runners matched to there size as figured earlier from my equation, or if going individual TB simple match the size to the size of the feed pipe from the intercooler to the plenum being on average on most fast road car builds upto 350bhp is 2.5" piping so being 65mm.

So all your diameters stay constant of pipe size from turbo to plenum, and same within the runners. Only area that has a reduction slope effect on size is the plenum itself as to give equal supply pressure to all four trumpets to keep devision of that mass volume equal.

Infact to reduce on that veriable effect of feed pressure and smooth its devision would be to centrally supply to the plenum when your making it rather than have it go in on one end. Then you can have a more gradual taper both sides then. Least then any incorrection you have would be shared by half to both outer cylinders rather than just given to one end cylinder making it least noticeable and least of a chance towards causing damaging or ill effect on mixture diffrence towards det.

Would guess this plenum tapper angle either side of the in feed would only be settable truely through putting individual widebands on all 4 exhaust runners to get spot on feedback to set it spot on of angle, but id think a mild taper of 10-20° should get you to in the ball park of ok for anything road based of power upto a range most wouldn't be surpassing. Plus any inaccuracy within this angle would not be causing much effect toward power lose being you on refining devision of the air mass not supply volume that would be really noticeable of very fractional afr/VE differential between cylinders. 👌 would be within the real fine art stages then of refinement to nail that in perfect on the engine.


Measure and make sure there is no pressure drop across the throttle body.

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