Forum » Off Topic Discussion » Throttle body diameter effect on restricted naturally aspirated engines

Throttle body diameter effect on restricted naturally aspirated engines

Off Topic Discussion

If it's not really about tuning or wiring. Then it belongs in here.

= Resolved threads

Page 1


I have been researching a lot about if the throttle body diameter has an effect on restricted NA engines but many opinions so far. Some say it wouldn't matter because the restriction is after the throttle while others claim that larger diameters will result in poor throttle response/control.

Any suggestions ?

try both to find out the truth.

Too small will kill power as the engine won't be able to draw enough air through it/them. Too large will make it difficult to control part throttle, especially at lower rpm, as even small butterfly angles will allow a significant amount of airflow.

Check this out -

Hi Martin,

I have done a bunch of testing on throttle bodies and sizes, there are two types of throttle body set up,

- The first is you are just trying to reduce restriction and get the best flow,

- The second is where you are trying to harness the pressure wave to get max cylinder fill,

So to the first, the goal is to reduce drag and if this type of setup is what you want then the butter fly can be worked around by just adding diameter. For say a 0.45 litre per cylinder this ITB might be 48 to 50 mm depending on target rev & hp.

The later is far more complex, for a 0.45 litre volume and a NA race engine wanting max torque at 6k to 7k you would run a 42 mm ram pipe and may just jump up to 44 mm throttle body then back to 42 through to the engine. What we want to do is create a over pressure of say 6 psi at the valve when the intake valve shuts. That way we get a VE greater then 100. The maths are 1/2 the mass times the velocity squared. So we are attempting to keep he mass up but have a average velocity of .6 of the speed of sound. To get the velocity up we create drag in the intake pipe by reducing its size and that causes a negative pressure in the ram pipe at the start of the cycle. That then causes the air to increase velocity and rush in.

What size engine are you talking Martin?

Kinda - I've never heard it described that way, but what you're describing is using the velocity head of the incoming air to form a pressure head at the valve, sometimes referred to as 'ram tuning'. However, that's only part of the intake requirement as the length to take advantage of the pressure waves in the intake is also important to get the positive pressure at the valve at the right time, using the reversal from the throttle bodies' opening. They are not separate designs, but are complimentary factors and both are used. The basic process is exactly the same as used in correctly designing an exhaust manifold.

Get everything right - intake, head, cam's, exhaust and you could see as high as 150% VE at the torque peak, but it can result in a 'peaky' engine with a narrow power range.

I have done a bunch of work on intake now I have moved to exhaust and creating suction.

Attached is a sample of the compiled data from one round of trials. My goal is to generate the a similar neg pressure as the older F1 engines. My aim is for a 2 litre Subaru NA to run to 11,000 rpm and put out over 300 engine hp. The heads are specials and flow 420 cfm on the intake, the bore is 99 mm and the stroke is 65 mm. In a perfect world I would like to see 12,000 rpm. Its a very serious project.

Attached Files
  • Tappered-Exhaust-compared-to-F1-egine-degree-based.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

Firstly, thanks for your opinion guys and shared info.

The engine is 600cc from Honda CBR RR and it will be equipped with complete custom variable intake system. As the manifold will be designed ,the plenum volume/geometry, fixed intake runners length and pipe diameters are studied. Plenum volume between 3 and 4 liters is going to be used because of packaging and the max length of the runners will depend on the final design of the plenum. So, a lot of things could vary ... Originally, the engine is equipped with ITB's with 38.1mm ID.


Anthony, sounds like a very cool project, get it right and I would expect you to be getting very close to 400hp, maybe more if the bucket diameter allows the camshaft profiles. Going to sound insane.

There was a Greek guy, Papadopoulos I think, who had a destroked and insanely developed N/A Coswoth based engine that was getting over 400hp and reving to 12k - might be worth checking out, if you're unfamiliar with the build, as I know there have been many articles on it. You may even have a better head and so more potential - regardless, if the Honda K guys can get 380+ on their long stroke/small bore 2 litre engines, you should be able to get very close to it, if the follower diameter allows the cam'ing to suit

Gord why is the cam bucket so important, was the Greek guyon pump gas or E85? Thanks for the supporting comments, I have been doing a bunch of testing using Kistler pressure transducers. Josh and Kelfords has been great working with me to get the ideal cam. What I think is a real surprise we may have to reduce the lift on the exhaust cam as its letting the gas out too soon, need more back pressure in the first part of the exhaust to create the neg 7 psi I am chasing. So far the numbers say I could get neg 7 psi on IVO and plus 7 psi IVC which if I get it at 12k will be a game changer.

My thoughts - and I could be quite wrong, so definitely check rather than taking it as gospel.

The bucket/follower diameter limits the aggressiveness of the cam' profile - lift and/or duration - as too much will run off the edge of the follower. It's why the NASCAR Chev' engine was allowed to use the larger Ford diameter flat followers. Kelfords are VERY smart people, they'll see you right.

I don't recall exactly what the Greek guy was running - but they do use a VP race fuel. On that, from a different post it seems you're using a rather low 10:1 If so, it seems rather low and even on 98 RON I'd expect around 12.5 and on E85 13:1 or more?

Not many realise the expression "tuning" actually goes right back to the early steam engines, when they 'tuned' the inlets and exhausts in exactly the same way you're doing it - except they were working with pipes 30 metres, or more, long.

With the tuning you're trying to do, you need a rapid opening to get a stronger signal, so I'd suggest reducing duration rather than lift. High initial acceleration will aid that initial lift and signal which is where the follower diameter comes in again. Another option is to play with pipe lengths - I assume you're using a dyno' set of exhaust manifolds that allow easy insertion of different length pipes and/or slip pipes - think trombone - that allow easy and rapid length changes?

If you haven't done so, I'd suggest reading - there are others, but that is a good'un. Note the part on collector design and taper.

I think you may already have a copy, but if not, I'd recommend - - reqarded by many as a seminal work on the subject.

Thanks for the links I will read through them with interest.

For now we are 10:1 but the aim later is to be able to move to 13:1 on pump gas provided we can clear all the hot gas from the cylinder between cycles. I am still trying to get my head around the effect of higher VE on compression ratio. What happens if say I am at 13:1 and I get a VE of 120%??? What about 130% VE will that then cause problems. Still trying to understand it and do some math's on combustion temp before lighting it up.

It might interest you to know I set up a test, 2 meter long exhaust pipe from 1 cylinder 2 pressure transducers 2 meters apart. The speed of the exhaust gas went down the pipe at 1,200 m/sec the returning pressure wave was at 300 m/sec.

Also it seems the negative part of the pressure wave on the exhaust is 40% of the period length. So expanding that I think I need the exhaust to be neg for about 100 degrees to get the best benefit. So as a result I will need 250 degree of solid exhaust flow to get max benfit on the suction to the intake.

For now this is all just theory so will need to see how it unfolds. Thanks again for the help.