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Deciding which size of throttle body to run

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I need to replace the throttle body on my 2L B series engine in my Honda EF as the one currently fitted is causing high idle problems.

Engine is a built high compression 2L and has a bigish intake manifold fitted, currently making around 245hp, expecting around 250 with new map due to some changes. I'm not worried about the tune as I've just changed to Hondata S300 and it will therefore be remapped after fitting a new TB.

Problem is I have no idea what size is best to run, my options are 66mm, 68mm and 70mm, some are tapered and some are straight bore. Most people seem to think that 70mm is best but most people are idiots and assume biggest is best. From what I can see a the 66mm is actually the best but I've heard of good gains from people running bigger on less tuned engines. The 66m one I am thinking about is 68mm at opening and tapers to 66mm at the plate and inside. Don't want to get this if I'll get abetter power curve from a 68 or 70mm.

So how do I decide what size is best for my build? Its mainly a road car so I want a good powerband and midrange but obviously a bit more top end is good on a high revving vtec.

From the research I had done its been said you do not want the airspeed to be going over 300fps, so with some calculations I came up with this chart... Not 100% sure on its real world applicability so take it for what its worth. I allowed for 30% drive-train loss so the figures are quite conservative.

2.25" or 57.15mm up to 250rwhp

2.50" or 63.50mm up to 300rwhp

2.75" or 69.85mm up to 350rwhp

3.00" or 76.20mm up to 400rwhp

3.25" or 82.55mm up to 475rwhp

3.50" or 88.90mm up to 550rwhp

3.75" or 95.25mm up to 625rwhp

4.00" or 101.64mm up to 725rwhp

Seems to tie in with anything I've found, found some calculations and they all came back with the result that a even a smaller TB than the stock one would be best, seems even the stock TB is bigger than it needs to be.

I might give the 66mm a try on my hunch that it will result in a better mid range and perhaps some gains at high rpm also. It starts 68mm and ends 66mm, I believe this Is good to slow the air down as It enters and speed it up after the throttle plate. I think the bigger throttle bodys are only available because companies know most people think bigger is better and therefore they will sell well. Also it will fool people into thinking their engine is more responsive because its letting more air in for the same amount of throttle movement as a smaller one would.

Am I right in thinking that a tapered TB is going to result in better performance than one that is straight bore the whole way?

Was hoping more people would have input into this. It seems to be an area most people don' understand properly, myself included.

On engines I mostly deal with the TB is the least of your worries because the intake intself is the restriction so it is no use going to a bigger TB. That being said I treat TBs like I treat carbies. Match the airflow capacity to the engine's intended purpose. Use a big TB if the intended purpose of the engine is high revs, use a small TB if you want low down streetable pulling power with excellent throttle response. I tend to work on the conservative side.

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