50mm throttle bodies....

[172lover]

Well lads

I'm currently fitting 45mm throttle bodies to the Williams...


I've some lad wanted to swap my 45's.....with his 50mm jenvey's


I've it in the back of my head... That the 50mm are overkill for the engine?
(although would sound unreal!)


The spec on the engine is standard enough....Kent cams(rn2002),Kent pulleys,customish 172 exhaust manifold, double valved springs.

 

[Cup_Phil]

Too big

 

[172lover]

Cheers Phil,


Any other opinions about 50mm?

 

[Cup_Phil]

Factors influencing size are; Power output, RPM, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.
Choice of bore size is a balanced compromise resulting from the following;
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.
Basic references for BHP per cylinder, assuming ca 120mm from butterfly to valve head and a max of 9,000 rpm are;
Up to 30 BHP - 30mm, up to 33 - 32mm, up to 39 - 35mm, up to 46 - 38mm, up to 51 - 40mm, up to 56 - 42mm
Up to 65 - 45mm, up to 74 - 48mm, up to 80 - 50mm, up to 87 - 52mm, up to 93 - 54mm.
These power figures may be increased by up to 10% in a purpose designed and well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.

200bhp - 4 cylinders = 50bhp per cylinder.
40-42mm size.

Though saying that I must admit I didn't read your original post correctly - the standard jenvey kit for the clio is a 48mm tapered itb
Depending on what power you are aiming for 50's might be better BUT you'd need a matched inlet manifold, as a step in the manifold wall would be a nightmare

 

[172lover]

Thanks for that quote Phil,

Not got any bhp in mind...Nor am I chasing figures,
Just fitting itbs for fun

The inlet manifold and trumpets are coming with the 50mm

I didn't think the Clio kit was 48mm standard...(thought it was 45mm)
50mm doesn't sound too bad when you compare it to 48mm..

Not sure yet still :/

 

[Chip-mk1]

Too big which will lose you throttle resolution, and almost certainly some low down torque as well (how much depends on the taper of the two manifolds you are comparing and the distance they are going to sit the plates from the head).

 

[172lover]

Rang a few fella's big in rallying..
got two saying go for it...
but then got a few more saying don't do it "won't get you anymore power"....aka will have too much air going in for the engine,

Think I better stick with the 45's

Thanks chip and phil

 

[Chip-mk1]

It wont have "too much air going in for the engine" thats a really silly statement, whoever said that you dont ever want to take any notice of anything they tell you about cars IMHO.

You want as much air as possible in the engine, but the engine spec itself will limit this before a 45mm throttle will, so 45 or 50 will both provide the same amount of air at full throttle and peak power, the problem is that a loss of gas speed at low rpm will result (as I said depending on manifold design and throttle plate placement) in LESS air entering the engine with the 50s, and hence less torque.

 

[Cup_Phil]

The standard ones are 48mm tapered - I believe they taper down to 45mm

As chip said it's got nothing to do with "getting too much air in" - the engine limits that, 45mm are still plenty big enough to flow more air than the 2litre engine will.