Throttle Bodies

[Toddy 7]

After some advice regarding throttle bodies.

Throttle body diameter

 

Everybody seems to choose 45mm bodies, which I am guessing is because they liken them to 45mm carbs but aren't the carbs fitted with 36 or 36mm chokes therefore 40 or 42mm bodies would be ample.

 

I know Dave Andrews states that 42mm bodies are good for over 250bhp.

Throttle body length

 

Most people seem to choose TB or TH jenveys which are 118mm and 91mm respectively, what is the criteria that people have used to make a decision in which length?

 

Trumpet Length

 

What trumpet length have people choosen as I guess this is the final factor in deciding what is the optimium tuned length, is this best waiting until you have been to a rolling road and found the best suited trumpets.

 

Airbox

 

Anyone have experience of an airbox on ITB's in a 205 and did it make a significant difference to noise and performance.

 

Any comments welcome

[Baz 421]

As for sizes, TB's and Carbs differ in size choice, i forget why i'm afraid! But i know the same theories don't apply with TB's as they do Carbs!

[Sandy 189]

Throttle body size in a simple sense can be chosen fairly simply, by expected power. But choosing the optimum size isn't that simple. The overall tract design and its relationship with the rest of the engine geometry (particularly port/valve/cam relationship) needs to be considered. The basic theory as given on Jenvey's website etc, is that you choose the smallest practical TB size and taper the tract for best flow. Which is a fairly reliable route to a reasonable result. However, to get the best overall result is rarely going to end up with that arrangement, in my experience, particularly the issue of taper, which seems to serve flow well, but often compromises the overall result due to other issues.

As an example, my 2.2 Honda engine was making around 235bhp on 50mm bodies. When I fitted 45s, the top end reduced noticeably, but the mid range flexibility improved. On another engine, I've seen a healthy, happy, revvy 266bhp on 45s. The difference is the way they breathe. The Honda engine is designed to draw in the charge in a shorter, higher amplitude "lump" if you like, where as the other engine was designed to draw in a longer more progressive flow. Smaller bodies on the Honda engine capped the peak flow and hence power, and bigger bodies on the other engine would have narrowed the power band by spoiling the gas speed and inertia at medium engine speeds.

I think 42s on a 250bhp engine could be regarded as an exception, I haven't seen that particular case though obviously, it depends on the overall method the engine designer/builder employs. A friend of mine, who's one of the best Vauxhall engine builders out there, favours largish bodies, but it gets great mid range, by exceptionally careful cam design and exhaust tuning. He's ended up down that road, because most of his engines go in RWD Escorts, which have very limited room for induction, so the inlet tuning potential is very limited in that case.

 

Getting back to specifics here, if we are talking about 205 engines, my simplistic suggestions would be as follows:

XU 8v

40mm around 160bhp

42mm around 180bhp

45mm around 195bhp

 

XU 16v

45mm 190-250bhp

48mm 230-270bhp

50mm 270bhp+

 

Overall tract length needs to be tuned for the best effects, but it seems all of the best results I've had in terms of grunty deliver on the XU 16v, has been with set ups in excess of 360mm from trumpet mouth to valve. I've had to make my own inlet manifolds to achieve that, because the Jenvey/Longmans ones point the bodies at the bonnet and so effectively limit length to about 330mm.

 

I'm not generally a fan of airboxes with a single side inlet, a large front panel filter shouldn't cause much cylinder interference, but the side exit boxes need careful development, if you're not going to bin the advantages of TBs.