Assuming two identical engines were running the following inlet systems:
1. Jenvey 45's to airbox and 3" diameter cold air intake pipe with conical filter at the end.
2. 45mm diameter inlet runners of a similar length to the Jenvey bodies to similarly sized air box using a 3" single TB and then a similarly
sized cold air intake pipe with conical filter at the end.
Would I be right in thinking that both should yield similar performance gains over the engines stock inlet assembly?
Cheers
Power maybe the same, but response will definitely be worse with the single throttle body due to the larger volume of air between the butterfly and the valves.
I have always wondered about this myself and in the absence of real data I am still curious. The actual extra volume with a single TB is probably 2 or 3 litres worth or about 2 rpm of engine induction depending on engine size. I have a sneaking suspision this could be an old wives tale from the days of carbs when fuel drop out was a big issue with single remote carbs vs Webers. I am prepared to be educated however.
Maybe... wife had a corsa 1.2 petrol and round the carpark throttle inputs would be delayed by a noticeable amount, but this had a large post butterfly volume, small capacity and low revs. When the engine is spinning at 6k maybe you would not notice at all?
If the two setups were identical but one was itb and one a single throttle body, the difference should only be throttle response.
In the real world you'd never get two setups that were that close in design to make a real comparison of the two.
What they use on race cars should answer your question.
I large TB would be lighter than a set, so why do they use individual ones.?
The only answer can be, better performance.
Depends on the race car 
Touring cars are single throttle body.
Perhaps they don't have the budget.
They don't use one on any Japanese high performance four-pots that I know of.
Backacha.
[Edited on 5/3/12 by Confused but excited.]
i thought it was part of the rules for the touring cars - to do with the restrictor they used to run (i haven't looked at the rules in donkeys years so could be completely wrong).
quote:
Originally posted by Confused but excited.
They don't use one on any Japanese high performance four-pots that I know of.
Yeah it's part of the rules. Lol.
ITBs are best, no denying that, but that's not to say single throttle bodies aren't capable of being almost as good.
As said, both systems should produce the same power, assuming all is equal. But due to a much smaller throttle to valve volume, the ITBs will give a more responsive engine.
It's way more complicated than that I think.
Yes, single throttles do have some level of delay over ITBs but that delay is relatively small - as mentioned it's really only a couple of cycles
worth and at higher revs that'll be less noticeable. ITB's tend to be matched to the port size, which in turn (when all added) is
significantly larger than most single throttles tend to be. ITB's tend to produce a nice amount of turbulence right before the injector which can
help or hinder depending.
What needs to be noted though is that ITB's prevent the use of variable intakes which, on non-VVT engines means limiting performance tuning to a
fixed RPM range. Even on VVT engines OEMs run up to 3 stages of variable length intakes. Many OEMs use multi-length runners and variable resonance
intakes to broaden the torque curve. On a track car this may be fine as you may just match your intake size to the RPM band you intend to stay in, but
on a street car that's limiting. Everyone loves removing variable intakes, especially those with throttle plates, assuming they're robbing
them of power, but ultimately they generally help across the range - that's why they were installed. Add to that the fact that you can, depending
on what intake model you subscribe to, use the plenum and intake pipework as a helmholtz resonator tuned to work with the cyl volume and runner length
for more gains, removing that may be a negative. Depends how well the original system and new system was tuned I guess.
I considered going ITB on my V8, which I might do still for grin factors, but I expect a performance drop when I do.
It's an interesting debate, and I think it's horses for courses.
[Edited on 5/3/12 by coyoteboy]
quote:
Originally posted by scootz
quote:
Originally posted by Confused but excited.
They don't use one on any Japanese high performance four-pots that I know of.
Like the F20, EJ, 4G63, or the 2ZZ-GE...
ITB's have two big downsides, one is turbulence as stated by Coyoteboy and the other is that they are restrictive due to the turbulence caused by
all those throttle plates hanging in the air stream, both mean that you most probably could use slightly smaller inlet runners for the single throttle
system which would restore any lost response.
I think that for a road car a well designed STB could save lots of weight and complication, without any discernible loss of power or drive-ability,
and a big saving in cost - the sticker is that design will most probably be a lot more complicated and critical in the STB case than the ITB case
Forgot to mention that ITB might give less of a pedal feel than STB due to extra friction so feel less responsive.
[Edited on 6/3/12 by Ivan]
quote:
Originally posted by Confused but excited.
Scootz, read my last four words again. I am oooooold.
Got grandkids dude.
Trust me, I'm not exaggerating. 
The big advantage of ITBs is being able to delete everything before the intake trumpet completely giving minimum restriction and then tune the inlet
length. They also overcome the plenum issue of uneven airflow.
If youre going to fit ITBs and then put a plenum over the trumpets with a 3 inch pipe leading in then I suspect most of the potential gain is lost.
For my part I made some grp inlet runners with trumpets and a plenum for my zetec which Im keen to try when i get it running!
The volume in a plenum before the throttle bodies will be closer to ambient pressure.
If the extra volume is after the throttle bodies it will be at a much lower pressure and take a finite time to rise if the throttle is snapped
open.
Strangely in the US some engine tuners clung on to single point injection for a long time.
oh for outright throttle response theres an physical argument, but for want of a couple of litres of air its not really as big an issue as people make
out.
Americans still cling to carbs. in discussions about best I find it most productive to completely ignore them as a nation.
One thing that I don't think has been mentioned is cylinder to cylinder interference with a STB and plenum. I am no expert but have read several times from those that are ( Dave Walker of Emerald among them ) that with high overlap camshafts getting an acceptable idle is nigh on impossible. This is a special case of course and not generally relevant but does impact on emissions and MoT as well as general driveabillty in traffic. Can be overcome if you have got VCT of course but that is another ball game.