There's been quite a bit of debate about whether a bigger TB can gain power on a tuned K20. Well, I've just got some very
good results on my FD2
First off, current engine spec of car is:
Toda A3 cams
Mugen air box
Hondata mapped by Sam at TDI South
While car was at TDI South having Toda A3 cams replaced (see Toda A3 cam thread) I consulted Sam as to whether he thought it
was worth it fitting a bigger throttle body and perhaps a better air intake pipe from Mugen box to TB. He said that it could
be but to be sure, I really needed to measure the pressure drop in the manifold during some full throttle runs. The theory is
that as the engine asks for more and more air, the air intake system and TB should be able to supply it unhindered. That
being the case, the pressure in the manifold near peak power should be within 20mb or so of the pressure at idle and if so, there's
no point in switching to a bigger TB. But if the the air intake system is struggling, you will see a bigger drop in pressure,
which indicates there are gains to be had from a larger TB.
Obviously, it made sense to do some testing as I didn't want to waste time and money for little or no gains. Fortunately, the
datalogging feature on Hondata makes it very easy to graph all sorts of parameters, including manifold pressure. So I did a
number of WOT runs and this is what I found:
Basically, there was a pressure drop of around 1.1PSI at near maximum power output, which equates to about 74mb - signficant.
That being the case, I decided to go ahead and fit a Skunk 72mm TB:
I also decided to maximise airflow from the Mugen box to the TB inlet. I did this by sourcing some 87mm internal bore
aircraft grade aluminium piping with a 45-degree angle bend - looks very much like this:
This was secured to the Mugen box outlet using a 90mm internal bore silicone coupler and at the other end, by using a
silicone coupler that smoothly reduced from 90mm to 80mm - looks something like this:
In summary then, the air flow from the Mugen box is now mainly 87mm internal diameter reducing to 80 just before the TB. When Sam and Dennis mapped it all up, the result was this (233.5bhp, red plot is power after, grey before - black plot is torque after, grey before). Please remember, all these figures are AT THE HUBS:
As you can see, after 6300rpm a sustantial increase in power and torque all the way to the redline
There's also more torque low down (which surprised me) apart from a very slight loss between 3400 and 3900rpm. How does the car feel? In a word - phenomenal. Really, really urgent over 6Krpm - really quick in fact. Also drives beautifully at all other rpms and on part throttle. However, note that the extra power and torque don't really make themselves felt until after 6300rpm at which point my car is producing around 187bhp at the hubs (which according to which flywheel conversion figures you use is around 215bhp at the fly). This means that to get any noticeable benefits, you're probably going to have to be making quite a bit more than this to start with - ie this should be one of the last mods you do, not one of the first!
PS - will post some actual pics of instal tomorrow when the rain has hopefully stopped!!
OK, here are a couple of engine bay pics. You can see that ideally, the pipe bend would be nearer to 30 degrees rather than 45 degrees but it's not far off and importantly, it works!: