Get a cuppa, and take a seat...
Much has been said on boring/honing your throttle body to increase the magic power... Its something which most claim will realistically only release 1 or 2 bhp from your engihe, about the same as a polish, or a new set of tyre valve covers. But anyway, that's all 'opinion' so with limited knowledge I thought I'd see what I could theorise.
The C and X 25XE was used in fwd applications, in Cavs, Calibras and Vectras as well as rwd. On paper, in the brochures these produce the same power, though there is debate that the rwd engines always made a little more power, due to their superior intake system. However, for the purpose fo this exercise, these engines are 'the same' power outputs that's what Vauxhall claim.
The fwd engines use a single butterfly valve, whereas Omegas use a twin affair, of course. On top of that, the multiram intake should smooth out the torque curve, while not actually adding power, makes for a more driveable engine. Doing the basic maths the fwd engines have, when enlarged, a bore of
TB bore fwd V6 68mm
sq. area 3631.68 mm
2TB bore rwd V6 56.5mm
sq. area 2507.19 mm
2 (but there's two of them, so...) 5014.38 mm
2or, rwd engines have what seems a massive 38% larger air intake surface area. That means nearly two-fifths more air can be sucked in (well... I say no, because the engine will only ingest 6 combustion chamber'sworth of air at any one time, but it is breathing 'easier' by 38%, shall we say?)
The fact that there's a barely perceptible difference between the engine outputs of the fwd and rwd 2.5 engines means, surely, this throttle body enlarging must account for....well, nothing?
Ok, let's make a theoretical assumption - the rwd V6 throttle body gives a whole
10 bhp extra over its fwd brother. That would equate to...
Enlarged TB Diameters...
68mm - 56.5mm = 11.5mm difference
11.5mm / 10bhp = 1.15bhp per mm of throttle body diameter increase.
So the rwd standard 54mm throttle body which can be enlarged to the above mentioned 56.5mm means
2.5mm difference diameter, or 2.5 x 1.15 = 2.9bhp difference.
That 2.8bhp extra based of, remember,
if the fwd and rwd engines differ by 10 bhp. If that is only 5bhp, that's 1.44 bhp, if it's only a couple of bhp we're down to
half a bhp!!
I'd welcome any input on this, with regard to anyone with a better grasp of GCSE Maths than I (which I suspect is most people
) to pick up on any errors, or even if you think there's flaws in my theory. Rather than doing it for real, thereby allowing placebo effect to take over I thought I'd play around with a notepad and biro.
You can go to the toilet now!
) he isn't especially close to me, but even so he only went and ruddy delivered the things to within a few mile from my house!! Had a natter, did the bloke thing of looking at each others' engines, then went our separate ways. 