OP
OP
pollyp
Club Member
Not my work but makes sense... relating to our TB discussions:-
Bigger Throttle body nonsense…
I love my dyno, it’s great.
It doesn’t suffer from any placebo effects, it doesn’t read biased rubbish on the internet, it doesn’t try to sell you anything or try to convince itself that its most recent purchase was not a total waste of money and it doesn’t flatter with “suits you, Sir” lies; it just measures what is being produced and shows it, warts an’ all.
So, now that’s out of the way, let’s look a little bit at HOW engines breathe, since that’s what induction changes seemingly set out set out to improve…..
Suck, squeeze, bang, blow right? Yes of course, mostly.
However, imagine that a given engine has a cylinder capacity of say, 3000cc, so each revolution it would DISPLACE 1500cc (1.5 litres), and that it’s running at 6000 rpm; that’s 100 revs per second so 50 CYCLES per second and thus, 150 litres per second @ 100% Volumetric Efficiency (VE). Think about that, a hundred and fifty litres per SECOND.
Well, given how fast that is all happening the VE isn’t going to be anything like 100%, is it, because there’s so little time to fill and empty the cylinders, right?
All the little restrictions along the way; air filter, trunking, intake silencer, throttle body, plenum chamber, intake runners, ports in the cylinder head, past the valves (which are closed for more than half of the time) and that’s just to get TO the cylinder, after all that it’s still got to get out….
SO…. what would you realistically expect the VE to be in such circumstances? Well, if you were to put a huge vacuum cleaner sucking at an unrestricted rate of 150 l/s (saving the piston’s uppy-downy motions from doing the job) pulling air through to the cylinders you’d get, perhaps 75% of the potential unrestricted flow. In that case there’s 25% of the potential flow lost to the restrictions.
You’re never going to get ZERO restriction because there’s far too much other necessary stuff in the engine and engine bay to fit filter, trunking, manifold and ports of sufficient proportions to have zero restriction.
A modern engine is pretty good out of the box and you wouldn’t ever expect as much as a 5% improvement in flow without drastic changes, and those very same drastic changes would likely make things worse in other ways. So, best ways up you might see 78 or 79% of the potential flow.
How come then, a typical modern high-performance road car engine has a VE of as much as 120%? Yes, a 3 litre engine would breathe as much as 3600cc in two revolutions!
With its silly uppy-downy pistons, pulling air past valves that are only open for typically 36% of the time, through ports that often have rough surfaces and uneven profiles, through often convoluted trunking and in from the outside world through a filter that only has open areas of a few microns. It cannot possibly be, surely? That’s over 50% more than the possible flow when the valves are only open for 36% of the time and the piston is only on the induction stroke for half of a revolution. “Get the ducking stool ready people, something isn’t right…..”
Well it can and it does, using PRESSURE WAVES. Tuning pressure waves is what it’s all about. Camshafts, exhausts, intake systems SHOULD all work absolutely together to harness the highs and lows of the pressure waves to best effect; so effective in fact that it amounts to a very significant supercharging effect.
Flow plays a very low second fiddle to pressure waves when it comes to tuning, because the difference that a loss of wave amplitude makes is far greater than the potential flow ‘increase’ from having a larger diameter and guess what? When the diameter of a pipe increases, the amplitude of the waves within it reduce because the molecule compression occurs over a greater area (the surface area, if you will, of the pipe section), so while the static flow may be imagined to be greater, the dynamic flow is actually considerably less.
For example, just look at a GT or F3 airbox - the airbox is huge to harness the pressure waves but the inlet FLOW is through a tiny little inlet restrictor, and despite this the engines still make PLENTY of power.
A flat six engine (all sixes actually for a specific reason that we don’t need to entertain here) is particularly well suited to harnessing the pressure waves that travel back and forth into, as well as across, the manifold if allowed to. They can make a huge difference to the VE at some engine speeds. This is why the (not stupid) design engineers at Porsche have gone to such great lengths to make best use of the pressure waves by employing various flaps in the system to either block or allow passage of pressure waves within the system.
When developing their engines manufacturers will do many, many hours of simulation work. After that they will make many test pieces, probably at least twenty different head casting designs, fifty camshaft profiles, piston crown designs, valve head shapes, compression ratios, manifold designs, you name it. At each engine test, a zillion sensors and probes are mounted all over the engine measuring everything that can be measured.
DO YOU NOT THINK THAT AT SOME POINT IN ALL THAT IT WOULD HAVE BECOME APPARENT TO THEM THAT IT NEEDED A BIGGER THROTTLE BODY?
No, of course not, they’re all stupid those Germans, right?
It takes Fred in his shed with probably zero engine design expertise, to think “Gad Daim, I can fit me a bigger throddle on there an’ life will be reeeeeal gewd….”
If only things were that simple…..
In the vast majority of cases BIGGER THROTTLE INTAKES DON’T WORK so save your money.
In every single Porsche case that I have EVER had on the dyno, NO bigger intake has made an improvement, and almost ALL has made less area under the curve than stock.
When a vendor selling parts says “There’s a 25 bhp difference with this Big-Boy Carlos Fandango intake Sonny Jim” just consider whether he means UP or DOWN!!!!!!
very fascinating, thanks for sharing Matt
this applies to both NA and forced right?
