Suzuki Swift G13B throttle body

[UgO_ssgti]

Hello all.

I own a Swift GTi with the stock G13B engine and I wonder if enlarging the stock 45mm throttle body to 48 or 50mm would bring any benefits on the high rpm range?
I think that on the low rpm range, would became worse because the velocity of the air will drop because there will be a a larger area. Maybe this is a useless mod on a stock engine- right?
BTW, is there any proportion between the throttle body area and the displacement and max rpm?
Thank you for your time.

[Guy Croft]

Short answer is NO! Read on:

Well, a 45mm throttle body will flow about 180cfm at 10" test depression on the flowbench. What does this mean in real terms? Well only one cylinder at a time is on induction, and even a very highly developed head (I mean by me, because I know them well) from an engine like yours will only flow about 110-115cfm on the same rig.

Sure, on-engine the ports will be moving a lot more air than they do on the flowrig but so will the throttle body and at real cylinder-valve pressure ratios (ie: far greater than the modest 10" mentioned above) and the flowrates will vastly higher - but that's not the point. We are comparing relative flowrates - the ability of the throttle body to feed the port that's inducting. So straightaway we can see that the existing throttle body is plenty big enough to feed that intake port.

In fact, based on steady-state flowrate alone, it's way too big! Some Formula (restricted) engines at 2 liter, 16v will develop 200bhp even with a single throttle body and a restrictor of less than 28mm diameter.

So why is it made with that rather large size? The reason is that when the throttle is opened the whole plenum has to fill to atmospheric pressure to feed the engine. That's quite a large volume and it takes time. The bigger the throttle body the faster this takes place. That's the good side. But make it overly large compared with what is needed to provide a good torque response, well, then there can be a significant loss of fine-throttle control that can actually make the car more difficult to drive. In other words it can make the engine rather 'twitchy'. Bit like having a flywheel that is way too light. The reality of 'going bigger' is that the opening area increases as the square of radius, so even slightly bigger has a rather 'disproportionate' effect on the change of condition downstream of the throttle plate.

It is actually pretty rare (I have never heard of it) to get a power gain or improvement in the torque curve from a 'bigger throttle body' than was supplied for the original engine, there's plenty in hand to even double the power on most of them, though of course I know that tons of rather, er, 'optimistic' stuff is written about it on the internet about how important it is to have one!

Going to multiple throttle bodies, well, that is a different matter altogether and something that really does work.

GC