Throttle Body and Fuel Injectors

[XanderWiFi]

Another info thread as I was carousing things on MPRacing.org. How does the throttle body and fuel injectors correlate and the affect things? The larger throttle body allows more air flow and the fuel injectors allow more fuel and obviously these things equate into more power. But how do they interrelate with the other components of the engine and whatnot?

I remember that someone told me once that you only need larger fuel injectors if you turbo charge your engine. Just looking to hear all the pros and cons and information for installation so it can be an easy search thread in the future.

[XanderWiFi]

nothing? Not one post? It's for the kids! Lol

The throttle body is obviously the first point where air passes into the engine, so you want to keep restriction to a minimum. But larger is not necessarily better. Velocity is just as important as volume - what you really want is to maximize the air mass travelling through the TB (via the Mass Air Flow sensor - isn't that convenient?). But throttle body shape can also play a role in velocity - an oval shape has been shown to increase CFM (in some cases) over a regular round TB. You also need to realize that your engine is mechanically limited as to how much air it can use at 1 ATM, so there comes a point where installing a larger TB doesn't help, and can actually hurt you through a loss in air velocity, plus you can run into other driveability issues.
Fuel injectors are pretty simple - it's basically a valve with a small electic motor (to oversimpilfy it greatly). The fuel enters the fuel rail, to with the fuel injectors are attached. Since there is a constant flow of fuel to the rail via the fuel pump, pressure builds in the rail. Now on return-style fuel systems, there is a pressure regulator which keeps pressure under control and returns excess fuel to the tank (hence the name return-style fuel system). When the car's computer tells them to (this is based on many inputs and calculations from sensors all over the engine and transmission) they open a small valve (this happens very fast; injector events are timed in milliseconds), which squirts the pressurized fuel into the intake port, where the fuel mixes with the incoming air. But in general, the fuel/air mixture needs to be kept within a certain range for maximum performance. Too much fuel and the motor bogs down, too little and it knocks (or even worse, you get detonation). It a fine line that frankly I don't have enough time to type out a full explanation. But that's the basics.

[BlackJack]

well, you TYPICALLY only need larger fuel injectors if you are running a power adder (turbo, S/C, nitrous), unless you are running an extreme normally aspirated setup (higher compression pistons, etc). Otherwise if you're just doing a basic port and polish, CAI, and exhaust, you don't really need larger injectors.

[WhiteV6]

If you have 200whp, larger injectors are going to help. Without tuning, larger injectors are worthless, so you have to add that into the equation. A 62 or 65mm throttle body requires the upper intake to be ported to match it.

Originally Posted by BlackJack

well, you TYPICALLY only need larger fuel injectors if you are running a power adder (turbo, S/C, nitrous), unless you are running an extreme normally aspirated setup (higher compression pistons, etc). Otherwise if you're just doing a basic port and polish, CAI, and exhaust, you don't really need larger injectors.

I'll agree with this.
My motor has a ported top-end, a cam, headers, and is just north of 10.1:1 on the compression. Even with 28# injectors, we had to dial them way back since the car was running so rich. I might have been able to make do with the stock 22.5# injectors.

[WhiteV6]

nope, you will make use of those injectors.

Originally Posted by WhiteV6

nope, you will make use of those injectors.

I hope you are right, because it ran way rich at idle and cruise until they were scaled back. No WOT tuning yet though, so maybe that's where I'll use 'em.

[BlackJack]

(I smell a thread-jack) ok so what overall tuning method are you using? Did you start off with scaling your entire range of IFR's by calculating the value by total percentage of difference in injector size? That's a good place to start, then you can start working a VE tune, and finish up with MAF. Once you have your fuel basically dialed in, you can start playing with timing and fine-tuning your fuel tables.

[AleroB888]

I'm not an expert as is WhiteV6, but my thought is an increase in velocity is not so important until approaching the combustion chamber in the lower intake manifold. There it also affects atomization. Up to that point, it is lack of the restriction of flow that is most important, which leads to pressure drop and power loss. The MAF sensor and throttle body can be larger, and may be a benefit, but could be a detriment if they create any turbulence. More velocity through the throttle body doesn't hurt, as long as there are no turbulence or frictional losses that result from it, but since the air slows down again in the upper intake manifold, my thought is that velocity at the throttle is not a benefit, whereas the slight rise in pressure (less vacuum) from a larger throttle can be.
I am now using 36 lb. injectors with the rest of the car basically stock, (as my intercooler is being reconfigured) and all I changed was the IFR constant. It idles and runs with no issues. The most basic thing is to get a correct idle. Then make one change at a time and analyze the result.

[BlackJack]

true to a point, but lets not forget that under an N/A setup, the velocity is somewhat driven by intake diameter, where with a moderate size path will allow the pistons to apply that needed vacuum to get the air moving. If you were to apply the same amount of vacuum to a larger diameter intake path, it would have a lesser effect.