Port Fuel Injection Vs. Direct Fuel Injection. Seriously, why do I care??? :P


Today’s discussion is about 2 different forms of fuel injection systems and why this actually does matter to YOU. Yes… YOU. The one that really doesn’t care about this. The one that doesn’t WANT to care about it and and in all likely hood will not read this post all the way through and end up in a shop sooner than later with a rather expensive bill. Do I have your attention now? 😀

So lets start with what the heck are we talking about here? Fuel injection is the system that gives the engine the fuel it needs to run. It’s responsible for giving the engine just the right amount of fuel at just the right time. Too much fuel, too little fuel, or fuel at the wrong time or delivered in the wrong way are all bad things for your engine and if any of these things happen, you will know it in the form of a rough running engine, low power, or an engine that just simply will not run at all. Now that we have that out of the way, lets talk about the 2 major kinds of fuel injection and just exactly why it even matters to someone who really just doesn’t want to think about it.

PFI (port fuel injection), which is the older system, and GDI (gasoline direct injection), which is the new technology, are the two kinds of fuel injection systems in use today on gas fueled engines. The major difference between the two is the placement of fuel injector and the fuel pressure required to operate the system.

In a PFI system, average required fuel pressure is anywhere from 35-60 PSI depending on the system. This system uses one fuel injector per cylinder. So if your engine is a V-6, you would have 6 fuel injectors. if it was an inline-4 cylinder, you would have 4 injectors, etc. The fuel injector is an electrical on/off valve if you will that when electrical power is applied, the internal valve opens and fuel is sprayed out. When the electricity is removed, the injector valve closes and fuel spray stops. This on/off happens very quickly. The amount of time the injector valve is on, or open, is measured in milliseconds. Sometimes single digit milliseconds. For reference, there are 1000 milliseconds in one second. That’s quick! The PFI fuel injector is located in the engine’s air intake manifold and is directed at the back side of the cylinder’s intake valve. We will discuss why the injector’s location is important a bit later.

A GDI fuel injection system is quite similar to a PFI system. Like the PFI system, the GDI system uses individual injectors for each cyllinder, they are electronically controlled and function much the same as the PFI injectors. The differences are that a GDI fuel injector needs upwards of 2000 PSI of pressure under heavy load operation. This requires a special mechanically driven fuel pump to produce that much pressure. The other significant difference is the injector location. Unlike the PFI system, where the injector points at the back side of the intake valve, the GDI injector is aimed directly into the combustion chamber, or cylinder as it’s more commonly known.

The PFI system on the left has the yellow injector above the valve. The GDI system on the right has the yellow injector below the valve, directly into the cylinder.

The injector’s location change is significant because fuel itself is a great cleaner. Because the injector in an older PFI system sprays fuel directly onto the intake valve, as seen above on the left, it is constantly cleaning carbon and build up off of the valve. The newer GDI injector is located after the valve as seen above on the right, directly in the cylinder therefore no fuel ever touches the valve and this allows large amounts of carbon build up to accumulate. When the carbon build up gets to the point that it interferes with the valve’s ability to seal on its seat, engine misfire and roughness begins to occur. If the build up gets too bad, the only way to remove it is to completely remove the intake manifold and use special media blasting equipment to blast the carbon build up off the valves and intake ports. In extreme cases, the cylinder head will need to be removed to remove the valves and replace them. This can be very expensive and is completely avoidable with regular induction cleaning services.

This photo shows on top, extreme carbon build up on the valves. On bottom, you can see what clean valves should look like.

We recommend on a GDI engine to have this service performed every 15,000 to 20,000 miles. Yes, that’s right. Carbon can accumulate to the point of causing runnability issues as soon as 40,000 miles. GDI systems offer greater fuel economy, more precise fuel delivery and more engine power. The trade off is some extra maintenance needs to be done to keep this system in top operating condition.

If you are unsure if your car or truck uses a GDI or PFI system, we are here to help! Visit www.auto-go.net or call us at 417-272-0091 and we will be happy to discuss this with you and determine your engine’s specific maintenance needs!

Now you know!

Premium Fuel… What you need to know!


I get people coming into the shop and talking with me all the time about their cars.  One of the things that honestly doesn’t come up very often is fuel.  I’d venture to say that most people think that premium fuel is simply better.  In a way I suppose it is but maybe not in the way you would think.  I’m here today to debunk the premium fuel myth and maybe confirm some suspicions that you might have about it.

So lets first get into the difference between regular (87 octane), premium (90-93 octane) and we will even cover a bit about E85 (yuk…). The main differences between premium and regular fuels are their resistance to self ignite and the additive package that the fuel manufacturer adds.  Regular fuel usually has very few if any additives that help to keep your fuel system and engine clean while most premium fuels will have detergent additives that honestly in my opinion are just marketing ploys to get you to spend the extra money on it.  Gasoline in itself is a great cleaner!

So what does this all mean and how does it apply to you and your vehicle?

I thought you’d never ask!!  You see, engines run by burning a compressed air and fuel mixture.  Air is drawn into the engine by pistons moving downward within a cylinder, similar to how one of those tube style water guns draw in water as you dip the tip in the water source and pull back the handle. While this air is being drawn in, a small mist of fuel is sprayed into air stream through the fuel injector and drawn into the cylinder with the air.  Once the piston is at the bottom of it’s stroke and it’s drawn in all the air and fuel that it can, the valve that let the air and fuel in closes and the piston heads back up in the cylinder squeezing (compressing) the air and fuel because the valve is now closed and nothing can escape.  This squeezing and compressing (known in the field as “compression”) creates a lot of heat and if enough heat is created, the air and fuel could ignite without the aid of the spark plug and before the piston gets near the top of its travel.  That’s where the fuel quality comes into play.

The fuel absolutely cannot ignite before the spark plug ignites it.  If it does this will cause a condition known as spark knock, or engine ping and that condition can be very damaging to an engine if left to continue.  Higher performance engines create more compression.  In other words to get more power they squeeze the air and fuel more.  Some production engines can have a compression ratio as high as 10:1!  That means if you start with 10 units worth of volume it will get squeezed down to just 1. It’s that extreme amount of compression and therefore heat creation that requires the use of premium or higher octane fuel so that spark knock doesn’t happen and tear up the engine.  Premium fuel resists that urge to ignite under high compression and heat better than regular.   For most gas engines on the road today, regular fuel is just fine and honestly using premium fuel will give you almost no benefit.  However, if you have a performance vehicle, and on your fuel door you see a sticker warning you to use premium fuel, you should.  That’s really it!  There isn’t any other reason to run it!

So the bottom line is this:  If the engine in your vehicle requires high octane fuel, use it.  It’ll run on regular, but you might unknowingly be doing damage to your engine.  If your vehicle doesn’t require it, don’t use it.  It’s not worth the extra cost as it will not provide any noticeable performance gains or power.  It MIGHT help keep things a little cleaner in there, but it is certainly no substitute for professional fuel and induction cleaning services that should be performed regardless of fuel type used at least every 20,000-30,000 miles and even sooner on a GDI equipped engine as there are places inside your engine that will suffer from carbon build up that do not come in contact with fuel to keep them clean, but that’s a topic for another day!  One more thing I should mention, if I don’t someone else will, some premium grade fuels do not have the “up to 10% ethanol” in them that pretty much all regular grade fuels do.  Honestly, I haven’t seen that much problem caused by the 10% ethanol in regular so I wouldn’t be worried about it.

Oh yea, I almost forgot… E85….  My professional opinion?  Stay away from it!  Especially if your vehicle is not a “Flex Fuel” vehicle. It’s tempting to use as it’s usually quite a bit cheaper, but I can’t tell you how many vehicles over the years have been towed to my shop after filling up with E85.  That’s right, some cars won’t even run on it!  And the ones that are designed to run on it see anywhere from a 3-7 MPG drop versus using regular.  E85 fuels can contain up to 80% ethanol which does not burn anywhere near the same rate as gasoline.  Flex fuel vehicles are equipped to handle this difference with extra sensors and programming to make the adjustments to the engine when this fuel is detected but fuel mileage will still suffer.

Till next time thanks for reading and as always you can find more info about my shop Auto Go Automotive Here!