EFI injector location question
 

I hope this isn't too dumb of a question, but........

When considering an EFI solution, is it better to have the injectors inject into the throttle bodies, or is it better to have injectors inject into the tapped ports in the head that were originally there for the MFI injectors?

Here is my idea.

http://forums.pelicanparts.com/porsche-911-technical-forum/809214-mfi-efi.html

The answer will ultimately be based on what parts you have and what you have to buy. If you have an existing MFI and want to upgrade to EFI to drive the fuel you can use the head adapters. This works really well and retains the factory ITBs when you have a unique engine build or an ailing MFI injection pump.

The aftermarket ITBs (PMO/Jenvey/Borla etc) are great if you do not have MFI ports or you are starting from scratch.

There are aesthetic differences in each if that is a concern.

Thanks Jaime,

My MFI throttle bodies have been off the car for a couple of decades, are badly corroded and I don't think they're usable. So, I'd be starting from scratch. So, I'm debating the benefits of using throttle bodies with injector ports (Like the PMOs) or using the head adapters and throttle bodies without injection ports. Is there a performance or reliability difference between one or the other?

IMO, the closer the injector to the intake valves, the better. Of course, sometimes there's constraints that prevent that from happening, and that's ok. You'll just have to tune for the arrangement. Farther away results in more evaporation. In steady state cruise, this is not too much a factor, it's during throttle changes, this plays a bigger role.

Read up on X-Tau for MegaSquirt, this is explained in depth, but a short, easy read.

I do not think you will see any functional difference between the two setups. The difference between injector placement on the two systems is approximately 70mm. At the time period of fuel travel at 50m/s intake gas speed you will not notice a 100ms or difference in atomization time.

As Cory mentioned there is a component of x-tau which is related to a certain amount of fuel sticking to the intake port and the time period at which it evaporates. Both the TB and MFI injector locations do a very good job of projecting fuel directly into the airflow rather than at a sidewall. I do not enable X-tau tuning on ITB setups.

Thanks for the interesting info. Given that either setup should perform about the same, is there then benefit to going with injection into a throttle body instead of into the head port in the interest of keeping the injector out of the dirty combustion chamber?

I'd think the further away from the intake valve the injectors the more you have to take warm-up enrichment into consideration: As the walls of the intake track are cold they will load up with vaporized fuel and "steal" it from the mixture formed. As a result you'll run lean during warm-up and you need to compensate for that.

As the engine warms up the amount on the walls will be less and eventually the effect can be neglected.

If you are close to the intake valve the area that causes this effect is smaller and the warm-up enrichment doesn't have to be as substantial.

The timing and amount of the enrichment is mostly done via the temperature sensor (CHT) and sometimes via a timer as well. However, the exact relation between CHT and required enrichment is far from linear. It really depends on how the components warm up. So I would always get as close to the intake valves as possible to reduce complexity of warm-up tuning.

Ingo

I don't follow what you mean by injecting in the combustion chamber? In that case, that's called 'direct injection', and requires pressures far higher than traditional pumps.

There's a HUGE drawback to DI, your intake valves will be gunked up with carbon build up since the valves do not get cleaned by the fuel injected by the injector. Requires walnut shell blasting every now and then to keep the intake runners clean restoring airflow (hp).

My mistake- I was talking about injecting where the injection point occurs on an MFI engine. I now realize that's not into the combustion chamber, but rather just behind the valve if I'm understanding correctly.