Enter your username:
Do you want to login or register?
  • Forgot your password?

    Login / Register



    Results 1 to 10 of 10

    Thread: EFI FAQ

    1. #1
      Join Date
      Aug 2004
      Posts
      582

      EFI FAQ

      fuel injection FAQ

      1. What type of system should I get? TBI? MPI? SFI?

      The answer is different for every person. I will try to lay out the advantages and disadvantages of each system, and from these you should be able to determine which is the best fit for your needs and wants. This depends heavily upon your budget.
      a. TBI.

      A. Overview: TBI is short for Throttle Body Injection. IT is also sometimes referred to as single-point injection (since all fuel is introduced at the plenum, as opposed to multi-point systems which have fuel delivered in individual runners). It is usually a low pressure (less than 15 PSI) system, but there are exceptions.

      B. Advantages: TBI is almost always the lowest price system. First, it does not require a custom intake manifold. Most TBI throttle bodies will bolt to any common 4bbl intake, either directly or with a basic adaptor plate. TBI also has few components (only 2-4 injectors, usually no Intake Air Temp sensor, etc), which also lowers cost and complexity. This also makes it the easiest system to install. Because TBI has no fuel rails, it offers the most “retro” look. Once the air cleaner is installed, it is difficult to tell that the engine is fuel injected. Upgrading can be very cost effective and simple. If you want to run a larger intake (from a Performer to a Performer RPM, for example), you do not have to pay to have it modified - it’s just like the engine was carbureted.

      C. Disadvantages: Because TBI delivers fuel at the same spot as the carburetor it replaces, it is vulnerable to any manifold inefficiencies (puddling, unequal distribution, etc) that the carburetor was - but since it is not relying on a venturi for fuel flow, typically a calibrator can often “calibrate around” these problems. TBI has the narrowest “dynamic range”. In a nutshell, it is harder to get a high rpm, high hp small block (which uses very little fuel at idle but a lot at WOT) to perform well under all loads. Forced Induction engines also have a large dynamic range requirement, making TBI an uncommon choice.

      D. Other notes: Despite the concerns listed above, TBI is not to be underestimated, as it can provide plenty of power and very good drivability. However, the more extreme your engine, the less likely you will have success with TBI. Roughly 450hp for small blocks and 550hp for big blocks is about as much as one would want to attempt, unless you are very experienced. For EFI newbies, I would probably subtract 100hp from those numbers if you will be attempting to calibrate yourself for the first time. Contrary to popular belief, a properly sized TBI will not cost you any power when compared to a carburetor. However, don’t expect to gain power either. As you know, power is airflow, so unless your carburetor was restricting airflow, a throttle body will most likely not improve flow. What you will gain is fast, reliable starting, much improved cold drivability (and actually a general improvement in throttle response) and maybe a little bit of economy, depending on how your carb was set up and whether you run closed loop (more on that later). Despite being a low pressure system, TBI will not save you much on the fuel system, assuming you do it properly. Yes, the pressure is lower, but this does not mean you can use carburetor fuel line. Circulating fuel systems aerate the fuel and lead to rapid breakdown of el-cheapo fuel line. Good EFI quality rubber line costs $4-5/foot. Also, high quality, full-circle hose clamps should be used.
      b. Batch Fire MPI
      A. Overview: As mentioned above, MPI (or MPFI) gets its name from the fact that fuel is delivered at multiple points, with usually one but sometimes two injectors per cylinder. MPI is always a high pressure system, with common operating pressures being 45 and 58 PSI. Batch fire means that the injectors are fired in groups. The arrangement varies; sometimes an entire bank is grouped, sometimes 2 injectors from each side are grouped, and on 4 channel systems the injectors are fired in pairs. The important thing to remember is that the injectors are not in sync with the engine. Some injectors will be spraying on an open valve, and some may be spraying on a closed valve.

      B. Advantages: MPI almost guarantees equal fuel distribution (however, if you manifold does not provide equal air fuel distribution, the benefit is lost). Because it does not have to wet an entire manifold with fuel, typically throttle response will be improved over TBI, or equal with much less calibration effort. This can be an important consideration for a novice, but not a deciding factor if you will be hiring someone to calibrate for you. It can also produce a small increase in economy. Batch fire MPI has a wider dynamic range, making it more suitable for higher hp/forced induction engines where idle quality is still an important factor.

      C. Disadvantages: MPI requires a custom or modified intake, which increases cost and complexity. It has more components, though most are pretty reliable so it isn’t a big problem.

      D. Intake Selection with MPI: MPI offers only the potential for increased power. I say potential because there are many factors that play into making power. Bolt a stock TPI system onto a 427ci small block, and you will lose power. Again, power is airflow, so manifold selection can be important. The fuel injection system is simply matching the measured (or calculated) airflow with fuel flow. But it’s the manifold, heads, cam, and exhaust that dictate airflow. So, if you are going to convert a standard dual plane or single plane intake to EFI, don’t expect much higher hp numbers than you would with a carb. However, especially with a single plane, the increased adjustability of EFI allows an experienced tuner to gain back a lot of lost torque down low that a carburetor could not because of it’s mechanical constraints -- but peak power numbers would remain comparable to a well-tuned carb. Simple physics dictates that a Super Victor 8000rpm single plane intake will NOT make as much low end torque as a Performer RPM regardless of the EFI system that you are using. In my opinion, the whole point of going to MPFI is the ability to use an intake designed for dry flow, such as a Stealth Ram, Super Ram, or TPI. But, the conventional intake does allow you to maintain a little bit of the “stock” look (though not as much as TBI, which does not have the fuel rails). Choosing an intake follows the same process as choosing any other part for your engine. What is its intended usage - Racing? Driver? Is the engine smaller than 400ci and you’re looking for gut-wrenching torque? A TPI may be in your best interest. Looking for higher RPM performance? Consider a short runner intake like the stealth ram. These are generalizations, and I don’t expect this document to be able to tell everyone what their perfect intake is. The answer is you have to research. An EXCELLENT resource for people who are really experimenting with SBC EFI is www.thirdgen.org. They’ve tried all these combos and more.
      c. Sequential Multipoint Fuel Injection
      A. Overview: Sequential Fuel Injection (SFI) is almost identical to batch fire MPI. In fact, it is another type of MPI. What sets it apart is the ECU, which controls each injector independently, syncing the injection events with engine position.

      B. Advantages: SFI has the widest dynamic range of the three systems, making it the most suitable for extreme high RPM or high boost engines. The individual injector control allows changes to fuel delivery to happen even more accurately in the event of acceleration or deceleration, for further improved throttle response and simultaneously better economy.

      C. Disadvantages: SFI REQUIRES some sort of sync signal, which must be generated from a camshaft, not a crankshaft. So, in addition to upgrading your intake, upgrading your distributor (which is optional on the other two), is now mandatory. Also, many SFI systems ALSO require a high resolution crank wheel be installed (60-2, 36-1, etc). With the exception of the dynamic range, the advantages listed above are VERY small, and without careful (read: expensive) calibration on a dyno, they will not be realized.
      2. I installed EFI solely for economy. Why didn’t I get a huge increase in mileage like I heard about?
      Many people associate EFI with huge economy increases, but what most don’t realize is that injection became popular about the same time as overdrive transmissions. It’s overdrive that gives most modern vehicles their bulk of their economy. Yes, EFI helps, but OEM EFI systems have hundreds if not thousands of hours invested in calibration. If you’re really devoted, your system may get 20. So, your calibration will most likely not be perfect. Do NOT expect huge increases from EFI. I would say 5-10%, depending on the condition of your carb. My truck went from 10 to 11mpg. An improvement? Yes, 10% in fact. Am I now a friend of GreenPeace? Hardly. However, I expect that my upcoming conversion to a 4l60e will get me into the 15-16mpg range. If economy is what you are mainly focused on, consider doing a transmission swap first.

      3. Should I go open loop, closed loop, or wide band closed loop?
      This one is also not cut-and-dry. First and foremost, we need to recognize a normal oxygen sensor for what it is, primarily: an emissions device. A typical oxygen sensor does one thing, and it does it very well: It allows the ECU to maintain precisely 14.7:1 air-fuel ratio. Why is this number so important? Because catalytic converters only work at a 14.7:1 ratio (plus or minus 0.1:1). 14.7 is not the best for power, or economy, but best for post-catalyst emissions. Depending on the engine, best power is usually between 12 and 13:1, and best economy is typically between 15 and 15.5:1.

      Does this mean an EGO is useless on my hot rod? No. Firstly, 14.7 may not be optimal for economy, but it’s pretty close, and probably closer than your carb is currently, so it does offer a potential small advantage in fuel economy. More important, it allows for constant system monitoring. In other words, it fine tunes for you, so you’re not setting the idle jets when temperatures start to drop, for example. So a normal oxygen sensor, when compared to open loop, offers the primary advantage of self tuning, with a possible improvement in economy. And, one big motivation for fuel injection is not having to tamper with it all the time like you would with a carb! If this is important to you, consider an oxygen sensor.

      However, constant monitoring is NOT to be confused with self tuning, especially at Wide Open Throttle. As mentioned above, a normal oxygen sensor can only show you 14.7:1. Anybody who tries to tell you otherwise is lying. Oxygen sensor output is heavily tied to EGT. In other words, if you change your spark timing, your EGO voltage will change, even though the air fuel ratio may not have changed at all.



      If you do not have access to a dyno, have limited calibration skills, and a unique engine combination for which nobody has an existing calibration, a Wide Band Oxygen sensor should be in your future. When coupled with an ECU that can use it for closed loop, your ECU can automatically target a desired A/F ratio under any circumstance, including wide open throttle. If your ECU does not have wideband closed loop, then the sensor will simply tell you whether or not you are about to blow up your new engine. However, even with closed loop, it’s no guarantee that your engine won’t self destruct before the ECU can compensate. Closed loop is still not a replacement for a good calibration! But, it’s probably faster than you are, especially in the event that something happens in your engine (localized air leak, stuck wastegate, etc) that could potentially cause a lean condition

      If your engine is being calibrated by a professional, and you don’t plan on making many changes to your system, there is probably very little reason for you to purchase a wideband for yourself, other than to have another gauge on your console. This is especially true for naturally aspirated systems. With forced induction, it’s good peace-of-mind and probably recommended. Otherwise, if the calibrator calibrates the majority of your system to 14.7:1, you can use a normal oxygen sensor for feedback and self-adjustment at much lower cost. Most systems will take learned changes and apply them to WOT, so you will not be entirely out of luck in that area. If your engine has a “radical” cam or other components, requiring a calibration that is very seldom at 14.7:1 (even at part throttle), and you want feedback, then you will have to buy a wideband. This should be discussed between you and whoever will be calibrating your system.

      4. What size injectors do I need?
      Sizing injectors is one of the easiest things to do. The equation is very straight-forward:

      INJECTOR SIZE = (HP * BSFC)/(# OF INJECTORS * MAX DC %)

      Where
      INJECTOR SIZE = individual injector flow rate, lbs/hr
      HP = Your engine’s estimated power
      BSFC = You engine’s estimated brake specific fuel consumption (lb/hp*hr)
      # OF INJECTORS = The number of fuel injectors your system uses
      MAX DC % = The maximum duty cycle your injectors will operate at

      Some rules of thumb: Most typical V8’s use a BSFC of 0.50. More modern engines (LS1’s, LT1’s, or very well-built engines) can use a BSFC of 0.45. Most forced induction engines should use a value of 0.55. The maximum duty cycle that an injector should be operated at is 80-85% (use 0.80 to 0.85 in the equation)

      Example. You have a 383 that makes 450 hp at 5500 rpm. You will be using a multipoint system with a converted carb intake
      INJECTOR SIZE = (450 * 0.50)/(8*0.80) = 35 lb/hr.

      5. What size fuel pump do I need?
      At first glance, this is an easy one. You need a fuel pump that will supply more fuel than your injectors can use at the rated system pressure. That last bit is underlined because most fuel pumps have published flow rates taken at ZERO pressure (open flow). Anybody who’s kinked a garden hose knows that the more you restrict the outlet and raise the pressure, the lower your flow rate will be. Therefore, you need to find out what the fuel pump’s flow rate is AT YOUR SYSTEM PRESSURE. For TBI, this is typically 15 PSI, and most multipoint systems are at 45 or 58 PSI. An extra word of caution for forced induction people: Almost always, the fuel system pressure will be tied to manifold pressure, so if your fuel system pressure is rising, your pump will need to be large enough to compensate.

      The second sentence says the pump flow must be greater than the injector flow, not equal to. So, if you have 8 30lb injectors (240 lbs/hr), your pump needs to supply more than this. How much more? A common figure is 50% more (360 lb/hr in the example), but you can go lower. Just realize that the less margin you provide, the more susceptible you are to pressure drops from clogged filters, voltage drops, and the like. I would not go below 20% personally.
      Last edited by Ralph LoGrasso; 08-19-2005 at 09:16 PM.


    2. #2
      Join Date
      Aug 2001
      Location
      Wilton, CA. (Sacramento)
      Posts
      2,995
      Country Flag: United States
      Boy did I misread your title! Very good info sir!

    3. #3
      Join Date
      Nov 2004
      Location
      NW Arkansas
      Posts
      1,742
      Quote Originally Posted by camcojb
      Boy did I misread your title! Very good info sir!

      Me too I was scared to open it

      Fuelie fan I think that is the longest post I have ever seen!!
      KENNY DAVIS HOTRODS

    4. #4
      Join Date
      Jun 2002
      Location
      Long Island, NY
      Posts
      11,320
      Country Flag: United States
      Fuelie, I stuck the thread up top. Thanks for your help on this!

    5. #5
      Join Date
      Aug 2001
      Location
      Wilton, CA. (Sacramento)
      Posts
      2,995
      Country Flag: United States
      Quote Originally Posted by Ralph LoGrasso
      Fuelie, I stuck the thread up top. Thanks for your help on this!

      Thanks Ralph. What do you think about making faq in capital letters?

      Jody

    6. #6
      Join Date
      Jul 2002
      Location
      Mesquite, TX
      Posts
      4,901
      Country Flag: United States
      I've provided him an update to this one with the formatting updated (and a couple of spelling corrections, sorry Fuelie). I don't have the access to update it myself.

    7. #7
      Join Date
      Jun 2002
      Location
      Long Island, NY
      Posts
      11,320
      Country Flag: United States
      I updated the post with Derek's formated version. Many thanks to both Fuelie Fan for writing this, and Derek for formatting it. Hopefully we can continue to add to this thread, and make the FAQ more and more in-depth. Thanks again guys!

    8. #8
      Join Date
      Aug 2004
      Posts
      582
      sweet, thanks, I can be a sloppy typist. I have an especially bad habit of, when in a rush, leaving out key words like "not" which can totally change the meaning of the sentence!

    9. #9
      Join Date
      Feb 2006
      Location
      Columbus, GA
      Posts
      132

      Great Info!

      Wow, what an excellent post! I pasted that to a word and and saved it for future use. Easy to read! One thing I was hoping to come across was fuel line feed and return diameter. I'm getting ready to plumb my 502 RanJet to a Ricks tank and need some direction.
      1970 SS Camaro, 502 RamJet, TKO600 Tremec, 3.91 Moser 12 bolt/Detroit Tru-Trac, C5/Z06 Brakes.

    10. #10
      Join Date
      Aug 2010
      Posts
      30
      great info





    Advertise on Pro-Touring.com