hi-tech FI discussion

[Guest Nic-Rebel450CA]

It is my understanding that Single-Point Fuel Injection (SPFI) has advantages over Multi-Point Fuel Injection (MPFI) due to the greater atomization time as the injectors are further from the intake valve. It is also my understanding the MPFI systems "run out of breath" at the higher RPM range. Logical deduction from that would be that the lack of atomization time is making it difficult to burn the mixture. Furthermore, a viable solution for high RPM applications would seem to be the use of an SPFI setup.

 

My question is this:

Would it be possible to make a MPFI system setup to take advantages of the SPFI style setup? Could you take a MPFI manifold and weld the points closed where the injectors go, and cut and weld in new points further up the runners or right where the plenum meets the opening to the runners? Or, could you make a Hilborn style setup with the fuel injectors before the butterflies essentially making a setup with a SPFI for each cylinder?

 

Technical responses would be greatly appreciated, though speculatory responses are welcome as well.

[Guest Nic-Rebel450CA]

Also, is there anyone in the Sacramento or Bay area that has a TPI setup that would be willing to try such a setup with runners with the injectors relocated? (A current TPI setup with runners as a separate component from the manifold and throttle bodies would be necessary, something like the TPI setup for GM V8's).

[Afshin]

It is my understanding that Single-Point Fuel Injection (SPFI) has advantages over Multi-Point Fuel Injection (MPFI) due to the greater atomization time as the injectors are further from the intake valve

 

From my understanding, this is incorrect. The reason that manufacturers spends the extra money to make MPFI over SPFI is that by atomizing the fuel into the air stream closer to the combustion chamber the fuel stays atomized. The primary problem with SPFI is that the fuel is atomized far away and they collects against the intake manifold walls causing droplets. This then leads to inconsistent air fuel mixture..

With the high fuel pressure you don't need that much distance to get full atomization and as distance increases you get droplets forming = less atomization and less consistency, efficiency, power and mileage....

The next wave is direct injection into the combustion chamber(used mostly in diesel for now). Also you can control the injector pulse to match the intake valve opening (not that the stock system does that) allowing for atomization at the right time for each cylinder as opposed to spraying during all cycles which would again maintain better atomiztion. Can't achieve that with SPFI

 

the MPFI systems "run out of breath" at the higher RPM range

I would expect the opposite for the reasons mentioned above

[Guest Nic-Rebel450CA]

I would expect the opposite for the reasons mentioned above

 

Isnt this an issue that is repeatedly being brought up by LS1 drivers? What is your basis for this particular conclusion?

 

I am also almost certain about the fuel atomization time. I have read a number of writeups talking about this issue. If you have some technical specifics, please share.

[Afshin]

Hi Nic. I went back and dusted off my F.I. book by Jeff Hartman and he clearly states what I already wrote earlier that the fuel has the tendency to collect on the intake walls causing droplets (otherwise the increase distance would make for better atomization). Also he said that since that designing the intake for equal airflow does not make for equal air/fuel delivery. The reason is that the dynamics for the fluid and air are different (heavier fuel tends to gravitate to the outside of the manifold bend) and hence you get uneven air fuel mixture in the different cylinders.

Also Edelbrock's co. claims 30hp and 40 ftlb torque gain on a 5.7 liter chevy motor when switching to MPFI. Also Holley pro injection on their tech info page clearly say the MPFI provides better performance and economy over SPFI and that direct injection is the best.

Again, all the cheap penny pinching manufacturers spend a lot more money using MPFI over SPFI because it is more efficient. There is no question about that. No professional race car, ferrari... or any car that I'm aware of use SPFI. As far as I can remember they were used early on when the companies were too cheap to switch from carburetion to MFPI, so they went with a hybrid which is the SPFI.

I don't know what the LS1 drivers are talking about, I can certainly be missing an important point. However I can't see the MPFI which is used by all race cars running out of breath at high RPM compare to the SPFI.

[Guest Nic-Rebel450CA]

Professional race cars do use carbs though, and SPFI is basically like a carb setup except that it has better atomization and a more even mixture.

 

Here is a thread that makes referrence to breathing issues with TPI:

http://www.hybridz.org/phpBB2/viewtopic.php?t=1267

 

Basically I am just trying to find a way to make an EFI setup keep up with my engine. I plan to have a 9000 rpm redline.

[fl327]

i can only see two ways to make a fuel system good for 9k

 

single plane carb setup (easiest way i can think of)

holley super ram or whatever its called (rusty on my v8speak)

i like the idea of an injector per cylinder, pointed in close proximity to the intake valve-its eficient and its great for performance.

forget tpi getting more than 5k rpm with all the bells and whistles.

either way efi that can support 9k on a v8 is bucks.

[Guest Nic-Rebel450CA]

How about a Hilborn style setup with very short stacks?

[johnc]

It is my understanding that Single-Point Fuel Injection (SPFI) has advantages over Multi-Point Fuel Injection (MPFI) due to the greater atomization time as the injectors are further from the intake valve. It is also my understanding the MPFI systems "run out of breath" at the higher RPM range. Logical deduction from that would be that the lack of atomization time is making it difficult to burn the mixture. Furthermore, a viable solution for high RPM applications would seem to be the use of an SPFI setup.

 

Sounds like "marketing technology" spewed forth by companies selling carb replacement fuel injectors. None of the above is true for reasons listed by Afshin and many others.

 

I don't know where to start to direct you to the correct information about the advantages of multi-point fuel injection over single point. My mind has been boggled. There isn't one place (except maybe cost) where single point fuel injection has an advantage (is better) then multi-point. I guess the only thing I can say now is that I know of no automotible manufacturer anywhere (except maybe Trabant or Yugo) that is currently running a single point fuel injection system on their 2003 model vehicles.

 

Regading what racers run - check the rule book before deciding the reasons why things are done the way they are.

[Guest Nic-Rebel450CA]

Regading what racers run

 

I was just mentioning that racers do use carburetors. I was just stating it as a point that a carbureted setup can be used in racing applications.

 

Honestly I dont really care if SPFI or MPFI is better, I am just trying to figure out how I can use EFI in a high RPM setup. All of the information I have obtained supporting either form of EFI sounds completely rational, however, I havent heard anything as to an explanation for why MPFI runs out of breath at the higher RPMS other than information that I have found that indicates it is because of lack of atomization time.

[johnc]

The fuel coming out of the end of a modern fuel injector is very well atomized. Much more so then fuel coming out of a carb. There may be a concern about creating a homogenius mixture but proper combustion chamber design and positioning of the injector will eliminate that concern.

 

If, when you're discussing MPFI running out of breath (actually, not delivering enough fuel) at high rpms, you're refering to OEM systems, then the reason is: the injectors run out of duty cycle time (meaning the pulse injector is full on all the time). OEMs specifiy injectors that are specifically sized for the application. They will only flow so much fuel and the OEMs probably specifiy a 10% cushing to handle additional false air as the vehicle ages.

 

The injector duty cycle issue also applies to OEM SPFI systems. Those two or four injectors at the top of the manifold can, again, only flow so much fuel.

 

Going to a larger injector that can deliver more pounds of fuel might allow an OEM MPFI (or SPFI) system to handle the rpms you're looking for IF you also reprogram the ECU, ensure the fuel delivery system can handle the additional flow, and do some other tuning. But, I think it might be cheaper in the long run to start with an aftermarket MPFI (or SPFI) system then to try and modify an OEM based system.

[Afshin]

OK, I think I may have figured out the confusion here. First to be clear, MPFI is superior to SPFI. It is more efficient and produces more power. This is fact, even the 5.7 l chevy motors output jumped significantly when they switched from SPFI to MPFI. Read any technical article and you will see it clearly described. Also when I was referring to racecars using MPFI, I was referring to formula 1 and Le Mans type of multimillion-dollar cars. Ferrari Enzo (redline 8500), Honda S2000 (redline 9000), Porsche carrera GT (rival to Enzo) redline 8000, Modena, new ford GT40…..all use MPFI because it is better, provides more power and sustains high RPM’s reliably. As Jon mentioned, the only benefit of SPFI is lower cost.

The problem you guys are referring to relates to chevy’s factory TPI (tuned port injection) or even Fords factory set up where they used long narrow intakes runners and set up the cars for high low end torque. This was to make the factory heavy automatic car fun to drive. This worked well but the cars did not breathe well over 4500 RPM. The long narrow intakes they used increased low end torque (high velocity and high momentum creating high pressure at the intake valve), however they minimize flow because they are long and narrow, which limits high rpm performance. As such people with moded cars/engine have to switch the intakes for better flowing ones and then EFI for further gain (still MPFI as long as you were not broke). Also post 1990 EFI setups using MAP sensors can not accommodate much customization since they can’t account for differences in vacuum or anything that affects factory-assumed relationship between manifold pressure, engine speed and engine air flow like MAS/AFM can (to a certain point off course), hence they perform poorly when you change cams, intake manifolds.. if the PROM is not remapped. Again, none of this has anything to do with MPFI vs SPFI. I hope this helps.

[Guest Nic-Rebel450CA]

Ah, I see. Thanks for clarifying that. I guess the problems that I have heard are specific to the TPI and not MPFI in general. I just got an email back from Edelbrock telling me that they have seen their pro-flo EFI setup run to 9500 RPM which should be good enough for me. Now for the budget part

[Guest Perry]

GM TPI = great low rpm truck motor.

 

Usually racers use carbs due to rules or cost but... carbs can be made to work great at high power levels because higher port velocities help out a lot with atomization issues and racers aren't as concerned with mileage penalties from running a little rich to compensate for settled out fuel or uneven mixture distributions to different cylinders.

basically you can build a multipoint to flow the air you need without much concern about mixture seperation or distribution problems. It takes a lot less work to get a good performing setup and can provide low rpm and mileage benefits over a typical well perforing carb setup.

Perry

[Guest gtmattz]

i posted a question similar to this before.. i was wondering if a tpi intake would work on a 327 block, grumpyvette reesponded with statements correlating the previous posts that explain how the manifold was set up for low end torque.. if you want a mpi system that can rev look at this

 

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=109770&prmenbr=361

 

I am planning on bulding a high redline 327 and this is the intake i am currently saving $$ to buy.

[randy 77zt]

most port fuel injectors are aimed at top of intake valve.fuel is shot on top of hot intake valve turning it into a hot vapor.atomised droplets dont burn as good.ever read about the pouge vapor carburator?or watch the smoke rolling off of a log on a campfire finally catch?for racing low speed driveability is not a concern.i have seen pictures of an offshore boat race engine with the injectors spraying down the middle of velocity stacks form the top.the use of intake valves to vaporize fuel is what causes deposits on them.gasoline has been reformulated to be used in this fashon of a gasoline formulation from the 60's.