Author Topic: Basic efi q's  (Read 5179 times)

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Offline Vishnuatepork

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Basic efi q's
« on: May 25, 2011, 01:26:54 PM »
I am still working on my Suzuki VX800 project. Its slow going, ugh!

The engine is in different guises in about 10 other bikes (the marauder in 3 generations, the intruder series, the c,m, s 50's, volusia,  etc, ).

The original was a carb iteration, as is mine, the latter are all efi.

I was looking on an online store, and seeing higher flowing injectors for `busa's.  In my never ending understanding of engines, would a higher flowing injector alone give you more power?  When are they used/required? I am seeing some that claim 8 holes, some with 12, is it all to assist in creating a finer atomization/spray?

Cheers
Alan
suzuki vx800, 41mm FCR, shrink ported head, mega cycle cam, singh grooved head, custom headers, Supertapp exhaust, Ignitech ignition module, vs1400 drive hub, nology coils, magnecor wires, 12:1 cp pistons, carillo rods

Offline Dean Los Angeles

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Re: Basic efi q's
« Reply #1 on: May 25, 2011, 04:42:35 PM »
The engine is an air pump. Increasing the air flow increases the horsepower. All of the work you do on the engine is to improve the amount of air you stuff in.

Pumping enough fuel isn't a problem, you can do that with a hose.

The correct ratio of fuel to air is one of the tricks to horsepower. Finding that ratio through the full rpm range can be a problem.

Even if you get the fuel into the chamber you have to burn all of it. A hose would result in large droplets that won't mix with the air.

The improvement between a carburetor and fuel injection is getting the fuel/air ratio better through the correct mapping. With a carburetor there are mechanical means of doing that that are not as efficient. The fuel injector is far superior at atomizing the fuel.

There are books written about swirl patterns in the chamber to mix the fuel and air efficiently. There are other books written on the subject of igniting the fuel in ways that aid complete combustion.

Heck, there are phd's on the subject.
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Offline Vishnuatepork

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Re: Basic efi q's
« Reply #2 on: May 26, 2011, 11:59:52 AM »
Dean & Scott,

Thanks for putting it in simple terms like that.

I think the only follow up question I have is, about the injectors them selves.  I have seen with 8+ holes in the nozzle.  Is that for volume or atomization?  If I understand things correctly, the smaller the fuel droplets the better the burn (at the same time a vapor may burn best, but thats a different beast altogether). The droplets burn from the outside in, and that takes time, so the smaller the droplets the better.  Would switching out to an injector with smaller/more orifices great more efficiency(power) with the regular setup? Or am I just reaching?

You mention books?  Could you cite, some I am intrigued!
suzuki vx800, 41mm FCR, shrink ported head, mega cycle cam, singh grooved head, custom headers, Supertapp exhaust, Ignitech ignition module, vs1400 drive hub, nology coils, magnecor wires, 12:1 cp pistons, carillo rods

Offline johnneilson

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Re: Basic efi q's
« Reply #3 on: May 26, 2011, 08:56:51 PM »
Alan,

Injectors are sized based on amount of fuel flow required. Typically in automotive the rating is based on 43 psi and 80% duty cycle.
By adjusting the fuel pressure you can adjust the amount of fuel (within reason). This is common in situations where adjusting the fuel curve is restricted.

Take a look at some of the injector mfgs websites, most have sizing calculators and will explain the basics.

I wouldn't worry about the amount of holes, it is more probably a flow function than atomization of fuel.
This is not like running a carb with large fuel ports because of weak available signal.

This reminds me of an old Carter AFB carb I had, made good power but always resembled a flushing toilet.

J
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Offline hotrod

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Re: Basic efi q's
« Reply #4 on: May 26, 2011, 09:00:40 PM »
Quote
Would switching out to an injector with smaller/more orifices great more efficiency(power) with the regular setup? Or am I just reaching?

Yes - sorta - sometimes!

The question depends on too many other unknowns to answer specifically. In general terms you are correct, better atomization helps in some respects (like fuel mileage) and may help in power production if the fuel being used is not completing the burn in the time available before the exhaust valve opens.

But -- that is not the whole story, for maximum power you need to have peak cylinder pressure at the correct time (typically about 12-14 degrees ATDC), fuel burn speed and atomization plays into that peak pressure timing just like ignition timing. You need to get all parts of the system pulling in the same direction. Sometimes you get non-intuitive results where you do something that should improve performance but it causes other issues that more than counter act the gains.

There are two common fuel injector types a pintle style injector that uses a small pin as a valve to open the hole for the injector, and form the spray cone, and a disk type that uses a small disk covering a series of small holes that controls the spray when the disk is lifted by an electro magnet. In general more and smaller holes produce better spray patterns with finer spray droplets. In the turbo import world you can see the effect of this on fuel economy. One common modification on some designs of disk type injectors is to remove the end cap that has the fine holes in it. This uncovers the actual opening which is just a single hole covered by the disk. This "de-capitation" modification is used on the turbo charged Subaru WRX injectors and increases the flow from a stock 440 cc/min to 810 cc/min. It is a cheap easy way to get a very large injector, but your low load fuel mileage generally drops because with a flow rate that high, the low load opening time is so short that the fuel is not well atomized, and the injector pulse times are so short it is hard to make fine adjustments in fuel flow. At high power fuel flow there is not a lot of difference in power production between the modified injectors and a conventional multi-hole design of similar capacity but a few dyno operators say that they can just see a very slight advantage in power production using very expensive high flow multi-hole injectors.

The question is are they really seeing a very small power increase or are they seeing an improvement in cash flow from selling the expensive injectors and "think" they are seeing a power advantage. The gain is so small many folks are not willing to spend the bucks necessary to find out if it is true.

Second in some applications they use an injector that sprays a solid stream of fuel directly onto the head of the hot intake valve, cooling the valve, flashing the fuel to vapor and pre-heating it just prior to the intake valve opening and the engine sucking a curtain of fuel into the cylinder as the intake valve comes off the seat.
I have heard that this system of direct port injection was/is used in the Indy champ cars. If it helps keep the engine alive by cooling the valve, and gives equal or better power than a normal injector it may have other payoffs than just power production. It might allow them to use mixtures and timing that produce more power but would burn valves using a normal injector.

Bottom line the only way to know for sure is to test different injector designs on the dyno and see what combination works best for you.

Fuel pressure also plays into it, as in general higher fuel pressures give better atomization up until you get to around 75 psi then you start to have injector reliability problems over coming the high fuel pressure, and a weak injector or weak supply voltage to the injector can cause an injector to not fully open under high load, leaning out a cylinder and killing the engine.

Most street cars run fuel rail pressures of 43 psi at idle and if boosted ramp up the fuel pressure in accordance with the manifold pressure to maintain that differential pressure between the fuel supply and manifold pressure.

Within limits you can increase fuel flow (make the injectors act larger) by increasing the fuel pressure. Fuel flow increases at the square root of the increased fuel pressure. Increase fuel pressure 2x stock and the injector will flow 1.414 x as much as stock.

If you only need a small increase in effective injector size fuel pressure is your friend you get both more flow and better atomization.

Larry


Offline Vishnuatepork

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Re: Basic efi q's
« Reply #5 on: May 28, 2011, 02:08:04 AM »
Larry,

thank you for a well written explanation.  I know the way I type it would have taken me close to 3/4 of an hour.  Know i appreciate your time.

Scott,

Wow your generosity is boundless.  Thanks for the offer, but I dont think I'll be getting to the salt this year let alone the Mojave which is where I was aiming for.  If you could send me the tittles  I would be curios to look at various books.  i must admit though I learn so much from this forum that i dont know if a book would ever live up to you guys!

thanks for all you answers!

Cheers
Alan
suzuki vx800, 41mm FCR, shrink ported head, mega cycle cam, singh grooved head, custom headers, Supertapp exhaust, Ignitech ignition module, vs1400 drive hub, nology coils, magnecor wires, 12:1 cp pistons, carillo rods