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I am going to try an EFI conversion on my 2 stroke.

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I am tired of carbs and their quirks. i have quite a bit of experience now tuning my subaru ECU (tuning in different turbos, converting to speed density from MAF, tuning different injectors and ignition tables...)  for several years and really like EFI when it can be accessed and adjusted. im also out of my mind and like punishing myself with stupid projects that take way too much time. :lol: so i ordered a microsquirt ECU and am super stoked and making my yz250 EFI :lol:  i know i can get something running and working but i would like to make it as elegant as possible. i might make a throttle body that fits in place of the keihin perfectly but would use an off the shelf TB if there are any that are narrow enough and would allow me to make adapter flanges without ending up longer than the carb. the fuel pump and regulator are another "issue". there are lots of pumps available but most of them use quite a bit of power. what do EFI 250s and 450s use? do they have the regulator in the tank or is it like a car with the regulator after the injector with a return line to the tank? too bad there are no mechanical pump options. i also need to figure out a good starting point for injector size. i have some subaru 400cc injectors but i have a feeling that is too much. from my googling it seems most 2 storke EFI setups end up using injectors with similar CC/min to the engine CC. 

 

so fire away with suggestions of which bikes i should use parts from. if anyone has measurements of throttle bodies from things such as a yz250f or something. 

Edited by MCRIPPPer

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Why would the 400cc injector be to much ?

it will flow 80hp crank at about 4 bar , so you will be running at about 60% duty cycle , maybe a 300cc injector might be slightly easy to control ? 

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i know it will work with the 400cc  (might actually be a 425?). it might be hard to get good low end control (but maybe not we will have to try. ) does anyone have the length and bore of a keihin throttle body used on 250s? im thinking about buying one and using the guts in a custom made body that fits the stock carb position. 

 

i have plenty to play with for now. i need to make a 36-1 trigger wheel to press on a spare flywheel and i have a spare stator that i rewound with 18 gauge to test power output. i plan on using MS for spark so i wont need any ignition coils on the stator. im going to spin it on my mill (0-4500 rpm test). might need to go down to 20 gauge to fit more turns on it. 

i will also need to get out the timing light and build the timing table in 2d to match the CDI curve. i want to figure out a way to spin the flywheel 9-10krpm on the bench so i dont have to rev the tits off the bike to collect data. :lol:

Edited by MCRIPPPer
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well my stator is a bit weak. it is able to light a 55w bulb rather brightly to 7vAC but needs 4000+rpm to do it. i need more turns of wire to get reasonable voltage at 1000rpm. im hoping for at least 10v at an amp or two at 1000rpm. im going to wind it with 22awg as many turns as will fit, and see how it does. if it works ok i will run it for a while at high rpm with load and see how hot she gets. 

 

ran some numbers and the subaru WRX 420cc injectors might actually work perfect. 

Edited by MCRIPPPer

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thanks i will take a look.  👍

 

something i just realized is that my fuel injector will act smaller since im going to run premix fuel through it and the oil will hog some of the volume. 

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28 minutes ago, MCRIPPPer said:

thanks i will take a look.  👍

 

something i just realized is that my fuel injector will act smaller since im going to run premix fuel through it and the oil will hog some of the volume. 

Makes sense, it is just a big jet after all 👍

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You are going to need at least 100 watts of electrical power to get anywhere close to what you need to power an EFI system. That's going to require a flywheel/rotor considerably larger than whats on your YZ250. There's just not going to be enough magnetic field strength, or room for stater coils.

 

Injector sizing is going to be based on fuel mass flow. Injectors are usually rated in how many pounds or kilograms of fuel they will pass per hour at a specific pressure rating. The fuel flow rate for the injector is based on the maximum hp of the engine. Another consideration is the injector 'slope'. This how linear the flow rate is when the injector is opened for a very short period to open 100% of the time. If the fuel delivery at idle requires a very short opening time, the injector will 'peg out' before full horesepower/rpm can be reached. This is why some fuel injection applications use dual injectors. for each cylinder (Ducati's come to mind).

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100w is simply not true. 30w is a number more commonly discussed by people who have converted bikes. if you add the power of all the parts it comes out to between 15-30w depending on how fast the pump needs to run at a particular time. but 50w would be nice to have. obviously it will be very borderline and will need to be carefully designed. 

Edited by MCRIPPPer

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4 hours ago, 2grimjim said:

You are going to need at least 100 watts of electrical power to get anywhere close to what you need to power an EFI system. That's going to require a flywheel/rotor considerably larger than whats on your YZ250. There's just not going to be enough magnetic field strength, or room for stater coils.

 

Injector sizing is going to be based on fuel mass flow. Injectors are usually rated in how many pounds or kilograms of fuel they will pass per hour at a specific pressure rating. The fuel flow rate for the injector is based on the maximum hp of the engine. Another consideration is the injector 'slope'. This how linear the flow rate is when the injector is opened for a very short period to open 100% of the time. If the fuel delivery at idle requires a very short opening time, the injector will 'peg out' before full horesepower/rpm can be reached. This is why some fuel injection applications use dual injectors. for each cylinder (Ducati's come to mind).

some are rated in lbs/ min or hour some are rated as cc , cc is easier to work out max flow rate in hp . formula to work it out is max cc drop the zero off the end and x number by 2 and that is max flow at the crank for each injector . so 420cc (42 x 2 = 84 hp) , then you should times by .9 for 90% max duty cycle . so 84 x0.9 = 75.6 hp is safe

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Don't waste your time unless you do it properly, inject at the transfers.

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Things to consider:

Fuel pump: I recommend snagging one thats for another EFI motorcycle of roughly 2X hp capability of what your expecting. They dont pull to much more and will be basically what you need

Injection: For the injector/injectors I'm going to make the assumption that you're going to need at least 50% what you think you need for the HP you're going to make if it was 4T . Injectors from a powerful V twin bike might work or pretty much whatever. I think you could just look at hp/per cylinder and if its around 80-100 I think you'll do ok.

 

Otherstuff: Vacuum based fuel pressure regulator might work ok, oil takes up 3% of the fuel if that so I wouldn't worry about it with sizing just maintain the mix for fuel ratio.

 

Big issue will be power at idle, you will almost certifiably need a battery to run the setup unlike the 4Ts which I don't think need them correct me if I'm wrong.

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9 minutes ago, Swes said:

Things to consider:

Fuel pump: I recommend snagging one thats for another EFI motorcycle of roughly 2X hp capability of what your expecting. They dont pull to much more and will be basically what you need

Injection: For the injector/injectors I'm going to make the assumption that you're going to need at least 50% what you think you need for the HP you're going to make if it was 4T . Injectors from a powerful V twin bike might work or pretty much whatever. I think you could just look at hp/per cylinder and if its around 80-100 I think you'll do ok.

 

Otherstuff: Vacuum based fuel pressure regulator might work ok, oil takes up 3% of the fuel if that so I wouldn't worry about it with sizing just maintain the mix for fuel ratio.

 

Big issue will be power at idle, you will almost certifiably need a battery to run the setup unlike the 4Ts which I don't think need them correct me if I'm wrong.

Good info. The power needs at idle might be balanced by a large capacitor. With a 4S, you have two spins of the flywheel and the extra unused power is stored by the cap. You are not going to have that luxury with a 2S. You may have to have a fast idle. There are only so many watts available at idle. It is possible you will be forced to have a small LiFe battery for idle assuming your power use at rev is less than what is generated so as to keep the battery charged. Also keep in mind, power generated is AC and you will be converting to DC and you loose on the conversion though a full wave rectifier will minimize it.

You do not know until you try it. Good luck!

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it will be interesting to see what happens. worst case i can put the EFI on the XR600. 

if i can get the power valve controlled by the ECU i could use the gear off the crank to run a fuel pump of some kind. that wold be trick. im thinking maybe stealing the gearotor form a walbro and making a new housing for it. 

i have been thinking about it and i will probably end up running it off of a 3s 4Ah lipo battery since i have a bunch of them. then once its in a working condition i can start trying to streamline it. 

Edited by MCRIPPPer
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12 hours ago, uniflow said:

Don't waste your time unless you do it properly, inject at the transfers.

sounds like a great idea, but how would the bottom end be lubed? i dont think oil injection is in the cards for this project. i guess it could use two injectors in the transfers and one in the reed block. would still oil the motor but not be as prone to loading up?

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you might be supprised as to how the oil gets around the crank case. 

With just B port TPI and oil feed in the fuel at 40 to 1 through the B port TPI injectors in our YZ250 we have had no bigend failures. Years of running since 2013. To be fair after one season I got nervous and fitted a very crude oiler. Solenoid open or shut through a very small jet in the throttle body. Through the LINK ECU I controlled this solenoid so above 60% load it was open and below this the solenoid was shut. 

Don't worry about oiling, but do worry about where you put your injectors. Transfer mounted injectors are the way forward ( KTM, Husqvarna). There is a good reason you don't see successful throttle body injection on twostrokes. Have a think about it before you rip into spending time and money on stuff that has already been proven a dead end. 

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Why not try an eco trons efi kit? It has everything all in one. Just order a 38mm throttle body for a 250 2 stroke.

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been there done that, they don't work on a twostroke, don't waste your money.

Check out Kiwi Biker forum, ESE thread under Buckets, the Ecotrons disaster has all been documented there. 

 

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so you ran port injection with 40-1 mix + an oiler in the TB? or one or the other? is throttle injection really that bad? it seams like it would be better than a carb. 

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