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By bmw_r35

Im starting efi project to BMW r35, its 350cm3, one cylinder, four stroke, 13.5hp at 3500 rpm

Getting no2c, 30mm throttle body with tps.
Air cooled, so clt from spark plug. Iat between tb and air filter.
200cc fuel injector is probably right size.

I have some questions
1) about fuel pressure, was thinking 3 bars, do I need pressure to be difference between map and fuel rail or just simply 3 bar to fuel injector?
2) What would be recommended fuel pump, preferrably only 1 amp
3) about ignition, is one hall sensor from camshaft enough?
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By jonbill
1) constant 3 bar is fine if that suits the injectors
2) oem?
3) one hall sensor is enough, one tooth isn't : what trigger pattern do you have?
By LAV1000
200CC might be to big especially at idle and low rpm, aim for a 150.

2. Try to get a fuel pump from a small efi motorcycle or ATV.

Think you also need O2 wideband sensor and controller to tune your setup.
select the injector to match the HP, not the CC

I think it's feasible to run fuel only from the single tooth cam. treat it as distributor.

you will need a fuel pressure regulator ideally with a map connection

tps for ae. though could possibly work off of map
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Agreed. 14hp on gasoline/petrol needs about 86cc/min (8.2 lb/hr), and that still leaves room at 85% duty cycle. Larger injectors are a conflict, and only used in some bikes using very high rpm, where there isn't enough time to inject before the cycle has passed. I don't think that's you. ;)

The farther you drift from best flow rate, the more compromises you face. While you can force almost anything to run, the results are incrementally impacted. Use what you must, but otherwise use what you should.

Both MAP-referenced (MRFPR) and fixed-pressure regulators can work, however there are advantages to MR, such as better lower-load control and flatter tables as they try to sustain the rail/manifold pressure differential. Additionally, adjustable MRFPRs can let you dial-in the best compromise in pressure range and flow for your tune.

Fixed-pressure regulators were primarily adopted for multiple performance reasons — emissions performance and production cost performance — not for their power performance capabilities. While their appearance of simplicity may be attractive; the trade-offs are a host of issues that OEMs are willing to deal with for the benefits in regulatory compliance, and a bit less groaning by the accountants. :lol: That said and again, use what you must, else use what works best for your application.

Search the forums for multiple options on low-power fuel pumps for EFI scooters, bikes, carts and ATVs. Some info for calculating required flow was posted here. Another option is to run the smallest common pump you can find, and if oversized (it will be) control the power with either a Triumph-style idle-speed resistor (lowers pump power at low throttle), or a similar scheme run from the VVT output providing lower pump PWM duty cycle at lower loads. Have fun and enjoy that awesome R35!
By bmw_r35
Thanks for replies.

I was thinking hall or optical sensor from cam with one trigger point at 40 degree before tdc, would it work?

I get injector 150cc or lower, maybe in future higher compression (6:1 to 8:1) for more power
Wide band a/f sensor
MAP-referenced fuel pump, that 12w might be an option, even when it is fixed pressure
By LAV1000
bmw_r35 wrote:
Wed Jul 21, 2021 6:18 pm
Thanks for replies.

I was thinking hall or optical sensor from cam with one trigger point at 40 degree before tdc, would it work?
That only works for injection not for ignition.
Do your self a favor and find a way to mount a toothed wheel to crank (preferable) or even cam.
Injection does help to improve an engine's behavior but the biggest gain is precise ignition timing !

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