Help with building your Speeduino, installing it, getting it to run etc.
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By PSIG
#49473
Air bubbles (different from vapor bubbles) are still a big problem with deadhead rails. This makes fuel system design especially important, in order to avoid and eliminate any entry of air, from micro-bubbles to fuel slosh ingestion to assembly entrapment. Your system must avoid entry of air or the performance will suffer and enleanment damage is possible. Fuel pumps must have cooling-flow bypasses or PWM control, or both (see my image below). There are many styles of returnless and semi-returnless systems, and they tend to be more complicated to deal with the side-effects, but are tolerated for the emissions benefits.

While obviously air replacing fuel will affect running, this has caused engine damage in situations you can imagine, such as a trapped bubble building at one end of a rail causing one cylinder to run intermittently lean as the air passes-through that one end injector. If you are not positive your system can run air-free, or does not have compensation features for deadhead effects, it would be generally recommended to use a return system in order to avoid the various issues that can accompany deadhead rails. Engine Specific Output is a factor, with stock engines likely able to cope better than performance engines that are closer to the edge.

Note in the module, a bypass serves multiple functions, powering a low-pressure venturi pump for fuel pickup to the air-separation sump area, while also providing bypass flow for pump cooling. Modules like this can be used for return and deadhead (semi-returnless and returnless) systems:
Image
User avatar
By jonbill
#49924
well... a few weeks have passed in the real world and I've come to realise that my problem is still there with heat soak. having a return from the fuel rail hasn't made much difference.
Another thread raised the suggestion that injector opening times rise with temperature, so I think my next move is test that and bring the IAT into the engine bay by the injectors and see if I can use IAT correction to sort it.
#49942
Connecting a fuel system as pictured solves any gasoline related problems.
Supposed your fuel regulator is working as intended and connected to the manifold (for precise adaptation to the pressure at the injectors).
So the pressure should drop at idle (low pressure in manifold) and rise when on full throttle (pressure near atmospheric ).
Maybe this connection does not work?

Then I would monitor voltage at the positive feed to the injectors during running and warm up.
If voltage fluctuates, this has to be compensated for in the tune. If you do not have a data sheet for your injectors, try some "usual" settings. Lower voltage = longer injection times. A working generator with a decent battery should settle around 14 volt. Many generators even vary output voltage with temperature. As the generator takes it´s own temperature as a reverence, output may vary with engine temperature.
Modern generators even have sense lines, that compensate for differences between the regulator output and the battery pole.

There are quite some things to watch, that where no issue with carburetors. The designer of an injected engine system has to make all of his homework. Funny enough, everyday driver need more perfection than amateur class race cars.

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