LPG and boosting.

[Pinger]

Figuring that I'd better ask any questions I have now in case the forum closes and I lose access to those who know... I have a question about single point systems on super/turbo charged engines.
My understanding is that what is required/done is that instead of the reducer's reference port being open to atmosphere it is instead connected to the inlet manifold where it 'sees' boost pressure (and vacuum?) and thus compensates for the increased air density provided by boosting. Is this correct? If so, what are the implications of the reducer seeing vacuum? Is it better to reference upstream of the throttle plate (or is that what is done)?
is there a limit to what level of boost can be tolerated (apart from the obvious capability of the reducer to supply sufficient gas)? Is the reducer affected by pressure pulsations (should they exist) at its reference port and if so, how is that dealt with.
TIA.

[LPGC]

No problem with single point systems on draw through setups where the mixer is at the atmosphere side of a turbo or supercharger... Or at least nothing that prevents a draw through system working using standard single point mixer system reducers.

The problem with single point systems on push through setups where the mixer is between the turbo or supercharger and the throttle is that boost (and to some extent the slight extra vacuum in most none boost situations caused by the turbo/supercharger) overwhelms the usual (as would be in case of normally aspirated engines) delicate balance between air flow through the mixer affecting the vacuum presented to the reducer outlet. On a normally aspirated setup the airflow through the mixer causes a slight vacuum at it's gas feed pipe which is fed from the reducer, it is only that slight vacuum that causes the reducer diaphragm to flex and causes the reducer to output gas to the mixer. The vacuum created by the mixer is only going to be a small fraction of atmospheric pressure even at full engine load. On a blow through system with the mixer under boost conditions there'll be positive pressure even on the line from the mixer to the reducer, hence a normal single point mixer system reducer won't work because any slight vacuum they need will be completely overwhelmed by the boost pressure.

There have been available single point mixer system reducers that have another line that connects to the air ducting between the turbo and mixer, instead of the reducer seeing atmospheric pressure as the zero point it then sees the boost pressure as the zero point and the reducer diaphragm (and hence reducer output) is again affected by the vacuum created by the mixer versus the zero point. The zero point reference pipe connection on a single point system reducer designed for turbo/supercharged applications does much the same job as the manifold vacuum reference pipe on a conventional multipoint injection system reducer. You need such special mixer system reducer to work on blow through turbo/supercharged applications. You could use any mixer system reducer if you use a draw through system but then most of the induction system including the turbo/supercharger contains air and gas at the optimum mixture for any ignition source such as a backfire to ignite it all. The extent of a backfire on push through system is only the air and gas in the manifold burning but on a push through system it's the manifold and all the ducting up to the inlet side of the turbo/supercharger.

[Pinger]

=

There have been available single point mixer system reducers that have another line that connects to the air ducting between the turbo and mixer, instead of the reducer seeing atmospheric pressure as the zero point it then sees the boost pressure as the zero point and the reducer diaphragm (and hence reducer output) is again affected by the vacuum created by the mixer versus the zero point. The zero point reference pipe connection on a single point system reducer designed for turbo/supercharged applications does much the same job as the manifold vacuum reference pipe on a conventional multipoint injection system reducer. You need such special mixer system reducer to work on blow through turbo/supercharged applications.

Thanks LPGC - appreciated.
I've picked out the above as it applies to what I intend - a blow through system with zero vacuum (blower scavenged 2 stroke) and avoiding the explosion risk of a draw through system. What I'm not understanding though is the detail differences with a ''special mixer system reducer'' compared to say, my R90 reducer which has its reference port communicating with atmospheric pressure but could - to my mind - have that port communicating with downstream boost pressure. What am I missing?

[LPGC]

The R90 just has a hole in the back plate for atmospheric reference, I'm not sure whether or not if you adapted your R90 to connect a pressure reference pipe to that hole and made sure it (and the back plate) were all sealed up it'd be the same as a turbo spec R90 or not. Someone like Dai (@Classicswede) or Mick (@TinleyTech / @LPGAutosupplies) might have the answer. I don't remember the last time I had a play with a single point mixer system on a blow through setup but from a tech point of view it's easy to imagine them being more difficult to set up than a naturally aspirated setup... I'm thinking about the potential for minor aspects to probably affect fuelling by quite a lot, such as angle of the boost pressure reference 'spud' in the boost pipe and where in the boost pipe it is placed, any boost that hits the reference before hitting the mixer will have the effect of leaning the 'tip-in' mixture and vice/versa. Also the 'dynamic range' of engine loads and fuelling are wider on a boosted engine (the amount of air and fuel it needs at no load such as idle versus the amount of fuel and air it needs at high rpm full load, divide the latter by the former gives what we could term the dynamic range of air and fuelling needs), the wider that range the more difficult it can be for the mixer and reducer combination to have linear relationship between airflow and gas flow.

[Pinger]

Thanks LPGC. I think my intended project is looking for a parking space...

[robertXX]

yes it works , my astra started off as single point blow through .

viewtopic.php?t=8781

[Pinger]

yes it works , my astra started off as single point blow through .

viewtopic.php?t=8781

Thanks - I've just read the linked thread - very helpful.

[Pinger]

deleted.