LPG Discussion Forum

thanks gilbert!


im wondering if classic swede will come on here and give me an idea if how much bigger a venturi i can get away with in percent terms if i go twin feed from the vap to the globes. i think he said in an earlier thread 34mm to 40mm .which is a 20% ish increase , so then my calced 22mm to 25mm would go to a 26.5 to 30mm....so a twin feeder may be a go-er .


robert.

I wouldn't try and get away with anything, you have enough unknowns already not to flout the rules of thumb.
Have you pondered what effect pulsing your mixers has? All the guidance is about mixers handling a "continuous" airflow, smoothed out by a multi-cylinder manifold. You've got a bunch of mixers getting occasional pulses, coupled together upstream at a supply buffer - which should at least isolate one mixer from another. The ram air pipes could even result in positive pressure trying to get back up the gas pipe, it is difficult to predict behaviour here. I'm sure it can be made to work, but you might get weird effects.

Just turn up some plastic plugs to at least trial the rule of thumb venturi size, fiddle afterwards when you have a baseline.

Will post later tonight when I have some more time to give a full reply

Better late than never.

By using both top and bottom outlets you can get away with around 25% bigger venturi in your mixer.

With the setup you have with a feed to each cylinder rather than a common point vacuum pulsing can be a problem especially at lower air speeds. Probably the most important thing is that you have enough distance between the air entry and where the gas entry is so that it does not escape to atmosphere at low rpm

You mean he's got to make the ram pipes even longer?????

Have the smaller venturis been fitted yet?

Not yet Simon. I want to find out what difference a 2nd gas input does first

That could imply a lot of extra plumbing to your mixers though Rob? Unless you just run a 2nd pipe from reducer to bulb, but would that have the same effect as Dai meant?

Can understand not wanting to fit smaller venturis but this would seem easier than plumbing (in first case / question above), and Dai did only say 25% smaller venturis... If your engine is as lean as it seems at the moment, the 25% more gas Dai mentioned probably won't be enough... Suspect it idles due to opening the idle bypass(s), even with reducer set to max sensitivity the signal nowhere near strong enough to otherwise have the reducer delivering enough gas.

Simon

I'm sure teh ram pipes are long enough. there are ways to make the venturi work better. Not wanting to trawl through the entire thread can you show me the venturis you are using. need to see shape and all the holes in them

here you go ,
































i have raked out a plethora of pics trying to show the inlet side of the venturi , hope they help .(or send you to sleep )


regards
robert

Shame you junked the body from the chassis originally - with LPG on it you could have had a unique London Taxi and avoided the Congestion Charge while charging tourists hansomely for the privilege!

I've literally just found this thread and had a very quick skim read of it (and your build thread too) and looking forward to the finished article. I've run V16s on gas but that had 2 mixers you could have a party in and two turbos each the size of dinner plates! That was just the intake trunking! They kicked out about 1830bhp but didn't go anywhere - they each powered a 1MW (mega watt) generator to produce electricity from (landfill) gas.
Your project is a lot more exciting though!

LPGC wrote:That could imply a lot of extra plumbing to your mixers though Rob? Unless you just run a 2nd pipe from reducer to bulb, but would that have the same effect as Dai meant?

Can understand not wanting to fit smaller venturis but this would seem easier than plumbing (in first case / question above), and Dai did only say 25% smaller venturis... If your engine is as lean as it seems at the moment, the 25% more gas Dai mentioned probably won't be enough... Suspect it idles due to opening the idle bypass(s), even with reducer set to max sensitivity the signal nowhere near strong enough to otherwise have the reducer delivering enough gas.


Simon

yup i agree Simon , i think i will try the extra link to the globes ..and see what that does . next thing may be to put a bolt with a big ball bearing on it into each venturi to create the area of a correct venturi diameter .and be adjustable .



Dai, thank you for the input ,very useful.



Laird,yes , it has also been suggested that , if the car is still down on the v5 as a taxi , i may be able to use the bus lanes ,and hoon down the lanes with impunity ....seting off car alarms .

Adding the second pipe to bulb will improve things with minimal extra pipework.

Venturi length is good to work well. A shorter venturi needs to have a smaller ID. The only thing I would change is the entry is better with a radius rather than the taper they have but the differnce only minimal.

If adding the second pipe does not work then just as a test use some good tape to block off have the air inlets just to see if the extra suction from that works.

robertXX wrote:next thing may be to put a bolt with a big ball bearing on it into each venturi to create the area of a correct venturi diameter .and be adjustable

Novel and interesting method but still a fair bit of work mate.. and I reckon will almost certainly work. Might also give the advantage of allowing mixture adjustment of each cylinder using a colour tune maybe... maybe more advantageous than first expected in case your bent copper tubes have varying restrictions to gas flow to mixers.

Simon

exactly simon ,not sure whether to try it on a longitudinal bolt so it can move in and out of the venturi , or a lateral bolt through the sidewall ,that can move off centre .

thank you dai ,like the tape idea , but would prob get v to stand there with a long sheet of ally !

robertXX wrote:exactly simon ,not sure whether to try it on a longitudinal bolt so it can move in and out of the venturi , or a lateral bolt through the sidewall ,that can move off centre .

thank you dai ,like the tape idea , but would prob get v to stand there with a long sheet of ally !

One idea - Maybe a length of bar running topside (or bottomside) across the front of all venturis (spaced in front of venturis maybe an inch and half), drill holes in the bar inline with middle of venturis, a drop bolt through each drilled hole with locating/locking nuts each side of bar on each bolt... So now there are secured bolts hanging an inch and a half in front of the middle of each venturi (and even if holes not quite drilled in centre, holes could be elongated laterally to allow drop bolts to be positioned centrally, bolts won't shift when nuts each side of bar tightened). Weld a nut at the bottom of each bolt (at 90degrees to the bolt), then the balls welded to another 6 (12!) bolts can be screwed through the nuts (locking nuts at one side or both side of welded nut)... Should allow perfect positioning for all balls. Takes some balls and time though!

I think I'd be looking at smaller venturis / making restrictors for venturis.

Simon

testing delayed due to family stuff ,BOOO..... resuming in oct ....

Bah, making us wait! Robertxx, big bloke or is that your 'otherwise / pawn name', member of any other websites?

Simon.

simon pleaase! leave my member out of it !!

Hehe!

Ok, well, seriously speaking then... I reckon smaller venturis will be the way to go and Dai's suggestion of twin outlets will help.

But I do have a nagging doubt... Having converted such as BMW M5's etc i.e. vehicles which have individual throttle bodies per cylinder (using sequential kit), I have noted vacuum readings that are lower (to be clear, higher absolute pressure) than would be expected if the engine had an inlet manifold - The M5, for instance, has separate throttle bodies, inlet trumpets, but it does by design have a common vacuum connection to each cylinder. On that common vac line the engine idles at around .5 bar, where if the engine had a manifold would expect the figure to be around .3 bar (and I know cam choice etc might raise the .3 bar a bit, but not that much, engine hardly struggles for breath at idle like a race engine might).

I put this down (especially at low engine rpm) to any vacuum that might be present at one venturi (on it's inlet stroke) simply being negated by the fact that at another venturi (might be on it's power stroke at the same time) effectively being an open ended pipe to atmosphere (at low rpm). In terms of sequential systems, this just causes some complications regards mapping and setup, in terms of a mixer system which relies on slight pressure depression in that vac line for fueling, I reckon this could cause problems.

This could imply even smaller than calculated venturis need fitting (only 60% the flow rate by example), or other radical measures.

6 People each suck on a length of hosepipe, each hosepipe sealed to a tin can... Put an airflow restriction on another hosepipe to the tin can and there will be a slight vacuum in the tincan (enough maybe to suck gas out of a reducer). But if only one person sucks on their hosepipe at a time and all the others let the pipe fall out their mouth between sucks, no vacuum in the tin can. With a manifold, engine valves are at one end of each pipe and the tin can at the other while the people are not sucking... there is still a vacuum in the tin can all the time. Without a manifold, there is a really wide whole (mixer venturi) at one end of the other 5 pipes... Not much vacuum in the tin can (pipe between reducer and venturis).

12x Blos LPG carbs or some Impco arrangement might address...

Simon