Advice needed re suitablity of Impco 425 lpg/propane carb for a particular engine

[allyoop]

Hi Guys, my name's Al and i'm a Mechanic here in New Zealand I'm hoping to get some knowledgeable/experienced advice regarding the most ideal components to convert my car to LPG (Propane)(DEDICATED LPG-I.E NO PETROL SYSTEM). I'm mainly just wanting to get it off petrol and onto LPG (Propane) as the fuel bill is really painful, and as it is one of a few cars I own, i'm not so much wanting to make a project out of the conversion as just get it on LPG in a fairly straight forward, practical (and hopefully cost-effective) way (as compared to making it some sort of a testbed to achieve the ULTIMATE possible system). (although having said that, I don't really want to lose any power or practical'user friendliness'just for the sake of a bit more money or labour).
The car in question is an Automatic 1964 Mark Ten ('Mk X') Jaguar with 130,000 miles (208,000km) on the original 3.8 Litre (230 cubic inch) straight-six Xk engine which is in fairly good shape with cylinders ranging between 135-150 p.s.i compression at operating temp). In terms of fuel/air requirements, i'd say this engine is more likely to have the needs of a significantly larger capacity engine-especially for it's era-, because it has (from factory) three x 2-inch (50mm) S.U carburettors (each one with it's own manifold to cyl head feeding 2 cylinders), twin overhead camshafts, straight-port cylinder head (alloy), hemispherical combustion chambers, largeish valves (i'm not sure of the exact sizes however I think the inlets are around 2 inch, and it's a fairly heavy car at just under 1900kg (4200 pounds), and from new, with their 3.54:1 rear axle ratio, the had a 17 second quarter mile time and a top speed of over 120 m.p.h (about 200 km/h)(and would likely be even faster when not on early 1960's tyres..), anyway, with those figures and that weight they clearly put a lot of fuel and air through the cylinder head (but despite that they aren't terribly rapid cars 'off the mark', so I don't want to give away any significant amount of responsiveness).

Other pertinent factors are that I may (at some point in the future) fit a 4.2 litre Xk engine (also with factory three x 2 inch S.U's)from a 1967 420G Jaguar, and am seriously considering converting to manual transmission (5-speed with overdrive)at some stage.

In terms of LPG carburettor/mixer I figure that I have the options of:

A)Three separate 'spray ring'-type mixers (one attached to each S.U carb, and an Italian-type regulator/convertor with a 3-way split/power valve leading off to each mixer (although I tend to think this setup is likely to be hard to keep in tune/carb balance-esp as everything is 50 years old), and also I suspect that the three mixers would have a combined effective significant restriction on the inlet air (?)

and:
B) (This is my basic preference, due to relative simpleness, combined with my having been served very well by Impco Equipment for many years), Attaching an Impco carb/mixer to the end of the long, one-piece alloy inler air 'horn' which bolts along the outside of the three S.U carbs (and acts as the common attachment for carbs-to-air filter element, via a conduit), this alloy housing has it's inlet opening (around 85mm)(3.25 inches) at one end. Now there isn't a lot of clearance at that point, but it looks like an Impco 425 could be got in there (but not an Impco 300-due to larger diameter (?) (i figure a 225 would be just plain too small to do this engine justice at top end of revs (?). It seems a 425 is also going to work ok when mounted vertically.

I have other questions about other aspects/areas of this conversion, however, for the moment, I'd really appreciate and solid opinions based in fact/knowledge/experience about what would be my best approach to this induction/carb/mixer area for a start. (and if you're still reading, i already appreciate it!),

Cheers, Al.

 

[allyoop]

p.s, I should have mentioned, the intended useage of the car is a combination of normal city driving and out of town/open road ('freeway') driving, and I tend to like driving 'hard' (i'm always in a hurry), and I'm hoping to achieve a trouble-free, reliable result without too much ongoing maintenance (other than the usual keeping the ignition system in top shape, keeping filters clean, etc, etc), and I have a 'Constant Energy Ignition' distributor and amplifier (was factory fitted to a 1980's series 3 xj6) with the mechanical advsance regraphed to suit LPG, ready to go into the 3.8 engine.

As I mentioned prevously, there are a number of other points i'd like to get opinions/advice on in connection to this conversion, however, for now i'd like to 'nail down' my choice of LPG carb/mixer arrangement.

Thanks again guys, Al.

 

[BigClive]

Only B) seems feasible to me. I would stick to petrol and get a smaller car for running about. I like triple polished 2" S.U. carbies on XK engines too.

 

[allyoop]

Cheers Clive, However I do want to make this car my daily transport and to do so on petrol would be unpractically expensive (not to mention that for a number of reasons i prefer LPG over petrol as a fuel anyway).

 

[franzh]

While not exactly the proper engineering topic for this forum, lets see if we can work through this project. The Impco 425 is a moving air-valve air-fuel mixer that modulates gas flow depending on the relative airflow through the mixer body. The mixer operates by a pressure differential above and below the air-valve which lifts the valve assembly. LPG is drawn into the mixer from a vaporizer which holds approximately 1/2" wc back until the demand requires the fuel.

The 425 is more ideally suited for engines in the 4.0-8.0 Litre range and may be a bit oversized for smaller engines. Starting and low speed performance may be an issue. The 225 may be a better candidate if drivability is an issue and it usually is.

All mixers operate by generating a pressure differential created by either a restriction or a venturi. If the venturi is small, low speed and starting is improved, at the cost of some higher end performance. If the venturi is larger, low speed and starting may be an issue. Installing three venturi rings on your carburettors will indeed be difficult to keep in tune as they will draw from a common vaporizer. Mounting a single venturi or air-valve on a log-type manifold generates a condition where individual cylinders have different air-fuel mixtures due to airflow dynamics inside the log.

Hope this helps a little.

Franz

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[allyoop]

Thanks a lot for your help Franz,

I have calculated the cfm requirements of my engine as per the(seemingly somewhat unrefined)equation supplied by Impco (I may be missing something but I say this because the equation is on one hand based purely on engine capacity, with no regard for valve lift, combustion chamber efficiency, etc, and, for instance, the engine in question has a horsepower rating of only about 10% less than another car I own which has a 5.8 litre (351 cubic inch)engine), and anyway, it comes out in the early 300's, and while i realise that my engine is technically (and, of course, actually) significantly smaller in cubic capacity than that normally considered ideal for use with a 425, I was hoping that i'd be able to succesfully use a 425 as it would fit in the physical space between the inner fender and the inlet end of the common air horn which runs across the inlets of the 3 carburettors, (whereas an Impco 300A looks too big to fit into the space available without some remodelling work), and because I strongly suspect that it displaces air in a volume which belies it's cubic capacity when compared to most other engines, due to the points which I mentioned initially, which stem from it's racing heritage, and the fact that I tend to drive hard and spend a fair bit of time at wider throttle openings. (there is also the consideration that i may well be replacing the 3.8 litre (approx 230 cubic inch)engine currently in the car with a 4.2 litre(approx 260 cubic inch)version in the future and would like, if possible, to avoid having to make the originally purchased air valve redundant at that point).

Another point which I always like to take into account is that while i'm sure a lot of setups go ok, or even very well at higher rpm's/power output situations and may even be impressive in this area, I really want to avoid a situation whereby there was even more potential but it was never realised simply due to an air valve which is just beyond it's limits at that top-end.
And while i'm aware of the possible starting and low-end running issues which can come with too big of an air valve, I have always had satisfactory performance in these areas, at least in part by having excellent: ignition systems (and i have an electronic distributor and amplifier from a 1980's series 3 xj6 being regraphed to suit LPG for the engine i question), excellent high-output ignition coil and secondary ignition insulation, suitable gapped. heatranged and maintained spark plugs, no inlet vacuum leaks, high performance starter motor with good battery, earth, and cables, etc, and this approach has served me very well over decades of dedicated LPG use in engines from the same era as the one in question.

However, if a 425 is indeed to large of an air valve for this application, then is there a reason that you didn't mention the possiblity of using an Impco 300A? (despite it's physical size being a potential issue for my application it would seem (on the face of the Impco cfm chart calculation at least) to be ideally suited cfm-wise.

Of course there are still the matters of which lock-off and convertor (regulator) are most suited, but for a start i'd really like to arrive at a choice of air valve first and deal with the others next.

Any further help/advice much appreciated, Al.

 

[sierra4000]

Multi carburetor engine is always problematic for lpg conversion.
also Selection of venturi size is critical because, though with small lose some top end
With a large diameter venturi is the loss of low end torque but for low air speed through the venturi lean mixture = also the risk of backfire!!!
Backfire is greater when the volume air-fuel mixtures from mixer to intake valve is large.

Radek

 

[franzh]

I am not particularly a fan of the 300, mostly due to the susceptibility to vacuum leaks and its size. However, there are two sizes of the 300-A, the smaller of which works relatively well on engines in the 3.0-5.0 L range.

Franz

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[allyoop]

Thanks guys, Franz, from what I've read here I think i will fit a 225 and attach it to the end of the alloy air 'horn' which runs along the outside of, and attaches to, the 3 2 inch s.u carburettors. I did initially have concerns that a 225 might be found wanting in regards to it's ability to supply this engine and/or the 4.2 litre xk engine which I may well fit in future at maximum output(and I really wouldn't want to limit full power even in the slightest), however I figure that you have a lot more experience than I have and would say if you thought that this was a likely outcome.
Can you suggest which specific model of 225 is better suited to this application? (including in regards to 'fb' etc type)-I don't intend to get too elaborate with this conversion and would like to keep it quite simple and would like to limit the electronics to the aforementioned distributor ampilifier and the stereo.. Would you also suggest a specific converter/regulator (i have an 'Aussie B2' on a 4.1 litre Ford Falcon with good results, but would get an Impco one if there's no real advantage in a B2 in this situation) a recommendation for a lock-off would also be greatly appreciated.
Lastly, i'd really like to get hold of a hard copy (as opposed to a download) of your book 'A diagnostic guide to Alternative Fuels', how can I go about securing a copy? Thanks again,

Al.

 

[franzh]

Al:
The last part of your question is against this forum's policy. Take a look online for other means of contact.

Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.