You can get a14force e15et pistons already heat proof coated for this application he has some for sale as he is going ca18det supercharged. Send him a PM.
You can increase deck height depending where you are but NSW and Vic have spray welding facilities at good workshops in which iron blocks are saved from over milling of the deck over the years from too many rebuilds, its a common practice for expensive industrial engines and rare heritage classic engines like ford, cadillac and other old world donks. It doesnt cause any distortion to the block as is a type of spark particle deposition and fusion. This will allow longer rod stroke ratio using Honda conrods as Finlands world record holder has doneor just use stock a15 rods prepped with e15et slugs.

Sump needs the sides modded as the longer stroke a15 cranks counterweights hit the sides and sound like a machine gun. Get a15 sump or mod the a14 sides to copy and A15 side dimples.

so there is no way you can use the exa pistons with an a15 crank?
ive also read that the a14 sump wont suit an a15 crank is there any way to get around this or do you need to buy an a15 sump?

Ruffin, A14Force has some awesome e15et pistons already heat proof or teflon coated for this kind of application and takes stock a15 rods and crank with little mod to the tops of the pistons as there is plenty of meat. The exa guys do this when using the E16 88mm stroker crank rather than the 82mm e15et crank to increase capacity.

thanks for all the info, im meeting with my engine builder some time this week, he has a fair amount of experience with lpg, but ill show him this as most of that went over my head.
as for the pistons, does anyone know if the nissan exa turbo pistons will be ok if i go for an a15 crank and rods? im struggling to find out any info on the exa engines.

The gra stuff is good for most applications, but it is a fixed venturi, so it can be hard to get any sort of a good result when the carb is sized larger than ideal for the app, and sometimes even when sized ok, the metering rod thickness you need for just off idle is thinner than it might otherwise seem, then as rpms increase, the signal strength comes up and it goes very rich with the same thickness metering rod, meanign you sort of chase your tail a bit with it.

On the other hand, impco mixers are variable venturi - like the lpg version of an SU carb. Teh air valve moves up and down and maintains signal strength across a variety of engine rpms, engine size ranges and throttle positions. They are a very flexible design, and can be used in blowthrough apps.

It's a little bit hard to see from the picture, so I'm not 100% certain but it looks like the convertor balance pipe is routed to the air filter. This will make it lean out under boost. It's where it 'should' be for a non turbo app. For blowthrough, you need to run that hose from anywhere in the plumbing after the turbo compressor outlet and before the gra mixer (and the same would go for an impco mixer too!) . It'll lean out and eventually lpg would stop flowing altogether.

Mixtures and detonation with lpg is a little bit complicated (sort of) . Richer mixtures 'only' work for petrol engines (or methanol or e85 etc etc) because the extra fuel sucks heat out of the air as it vapourises. It doesn't get burned, it is added in there almost solely for it's ability to cool the intake charge a little bit more. So going a 'lot' richer than stoich on lpg (unless it is direct liquid injection) has no cooling effect so no anti-detonant effect either. What 'can' happen is that around stoich, although you have (in total) the right ratio of oxygen and propane molecules to burn completely, the lpg and air isn't mixed 100% perfectly, so not all of the air can get at the available propane and support complete combustion, using up all the available oxygen. So if you go a bit richer (with lpg) then there's a bit more propane, and you utilise just that little bit extra of the available air. So the total heat energy created (compared with running at 'true' stoich) is higher so combustion pressure a little higher, more heat to deal with, and so it's a little bit closer to the detonation (or pre-ignition) threshold. If you lean the lpg out leaner than stoich, then less and less of the available air (or the oxygen molecules within it) is used for combustion and less propane is burned, so less heat energy and thus lower combustion pressures, and slightly less chance of detonation. BUT of course, that also means less power is produced.

So you generally always want to make use of all the air you cram in there (if you don't what's the point of running X amount of boost at all?? :) . If you find it is detonating (and of course you can't let it do that for long!) then although you 'could' lean it down until it stops, you'd actually end up with a better result by running less boost, but burning all the available oxygen you do feed in there. The reason there is simple enough, less boost = less intake charge heat, so less boost but all the oxygen burned is safer than more boost, and a hotter charge, but then only burning/using 'most' of the available oxygen (if that makes sense!)

You'll also tend to find that since lpg is metered in as a gas not a liquid (carbies feed in fine droplets of fuel, but it is still a liquid and it has to change to a vapour state before it will ignite) it doesn't need to vapourise, so it will ignite with less delay. Meaning that (esp under boost) you'll tend to want to run less total ignition timing with lpg. Considering some high boost petrol engines run less than 20 degrees total timing, it's easy to run what seems like 'not that much' but is actually 'way too much' on a boosted engine on lpg.

With regard to boost levels, you'll tend to see some convertors start to seep (seep gas not coolant, and fwiw it will just blow away when the car is moving, so not an astronomical risk, but there is still SOME risk) at above somewhere around 10-12psi if they have the plastic covers. I'm not sure about the alloy covers beyond the fact that they are a bit better, I just have never tested how far. I think GRA used to advise never going above 20psi, but don't quote me on that. You'd probably want to look at perhaps 15psi as a safe limit (for the lpg system being able to supply it. What the engine will or won't handle boost/compression wise is a whole other story)

If I was in your shoes and couldn't get teh gra carb to run ok, I'd give serious thought to replacing it with an impco 200 mixer, which wouldn't take too much alterations to plumb in there, but that's just me.

peter zeckert has a15 cranks on his site

http://www.PeterZekert.com

http://datsun1200.com/userinfo.php?uid=305

the carby is a 350cfm, but has been choked to 200.
the motor came yesterday via frieght.
the bore is 77mm (is this standard bore size for an a14?)
i havent had a chance to do any work on it as it came late yesterday afternoon.
the pistons come from a nissan exa turbo aparently.
the bottom end is stock a14 rods and crank. i was thinking of going for the a15 crank does anyone know where to find them or are they pretty rare?
and a custom camtech turbo grind cam.
thanks nick

if you bored that block out to 83 mm you be getting papper thin in the bores, plus you would,nt get a head gaset for it .unless it was custom made an a nother thing the std rods small end don,t fit on ca 18 pistons

They are definitely not 83mm pistons, put a ruler on them to see they wouldnt be more than 79-80 mm as anything larger needs sleeves.

Do you know what cfm the carby is? I used to get a crappy idle stalling flat spots etc due to it being too big. I sent it off and had it re sized to a 300cfm which helped but the car was never 100 percent right on gas so I got rid of it.