1972 Chevy pickup converted by LPGC

[LPGC]

Just finished the conversion of this 1972 Chevy pickup, V8 with 3 on the tree manual box.

Fitted an Impco setup with 105 litre toroidal tank.

I had a look around the car a while ago and told the owner the (already fitted) custom exhaust at the rear would need to be modified to fit a toroidal tank in the spare wheel location which is underneath at the rear. The custom exhaust used a couple of 'cherry bomb' straight through back boxes fed from a Y just in front of the diff, the cherry bombs would need to be spaced wider apart to make space for the tank to fit between them. He was going to sort the exhaust before bringing it in, but when it came in for conversion the cherry bombs were still fitted in the problem position. The owner said 'Just take them off, I'll refit them where I want later' but I realised that after disconnecting them from the Y the exhaust gasses would blow from the Y just where the flexible brake hose connects to the rear axle, so I had to makeshift/temporarily refit the rear exhaust setup after fitting the tank anyway.

As it was before conversion

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Can only attach 3 files to each post, so I'll post more pics on the next post below...

[LPGC]

After conversion

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More pics below

[LPGC]

In the pic below you can see the LPG switch / fuel gauge but you might not realise that the chromed pull handle is also part of the LPG system. The vehicle is a 1972 model so has a carburettor fuel system, to which I've added an LPG carb which sits on top of the petrol carb. The LPG carb works a bit like an SU carb, a 'plunger' opens increasingly with increasing airflow. The chromed handle, when pulled, opens the plunger on the LPG carb so as not to cause any difference to airflow to the petrol carb that might otherwise cause the petrol carb to deliver richer than intended mixture when running on petrol.

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[c3h8]

I had a 1972 converted. 350 CI straight propane. Great truck and great engine to convert. Two comments on your truck. It may be an illusion but the regulator looks very close to the exhaust. That would not be allowed in Canada. Minimum of 8 inches (about 20 centimeters required here). Secondly, a really minor point but did you run out of short bolts for the regulator mounting. Overall though a very nice installation.

[LPGC]

I had a 1972 converted. 350 CI straight propane. Great truck and great engine to convert. Two comments on your truck. It may be an illusion but the regulator looks very close to the exhaust. That would not be allowed in Canada. Minimum of 8 inches (about 20 centimeters required here). Secondly, a really minor point but did you run out of short bolts for the regulator mounting. Overall though a very nice installation.

It was a while ago when I converted this for a repeat customer, I don't remember exactly how far the regulator is from the exhaust but I would think it's around 8 inches. Heh, I cut the bolt short after taking the pic. Thanks

[robertXX]

just out of interest simon , but , why not a mixer ring and a r90 ?

[LPGC]

just out of interest simon , but , why not a mixer ring and a r90 ?

The Impco gas carb setups are a lot less fiddly to set up correctly than a mixer setup, especially with today's R90's which are not quite what they used to be. They're also more lkely to be reliable and allow consistent starting on gas with a cold engine without any problems. At one time an option for converting American V8's that had Holley type carbs was to use an R90 and a mixer that was actually designed for a Mercedes application, it would mate to the same diameter mounting point between the Holley and air filter but the veturi size was a bit restrictive for most V8's so the conversion lowered power on LPG and on petrol. Other than that the only Holley size mixer that was much better was the old Spring Gas mixer, an Australian made bit of kit which became unavailable many years ago, the Sprint Gas setup replaced the pan type air filter. If the Spring Gas mixer was still available I would have considered it (with an R90 or similar reducer, probably a similar reducer these days...) as a less expensive option but the Impco gas carb would still have been the better system and I think for this customer's requirements it was always going to be the Impco.

[robertXX]

gotcha ,thank you .

whats changed on the r90 , and can the new ones be made to work as well as the old ones do you think ?

[LPGC]

TBH I don't see many mixer systems these days but they changed over the years.

The older ones seemed to be more sensitive in that you could probably get away with fitting a slightly wider venturi mixer (compared to a newer one), set the sensitivity screw so it was sensitive and it could still be set up to be linear and deliver correct mixture even with the relatively small vacuum signal created by the big venturi mixer. Come back to it the next day and it performed the same. With some of the later ones if you tried the same slightly oversize venturi it could be difficult to set the mixture for low loads, either a way lean or way rich mixture and it not seem there was fine enough control. Come back to the later one the next day and it might seem a gremlin had opened the bonnet and twiddled the adjuster screws overnight, of course it wouldn't be leaving it overnight that made the difference, it'd be aspects like temerature differences etc.

For the last few mixer system conversions I did on medium and fairly big sized engines I fitted a BRC vaporiser instead of an R90, the BRC was far better in terms of sensitivity and seeming to retain settings (or not be affected by aspects like temperature as much). The BRC probably couldn't be set to be as sensitive or linear as the old R90's but was like a breath of fresh air compared to the newer ones.

Not sure what changed internally on the R90s but there was a change in spec and performance of most OMVL reducers including the Dream reducers (for sequential setups) and the change with R90s seemed to occur not long after the Dreams. The Dream reducers went from being capable of flowing enough gas for 400bhp to around 150bhp in 1bar or around 250bhp in 1.6 bar guise around that time but this was maybe 15 years ago. Maybe internal changes occurred at the same time but we didn't see the newer R90's until a bit later than the Dreams because there were still plenty older spec R90s sitting on shelves, sequential conversions were much more common than mixer conversions by then.

[robertXX]

fascinating , thank you Simon , isnt it odd when something that works perfectly well gets changed to something naff .


i still grieve over the loss of my two good r90's , one of which was a bona fide turbo version with the factory pressure connection on the back plate , that got taken from where i was storing my stuff during the house move .



regards
robert