CNG wrote: Sat Oct 14, 2023 4:21 pm
Blowback Backfire Ring Valves
Anally retentive query. I've a collection of these in 80mm., yet never been able to source one in 70mm, and I've not had a backfire for years- ever less likely with a Leo/Gal, yet reckon a valve won't hurt. The usual suspects long sold out of anything in 70mm. Single-point is - let's face it - dead. I must be the last installing such things, in the UK anyway.
Blowback Backfire Valve.png
Finally, finally got a used one, found in Lithuania. I'm about to plonk it in, and was going to put the hinge at 12 o'clock. Then it occurs to me why would I want it shut in default, and add restriction before it opens to WOT?
Then 6 o'clock makes some sense. 3 or 9 o'clock leaves the flap's weight on the hinge, this over choking air-flow, yet more readily closed?
It's an LR. Can't imagine it makes much difference, what's received wisdom?
Sorry the late reply re 'blow back valve'.
To me the 'blow back valve' looks like a standard 'blow off' valve screwed to the inside of the outer part of a conventional mixer body.
Blow-off valves are normally closed, they are normally fitted to the outside of an air filter housing so that in the event of a backfire the door opens releasing pressure instead of the air filter housing exploding.
Not that fitting a mixer system to something with a weak (likely to explode) inlet manifold would be a good idea anyway but imagine if it were a blow back valve supposed to fit on the throttle body... It would further increase the chance of any backfire destroying one of the more important components on the vehicle, it would protect the air filter at the expense of increasing the risk of damaging the inlet manifold (or if the throttle were nearly closed it wouldn't do anything much because the throttle plate would likely make just as good a seal and the throttle plate wouldn't need to move to react to the pressure front). Much better that an air filter housing gets broken than an inlet manifold! Even on a carb vehicle with an inlet manifold that can normally handle backfires a functioning blow back valve might still see higher pressure in the inlet manifold than without it fitted.
If it is normally closed it would prevent the engine running on petrol or LPG if you fitted it at any point in inlet air routing. Also you wouldn't want those screws to come out and enter the engine
I think someone in Lithuania didn't understand the function of a blow-off valve designed to be fitted to an air filter... If it does turn out to be a blow back valve that's normally closed it hasn't come off a carb or LPG mixer equipped vehicle (not one that ran properly with it fitted anyway, and it would probably severely restrict the performance of a fuel injected vehicle)... I wouldn't fit that in the way/position it seems to have been adapted to be fitted. Even if it could be made to work as a blow back valve if set at a certain O'clock, the Landrover might be expected to run at various angles of O'clock off-road and any vehicle sees G forces that would effectively affect O'clock when cornering / braking.
Mick at LPGAutosupplies may still be able to sell you a blow off valve to fit into an air filter housing, may need modifying to suit a Landrover air filter housing.
You certainly won't be the last to fit a single point mixer system in the UK, every year I get enquiries for converting carb equipped vehicles and still do the odd one. There's a lot of classic cars and 90's campervan (such as carb equipped Peugeot/Talbot 2L campers) out there.
There are other oddity type components to mixer systems too, such as 'flap openers'. Bosch K-Jetronic (mechanical fuel injection systems) use a flapper in the intake air stream, as the flapper deflects with increasing intake airflow it progressively opens internal channels in the fuel injection air/fuel metering block to allow more fuel to the continuous flow fuel injectors. The system worked great on early (early 80s etc) fuel injected vehicles such as Mercs and Ford 2.8i engines and could even be made closed loop (with KE-Jetronic)... But the flapper was a restriction in air flow that messed up fuelling on an LPG mixer system and generally caused it to run too rich because the restriction caused a bit of a vacuum in the air ducting before the mixer. One solution was the 'flap opener', a device that used LPG pressure from the entry point of gas to the LPG reducer to move (kind of hydraulically / pneumatically) a rod that kept the K-Jet air metering flap open when running on LPG. Gas pressure whenever it was running on LPG because then the reducer would be getting gas pressure, automatically releasing the flapper to free movement to again respond to airflow when the reducer didn't have any gas pressure feeding it. If you fit a normally closed door in the intake stream then you're effectively building in a problem that the K-Jet flap opener was designed to overcome, worse because the normally closed flap won't be as likely to open anything like as linearly to incoming airflow as the K-Jet metering system.