nitro carburetor size question
nitro carburetor size question
(OP)
Hi,
Im building fancy watercooled vw nitro motor.It is basicaly stroker multivalve design.The principle idea ir to built up to 50% nitro naturaly aspirated angine, involving stone age management for fuel and spark, opposite to modern EFIs.I have done short motor, and have got points mag ignition from respectable builder, now approaching fuel system. The best for me would be carburetor fueling.But I know only harley people are running carb for nitro, usualy big s&s D supercarbs. The dilema is, I cant find a tip on proper carb size.Nitro is heavy fuel, and I need efective venturi and proper vacuum to draw enough of it atomised well.On the other hand, I need no choking restrictions for my small motor. So I need to calculate proper carb size.
I was thinking to built something like toilet injection before. But I found some info, and was suprised about it - that people manage to run nitro fuel on carburetor!
Motor will not rew high, in area to 6000 rpm max.It is like 212 cfm at max rpm draw.Head flows good.Cam is mild, 11:1 dynamic compression.
Im doing my best to gain my knowledge in carbs now.But I cant find good resource of info, how to calculate proper theory barrel/venturi size matching my cfm versus special gravity fuel.
Any tip would be very helpfull.
Best regards.
Andrius
Im building fancy watercooled vw nitro motor.It is basicaly stroker multivalve design.The principle idea ir to built up to 50% nitro naturaly aspirated angine, involving stone age management for fuel and spark, opposite to modern EFIs.I have done short motor, and have got points mag ignition from respectable builder, now approaching fuel system. The best for me would be carburetor fueling.But I know only harley people are running carb for nitro, usualy big s&s D supercarbs. The dilema is, I cant find a tip on proper carb size.Nitro is heavy fuel, and I need efective venturi and proper vacuum to draw enough of it atomised well.On the other hand, I need no choking restrictions for my small motor. So I need to calculate proper carb size.
I was thinking to built something like toilet injection before. But I found some info, and was suprised about it - that people manage to run nitro fuel on carburetor!
Motor will not rew high, in area to 6000 rpm max.It is like 212 cfm at max rpm draw.Head flows good.Cam is mild, 11:1 dynamic compression.
Im doing my best to gain my knowledge in carbs now.But I cant find good resource of info, how to calculate proper theory barrel/venturi size matching my cfm versus special gravity fuel.
Any tip would be very helpfull.
Best regards.
Andrius





RE: nitro carburetor size question
Nitro needs a a:f ratio of something like 1.8:1 vs methanol at about 6:1 and petrol at about 13:1 for maximum power.
I would use carbies of at least original size and probably a lot larger.
Your real problems will be fuel pump and carby fuel volume handling capacity and corrosive and toxic gas emissions.
Nitro needs to run so rich, that it seriously washes down bores and dilutes oil. It is also very hard to light, so you will need a much stronger than stock ignition and tight plug gaps.
Regards
Pat
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RE: nitro carburetor size question
RE: nitro carburetor size question
I can use either predator carb, also holley alky carb, but I dont want to be overcarbureted in size, because I dont have cc these carbs are built for.My motor will not make enough vacuum for those barrels I think, and heavy nitro fuel may not atomise good enough because of lack of velocity at big veturi diameters.It could work with blower draw thru maybe, but I want it to be N/A motor.Shall I give up idea on carb and go injection? Just take Harvey book and follow tables? Id like carb very much, but I dont know if it would work efectively for me.I dont find any info on internet on similar setup ever done, or any other guidance.
RE: nitro carburetor size question
Your problem will always be getting enough fuel flow without having to much air flow.
Can you get an alky version of the Holley 390 CFM with twin cathedral float chambers and the big SS needle and seat in each chamber.
If you could block of one side so that it became a twin rather than a 4 barrel, or if you could throttle down the chokes, you might get enough fuel flow. Maybe even make and install a plate that fits between the metering block and the body and diverts fuel from all fuel circuits to one side only and the other side is blocked off
Regards
Pat
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RE: nitro carburetor size question
RE: nitro carburetor size question
All the screw threads stripped, and the inside looked like putty that had hardened. The brass metering jets originally for gasoline were drilled out (no brass, the pewter carburetor body was drilled), plus the idle passages were opened too. I was told that this was common back then, give it as much fuel as it could handle, it would keep the engine cool.
Franz
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RE: nitro carburetor size question
RE: nitro carburetor size question