nitro carburetor size question

[ebola]

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

 

[patprimmer]

Is that 50% nitro in methanol or in petrol.

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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[JWaterstreet]

30 years ago I used to race enduro karts, and used varying fuels. Usually ran 50/50 petrol/Methanol, and didn't need different jets vs straight petrol as the carbs had large jets to start with and needles to adjust for the shift. We started experimenting with Nitromethane, and to use a 25%/75% nitro/alky mix, we had to use a larger jet size, but don't remember what we used. We couldn't get the engines to run on a 50/50 mix of nitro/alky. These were small 100cc two strokes. We tried even larger jets, but couldn't get them to run right. As it was, they needed to be started with a power pack that would spin them at high rpm to get them to light off.

 

[ebola]

It is methanol and nitro 50%.I know pretty much, what Im going to face theory-wise according to fuel properties, safety issues etc.Motor is built with high volume oil pump, block girdle,block filled to the deck, and tolerances to accept 50W oil.Ignition is Vertex race magneto modified by Spud Miller at fuelinjection enterprises, red malory coil and tested for highest output.
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.

 

[patprimmer]

You won't find better advice than Jim Harvey or Spud Miller re nitro tuning.

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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[ebola]

thanks patrpimmer, for ideas.

 

[franzh]

At a swap meet in the early 80's, I bought a set (6) of Stromberg carburetors that were originally on a nitro car from the early 60's. When I got home, I opened one of them just to see what was inside. Oops.

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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[BillClark]

i would go with mechanical fuel injection as there are plenty of guys that can get you set up safe (relative). consider that if you get it wrong even NA you will probabley be picking bearing material off of the crank journals (or worse). We run a top alcohol funny car and although I have no direct experince w/nitro I have seen some really bad stuff happen to a-fuel cars in the pits when nitro was allowed to acumulate in a cylinder (cylinder head blown off block when turning engine by hand with mag on)so please be careful.

 

[JWaterstreet]

The primary reason top fuel engines blow apart is not from the explosive nature of nitromethane, rather it is because they are always close to hydrolock from so much fuel. Nitro supplies it's own oxygen, so the traditional concept of stoich goes out the window; i.e. you can continue to go almost to zero air...