A very common 4bbl straight-propane system for a V8 engine is based on the Impco Model 425 carburetor and Model E converter. The Model 425 is a diaphragm-operated air-gas valve mixer design. This system works extremely well but has some limitations for high performance applications. The Model 425 carburetor is simply a Model 425 mixer mated to a 4bbl throttle body. The Model 425 bolts up directly to a square-bore throttle body and can be bolted up to a Quadrajet spread-bore throttle body with the Impco AA3-86 adapter. The Model 425 mixer is rated at 460 CFM @1.5" Hg. This rating means that this air flow will result in the air-gas valve reaching the top of its travel because it has generated13.8" WC (Water Column) of internal vacuum. Above this flow rating, the fuel mixture will gradually become leaner because the air-gas valve (or more simply, gas valve) cannot open any further. The Model 425 mixes fuel in direct proportion to air flow according to the shape of the cone of the gas valve. The only way to enrich the fuel mixture of a Model 425 mixer for more power is with the converter-mounted Impco VPV (Vacuum Power Valve), which adds more fuel when manifold vacuum drops below 3" Hg.
Another system used in the V8 applications in the past was based the OHG X-450 and OHG X-1 converter. Woodward also sold this mixer as the Streamline CA475 but due to a stoppage in production, this mixer has not been available for many years in either form. NGVINA still sells the OHG X-1 converter as the Streamline N-H420. The X-450/CA475 is a more complex mixer than the Model 425 as it also has the ability to vary the fuel mixture with manifold vacuum. The OHG X-450 can be mated to a 4bbl square-bore throttle body with a Gann A9-1 adapter or to Quadrajet throttle body with a Gann A9-7 adapter.
High performance gasoline carburetors often have much higher flow ratings than their actual airflow at their redline. This is because 4bbl carburetors are rated at 1.5" Hg of manifold depression, which means that the pressure drop (or the vacuum generated) across the carburetor at the rated airflow will raise a column of mercury 1.5" high. This also means that the pressure within the intake manifold is at this rated flow will be 1.5" Hg (or about 0.74 psi) less than atmospheric. Because volumetric efficiency (VE) is all about packing the combustion chamber with air and fuel molecules, keeping the pressure loss from the carburetor as low as possible will serve to increase volumetric efficiency by increasing the fuel mixture density in the combustion chamber. For example, in a 350 CID engine at 6000 RPM, the 100% VE air flow is 607.6 CFM. For this engine, a carburetor rated at 625 CFM @1.5" Hg, will have a pressure loss of about 1.418" Hg (0.696 psi) and a carb rated at 750 CFM will have a pressure loss of about 0.985" Hg (0.484 psi). The improvement in going from a carb rated at 607.6 CFM to one rated at 750 CFM will result in an increase in combustion chamber air-fuel density of 1.8%. Zero pressure loss at the carburetor (i.e., manifold pressure = atmospheric pressure) would increase the air-fuel density in the intake manifold by 5.3%, which the reason that oversized carburetors improve performance.
Assuming our 350 CID engine has a volumetric efficiency of 85%, the 460 CFM Model 425 will reach its rated flow at 5344 RPM. If it's spun to 6000 RPM, it will flow 516.5 CFM (with 85% VE) and have a pressure loss of 1.891" Hg. Similarly, the Model 425 on a 460 CID engine will reach its rated flow at 4066 RPM, assuming 85% VE. For general use, the Model 425 carburetor will perform well and very reliably. In any case, power will quickly fall off above the rated flow of the mixer as fuel mixtures gradually become leaner.
The great thing about using a propane carburetor is that the hot rodder who enjoys driving his car won't have to worry about parking his car on account of high gasoline prices. He gets a high octane fuel (104 octane rating for HD-5 Propane) that costs significantly less to use per mile than 93 octane gasoline with a minimal impact on power (about 5%). For those in northern climates, propane motor fuel does not become stale over the winter (or even over several years) so there is no need to add fuel stabilizer when the car is taken of the road. For cruising and car shows, using a propane carburetor will pretty much guarantee that you have the most unique vehicle.
To get the maximum power out of an engine, the density of the air-fuel mixture in the combustion chamber needs to be maximized. One way of doing this is to use a large carburetor with a small pressure drop at wide open throttle, as is commonly done with 4bbl gasoline carburetors. Impco-style propane mixers are more restrictive because of the nature of the gas valve that meters propane into the air flow. While it's possible to use an oversized mixer on smaller engines, the largest mixer generates 1.5" Hg of pressure drop at 460 CFM. The lower the manifold vacuum, the higher the fuel mixture density but using a oversized mixer runs the risk of poor low-speed operation and hard starting.
It is possible use a pair of mixers and Impco has a dual Model 425 mixer adapter to use with a single square-body throttle body (PN AA3-80) but this adds 5" of the height between the throttle body and mixers. Unfortunately, the air cleaner assembly for this setup is obsolete and you would have to fabricate one or find an aftermarket assembly. The AA3-80 adapter can also be fitted with adapters to use other mixers.
Megajoltman on the Moparts forum posted these photos of his dual OHG X-450 mixer setup mounted on a big block Chevy engine.
This is mono-fuel system using a pair of Model 425 mixers and a dual mixer adapter that the vehicle owner (Dodgen1) fabricated from a pair of AA2-46 adapters for his 440 powered 1975 Dodge W100 4X4. See My Toy for more information.
Technocarb's Off-Road Carburetor is now obsolete and this page is for historical purposes. Do NOT contact Technocarb about buying this carburetor.
The alternative to a diaphragm-operated air-gas valve carburetor is a venturi-design carburetor. In this case, there is no restrictive gas valve to reduce the flow capability of carburetor's venturis. Technocarb made the Off-Road carburetor, which is a 4bbl venturi-style carburetor suitable for high performance applications and has an integral throttle-body using a square-bore Holley flange. To suit a variety of engine sizes, the carburetor incorporates replaceable, pressed-in venturis. At this time, the following sizes (internal diameter at the vena contracta) and approximate flows with 4 venturis. Technocarb has not flow-tested their carburetors themselves so the following ratings are estimates.
- 0.75" (400 CFM)
- 0.91" (575 CFM)
- 1.00" (675 CFM)
- 1.10" (810 CFM)
In contrast to the Impco 4bbl LPG carburetor, the Off-Road carburetor does not become leaner above its flow rating. Because of the nature of venturi carburetion, the fuel mixture actually starts off a bit lean at low flows and gradually becomes richer as air flow increases. With the Open-Loop system (i.e., no feedback fuel mixture control with an O2 sensor), the full load fuel mixture is set with a load-block valve in the hose between the carburetor and converter. A nice feature of the Off-Road carburetor is its ability to use different-sized venturis in the primary and secondary barrels - in effect becoming a spread-bore carburetor. This allows the carburetor to have small primary venturis for easy starting and low-RPM torque while having larger secondary venturis for more high-RPM power.
Going back to our 350 CID engine with 100% VE at 6000 RPM, 4 x 1.00" venturis would have a pressure loss of about 1.216" Hg and 4 x 1.10" venturis would have a pressure loss of about 0.845" Hg. Without a gas valve to limit fuel flow, the engine may be run up to any redline without a lean-out of fuel mixture.
See Technocarb 4bbl Carb Kits for more information.