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QUESTION - Carburetors suck! But how much? And how much sucky do I need for my 4 cylinder Land Rover? REPLY - This is from Jim Allan, friend & all around swell guy who knows his way around both Land Rovers and writing. Subject: Performance Carbs for 2.25 To All Series Owners, Some of you may have seen the "Power Feed" article in the Summer Issue of LROI. A lot of good data was edited for space considerations so I thought you might like to see the missing parts. Print this one out and stick it into your magazine as a companion. Enjoy! Jim Allen Here's the part that tells you how to figure CFM requirements for any engine at any speed. CFM=3D rpm X displacement X volumetric efficiency Lets break the formula down. RPM: Usually the maximum rpm of the engine is used here but you can plug in any engine speed. DISPLACEMENT: In cubic inches, the size of the engine. 3456: This is a constant that converts the upper figure to CFM at 1.5" Hg. VOLUMETRIC EFFICIENCY (VE): This is the engine's theoretical ability to 128.6621 CFM=3D 4250 rpm X 139.5 cid X 0.75 If you compare this figure to the tested ratings of the stock carburetors, you'll find they match fairly closely. If you run one of these engines, you will also know from personal experience that the engine is definitely breathing hard above 3500 rpms. A slightly larger carb can enhance upper rpm performance. RE: Oil Bath Air Filters The original oil bath air cleaner has the theoretical airflow to keep up with the engine. In practice, I have felt a big gain in power by eliminating it. I was in a quandary on this question until Earl Davis, K&N's dyno guru, explained a possible reason. "It isn't just raw airflow capacity, " he said, "it's also velocity. If the air cleaner is slowing the velocity of the air, which could effect performance." Earl noted that the 90 degree elbow and corrugated hose might have a deleterious effect and recommended trying a smooth hose and an elbow with a shallower radius and/or moving the radius farther away from the carb by about 3-4 inches. Some owners have reported adapting the factory oil bath filter to the big Weber two-barrels and claim good results. I have to reserve judgment on this, having tested the combo with very poor results. At 3800rpm the engine fell flat on its face due to lack of airflow. Mike Pierce reported similar results when he was developing his Weber 2-barrel kit. Obviously, these folks never rev the engine much above 3500. Some Test Data They Didn't Print To save your calculator some wear and tear, I've run the figures for a 2286 from 2000 to 5000 rpm. Compare these figures with the airflow tests of the various carbs, manifolds and air filters. 2286cc Theoretical Airflow Requirements
SINGLE BARREL CARBURETTERS
TWO-BARREL CARBURETTERS
AIR CLEANERS
INTAKE MANIFOLDS
Comment by Bill DavisIt is true that a carb with a smaller venturi will normally produce more torque at a lower/mid rpm range because the velocity of the air/fuel mixture thru the venturi will be higher and hence you get better fuel atomization and hence better combustion. I have always taken the Jim Allen test with a grain of of salt since it only involved one vehicle at sea level. It also didn't factor in things like performance on inclines and side angles or in other words things important for off roading.
Note on highway driving RPMs from TeriannJim did an excellent job pulling together all this data, but I personally think actual engine running RPM s should be taken into account as well. Bigger is not always better. The engine torque curve is very steep below max torque. I drove my 109 with a tach for about 15 years and developed a sense on how the engine behaves at different RPM. While the engine can run smoothly under load down to 800 RPM or so there is almost no power there. If you are climbing a hill and need to down shift but want to keep your speed as high as possible you want to down shift somewhere between about 2400 and 2500 RPM. If you are almost at the top of a hill and you are feeling lazy, you can let the RPM slide down to around 2200 before the power starts to drop off quickly and you really need to down shift. The engine seems happiest cruising in the 2800 to 3200 RPM range. I'm guessing best fuel economy around 2800 RPM on level roads under most normal driving conditions (The slower you are going the better the fuel economy). During the 1960's the factory gave a peak power 4250 RPM spec. But during the 1970's they only provided a 4000 RPM spec. Running a fairly fresh SIII 8:1 engine with Rochester carb I found 3800 RPM about as high a shift point as I wanted to use for maximum acceleration. Most of the time I up shifted at around 3500 RPM. I think that while the peak HP may be at 4250, the power curve is mostly flat above 3700 RPM and the added RPM doesn't give you anything to write home about. From my experience the most important part of the curve is between 2500 and 3500 RPM or between about 80 and 115 CFM The Solex is a very good carb for this RPM range but probably starts getting in the way above 3400 RPM. One of the other carbs can give you a tad more power at the high end of the shift curve or if you are cruising the engine at faster revs than the engine is happy at. But otherwise the extra flow really isn't noticeable. A Zenith works fine through out the engine's RPM range as does a Rochester. I think the advantage of the Rochester is that rebuild kits can be had anywhere cheap, fuel economy can be as good as the other carbs, they are rugged reliable carbs that are easy to work on and they are cheaper than the Solex or Zenith. |
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