1986 Update Porting
*The following article is reprinted thanks to Dyno- Tech
Sled Owner: Terry Paine
During the winter of 1983, Yamaha published what are commonly referred to as "update porting specs" as part of a performance improvement for the then new V- Max.
Besides a recommended increase in intake and exhaust port timing, the specs included a fairly complex transfer port modification. The two rear transfers are enlarged sideways, to the port that they nearly meet in the rear on the cylinder. This was a modification that was well beyond the capabilities of most do-it-yourselfer port grinders. Also included in the update was a moderate compression increase.
This particular engine was ported by Fast By Gast of Grand Island, N.Y. (716- 773- 1536) the premier builders of record setting two- stroke Kawasaki drag racing motorcycles.
When using the stock 38mm carbs, the stock air box (which uses hot under hood air) must be altered to work properly. There is a baffle inside the air box which should be removed unbolted) to allow the engine to breath. With this done, the carbs will flow more air, and less fuel, and the engine will make more horsepower. The only function of the air box at this point is to reduce intake noise somewhat, and provide a mounting surface for the ignition coil.
Regarding the carburetion on the update port V-Max, the stock 38mm carbs are all the engine requires. Larger carbs will not increase airflow or horsepower on this engine. This is similar to a Wildcat, which developed the same airflow and horsepower with either 38mm, 40mm, or 44mm carbs.
This V-Max, with the early 1981 CDI and stator, Was tested with the engine in Chassis and the hood closed, on a60 degree dry day. The Carb Air Temperature, however, was 25+ degrees higher during our ten second acceleration test due to the inefficient under hood air intake
VP C10 unleaded 100 octane gasoline was used, premixed at 32:1. Why premix? The V-Max is plagued by Yamaha's absurd method of injecting the oil into the fuel line before the fuel pumps.
All test are run under the following conditions:
|310- 290 MJ 130 PJ|
|Data for 29.992 inches of HG 60 F dry air|
|Test: 100 RPM/sec Acceleration|
|Fuel Specific Gravity: .750|
|Vapor Pressure: 1.00|
With stock pipes, 310-290 main jets, and 130 power jets installed (winter safe spec), we achieved the following results.
Retaining the stock pipes and 310-290 mains, we disconnected the power jets to clean up the carburetion for our mid eighties F CAT. Interpolating the following data, we can see the power peak is at around 8400 RPM.
We installed our PSI twin pipes on the update ported V-Max with the following results.
Aaen twin pipes were tested next, and here are the numbers the generated.
Installing a set of Bender Racing's shortened stock pipes, (which they recommend only for F111 style porting) raised the power peak to around 8800 RPM, with a loss of torque and horsepower resulting. Terry had tried these higher RPM pipes in the field last season, before our dyno analysis, and the sled was actually faster. Most likely, Terry's original clutch setup had been wrong (I helped him with it), allowing the engine to overrev beyond the stock pipes' greater power peak. The shorter, higher revving, cut stock pipes, though producing lower power, had made Terry's wrong clutch setup work better. Here are the dyno results.
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