The engine of all Virago motorcycles is constructed in V-twin form and has no water cooling but has an air cooling. For cooling of engine an important role has the engine oil which, besides its lubrication function, has important role of engine cooling too. Therefore the oil quantity in Virago engines is greater than oil quantity in other similar motorcycles with water cooling.
The air cooling for such engines represent a big technical challenge because huge temperature has to be somehow dissipated. Because of its physical position for air cooling (it is partially hidden behind first cylinder), the rear cylinder is in worse position. As one method for alleviating the problem of lower dissipation Yamaha displaced rear cylinder aside in order to be more exposed to air and achieve better cooling. Second way to achieve better cooling is to use richer mixture of fuel and air (that's why front and rear carburetors have different main jets and jet needles).
V-twin
Yamaha Virago engines do have two cylinders (V-twin) with angle between them:- 60° for models xv125 and xv250,
- 70° for models xv400 and xv500/535,
- 75° for models xv700/750 and xv920/1000/1100.
As more powerful the engine is as bigger the angle between cylinders has to be in order to achieve better cooling. Also bigger angle makes more space for placement of bigger carburetor between cylinders. Smaller Virago models xv125 and xv250 do have the carb Mikuni BDS26 with diameter of 26 mm, middle Virago models xv400, xv500/535 do have carb Mikuni BDS34 with diameter 34 mm and biggest Virago models xv700/750, xv920, xv1000/1100 do have carb Hitachi HSC40 or later Mikuni BST40 both with diameter of 40 mm.
Having bigger angle between cylinders a better engine vibration compensation and lower bike's center of mass is achieved. That is important characteristic for cruiser type of bikes like Yamaha Virago.
On Yamaha Virago motorcycles every cylinder has one intake and one exhaust valve. Opening and closing of these valves are managed by the same camshaft.
The synchronisation between camshaft and crankshaft is achieved using timing chain and that is of crucial importance for engine operation. Since valves do control input of mixture into and exit of gases out of cylinders, they have to open and close themselves in exact moment in relation to position of pistons in cylinders.
The synchronisation between camshaft and crankshaft is achieved using timing chain and that is of crucial importance for engine operation. Since valves do control input of mixture into and exit of gases out of cylinders, they have to open and close themselves in exact moment in relation to position of pistons in cylinders.
SOHC on Yamaha Virago
There are two different mechanisms for synchronization of valves with crankshaft:
Yamaha Virago engines do use OHC mechanism, more specifically they use SOHC (Single Over Head Camshaft) mechanism for synchronization of crankshaft and camshafts opening valves - one camshaft per cylinder residing above cylinder head. Another synchronization mechanism is DOHC (Double Over Head Camshaft) having two camshafts over cylinder head (one for intake and another for exhaust valves). The advantage of SOHC over DOHC engines is that its mechanical parts are simpler, they do have less moving parts, so they are easier for maintenance, but in the same time are less sophisticated and less efficient.
- OHV (Over Head Valve) where only valves and rocker arms reside over cylinder head and the camshaft resides near crankshaft and movement is transfered to rocker arms with rods
- OHC (Over Head Camshaft) where the camshaft with valves and rocker arms resides above cylinder head, and movement to cmashaft is achieverd with timing chain or belt
Yamaha Virago engines do use OHC mechanism, more specifically they use SOHC (Single Over Head Camshaft) mechanism for synchronization of crankshaft and camshafts opening valves - one camshaft per cylinder residing above cylinder head. Another synchronization mechanism is DOHC (Double Over Head Camshaft) having two camshafts over cylinder head (one for intake and another for exhaust valves). The advantage of SOHC over DOHC engines is that its mechanical parts are simpler, they do have less moving parts, so they are easier for maintenance, but in the same time are less sophisticated and less efficient.
Comparison SOHC vs DOHC
SOHC on Yamaha xv535 Virago
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| Crankshaft and two camshafts on xv535 - SOHC |
The diagram depicts two camshafts (#1, #2) with sprockets (#8). Camshafts (front and rear one) are not identical! Each camshaft has its own profile and its own number is engraved on it corresponding to the cylinder this camshaft belongs to. At the bottom of diagram there is the crankshaft with axle which has smaller gears on both sides. These gears are used for transferring synchronization to camshafts over timing chains (#7). In order time chains to be always tense here are dampers (#12, #13) and tensioners (#9).
Note one interesting part oil baffle (#3) which resides on one cylinder only (rear one for xv535).
It's important to note that on xv535 tensioners tighten chains on front, and on xv750 model on rear side. That has an impact on ignition timing.
SOHC on Yamaha xv750 Virago
SOHC mechanism for xv750 is very similar to xv535 and has similar parts. |
| Crankshaft and two camshafts on xv750 - SOHC |
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