Some basic KLR650 engine calculations and specs will be covered here. In the case of the KLR650 the 685 big bore piston makes the engine a lot smoother but it does not in itself contribute to a major power increase.
KLR650 engine calculations vs 327 Chevy:
To calculate the amount of air an engine can move at 100% volumetric efficiency (VE): (Engine size x (RPM÷2)÷1728)
We use a 327 Chevy engine to compare to the KLR650 because there is only 1.149 cubic inch difference in size per cylinder. 40.875 vs 39.726 = 1.149
The KLR650 has a displacement of 39.726 cubic inch and delivers about 42 HP on the crank for an average of 1.05 HP per cubic inch. At 100 VE the KLR650 engine should move 80.464 CFM of air at 7000 rpm producing 0.522 HP/CFM.
A well build 327 Chevy engine with a set of Airflow Research Cylinder Heads will produce about 1.4 HP per cubic inch at 6800 RPM. Based on calculation the 327 Chevy at 6800 RPM produce about 57.75 HP on a 40.875 cubic inch cylinder whilst displacing about 80.42 CFM. That is 0.72 HP / CFM.
The KLR 650 cylinder head at +/- 325 CFM flows at 0.350″ of valve lift the same than the 327 Chevy’s Airflow Research Cylinder Heads does at 0.510″ of lift.
Calculate valve curtain area:
Curtain Area (defines the available flow area for any given valve diameter and lift): = valve diameter x 0.98 x 3.14 x valve lift
Curtain are for the 327 Chevy 2.08 inch inlet vales at 0.510 lift is 2.08 x 0.98 x 3.14 x 0.510 = 3.2643.
Curtain area for the KLR650’s 38 mm valves at .346″ lift is (1.496 x 0.98 x 3.14 x 0.346) x 2 =3.1857 about 2.5% less than that of the Chevy.
Again here is a marginal difference but not enough to warrant the 28% difference in power output.
Compression Ratio on the 327 Chevy is 11:1 compare to the compression ratio of the KLR at 9.5:1 a 15% difference
A 15% difference in compression ratio and a 2.5% difference in curtain area is still not enough to explain the difference in power output. That leaves rpm, timing and VE. Can the combination of timing and a higher octane fuel make up for the 15.75 HP difference? NO, but an increase in rpm will allow more air to move through the engine which in return would increase power output. Unfortunately there is not currently a camshaft, bucket and valve spring setup that will allow the KLR to safely increase rpm to 8 or 9000 rpm. Now what?
Sure timing control would be nice to have but unfortunately the KLR650 does not afford its owner that luxury. On top of that an increase in VE does not require a higher octane fuel and will allow the owner the continued use of 87 octane.
Port Volume can increase VE
Port volume for the 327 Chevy is 227-cc compare to the 128-cc of the Gen2 KLR650. Does that now mean that the ports on the KLR need to be hogged out? On the contrary, the author believe that the intake ports on the KLR650 is to big and needed to be reduced. By reducing the intake port, airspeed in the port will increase and thereby increase VE and produce a better cylinder fill. Better cylinder fill equals more power. Click to go to small port head mod.
|Gen 1 (651 cc)||Gen 2 651 CC||Small Port||685 cc EFI Project in progress.|
|Piston Speed at 7500rpm||68.077 ft/s||68.077 ft/s||68.077 ft/s||68.077 ft/s|
|Air displaced @ 7000 rpm||80.464 CFM||80.464 CFM||80.464 CFM||84.666|
|Combustion Chamber||64 cc||64 cc||64 cc||68 cc|
|Intake Port Volume||122.6||123.4||106||131.4|
|Head Gasket Thickness||0.012″||0.012″||0.012″||0.012″|
|Intake Valve Size
||38 mm||38 mm||38 mm||39 mm|
|Intake valve Lift||8.67||8.67||8.67||9.11|
|Opens||19 BTDS||19 BTDS|
|Close||69 ABDC||69 ABDC|
|Duration||268||268||268||235.75 @ 1 mm lift|
|Lobe Center Line||115||115||112||117|
|Exhaust Valve Size||33 mm||33 mm||33 mm||33.5 mm|
|Opens||57 BBDC||57 BBDC|
|Close||31 ATDC||37 ATDC|
|Duration||268||274||274||235.75 @ 1mm lift|
|Lobe Center Line||103||100||106||108|
|MAX Flow (+ carb +airbox)||283||283||322||NA|
|Max Flow (+TB + AirBox)||NA||NA||NA||400|
|Max Flow head only||322||312||342||440|
|Port Velocity||318 ft/s||318/ft/s||331 ft/s||330.73 ft/s|
TB = Throttle Body