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:

KLR650 Engine CalculationsCurtain 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.
Bore 100 100 102  102
Stroke 83 83 83  83
Rod Ratio 1.66 1.66 1.66 1.66
Piston Speed at 7500rpm 68.077 ft/s 68.077 ft/s 68.077 ft/s 68.077 ft/s
Cubic Inch 39.726 39.726 39.726 41.801
Air displaced @ 7000 rpm 80.464 CFM 80.464 CFM 80.464 CFM 84.666
Compression Ratio 9.75 9.75 9.75  10.5:1
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
 Inlet Throat
 Exhaust Throat
 Port Velocity 318 ft/s 318/ft/s 331 ft/s 330.73 ft/s

TB = Throttle Body