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Kawasaki KZ  900/1000 | 81-83 J1000 / 1100 |  83-85 GPz1100 Cylinder head information

Flow Data

KZ 900/1000



Gpz 1100 1983-1985

Stock CFM @10" Test Pressure - .500" lift




CFM / Sq. In  Valve  Area @10”




CFM / Sq.in Throat Area @10”




Intake Valve Size mm




Exhaust Valve Size mm




Intake Port Measurements /mm

Port Entrance




Runner Mid Point












Throat % Valve Diameter




Bowl % Valve Diameter




Cylinder head flow data is provided for each of the stock heads so you can see for yourself what the  performance differences and potential are.

Just for example, an 88CFM @ 10" of water head is capable of making 130 HP on an engine as small as 1045cc with the right cams and compression.

J Heads, in stock form, are not the big power makers that many people think they are. They require port work to bring out their potential, just like the KZ head does.

A late model [83-85] 2V GPZ head is a pretty good head in stock configuration. There would be a noticeable difference in performance using one of those heads on a J or on a KZ bottom end with the right pistons to correct the compression ratio, even without porting it.

The stock J Head would not provide the same power level as the stock, late model GPZ head.

A stock KZ head is rather restricted when it comes to making Horsepower. The runners have a nasty kink   that forms a double apex with the sort side radius. Air is turning down and also bending off to the side in two areas very close in proximity. While velocity in a stock KZ port is good [avg 300 FPS @ 28" of Test Pressure] right down the middle of the runner, the CFM capacity is relatively low. Proper porting will improve the CFM without killing the velocity. Improper porting will kill the velocity without raising the CFM.


KZ motors used 3 bolt cam sprockets while the J motors used 2 bolt cam sprockets.

It’s possible to use J model cams in KZ motors with modified KZ sprockets or use APE Sprockets designed specifically for this purpose.  They are available in both 30 and 32 teeth versions.

 When modifying stock sprockets, essentially the center of the sprockets need to be bored out to fit the larger boss of the J model cams. In addition you have to create 2 new mounting bolt holes and use allen head bolts with the allen head turned down slightly so it doesn’t touch the cam chain.

The only time it’s really worth pursuing that effort is to use 83-85 GPz1100 cams in a KZ motor. I’ve done that mod with 900 & 1015 motors over the years.

KZ 900 / 1000

83 - 85 GPz11

81 - 83 J Model

There are two different KZ1000 engines. The first is the Z1 type which was built between 1973 and 1980. The second is the 1000J, or "J model" as it is called, and was built from 1981 until present ( currently only available in police motorcycles). These engines are different, but some of the parts can be interchanged, and this has caused much confusion. We hope this makes everything clearer.

The first example is simply a big bore Z1 900. This engine came with a kick starter and electric starter. The cam chain tensioner hole in the cylinder block is a rectangle. The ccs cast on the cylinder is 1015. The later "J model" has no kick starter, and the cam chain tensioner hole in the cylinder block is round. The cc's cast on the cylinder is 998. This engine also became the GPZ1100 in 1981 and '82, and virtually everything between them is the same. (The '83-'84 GPZ1100 was changed). Cylinder blocks and lower engine cases are NOT interchangeable. You can not put a J model cylinder on an older style lower end, and vise versa.

Cylinder blocks.. Not interchangeable. If you have a J model block/GPZ1100 block you must use a J model or GPZ1100 cases.

Crankshafts... Early KZ1000 used a roller type cam chain and the crank has that style center sprocket. J model used Hy-vo style chain. J model cranks can be used in early lower end if the center crank pin is changed to one with a roller chain sprocket. The J model connecting rods are approx 1.2 mm shorter than early style rods.

Cylinder head.. J model head has larger valves and ports. This head can be used on earlier style cylinder and lower end by installing APE #CCIA cam chain sprocket adapter in head . How to tell which head you have.. Early style heads have four factory drilled and tapped 6mm holes in the center to bolt in cam chain idler gear. Z1 900 heads have 6 mm exhaust studs, KZ1000 8 mm. On the J model head, the four cam plugs at the ends of the camshaft areas are bolted in, on the early style head, they just push in and the valve cover holds them in. The J model / '81-'82 GPZ1100 cylinder head has a larger combustion chamber than the early style head. When these heads are used on the early style motor, a compression ratio drop of about 2 full points can be expected.

Pistons... All pistons have 17 mm pin holes, but the J model/ '81-'82 GPZ1100 pistons have the pin hole lower in the piston to make up for the short connecting rod. All J model and early '81-'82 GPZ1100 pistons are the same ( '83-'84 GPZ has 18 mm pins ) Therefore any '81-'82 GPZ piston can be fitted to a J model engine to make a big bore engine. Pistons for the J model / '81-'82 GPZ1100 can not be used in a motor that has an early style long connecting rods unless a spacer plate is used under the cylinder block to compensate. If early style KZ pistons are used in an engine that has the short J model / '81-'82 GPZ1100 connecting rods, a special short cylinder block has to be used.

We have tried to make this as clear as possible. When ordering your parts from APE, let us know which engine you have, or if you are mixing early style and late style parts, let our tech/order department know and we will see to it that you get the correct parts combination.

Differences between KZ & J Motors from APE's Website

Cavanaugh Racing Flow Information

Some of the  differences between  the 3  heads is as follows.