100 hp KZ650

Some time back someone posted that a 100 hp KZ650 was possible. I doubted it but, as they say in Washington, mistakes were made but not by me!

Actually, I learned that it is possible to make a Kawi 650 produce 100 hp. Here's how.

Take a stock KZ650 and replace the cylinder with one from a KZ750. Have it bored out to 810cc and install the Wiseco piston kit. Then acquire a GPz750 head. It all bolts right on the KZ650 cases. Have the head ported and add hotter cams if you like. Don't twist it over 11,700 rpm and you'll get at least a season out of this motor that will say "652cc" but be putting out an ungodly amount of hp!

By the way, this will not work for a GS650 with a GS750 cylinder and heads. The GS750 is a good motor but it doesn't make the hp of a GPz750.

There's just a bit more to it than that.

The only 650's that a 83-85 GPz750 head will bolt on to with no modifications are the 81 and later models.

And the 83-85 GPz750 heads are the ones you want to use.

People will tell you that the only differences between the KZ750 heads and the GPz750 heads are that the GPz750 head has different fin shape, no tach drive, and minor porting, in fact I have said so many times. But after working on both a KZ750 head and a 83-85 GPz750 head all afternoon, I can tell you for a fact that that is not true.

The combustion chamber is different on the 83-85 GPz750 heads, it has a "bathtub" shape similar to that of the 83-84 GPz1100, vs the round hemispherical shape of the KZ750 heads.

I don't know why I have never seen this mentioned anywhere before, surely I'm not the first person to notice this.

I don't have any GPz750 heads prior to 83, so I don't know what combustion chamber shape they have, I have only compared 81 and 82 KZ750 heads against two 83-85 GPz750 heads (and I don't know exactly what years they are, just that they are 83-85).

Oh yeah, the KZ750 heads have provisions for the cam chain roller of the earlier 650's, and the holes are drilled and tapped, the 83-85 GPz750 lacks the holes, but there are places there to drill and tap.

Wondering what kind of response this post will get

I wish I could have found one of those GPZ750 heads. I didn't have any luck so I just ended up with a KZ750 head.

Do you know that the combustion chamber cc's are between the KZ and GPZ head?

Not yet, but I'm thinking about cc'ing both (or maybe 3) heads.

I know what it says in the GPz750 FAQ about combustion chamber volume on the various 750 heads, but I won't believe it until I check for myself.

There's some info on getting more HP on a 650 here:
90 horse outa a 79 650??

There is a lot of mistaken info in that thread.

KZ900/1000/1100 carb spacing is the same as the 650/750.

650 cases don't have to be bored to let the pistons clear the crank, they need to be cleaned up to allow the sleeves to slide in (except on 81 and later 650's, they slide right in).

650 points set up will work on the GPz motor, if you are so inclined, but so will the KZ750 electronic ignition with mechanical advance (that's what we put on the GPz750 motor in the purple bike).

All KZ750 fours have a HyVo cam chain, including the 1980 model.

At least the 80 and 81 KZ750 cylinder blocks and heads have provisions for the roller cam chain of the pre 81 KZ650. Don't know about the 82 because I haven't had an 82 motor yet, but it "probably" does.


It's late and that's all I can remember at the moment.

Still thinking about CC'ing those heads.

I'd say there are mistakes in that thread. My XLS (1200cc w/ 10:1, step tuned pipe, 6700 rev limit, crane hi-4, etc.) has never had trouble walking all over kz's of all kinds. My brothers kz1000 will keep up till about 60, but his carb jetting is a little out of wack.

I've even smoked V-rods with this motor. But, the first time I rode my buddies kZ650 years ago, I was thoroughly impressed with the agility of the bike.

Is there a lot of interchangability with a KZ550 four?

I dont know what the 82 heads will actually cc but the 82 FSM shows 24.9 for the std 750 and 24.9 + or - .04cc for the GPZ as minimums. Just showing what I read, no arguement needed :laugh:

I wish I lived near you, Id invest some cash with my 82 GPZ 750 against your Sportster any time

mtkawboy wrote:

I dont know what the 82 heads will actually cc but the 82 FSM shows 24.9 for the std 750 and 24.9 + or - .04cc for the GPZ as minimums. Just showing what I read, no arguement needed :laugh:

You got me, I've never had a 82 750 four head at all, so I don't have a clue

CB/GS/XS wrote:

Some time back someone posted that a 100 hp KZ650 was possible. I doubted it

Actually, about 200 HP is probably possible for a 650. The current Kawi race bikes are putting out about 5HP/cubic inch without turbocharging, I recall it's about 220HP for the 750's (45 CID). It's just a question of how much $$ you want to spend.

Theyve made a little progress in 600's in the last 30+ years. A turbo on a built motor is the only way an old one will ever see 200.