boring cases 650

PS- I don't know if the kz750 has bigger valves or not? If so, it makes the picture look worse, for 12 hp.

From The Motorbike Archives-Kawasaki KZ750 Four (1980)

No magic was required to achieve the transformation of the KZ650 engine. Bore size swelled 4mm, while the compression ratio shrunk a half-point to a mild 9.0:1. Close attention to port shape and an increase in valve size—lmm for inlet valves, 2mm for exhaust valves—helped maximize the potential of the 750's 34mm Keihin constant-velocity carburetors. Kawasaki engineers believe that you need look no further than the increased displacement, larger carburetors and less-restrictive mufflers to account for the 750's horsepower bonanza.

According to the GPz750 FAQ, the GPz and KZ 750 have the same valve size. The '83 and '84 have a ported head from the factory. The specs are:

1983 GPz750 A1

---

66.0mm x 54.00mm (bore & stroke)

9.5:1 compression

86 hp @ 9,500 rpm's
50ftlb torque @ 7,500 rpm's

286* intake duration (38*/68*)
105* l/c

286* exhaust duration (68*/38*)
105* l/c

105* LSA

76* overlap

34mm Carbs

Could you assume that a 810 kit would be an increase of ~10% displacement leading to a ~10% increase in power (with pipe and jetting)? ~95hp for the GPz (at the crank)???

cafekz750 wrote:

PS- I don't know if the kz750 has bigger valves or not? If so, it makes the picture look worse, for 12 hp.

From The Motorbike Archives-Kawasaki KZ750 Four (1980)

No magic was required to achieve the transformation of the KZ650 engine. Bore size swelled 4mm, while the compression ratio shrunk a half-point to a mild 9.0:1. Close attention to port shape and an increase in valve size—lmm for inlet valves, 2mm for exhaust valves—helped maximize the potential of the 750's 34mm Keihin constant-velocity carburetors. Kawasaki engineers believe that you need look no further than the increased displacement, larger carburetors and less-restrictive mufflers to account for the 750's horsepower bonanza.

According to the GPz750 FAQ, the GPz and KZ 750 have the same valve size. The '83 and '84 have a ported head from the factory. The specs are:

1983 GPz750 A1

---

66.0mm x 54.00mm (bore & stroke)

9.5:1 compression

86 hp @ 9,500 rpm's
50ftlb torque @ 7,500 rpm's

286* intake duration (38*/68*)
105* l/c

286* exhaust duration (68*/38*)
105* l/c

105* LSA

76* overlap

34mm Carbs

Could you assume that a 810 kit would be an increase of ~10% displacement leading to a ~10% increase in power (with pipe and jetting)? ~95hp for the GPz (at the crank)???

Yes, I would think so. I just don't know if your on a level playing field. There are couple more things that are worth looking at.

Are the transmission gear ratios the same? How will I need to gear the bike, for my use?

KZ750 had a extra 1,000 rpm's. Where do you loose that 1,000 rpm's, that you are gaining?

I would worry most about the carbs. They are smog carbs with alot of limits. You may not be running a 810 as lean as a 750.

I read the articale that you linked to this post. Here is my answer to cam degreeing. Take an extra valve cover and cutaway part of the top. You will be able to degree the cams with the chain tight. That will take away the cam timing, guess work.

Some times you just don't know, until you give it a try. I think the 750 cyl. and the GPZ head would be a good platform to start a real build from. The carbs & cams are a way to get by, but not your best performance items.
Most of the time I don't get involed in 650 stuff, I mainly do 900/1000. Last week after coming to my concludions on kz650's and how they could run with sportbikes. I would use this platform (kz750 bore size) plus some real race mods as a weapon. I think the biggest thing is the cost, of the kz750/gpz extra stuff. Please remember, I am not talking about a daily driver for myself. People throw around 4mm over bore, like it's nothing, but it is..............

kzz1p wrote:

cafekz750 wrote:

Could you assume that a 810 kit would be an increase of ~10% displacement leading to a ~10% increase in power (with pipe and jetting)? ~95hp for the GPz (at the crank)???

Yes, I would think so. I just don't know if your on a level playing field. There are couple more things that are worth looking at.

I agree; a KZ650 is different from a GPz750 with a 810 kit, pipe and jetted.

Are the transmission gear ratios the same? How will I need to gear the bike, for my use?

The KZ750 and GPz750 have the same transmission, the KZ650 is geared slightly lower.

KZ750 had a extra 1,000 rpm's. Where do you loose that 1,000 rpm's, that you are gaining?

To gain the power, you are moving the power band up.
The KZ650 has 64HP@8500rpm and 42ftlb@7000rpm
The KZ750 has 74HP@9000rpm and 46ftlb@7500rpm
The GPz750 has 86HP@9500rpm and 50ftlb@7500rpm

I would worry most about the carbs. They are smog carbs with alot of limits. You may not be running a 810 as lean as a 750.

The KZ650 has slide Mikunis, the KZ750 has CV34 Keihins and the GPz750 has BS34 Mikunis. I know that there is a member (JR) with an 810 kit in his KZ750 and he is using the Keihins.

I read the article that you linked to this post. Here is my answer to cam degreeing. Take an extra valve cover and cutaway part of the top. You will be able to degree the cams with the chain tight. That will take away the cam timing, guess work.

That is a great idea. jimmybon did that to degree his cams in his Z650. Here is the link: cam timing tools/numbers

Some times you just don't know, until you give it a try. I think the 750 cyl. and the GPZ head would be a good platform to start a real build from. The carbs & cams are a way to get by, but not your best performance items.
Most of the time I don't get involed in 650 stuff, I mainly do 900/1000. Last week after coming to my concludions on kz650's and how they could run with sportbikes. I would use this platform (kz750 bore size) plus some real race mods as a weapon. I think the biggest thing is the cost, of the kz750/gpz extra stuff. Please remember, I am not talking about a daily driver for myself. People throw around 4mm over bore, like it's nothing, but it is..............

First of all, I am enjoying this exchange. I am still running my KZ750 as a 738cc with Keihin carbs that are jetted with a 4-1 pipe. I think the 750 has plenty of power for my needs at this point. I prefer handling over straight line performance. If you have taken a stock 750 into a corner and wind out 3rd gear to 10000rpms and decide to shift or back off the throttle; this really highlights where I think the scoot needs more help. With the chassis. Perhaps, with more power a person would spend less time at the limit, but is it fun to go slow on a fast bike or to go fast on a slow bike?:laugh:

My main reason for doing all of this is that my cylinders need to be bored and honed anyways, and stock oversize rings and pistons will cost nearly as much as the Wiseco ones. Plus I want more power, the 650 is great in the turns but lacking on the launch out of them.