KZ/GPz Engine Tech: important things to know

For anyone reading this - I have done some research and this is what I have found.
The 83-84 GPz1100 head is not a hemi combustion chamber like the older GPz's and most KZ's. The 83-84 GPz1100, the 84 KZ1100R, and the 84-85 ZN1100LTD all use a similar cylinder head design that should not be bolted to any other KZ or GPz models without changing the pistons as the minimum.
In detail, here are the differences I know of.
The 83-84 GPz1100 head (and the 84 KZ1100R / 84-85 ZN1100LTD) are by far the most coveted stock heads for KZ style engines from a strictly racing performance perspective. Because of this they are rare and expensive. These are not “hemi” heads! These have the largest valves and ports of any stock head. Valves are 38 mm intake and 32.5 mm exhaust. The intake valve guide angle is unique to this head so Kawasaki could use the largest intake valve possible. The combustion chamber is sort of a “twin hemi” chamber – two semi spherical pockets, one per valve, with squish areas on both sides. You cannot put this head on a 81-up “J Model” or 72-80 Z1 style set of pistons (both are hemi style chambers) because the piston dome will hit the squish areas of the combustion chamber / head. Stock chamber volumes I’ve measured are 49-50 cm3, exactly the same as the 1981 and up “J-Model” hemi head; even though many incorrectly believe the late GPz chambers are much bigger than the J model and will drop compression ratios. Another important fact is that the 83-84 GPz, 84 KZ1100R, and 84-85 ZN1100LTD all use 18 mm diameter piston wrist pins (versus 17 mm for all other KZ900/1000/1100 pistons) – so swapping pistons to use the late GPz head typically means replacing the crankshaft also! The 83-84 GPz head was reputed to be “hand ported” at the factory by Kawasaki, but I have my doubts on this. I have examined several stock examples of these heads and found none that show any sign of port work done, and some have had fairly major casting imperfections in the intake runners that should have been caught and corrected if the heads had been examined or worked on by a factory technician supposedly hand porting the heads!
All other 1981 and newer heads, including the 81-82 GPz1100 and 81-83 KZ1000R Lawson Replica, are basically the same “J – Model” head first introduced on the 1981 KZ1000J. In my opinion, unless you are money racing in a class where lowest ET’s are all that matters, you don’t really need a late GPz head! If you are going to modify your head in the usual ways – bigger valves, porting, big cams, higher compression, etc, these heads will work as well in street / race performance applications. All the “J Model” heads are fundamentally alike in port and combustion chamber design and valve sizes. So it really doesn’t matter what it is from – LTD, J, CSR, Police bike, etc. Basically they are all the same with detail differences only. Some (CSR and maybe some police models) have provision for a tachometer cable drive off the exhaust cam like the old Z1’s. 81 GPz1100 J - heads are different than the 82 GPz1100 in the way the fuel injection and throttle bodies are designed. Although I have never seen one myself, the 81 GPz is reputed to be like no other KZ-J head because of this (EFI plumbing ports are cast integral with the head). Stock intake valves are 0.5 mm smaller than the late GPz. All have 49-50 cm3 chambers – same as the late GPz.
I believe (soon to be proven right or wrong) that a J head can be used with late GPz style pistons with no change in engine compression ratio (CR). Spark timing might be adjusted for the different combustion chamber shape. I am currently assembling a 1395 cm3 big – block engine I’m building this year for myself. I am using a twin plug / ported / big valve J head (thanks go to Larry Cavanaugh) with late (83-84) GPz style pistons. Even though the pistons are sold as 10.5:1 compression ratio, my measurements and calculations give an actual 12.3:1 CR, which is helpful with high lift / long duration cams – bonus! A buddy is building an otherwise identical engine but using a late GPz head with single spark plug. I’ll keep everyone posted on which bike goes faster down the ¼ mile later this summer!

The only change I made to the stock std. J pistons, was to open the valve pockets a little for extra clearance for the 84 GPZ cams in the ZN head...
Boy, do she fly! :woohoo:

Kray-Z wrote: For anyone reading this - I have done some research and this is what I have found.
The 83-84 GPz1100 head is not a hemi combustion chamber like the older GPz's and most KZ's.

For reference

daveo wrote: The only change I made to the stock std. J pistons, was to open the valve pockets a little for extra clearance for the 84 GPZ cams in the ZN head...
Boy, do she fly! :woohoo:

Hey daveo,

Thanks for that photo. Nice job fly cutting the stock pistons!

I never checked the stock, low compression, J pistons for clearance! But having said that, I still think the piston to head clearance must be extremely tight with those pistons and the late GPz bathtub head...

daveo wrote:

Kray-Z wrote: For anyone reading this - I have done some research and this is what I have found.
The 83-84 GPz1100 head is not a hemi combustion chamber like the older GPz's and most KZ's.

For reference

Just want to add that daveo's photo is the chamber of the late model "GPz" aka "Bathtub" head...note the squish bands on the sides of the chamber and the separate valve pockets...

Kray-Z wrote: Hey daveo,

Thanks for that photo. Nice job fly cutting the stock pistons!

I never checked the stock, low compression, J pistons for clearance! But having said that, I still think the piston to head clearance must be extremely tight with those pistons and the late GPz bathtub head...

An extremely accurate, high-tech fly cutting system :lol:



...and for assurance of critical clearances :lol: :whistle:

daveo wrote:

Kray-Z wrote: Hey daveo,

Thanks for that photo. Nice job fly cutting the stock pistons!

I never checked the stock, low compression, J pistons for clearance! But having said that, I still think the piston to head clearance must be extremely tight with those pistons and the late GPz bathtub head...

An extremely accurate, high-tech fly cutting system :lol:

...and for assurance of critical clearances :lol: :whistle:

Awesome! Love it! Plywood fixture and a drill press - truly a masterpiece of DIY engineering my friend!

Did you check the squish band area - piston to head clearance - with your modeling clay also?

Kray-Z wrote: For anyone reading this - I have done some research and this is what I have found.
The 83-84 GPz1100 head is not a hemi combustion chamber like the older GPz's and most KZ's.

Sorry to quibble, but all Kawasaki KZ and GPz bikes use a hemi chamber. The later version just have an added feature that's commonly denoted a "bathtub" shape. Still a hemi though. Rob Muzzy was known to modify early KZ heads to add the bathtub detail. Anyone know if his work influenced the factory in adopting this in production?

BTW, Suzuki added the bathtub shape when they made their 650 line of bikes, which was the last of the GS's to hit production. The 650 made just as much HP as the earlier 750 bikes, in part because the combustion was more efficient.

Nessism wrote:

Kray-Z wrote: For anyone reading this - I have done some research and this is what I have found.
The 83-84 GPz1100 head is not a hemi combustion chamber like the older GPz's and most KZ's.

Sorry to quibble, but all Kawasaki KZ and GPz bikes use a hemi chamber. The later version just have an added feature that's commonly denoted a "bathtub" shape. Still a hemi though.

Definition of "Hemi" (spherical - BTW) - half of a sphere, as in "Northern Hemisphere". Must be circular (spherical) in shape. No doubts - get a geometry text book and look it up.

In all trueness, not even the mighty 392 / 426 Chrysler Hemi was a true hemi chamber. But it was close - at least it was spherical in shape (except the valves and spark plug hole(s) - which would be pretty tough to do). The ford Boss 429 was a "semi - Hemi" as it had flat squish bands on the sides. But both had mostly spherical shaped chambers.

One guy I went to automotive / diesel / machine shop school with thought hemi meant it had the spark plug in the middle. He thought my CBX (pent-roof chamber) was a "Hemi". Sorry Dave - way wrong, friend.

By using the term "Hemi" I was referring to the Z1/Z1000 and J head' chambers being basically spherical in shape. The GPz Bathtub head chamber is definitely not spherical...I don't get why it is called a "bathtub", either... but whoever did that got to it way before me, so I'm going along with it...

Kray-Z wrote: Did you check the squish band area - piston to head clearance - with your modeling clay also?

I did not measure the clay other than at the valve pockets. The pics below provide a glimpse of my process, and may offer an answer your question. I took a lot of pictures, since I was geeked about the project, having such limited experience with such things.

Before


After


Close-up

Been doing some reading on what constitutes a hemispherical combustion chamber and had no idea how some people kicked out the bathtub people from the club. Interesting, but crazy.

Nessism wrote: Been doing some reading on what constitutes a hemispherical combustion chamber and had no idea how some people kicked out the bathtub people from the club. Interesting, but crazy.

Thanks Ed. Happy to be in the bathtub club...but glad to splash-about in both. :lol: