KZ/GPz Engine Tech: important things to know

I had a problem with the 1136cc pistons rubbing on the 'squish' area when I put the '84 head on my '82 GPz1100 with the WISECO pistons, so we put in a .040 block spaces under the block, with t .020 gasket on each side before they were compressed, and it works fine - makes gobs of power with the '84 head and cams. I didn't mechanically figure out the compression, but it has a lot - I fill itm up at the premium pump, and do not try to lug it when it's hot.

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. 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.
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I've had no problem swapping a bathtub head onto my '81 GPz. And that's with it being milled .025.


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Nebr_Rex 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. 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.
!

I've had no problem swapping a bathtub head onto my '81 GPz. And that's with it being milled .025.


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Yep, I am definitely wrong on that one - I never considered using the stock KZ low compression pistons in a performance build. Always wondered who bought OEM standard and oversize pistons - now I know that some people obviously like using them.

For me, I start with 10.5:1 on anything I'm going to spend money on. Even the stock 83 GPz cams are better with 9.5:1 or higher to make best peak power. Cams with serious lift and valve overlap need squeeze to work. On the downside of high compression pistons and big cams is lack of clearances. In V-8 performance street / racing engines, our unofficial rule book recommended minimum 0.050" piston to valve and 0.050" piston to head clearance. We sometimes pushed that to 0.030" and 0.025" respectively on more serious engines if we had no choice with the cam lift / duration and CR, but some of those went bang / crunch / thud / scrap metal / start over again, too. Aluminum rods needed more ceiling room than steel rods.

Our local track #1 bracket bike is a GS1150 that uses 18:1 compression. He had his pistons custom made by J.E. Bike is 1500 and has run 8.30's in it's first season (normally aspirated). In it's second season, it was definitely "hurt" and only running 8.60's. We figure the engine has loosened up now and the pistons / valves / head are just starting to touch. I'll wager his pistons and valves have "witness" marks on them, likewise the head...the engine is up for sale if anyone is interested!

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.

Yep, crazy...

Hey Ed, do you know why it is called a bathtub? I can't see the resemblance...if you know, share the secret...

Oh, wanted to mention the nearest to an actual hemi combustion chamber I've ever seen outside of the exotic European stuff (Alfa Romeo, etc.), the SOHC 427 Ford from the mid - 60's. The chamber was completely machined long before CNC and very nearly a complete half dome. An awesome bit of engineering for it's time, given that it was basically a set of high tech (then) racing heads designed to go onto an existing engine block (the "FE" series) designed in the 50's (the first was 332 c.i.) and best suited to a farm tractor...or maybe a grain truck. Very tough engines, but a factory built 7000 RPM big block racing engine? No wonder they lost buckets of money on those.

Most of the Ford guys would give their right arm and first born to acquire a complete running "Cammer"... just to have it displayed on an engine stand in the garage. I held a "Le Mans" con rod and piston from one in my hand once, and saw a NOS bare head displayed at a swap meet once - cheap at $10 000. An old guy here (he is an ex Ford sponsored drag racer from that era) has one in a kit replica 427SC Cobra. It's dual carb and making about 750 hp, In a car that weighs about 2000 lbs., It moves like the automotive version of a squirrel...and the side pipe exhausts shakes windows and sets off car alarms 2 blocks away.

The bathtub nickname refers to the elongated part of the chamber where the valves sit.
The correct name for the design is Twin swirl.

zed1015 wrote: The bathtub nickname refers to the elongated part of the chamber where the valves sit.
The correct name for the design is Twin swirl.

Twin Swirl makes much more sense than "Bathtub"- but I guess Suzuki (TSCC) already had the rights to that one by then?

Thanks!

Also known as Active combustion chambers.
In this instance designed to produce a more efficient burn and maximise squish.

I started racing last year on KZ 1327cc bar bike and I want to start putting together one that is a little quicker so I figured I'd start with the heart and I have found a 84 GPZ 1100 head ... but I know there is a lot of different combinations so my question is when buying a set of cases or block and, crank, rods Pistons....what should I get the 80' and Below KZ or the J model ... The bike I'm running now is all 80 kz1000 stuff but I didn't plan on stealing parts from it anyways ...it will be N/A so id like to get the same 13.5to1 pistons but step it up to 14 or 1500cc
thanks for the help 

The squish area or "bath tub" shape on the combustion chamber is only useful with pistons where the dome goes right out to the edge of the piston (stock cast pistons). Pistons where the dome does not go out the the edge (stepped) will not require this squish area, in fact your giving up compression by having any shape on the combustion chamber edge using pistons with that top shape. That "squish" is extremely important on two stroke applications where the piston is pretty flat.