Gpz750 pistons with a kz750e head?

As the title of my thread states, I'm wanting to put the higher compression pistons from the gpz750 (or the Spectre, zr750, zr-7s...etc ) into my '81 kz750e. But I am concerned that the taller piston domes will interfere with the smaller kz combustion chamber. There is another thread with almost the same heading but asking the reverse. I didn't want to hijack that thread, so I decided to create a new one.
I have tried to research this as much as possible, but i'm at an impasse and i'm afraid of buying pistons that may not work, I decided to see if anyone can shed some light on this for me.

What I "know":
1: The kz750e piston uses 9.0:1 pistons and the gpz's use a 9.5:1 with a slightly raised dome that is obviously taller but also starts closer to the edge of the piston in relation to the kz.



2: The 9.5:1 piston heads all have a squish band integrated into the combustion chamber. (Among other improvements)



3. I'm aware of many folks using Wiseco 810 10.25:1 pistons with what I assume are taller domes in kz750's. But there is virtually no mention of squish band measurements or interference issues.

4. I know to put in the the corresponding 83-86 gpz750 cams in conjunction with the higher compression pistons for the full power potential. I'm also aware that the gpz exhaust cams have a smaller base circle and may not fit in the kz head without trimming valve stems...but I am still researching the benefit of running mixed gpz intake and kz exhaust cams for better mid-range torque. But that can wait.

A summary of my questions:
I'm simply unsure if the gpz squish band is machined to directly match the curvature of the 9.5:1 piston?
If so, will the taller dome of the 9.5:1 piston make contact with outer edges of the the smaller kz combustion chamber?
How does the Wiseco 810 piston dome compare to that of the stock 9.5:1? (Is it taller, but narrower...are they identical?)
Does the higher compression ratio of the Wiseco come from the dome of the piston or the +3mm of bore enlargement?

I'm super confused at this point and would like to plan the direction of my engine rebuild, so if anyone can help I would be grateful!

Thanks in advance!

Following

You may want to reconsider what you're trying to do. It seems like you're going to a lot of effort and expense to gain about 1 hp. :S

The KZ750E came stock with 74 hp. Using the calculator in the link below, bumping the compression from 9.0 to 9.5 will increase the horsepower from the stock 74 up to 75.1. That 1.1 hp increase isn't going to be noticeable at all. You probably could gain more power by simply performing a careful tuneup on the stock engine. Ed

www.wallaceracing.com/hp-cr-chg.php

I have to agree with Ed this seems a lot of brain strain and effort for the likely outcome.

While there may be some difference in on paper performance, I doubt in reality there's a huge difference in performance between your bike and a GPz750 any way - the difference is small enough to be negated by the differences between individual bikes i.e. a particularly quick and well tuned KZ750, could have the edge over the average GPz already. There is in theory 4 more HP at 9,500 rpm as opposed to 9,000 rpm. Other than the piston change, everything else will most likely just push the power higher up the rpm range, with a slight loss of bottom and mid range, which is more desirable on the road.

I have a set of GPz pistons and some other parts and thought about making my KZ650 into a 750 using another 650 motor I have and some of the GPZ750 parts, in the end I decided against it. If you really want more power perhaps the 810 kit is the way to go, I'm still thinking about that route if I ever decide to go big on my KZ650 motor, although there's more to it on the 650 than on a 750.

Are there aftermarket cam options for the KZ750?, it seems if there were some "drop in" cams like you can get for the KZ1000's that would be any easy route to scratch your itch for more than stock power, but given these engines were fairly high performance any way, you are not going to see much of a power increase without a serious rpm increase, which will likely make it less pleasant to ride, personally I don't feel the inclination to constantly red line these old bikes. Looking at the power the 2000 ZR7 produced is likely a clue as to what could reasonably be got out of these motors as it's basically the same HP, and that's some 20 years later, I'm sure Kawasaki would have squeezed some extra out if viable,

If you are thinking of combining the swap with a motor rebuild because your existing motor is sub par, then I'd say get the whole head and cams from the GPz, ZR7 or whatever you need, don't mix and match or perhaps just get a good whole motor.

These are all good points!
I failed to elaborate on my current engine and the plans I have for it, so I should fill in the blanks...

My current engine is a bone stock 1981 kz750e motor with 25,000 miles- 74bhp
Compression test shows 150psi on all cylinders and leakdown shows 10%
Leaking Upper oil seals at the head gasket are forcing me to dig into the topend.

Other 750 variations and options to choose from:
1982 gpz750r1 - 80hp from factory ported and polished heads with squish band, different cam profile, 9.5:1 pistons and freer flowing carbs.
83-86 gpz750 (zx750a) is 84bhp due to better designed head with squish band, .5mm countersunk exhaust valve to promote better intake flow (hence the smaller base circle on the exhaust cam)and no longer ported from factory, redesigned hotter cams, 9.5:1 pistons and freer flowing CV carbs.
Zephyr zr750 and zr-7s - 76bhp using same cams as the 1982 gpz750, Squish band head (without the fancy exhaust valve chamber or porting) 9.5:1 pistons, cvk32 carbs

Hopes for my engine;
I'm aiming for the some of the added power of the gpz750 but with the torque being lower down in the power range to make it more usable....
My cylinder head is very good condition other than needing a valve job I suspect,
so I"m trying to do any number of things to increase power without having to buy a used GPZ head.

What I have planned:
Fully rebuild head
cvk32 carbs from zr750 (already purchased and rebuilt...just waiting on how to jet them)
4-1 exhaust (already purchased Kerker)
Oil cooler ( already setup with zr-7 oil pan and zr550 oil cooler)
Higher volume zr-7 oil pump (still not sure if this is necessary)
zr-7 starter clutch
Updated CDI Ignition ( already have complete CDI ignition system from zr750 including pickups and coils already integrated into wiring harness)
zx750 Intake cam ( still not sure if mixing gpz intake cam with the stock exhaust cam will move the torque lower down in the power band like I want)
810 kit and/or zx750 pistons ......which brings us here

And will eventually go into this:


Hopefully that clears up my intentions....or complicates everything...I don't know anymore

I think if I was in the position of rebuilding a tired motor I'd go with the 810 kit, if I was planning for torque. Then I might consider the head/cam route if I was really looking for all out power - I think I'd settle for just the 810 kit, although I've never seen figures for the projected power increase. perhaps 10 hp with about 60cc more capacity and the higher compression? Perhaps someone running that set up can chime in. If you could get an extra 10hp somehow at about 9k rpm that would be a significant and noticeable increase.

Looks like you have the basis of a nice setup with that rolling frame.

Good thread, I'm following this too.

Recently I sold a zx750 head and cams with the 'bathtub' squish chamber, the (UK) guy was quite serious about taking it to Bonneville salt flats. I warned him about the base circle issue onthe later cams but he said there are ways around this...

When in doubt you need to get the blu tack or modelling clay on top of the piston and physically check things. Time consuming I know, but theonly way. Agreed above . cams and head flow matter more than CR.

Is that a KZ1000 MKII tank? Bike looks really good. Back in 1980, right after the KZ750E/H models first came out, we had issues with them bending valves, We sold a bunch of those that spring/summer and we had a lot of them in for bent valves under warranty. At first we thought it was operator error, then wimpy valve springs, but the issue was not enough valve relief in the piston dome. Kawasaki had at least 3 different piston configurations before they would stay together long term. Be careful if you go mixing and matching parts.

Is that a KZ1000 MKII tank?

It is a actually a kz1000e shaft tank. I just made a new rear mount on the tank and new forward mounts on the frame and it fits like it was designed to be there.

I am very aware of collision between the piston and the valves and have read every thread about measuring the squish band, quench area and valve pocket clearance that I can find. At this point I just need to be sure of what kinds of pistons are known to work or not to work before I buy something I can't return.
When I put zx550 pistons and cams in my zr550, I had to put in the matching pistons because the zx cams would cause the valves to hit the pistons if they were used with the stock kz550 or zr550 type.

I did find a 1982 gpz750 with the factory ported and polished head in a salvage yard. It is physically undamaged on the outside, but the bike has 40,000 miles on it....so its going to need a new everything. It is still bolted to the motor so the inside could trashed for all I know. I simply didn't want to replace my perfectly good head unless no other pistons will fit.
I also must tear apart my engine and have the bores measured....if they are out of spec, then a 810 kit will be my only option.
If not...I would like to formulate a definitive plan of action.

Interesting read Chuck, thanks .

When I put zx550 pistons and cams in my zr550, I had to put in the matching pistons because the zx cams would cause the valves to hit the pistons if they were used with the stock kz550 or zr550 type.

I did this too, z550 pistons with zx550 cams, thrashed the nuts off it into and past the redline with a heavily skimmed gas flowed zx head for 5 000 miles...see my 'thar she blows thread'. I was using slotted cams so it wasn't standard valve timing.

...but the bike has 40,000 miles on it....so its going to need a new everything...

Don't assume that, my 750R is about 43k miles and runs sweet as a nut.

Daftrusty wrote: ....... It is physically undamaged on the outside, but the bike has 40,000 miles on it....so its going to need a new everything. .....

What? 40,000 miles isn't really much at all on these bikes. My 1977 KZ650 has right around 60,000 miles, has never had the head pulled or any other engine work, and it still runs perfectly. RonKZ650 put more than 160,000 miles on his and it still runs although it's starting to get a little tired.

I wouldn't assume that the engine needs much of anything just because of the miles. Unless maintenance has been sorely neglected it should be fine. Ed

Yea, at 40,000 miles the engine may not need much work. The oil leaks are par for the course and will be a constant battle on a KZ650 or KZ750. However if you want to put the GPZ750 pistons in, I can't see that being any problem. The green KZ650 in my avatar has GPZ750 pistons even with a KZ650 head, so GPZ750 pistons in a KZ750 should be simple. I never had any luck with the Wisco kits as they were too high compression for pump gas. The GPZ750 pistons compression works perfect for me.