gpz750 quench chamber heads on kz750 pistons

I have recently bought a set of 1980 kz750 jugs and pistons to put on my 1981 kz650. I was originally intending to just clean up my 650 head with some kz750 cams on the kz750 jugs and pistons.

I am wondering now about what sort of compression this will result in and possibly getting a using a gpz750 head.

I like the quench chamber design and bigger valves of the gpz head but I don't know if the stock kz750 domes for the full hemi head will work with those chambers or what sort of compression this would result in.

I realize much of the gain from the hp increase in the gpz is also from the cams, I intend to use the kz750 cams with the gpz head though as I would still like to keep a broader power range so I am not looking for a full stock hp gpz gain with this setup necessarily.

This is the information I have found in searching for this so far for relative combustion chamber sizes.


Kz650 23.7 cc.

83-85 GPz750 25.9 cc

Kz750 H 24.8cc

Kz750 L 25.3 cc

Using www.csgnetwork.com/compcalc.html This calculator set to come up with 9.5 for the stock 650 and changing nothing but the bore from 62 to 66 I get 10.6:1 CR.

Then changing the combustion chamber size to 25.9 for the gpz head results in 9.6 CR.

I realize there are many other factors involved such as piston dome volume, Chamber volume removal from having the head decked and head gasket thickness but assuming these things are stock and equal what will contribute most to a well running engine? Head flow, chamber design with the quench head or compression ratio?

Has anyone who has been through this attest to piston dome shapes or volume differences between the kz650, kz750 and gpz750?

These are the specific pistons I have

This is the specific head I am looking at

Here is a better picture of the gpz chamber from another thread on this forum,

It looks to me like the piston dome has a decent size quench ring around the outside before the dome starts that should be relatively close to where the gzp head starts its hemi shape after the quench pad. I'm confident that i can carefully clearance that area if need be and then weight match the pistons I am just wondering if anyone else has been down this road before and if the gains are worth it or the real world trade off between the cylinder head design and compression ratio is in the end.

i would go with the gpz 750 head .have it milled .025- to .030 thous .and use the cams for the gpz 750 motor .

I agree with keeping all the parts together as a system: pistons, head, cams.

Not so sure about milling a 750 head. The valve seats are extremely close to the stock head surface. Mill the head more than a very minimal amount and you will hit the seats.

i think at.035 you will hit the top of the seat ,but if you do a valve job the valve will be lower.

I've read about the gzp head not being able to be decked much before. Any idea how much cc reduction or CR increase from .035" removal?

I understand that the gzp head and cams and pistons were designed to work together. I can't seem to find a picture of the stock gzp piston domes to match the quench chamber or any for sale for that matter. Does anyone have any more information or experience with them specifically?

Like I said I'm also aiming for a general overall gain vs a trade off for high rpm. More displacement and more compression don't take away from the lower rpm range, I have read the gzp cams trade low for high and the stock kz750 cams do not. I would add greater quench area and combustion efficiency to the list of size and compression for adding overall gain vs a trade off. I just don't know how well mixing the kz750 piston domes with the gzp chamber will work.

The H and E have 9/1 shorter dome pistons. Round corners on the fins of the block and head.
GPz, Spectre and maybe the L have 9.5/1 with a taller piston dome and bathtub combustion chambers.
These have the squared off fins and thinner piston rings.


.

Any idea about the actual dome volumes? Most of the other engines I've worked on have a large body of information about year and model differences and the piston dish or dome volumes and combustion chamber volumes. This stuff seems to strangely be somewhat vague for these engines.

I have not torn my 650 down yet to check it myself but does anyone know the stock deck height for these 3 engines? If I assume they are all the same with the same head gasket thickness the kz750 pistons must have a smaller dome volume to result in a 9:1 CR with a 24.8 cc chamber. With a .042" compressed thickness head gasket and 0 deck clearance the 650 @ 23.7 cc requires an 8 cc dome to hit 9.5:1 CR. The 750 with the same gasket and deck height with a 24.8 cc chamber needs a 5.2 cc dome to hit 9:1 CR.

If this is true then even with the smaller 650 chamber the kz750 pistons would result in 9.4 CR once again with the same deck height and gasket. That actually lowers the compression vs stock. The gpz head at 25.9 cc with the smaller kz750 dome piston would result in 8.65 CR. GPZ750 @ 9.5:1 CR = 7.6 cc dome.

From others real world experience do these conclusions sound reasonable or are there differences in the deck height and gasket thickness as well? If the deck height is greater than 0 then playing with base gasket thickness could make up for some of the compression loss. Otherwise the stock 650 head is looking the best right now using these pistons.

If I could deck the gpz .035" that could theoretically result in roughly a 3 cc reduction in the chamber volume to 22.7 cc. This would put it back to 9.7:1 CR with the 5.2 cc kz750 piston domes which would be nice.

You have an 81 650 which has the later hy-vo style cam chain. You have a 750 cylinder block and a gpz head and cams....

By the 810cc piston kit from Weisco and clearence your cases. The 810 build with a gpz head and cams is a known performace build and you have everything you need already except for the pistons.

No eBay APP ID and/or Cert ID defined in Kunena configurationNow I am wondering what the real differences are between the spectre and gpz engines?

Would I be better off using the jugs and pistons from a spectre with either a gzp or spectre cyinder head? I can find those on ebay.



The difference between the pistons appears to be minute. If this is true I wish that I had known this before I bought the kz750 jugs and pistons. I didn't really even know anything about the spectre before, makes me wonder what else I need to learn about.

I have kz750 cams not gpz cams. I realize the 810 setup is a proven setup but I'm not looking to go crazy spending money on this. The cost of the pistons and machine work to fit them is very different than a simple rehone and top end rebuild using easily aquired stock parts. It was sort of the cool old shcool hot rod ability of this engine(s) that attracted me to it in the first place. I assume lots of people have done these types of builds before the custom pistons were even around. If i was going to drop the $ for forged pistons they would be for boost. I am just wanting to cut my teeth on these engines with this build for now.

Alright then, I agree with above advice. Use the 750 top end. Run GPz cams if you can. The cam will have way more influence than the combustion chamber difference between the 750 and GPz head and even the bigger valves.

Hei timi, good that you have way above average knowledge of engine building and are asking the right questions but there are some peculiarities to the 750 motor.... while most of the info out there is about the bigger mills 900 + .
i've been through all this while building my vintage road racer which is a 810 kitted gpz750. quite a lot of info in my build thread

none of the stock kawaski pistons will give you the possibility to build a proper quench band that REALLY creates turbulance and accelerates burn rate. to get that you need to go down to 035-040 between the chamfered area on the head and the piston, not just at the nearest point but to have a relatively wide quench area of say 6-8 mm

I achieved this by actually welding the heads in the chamfer area and creating a flat quench band.
BTW, the cruzing image 810 kit form ebay is pretty cheap....