Recommended Headwork for Slightly Overbored GPZ110

Bozo wrote: Interesting especially the link to the Bathtub head. As some of you might remember I just put together my Frankinbike (GPZ1100B with all my spare bits).
I had no intention of making a rocketship but ended up with one. The porting without doubt was one of the reasons and the other was the 83 GPZ1100A cams. I was amazed at the low/ top end power it puts out when ALL else is stock (I had a J model head, pistons STD, Valves STD etc). It might not compare to some big bore bikes but it won't be embarrassed either.
I must add the bike weights 215kg wet (half tank of fuel).
So my recommendation, porting at least and decent cams for street use (not racing cams) BUT don't forget change the valve springs if they are old.
Also don't forget more power means a wilder ride unless other running mods are made (frame etc)

Thought I was reading about my own engine for a moment. :whistle: The exception being the ZN 1100 (bath tub) head, on which the valves were left stock too.
The head did have minimal factory porting originally done by Kawasaki, limited to the intake runners directly above the valves. I wonder what a quality minimal porting job on the head would do for the engine performance, or if it would even be worth having done for a street bike.
What would be the going-rate for a porting job on a J or GPz bath tub head, retaining the stock valves as well?

Daveo, I ported my GPZ head myself, I concentrated on the flow not so much making the port wider, I did not make them mirror shine, I purposely left them slightly rough. The valve seats were ported to the seat and both Intake and exhaust outputs were matched to what ever was bolted to them.
I run 36mm CV's from the ZRX (minor mods required), the response is incredible (its the pumpers). Some say these are too big but I dare anyone after riding this bike to criticize them (low and topend).
The cams haven't been dialed in and the way its going now, that can wait. At present I'm doing final touches to get good fuel economy without loosing power.
I have a fairly beat up 83 head in my shed and after reading this I have decided to remove the spiders and try to clean it up for the future

Edit: I stated a GPZ head porting I meant a Z1000J head which is fitted to Frankibike

Porting the stock 'J' Model head is a better option than the 83/84 Head Lower Compression, Low port Velocity Why do you think 83 / 84 GPZ 1100 Had DFI ? poor low speed response Stock 83 /84 head flows approx 84-86 cfm porting a 'J" head to those numbers by a Competent Porter ! to the same numbers way better for carbed street engine .

83/84 GPZ1100/Spectre etc bathtub style heads are actually higher comp ( smaller combustion chamber volume) than the earlier 'J' heads.

zed1015 wrote: 83/84 GPZ1100/Spectre etc bathtub style heads are actually higher comp ( smaller combustion chamber volume) than the earlier 'J' heads.

I was struggling with the connection to "DFI being used due to low compression".... :blink:

750 R1 wrote:

zed1015 wrote: 83/84 GPZ1100/Spectre etc bathtub style heads are actually higher comp ( smaller combustion chamber volume) than the earlier 'J' heads.

I was struggling with the connection to "DFI being used due to low compression".... :blink:

It Has NOTHING TO DO WITH COMPRESSION the reason was to improve operation at lower engine speeds.

If interested in reviewing my 84 bath tub top-end thread posted in 2015, see:
www.kzrider.com/forum/2-engine/596933-ca...-on-my-engine#658819

or zed1015's initial reply to that post which follows:

"Which engine do you want to fit it on.
One of the best heads to have for performance modding.
Bathtub/twin swirl head.
Same casting as GPZ Unitrack but probably shim over bucket instead of under.
Lower compression than earlier KZ9/1000 heads but slightly higher comp than the earlier twin shock GPZ and J heads.
Has steeper valve angle which allowed the larger valves to be used.
Be careful with piston to valve head clearances on assembly.
Depending on application you may need to fit different pistons or machine the valve pocket angle to avoid collision etc."

My question is, what effect does the unique combustion chamber (bath tub) shape have on the incoming air/fuel mix and outgoing exhaust gasses, compared with heads having the simple hemispheric shape?

The GPZ Bathtub head is of a twin swirl combustion chamber design which speeds up and increases combustion efficiency better than a plain hemisphere (simply search "Twin swirl" on google for more detailed info).
This equates to a fuller and faster burn which makes the most of the fuel available and decreases the amount of unburnt fuel lost on the exhaust phase.
Simply put. The twin swirl will produce more power than a plain hemi for the same amount of fuel.
This is why (as well as the bigger valves) the GPZ bathtub head is the one to have for performance gains.

Very interesting, so I should save my pennies once I have it running for a 1983 or 1984 GPZ1100 Head and cams? It will be a direct plug and play?

In the interim until I find that unicorn, does anyone know the max lift that I can get with stock valve springs if I decide to put in an aftermarket cam later on?

Edit; Added a word

DFIGPZ wrote:

750 R1 wrote:

zed1015 wrote: 83/84 GPZ1100/Spectre etc bathtub style heads are actually higher comp ( smaller combustion chamber volume) than the earlier 'J' heads.

I was struggling with the connection to "DFI being used due to low compression".... :blink:

It Has NOTHING TO DO WITH COMPRESSION the reason was to improve operation at lower engine speeds.

But thats exactly what you said....?

Porting the stock 'J' Model head is a better option than the 83/84 Head Lower Compression, Low port Velocity Why do you think 83 / 84 GPZ 1100 Had DFI ?

Kawasaki was developing its injection systems on production bikes way before the bathtub heads {1980}, they were simply the first Japanese manufacturer to do so {in the race for better tech and emissions control}. I have an old mechanic friend thats been doing carb conversions on injected GPZ1100's for years, NO problems with low speed running, its a common conversion, Kawasaki's first few attempts at injection were quite primitive, it was introduced so Kawasaki could be ahead of the USA's tough emission regulations, improve operation right across the rev range, as well as better fuel economy............

In 1980, Kawasaki was the first motorcycle manufacturer to recognize that in the following years, bikes would have to meet even tougher compliance laws. Fuel-injection, rather than carburetion, on an engine would be one way to get a motorcycle through emission testing

Further reading for those interested (with pictures!):

kzrider.com/forum/2-engine/602288-kz-gpz...hings-to-know#723639

P.S. Whatever possessed me to do such a thing to my motor? :ohmy: 12,000 miles w/o issues, and it loves regular gas. :whistle:

diego092409 wrote: Very interesting, so I should save my pennies once I have it running for a 1983 or 1984 GPZ1100 Head and cams? It will be a direct plug and play?

In the interim until I find that unicorn, does anyone know the max lift that I can get with stock valve springs if I decide to put in an aftermarket cam later on?

Edit; Added a word

Be careful changing the cams later to a higher lift without cleaning the valve stems.
A lot of the seals die because the valve stem gums up to a lower lift, then when a higher lift cam is added the seal goes on the gummed area causing the seals to fail.