The GPzephyr 810 turbo project

Document your ongoing rebuilds and restorations here

The GPzephyr 810 turbo project

Postby GPzephyr » 18 Jul 2020, 20:16

Hi all.

This thing was built by me the best part of a decade ago but since the original Zephyr Zone died a lot of information disappeared.

There are some pictures in the showroom section and I may be able to find some more from years ago that might be appropriate to put up too.

So I bought this in 2003 as a standard 750 Zephyr. Rode it around for some time and loved it but like most blokes in their 30's I wanted a bit more speed.
So I tried a few airbox mods which to be honest are not worth the time and trouble. Managed to get some '83 GPz750 cams which had noticeably more lift and duration. So fitted them along with a bit of porting and the bike was certainly more energetic for that. Made it a lot more rev happy to the point that it was running out of gears.
Not to shy away from a challenge. I did some homework and found the the gearbox casing dimensions were identical to that of the smaller GPz/Z 550 and that also includes the 550 Zephyr. So back to ebay and 20 quid later i had an old GPz550 engine to play with. Stripped it down and moved the entire six speed box into my 750. Worked great. And made higher speeds a bit more comfortable as it wasn't revving its tits off.

After a few years of that I picked up a Z750 turbo. It ran fine but had been built into a non standard GPz frame due to previous accident damage. I road it around for a while and eventually decided to do the engine swap into the Zephyr.
This is surprising straightforward as the engine itself is fundamentally the same as the 750 Zephyr lump albeit with extra stuff bolted on it. Its all this extra stuff that's the main ssue. Being fuel injected I had to find room for the fuel pump, plenum chamber, ecu and a mass of extra wires.
Needless to say in near stock form it was quick. The standard Kawasaki turbo is 112hp, which they had kept to that level so it didn't interfere with sales of the then new GPZ900r. However Kawasaki had put a special mode into the ecu which means that fueling could be adjusted by a simple variable resistor and boost was unrestricted. This is simply done by cutting a particular wire. Before that it would cut out abruptly at 12lbs of boost. Now I could do pretty much what I liked. Many thanks to the guys over at 750turbo .com for all the information.
So more money was spent ( this is a ongoing theme with this lol) bought a Garrett t25 from a Nissan Silvia and welded a new flange to the exhaust for that. The original Hitachi turbo tend to self destruct at higher boost. Bought a Power Commander to control the fuel better and it became a real beast to ride.

Then came the single sided rear end. This was again more straightforward than I thought. Bought as a complete rear including wheel off ebay. The shock needed a mount put in at the top of the frame and a pivot for the linkage at the bottom. The overall width was very similar to the Zephyr arm so only need new bearings and spacers made to get the alignment with the chain and sprockets correct. This helped me as I could now run a 180 rear tyre with which was becoming more important to handle the power.

Finally tha last major change was the ZX7RR front end. Again acquired off ebay. This wasn't such an easy change as it needed a hydraulic press to change out the steering stem for the Zeohyr one.

About 7 or 8 years ago. The bike blew a load of electronics due to a unprotected relay putting a spike into the electrical system. Cost me the wideband unit and the power commander. At that point other things were happening in my life so it got sidelined in my garage.

So forward to now.
I've got the old girl out again. Freed the brakes today. Rewired some things to make it a bit simpler. Had it running several times this last week trying to feed some fuel injector cleaner through the system. Definitely running better now. Treated it to a new wideband O2 thingy.

Things to do. Well the paint is pretty crap. And she smokes when hot. Got some new oil on the way and if that doesn't sort it then I suspect the turbo seals are probably gummed up. Not a hard job luckily but we'll see.

Let me know what you think and any questions

Posts: 13
Joined: 12 Jul 2020, 23:15

Re: The GPzephyr 810 turbo project

Postby ZoneAdmin » 18 Jul 2020, 21:15

Hi Chris

Your bike has an interesting history. I will look out for updates to your project.

Site Admin
Posts: 46
Joined: 09 Aug 2017, 13:52

Re: The GPzephyr 810 turbo project

Postby jrkk » 18 Jul 2020, 21:49

I'm excited on this.
Although, we have different models and there is not much common to share.
But always pleased to see these stuff.
Keep updating.

Posts: 148
Joined: 20 Jan 2018, 10:53

Re: The GPzephyr 810 turbo project

Postby GPzephyr » 19 Jul 2020, 10:05

Thought this might be interesting

Main layout of components.


a. Inlet pipe from turbo

b. Injectors. Bosch yellow top from a 2l Vauxhall car

c. Plenum chamber

d. Fuel pump. Walbro 255

e. Boost pressure sensor. Out of sight but it attaches to the back of the plenum

f. Fuel filter

g. Fuel pressure regulator.

h. Fuse box and relay set for main wiring loom. Under the alloy is the top shock adsorber mount.

i. Ignitor unit for ignition coils

j. ECU
Posts: 13
Joined: 12 Jul 2020, 23:15

Re: The GPzephyr 810 turbo project

Postby GPzephyr » 19 Jul 2020, 10:26


k. Main relay for the DFI

l. Battery. Uses a glass fibre mat electrolyte so it can be mounted flat

m. Fuel return pipe. Connects to a one way valve to return unused fuel in the rail to the tank

n. Fuel rail. This is a really tight fit to the frame.

o. Throttle body unit and throttle position sensor. The main throttle body is off a an early injected GPz1100

p. Dump valve. Fitted to relieve pressure on the turbo when the throttle is closed

q. Turbo. Garrett T25 off a Nissan Silvia. The small top exhaust vents when the waste gate opens

r. Ignition coils. Dyna 2.2ohm

s. Boost control solenoid. Not visible but switches between high and low boost via a switch on the LH switchgear. Toggles between 5psi for normal and 15psi
Posts: 13
Joined: 12 Jul 2020, 23:15

Re: The GPzephyr 810 turbo project

Postby GPzephyr » 19 Jul 2020, 10:33

As you can see from the above 2 posts the layout is fairly cramped. The close fit of the injector rail against the frame means that some of the wiring has to be routed over the frame at the back of the tank.
The fuel tank itself has to adapted to take a fuel return pipe and also the fuel tap itself has to be a gravity fed one instead of the normal vacuum type fitted to Zephyrs.

Got a Power Commander V coming in the next week. This will be modified and fitted up behind the ECU
Posts: 13
Joined: 12 Jul 2020, 23:15

Re: The GPzephyr 810 turbo project

Postby Fishrider » 26 Jul 2020, 03:35

Love the single sided swing arm. When others are chopping you went for the kitchen sink. Lovely job.
Posts: 52
Joined: 24 Mar 2018, 18:11

Re: The GPzephyr 810 turbo project

Postby GPzephyr » 02 Aug 2020, 09:44

Took the bike out for a quick spin a week ago. Just pottering about making a mental list of any issues. Came back with a shopping list lol.
Mainly electrical stuff. Idiot lights, engine temperature sensor and stuff like that. Been working through them all.
The engine temperature sensor is a homemade affair. Just uses a varistor which changes its resistance according to the temperature. Simple enough but i had it in a position where it just wasn't effective enough. So I soldered it to a bit of copper formed into a ring and it now fits nicely under No3 spark plug.
I did have it down the back by the tensioner but it didn't get hot enough so caused the engine to run really rich.

Need to make an idiot light display. My original Zephyr one is scruffy. So i had made a display using some leds but its far too dim to see in bright daylight. There is that to do and also to work out why the neutral light stayed on all the time.

Finally need to set up the suspension. Been riding smaller bikes for a few years and suddenly getting on the Zephyr reveals its issues. Steering is very heavy. Mainly down to the bikes weight. Might have to look at changing front preload or even going to some stronger springs.

Have a good weekend
Posts: 13
Joined: 12 Jul 2020, 23:15

Return to Ongoing Projects

Who is online

Users browsing this forum: No registered users and 0 guests