Swapping 750 Engine into Seca 650 - Keeping it Turbo!

Good Day! What a great resource this site is - rich knowledge-base (and thanks to those maintaining it!)

As my screen name would allude (and the title of this thread suggests), the goal is to swap a higher compression / larger displacement 750 engine, into my 650 Turbo (retaining the turbo setup...)

My Yamaha roots run DEEP! (I grew up with older brothers who had lots of different bikes growing up and some of my fondest early childhood memories were formed on the back of my oldest brother's XS 750, then on his XS 850 and eventually on his 1.1...)

I'm heavy into 2-strokes as well. I have owned everything from an RZ50 as a kid, to an RD350LC and I currently own an RZV500R, as well as a classic TZ250 race bike. I love the tripple Yammies as well (I currently have an FZ09 - among other things like a CBX, GS1100, couple of Guzzis, a Ducati 999r etc.)

Growing up in South Africa, the 80s Turbo machines were a thing of lore! Rode many CX Turbos, GPZ Turbos and XJ Turbos (but never a Suzi Turbo - strangely enough - they are not as common, I guess.) Anyway, I have this "Seca" thing (our South African models were simply known as XJ650 Turbos.)

The motor is tired and the opportunity to procure a 750 engine came up, so that's the current plan - keep the turbo configuration, but swap in the 750 engine! Should be a blast!

After a lot of general web searches (and constantly running up against posts here on the XJ Bikes forum), I decided to join, share and ask questions. I saw lots of posts discussing 650 to 750 NA swaps, larger displacement Turbo engine rebuilds and even 750 NA to 650 frame swaps - converting to NA!

I just couldn't find any clear "installing a 750 engine, into a 650 Turbo bike - and turbo-charging the 750 plant with the 650 turbo parts" threads easily. Unless I'm blind.

Anyway, happy to be here. Let the stoning begin!

 

Read 'Simmy's naked Turbo project' thread a few post below this one. May not have the specific info you need, but lots of good info nonetheless.

 

Hello @XJ 750 Turbo you are a kindred spirit.
I to envision a larger displacement Seca Turbo since at 1st and 2nd glance Yamaha seem to provide everything needed to bolt one together.
I acquired an XJ900 motor with intention of swapping it in to my Turbo.
As I began delving into this I discovered a few things which dissuaded me from a direct motor swap.

The Turbo crank assembly has special Babbitt bearing shells with a big oil groove.
The oil is drawn through the centre of the crank shaft (like all XJs), into these grooves then flung through a drilling in the connecting rod to splash oil to the underside of the piston.
The Turbo has forged pistons, will standard 750 pistons survive the heat without the oil splash? Who knows?

The knock sensor for the Turbo system screws into a boss at the front of the cylinder block between cylinders 2 and 3.
Several XJ cylinder blocks have this boss (untapped) but the XJ750 does not.
A skilled aluminum welder should be able to build this up in order to tap the threads for the sensor.

The Seca Turbo feeds oil to the turbo from a drilling into the main oil galley at the bottom of the crank cases.
This can be done easy enough, then tapped for the oil check valve.

The Turbo has a scavenge pump piggy backed on to the oil pump which sucks the oil from the turbo unit.
The Turbo's oil pan is a direct swap onto the 750 but the pump's sprockets and chain are different.
All the Turbo's stuff could be swapped over but you'll need to split the cases for the larger oil pump drive sprocket.

You'll want to keep the Turbo's cams and this will be a direct swap.
You'll want to use the Turbo's carbs and this again will be a direct swap.

The greatest unknown is the carb jetting.
I'm unaware of any other factory turbo using blow-through carbs.
If I was further along with my 772 turbo you could use the same jetting for a 750 but I'm not there yet.
What I'm intending to do is install a wide-band O2 sensor in 1 and 2 head pipes on my currently stock Yamaha 653T.
This will show the fuel/air ratio from Yamaha before I bore it out.
Then I'll be able to compare the data then swap jets to get the fuel ratio correct.
As I said, I'm not there yet. I need to get some shabby head pipes and weld the bungs in for the O2 sensors.

I have acquired all the parts for my big bore from 653 to 772.
Designing the cylinder block was a long process and having it built was expensive.
In my build thread it explains the process I went through to determine the CR.
Something you're not planning to do as you're planning stock 750 compression.

 

"As I said, I'm not there yet. I need to get some shabby head pipes and weld the bungs in for the O2 sensors."
I have a full set of original 750 maxim pipes, make an offer, for shipping I'm in Saginaw MI. USA 48601
Cheers, 50gary

 

Thanks @50gary I have a spare set of XJ700N head pipes.
Apparently the Turbo has smaller ID pipes, probably won't matter much just for the measurements but just to be sure I'm going to find some Turbo pipes.
The smaller ID apparently spins the turbo faster, that will make a difference for the main jet needle.
Yamaha already has testing ports at 6 o'clock but the sensor manufacturer says to mount them on the top half of the pipe.
Apparently any fuel or water in the pipe throws off the readings.

 

"The smaller ID apparently spins the turbo faster, that will make a difference for the main jet needle."
Maybe it spins it up quicker - as in reduces lag, but doubtful it will give more rpm, or boost?
For @XJ 750 Turbo - I might be inclined to drop the cr a touch in the hopeyour pistons might last longer. Did @Simmy remember to mention the 650 turbo uses the 900 clutch - not sure if this has any gearing implications...

 

yes you're right, quicker is a better way to say it.
Ideally the turbo would be mounted closer to the head like the Kawasaki GPZ Turbo.
Smaller ID just means less volume to pressurize, if you can't make the pipes shorter then make them smaller.
The Suzuki Turbo stuck the turbo even further downstream than the Yamaha, lag must be even worse.

the motor will be more likely to knock sooner, with the ignition retarding sooner, could end up with less power

 

Wow! Thanks for all of the great information guys! Lots for me to absorb here.

We own an automotive business which routinely engages in very custom work, so modifying machines is not foreign to us. I'm also fortunate to have a very broad-experienced classic motorcycle specialist (and a highly-skilled bike tech at that), working on this, or I might be discouraged.

We're waiting for the 750 engine to arrive and then we will reevaluate and can go from there.

 

No, less cr equals less likelyhood to knock. Assuming you maintain squish bands of course, which might not be that easy

 

I was agreeing with your post, using the stock 750 CR will increase likelihood of knocking.
In which case the Seca Turbo ignition will retard the advance

 

The big day has finally arrived!

The 750 donor bike was dropped off today and we cleaned it up a bit (The owner just stopped riding it and then it sat outside under a tarp for a long time, unfortunately.) We're going to pull plugs, change oil, do the basics to get the motor running off of a drip-tank (do a crank-case flush, check compression) and then yank the motor and start the conversion!

We have the old 650 mill out of the bike and we have been able to plot a path forward (probably a chain-split for the pump sprocket swap etc.) There are ways around every issue, it's just time and a bit of work-around. No deal-breakers from what we've seen so far.

 

Doesn't look bad for sitting outside.

 

Yeah, not bad - just lots of hardware surface rust, which sucks. I want it all nice, but we'll start with the conversion, validate the concept and then go from there to make it pretty. The air-box is still on it and the plugs were in the motor - it's all sealed up (so that's usually a good sign.) We'll probably stick a bore-scope down the holes while the plugs are out and see what the cylinder wall condition is like.

The 650 turbo was a faded burgundy model (hate that color), so I want to either change it to the standard silver (or a do a black homage to the 007 movie bike.) I've also considered making it a John Player Special (black with gold) homage. Either way, it gets debadged as a "Seca" and get's classic Euro / South African Turbo decals on it.

We'll try to work "XJ750 Turbo" into the mix there somehow with combination badging, or something.

 

My man Jason is making great progress here (yes, that is THE Seattle area Jason...!) We haven't encountered any snags yet. The oil pump and everything associated with it was a direct swap! (Some marks on the oil pick-up screen from a previous idiot's choices, but nothing bad.)

The 650 electronics like the stator etc are swapped over (we can always upgrade things later - once the concept is validated...) Still have to drill and tap for the knock-sensor here. Oil filter housing swapped. Still have to do a valve cover gasket and rebuild the Turbo carbs, but yeah - getting close.

Unless there is something bad on the horizon, this swap is relatively straightforward.

 

I'm curious how you do this? The 750 doesn't have the boss cast in for this.
Are you changing the head or at least installing the Turbo cams?

 

Success! We have a running XJ750 Seca Turbo!

Internally, we kept the engine all 750 (the head, compression, displacement - everything - except for the cam...) We did swap in the 650 Turbo cams and then we installed all of the 650 Seca Turbo electronics, ignition etc. The bike is an XJ650 Seca turbo (complete with 650 carbs), "driving" an XJ750 higher compression engine (except with 650 cams, turbo, oil feeds etc.)

On the 750 cylinder block (where that knock sensor boss would be on the 650 cylinder block), there is a single external bolt where we'll install a longer stud and position the knock sensor. See how it works. I do plan on taking the bike to the dyno to check AFRs and to listen to the motor with knock-box ear muffs - it's typically pretty easy to hear knock.

 

Wow, that's a great big jump in compression ratio from the 650T engine to a 750 engine, make sure that the turbo wastegate actuator is working 100% properly, or even an aftermarket boost controller to be able to dial back the boost. I'll assume you're using the XJ650 Turbo rubber intake manifolds, as they are o-ringed (seal to head) rather than gasketed to withstand the additional manifold pressures.

So I would tip-toe gingerly on that throttle until the knock sensor is hooked-up and verified working, and even then, due to the lack of the extra-duty features of the 650T engine (like Simmy noted, the crank and rod bearings are designed to keep everything super-lubricated down there to handle the extra work load).

Also, turbo piston rings were unique (although Yamaha supersedes the turbo ring sets to the stock non-turbo 650 ring sets) and I don't know if they were different due to the difference in the pistons (lands, etc.) or they were of some heavy-duty design/materials/other specs to be able to deal with the extra pressures.

Finally, the 650T clutch used much stiffer springs than the 650/750 models, for obvious reasons.

Sounds like an exciting (and certainly interesting) project.........make sure you don't blow the crankshaft out of the engine by getting too enthusiastic with it too soon.....!

 

Nah, figured we'd just zip-tie the wastegate shut and send it.

Thanks for all of the great feedback guys. We'll get this thing figured out.

 

Make a video when you do!

 

So, we have an issue with the engine leaking some oil out of one of the exhaust primaries - right at the collector - while not running (bike on the side stand...) Looking at it a bit closer; could it be drain-back coming from the turbo? Is there some sort of a check-valve that's missing, or that has failed?

On the automotive side of things that we do here, we always seem to fight for slope when designing turbo drain-back systems (back to the sump...) As much as a given turbo unit may have oil pressure coming in, it won't "push" oil out on the recovery side. They all NEED good slope for gravity feed / drain-back down to the sump after the turbo bearing.

In the case of the Seca design, the turbo sits down at the bottom (and basically, below the crank case!) From the turbo, it looks like it has a "dip" line to take oil down for the gravity / drainage (and it appears to be a double pump setup.)

Does one side act as a recovery system that picks drained oil back up from the little "mini-sump" tube, to push it back up into the recovery side of things...?)

 

If I look at your last series of photos I see the boss for the turbo oil feed undrilled.
This is where the check valve screws into the cases and stops oil flow when parked.
How are you feeding oil to the turbo?

Oil pooling in the turbo unit and filling the exhaust is a common complaint. Most suspect the check valve is not functioning. I tested mine and found it stopping the oil at rest like intended. What I discovered is oil seeping past the scavenge pump, allowing oil to pool down there. Rather than replacing the scavenge pump with new I installed a $4 check valve in the return line. Now I have functioning check valves to the turbo and from the turbo.
My bike does not smoke when started after sitting, which was the reason my bike was parked by the previous owner.

 

this it will need if all goes well!

 

Thanks for that information. We added the oil-supply after those pictures were taken.