Modifying xj 650 head for improved performance..

So here we go then, I know this will upset some purists, but when I see something that obviously looks wrong the tuning genie in my head won't let it go. So having aqcuired a stack of 650 parts, four bikes and a 750 engine from @Charlie J (thanks again btw), after stashing them away thoughts go to what to do with them all. No doubt two of them will get returned to original, not sure which two or how long that will take, since I allready have one candidate Seca for this waiting. Makes 7 XJs, if anyone's counting.
Now, it's easy enough to fit the 750 engine into one of them - some jetting and voila, instant improvement. But that still leaves the power hungry genie wanting more, and spying the insides of the mankiest of 650 heads gives me the itch.
Obvious things like zero squish, restrictive exhaust ports, and steps in the intakes look like candidates for attention. The valve sizes feel adequate (they wete the same size on the 750s), and by my head maths should be big enough for 100bhp... But I will be running these numbers through some software I have to see what the theoretical peak might be before committing to anything. Bigger valves could well end up just shrouding themselves in the head anyway, so not gaining anything...
Eventually, when tested, and if succesfull, I would hope to apply the same to the 750 head, so get a double whammy, but we'll see.

 

https://magazine.cycleworld.com/article/1980/7/1/yamaha-xj650-maxim-i

"Showing the Maxim's cylinder head to some riders not prone to forgetting about checking the oil, in fact some hardcore hop-up freaks, brought ooohs and aaahs of approval. There’s plenty of room in the head for 35mm intake valves, pointed out one. as long as the exhaust valve seat was sunk a bit. Plenty of beef, agreed another. It's a minature XS11, said a third.

As for the styling, it didn't bother them a bit as they fingered calipers and calculators, and scoped out the cylinder head. “This baby can be made to fly,” exclaimed one. They could see what potential the engine had, and didn't care what the rest of the bike was like."

The specs in that article claim 64bhp, so getting to 100 would be a pretty big hop, but who knows with porting and polishing, domed pistons to push compression to 12.5:1 or so, etc.. How long would an air cooled machine running 12:1 compression last? I think the easiest route would be to punch it out to 820cc and go from there.

 

100 bhp? I certainly don't think so. My point was the valves should be big enough for it...
In all honesty, more bottom/mid range torque with an extra 10 bhp is about what I'd expect without regrinding cams.
Then I would need more CR to take full advantage, another 10bhp? So that's about it, assuming the carbs aren't restrictive. And I'm not planning on regrinding cams - yet.
Interesting read there @XJ650inTexas .

 

I have no experience with hotrodding Yamaha inline fours but have fooled with small block Ford v8s and head work really doesn't make much difference at low rpm because the engine isn't moving enough air.

If the goal is improved low to midrange power then swapping the 750 jugs, crank, rods and pistons would be the way to go. They use the same cam and head (and carbs with different jetting, etc.), the 750 doesn't have a hp drop off at high rpm so the head doesn't seem to be a limiting factor. That tracks with what I've seen with V8s - take otherwise identical 302 vs. stroked 347 engines and the 347 makes gobs more torque below 5000rpm but the 302 may have more peak hp. With proper heads the 347 out runs it at every rpm.

 

I have no experience with hotrodding Yamaha inline fours but have fooled with small block Ford v8s and head work really doesn't make much difference at low rpm because the engine isn't moving enough air.

Your point re airflow is correct, but you ignore squish, or lack of it..

 

The XJ750 (allegedly) makes 20 more hp than a 650 with the same head and cams. An 820cc overbore may not be common but kits do already exist. If your goal is increased low end power in a normally aspirated engine there's no replacement for displacement.

 

No, I struggle to believe that figure, but there may be things going on that aren’t obvious. I’ve no idea what might release more power than the percentage capacity increase holds. But you’re right, there’s no substitute for ccs.
However, the job at hand right now is to see what can be done with the 650 head. Modding the 750 will need to wait, but hopefully info gained will be useful.

 

I goggled the 750 secas. Pre 83 got 82bhp, post got a revised engine, bigger valves, carbs, cams? And 87bhp...

 

1984 air-cooled engines in the XJ750RL Seca has one more cc (don't know where it comes from........), a 1mm larger intake valve (using the XJ700 air-cooled head), the exhaust cam from the XJ700 air-cooled engine (0.50mm greater lift), and a different TCI (which may have a slightly quicker advance curve ---- hard to say as Yamaha has this bad habit of providing non-directly-comparable info on advance curves). I guess all that could make an extra 5bhp, or maybe it's extra marketing efforts that account for most of it......

 

Slightly longer stroke gives the capacity increase, and CR hike. If the inlet valve is bigger and head different I would expect they may have modified the chamber (and piston) to increase squish, which gives improved torque throughout the range, and allows more advance. I guess I need to find out which version of 750 I have - the bits I had with the engine suggests it’s the later one.

 

Look at the front-center of the jugs, they have the cc's cast in there. "early" 750 engines were cast as 748cc, the 1984 RL engine has a 749cc cast.

And look at the underside of the cylinder head on the rear (left side, if I recall correctly), the casting # for the 1984 RL head is 5G201...........earlier 750's and 650 YICS heads have casting 5G200, while the 700 air-cooled heads are 5G202.