My built xj 750

These XJ's have short wheel bases as well. But honestly I'm not a big top speed Guy. To Me all the Funs between 30 and 90..

 

Oh and 1 last Mod I forgot to mention, The Clutch. After I Installed the Pod's on the Factory Boot's it would start Slipping like Hell around 5k. Tried just replacing Plates and Spring's but no Dice. Ended up having to go with 20% stiffer Spring's to get it working right...

 

nope

yep

Sorry to burst your bubble Jerrod but no way in hell is an XJ750 hitting 95 hp at the rear wheel.
What does it matter anyway, you're having fun modifying things, I respect that, in the end that's what counts.

 

No worries Brother, no Bubble has been burst. I guess all I can really do is explain My conclusion. As I said I'm Running Neck and Neck with a 2013 CB-650R. It's healthy with a Shop Tuned Exhaust, but we'll leave that out and just acknowledge it's Factory claimed 94hp. We're both 200lbs, the CB is a 440lb Bike, My XJ with a full Tank is 460lbs on the Nose. There's no way I'm keeping up with anything less than 95hp. I was honestly being Modest, I think the Real Number Is a little North of 100..

 

You don't have to believe Me Man, I'm not offended or Hurt in any way. But I will ask have you given your undivided attention to what a Naturally Aspirated YICS 750 Can make?

 

https://bikez.com/motorcycles/yamaha_xj_750_1983.php

 

Yep. I lost ten stones and my bike went like a rat up a drainpipe.

 

Disclaimer: I don’t know crap. However, I believe there must be theoretical limits to how much HP can be generated per unit of displacement for any given fuel.

a quick google seems to suggest that 1 HP per 10-15 cc of displacement would be expected for an N/A engine. So 82 might be on the high side of that, but not outrageous.

makes sense that in the real world you’d end up with 50-something at the rear wheel.

 

Is that faster than a raped ape?

 

I thought of something last night, and it may explain why I'm making a little more Power than the average XJ-750. When I Rebuilt the Head it was gonna be like a Month wait on a New Head Gasket so I Cleaned up the Old one and Re-used it. Since then if I Run 87 or 89 it knocks. I have to Run Premium now. I think using the Old "Thinner" Head Gasket Raised the Compression and that's why I'm getting a little more Power. I'm not positive, but it's the only Theory I have...

 

Apes get out of the way.

 

This comes down to what's called volumetric efficiency. Decently tuned, ie modified 2 valve engines struggle to get over 100%, but with tuned pipes and intakes matching cams it can work. 100bhp per litre is a decent figure, but of course hp is torque multiplied by rpm, so higher rpm get you bigger hp numbers - this is where bike engines gain hp numbers over car engines.

 

No not that much, at least from what I've Read you lose around 10 or 12hp through the Shaft, which does lose more than a Chain drive.

 

And a big Factor You didn't acknowledge is Compression Ratio. If you can increase an Engines Compression Ratio You can make more Power with the same displacement. This is where different Octane Fuel's come into play. Low Octane Fuel becomes unstable under High Compression and will Ignite in advance of the Plug while the Piston is still in it's upstroke. That is Knock. So Higher compression Engine's require higher Octane Fuel to avoid knock. But I've also Read that some modern Engine's have found a loophole using Direct Injection in order to Run Boost with Higher compression Engine's. But that's a different conversation...

 

You misunderstand the point I'm,trying to make here - I'm referring to generic bike engines - which rev higher than car engines, you double the rpm and the hp doubles, simple, for the same torque.
What happens after that is whatever. You are correct though, the shaftie loses more power than a chainy.

 

I'm not sure that I am totally following You. So try not to take offense to anything I say, I'm not trying to be offensive. By "Generic" Motorcycle Engine I assume You're referring to the Transverse 4. And though Your Theory seem's Logical, You can't really just raise an Engine's Redline. By the Time You achieved a higher Redline You would no longer have the same Engine. And just because an Engine is Revving higher does not alway's mean it's making more Power, it's really not that simple.

 

Going back to my original post:
Damnit, struggling with inserting quoted posts..
Anyway, for 2 valve car engines, 100 bhp per litre is a decent figure. You can get more, but generally even 1 litre car engines are limited to 7000 rpm. So for a 1000cc car engine, 100 bhp would be quite an achievement.
If you had a 1000 cc bike engine the rpm limit is likely to be 9000 rpm, maybe 10.
So the bhp jumps up, provided the thing can still breath at those rpms (that thing called volumetric efficiency again). Remember, hp is torque times rpm, so if the torque holds up, more rpm equals more power, it’s simple maths.
Doing things like fitting single carbs to your Yam 750 might be a good idea while using it to get to work, but if you want to race it, it’s a disaster. Modifying an engine, any engine, is about maximising torque first, then it’s about getting that torque to stay up over a wider rpm range, and keeping the whole lot from going bang of course.
The problem is that to get more air through an engine sometimes needs bigger ports, carbs, exhausts, and this loses you torque at the bottom end...

 

I understand.. You're talking about Flow. But Volumetric Efficiency isn't the whole Picture. For example raising the Compression Ratio increases Power and Torque without affecting Volumetric Efficiency at all. From what I've Read for every Point of Compression added you can realistically expect an increase of about 8% of the Engine's Gross Power Without Improving Flow. But there's certainly a downside. As the Compression Rises so does Heat, and that Heat will cause lower Octane Fuel's to Detonate/Knock...

 

And there's other things that dictate Engine capability like if the Cylinder's are Oversquare, Undersquare or Square. With an Oversquare Engine the Bore is greater than the Stroke, Their Peaky and make all their Power up Top. With Undersquare the Stroke is greater than the Bore, Their Torquie and make all Their Power lower down in the Rev range.. An R1 would be an example of a Oversquare Engine, Your typical Harley will be Undersquare...

 

You need the right compression ratio to match your cams - tame cams and high compression is a recipe for detonation.
Knock, as you say. But if you have wild cams with lots of overlap and not enough compression the engine performance will be pants.
And you’re right about the bore to stroke ratio, if you want hp you need an oversquare (short stroke). Yes it will be “peaky”, but the maximum power will be higher.

So, to make a 750 2 valve engine more powerful, what do you do?
Bigger, better flowing valves
Optimum porting, without “hogging it out”
Flat slide carbs
Tuned length intakes with free flowing filters
Free flowing exhaust, usually bigger diameter (more in needs more out, plus higher rpm needs more out)
More cam timing and lift (needs lighter valves and or stiffer springs)
Higher compression (this conflicts with bigger valves and more cam timing for purely mechanical reasons - things start getting close)
Lighter pistons, con rods, maybe big end journals
Optimised spark and fuel

All of this could turn a 750 into a 100bhp screamer, maybe. What did the OP do to his? I forget...
Oh, forgot an oil cooler and decent clutch, brakes, tyres, underpants.

 

The Problem with these XJ's in My experience is there's not really an Aftermarket. I have a Gen 1 CB-750 and You can find all kinds of goodies for it. But not much for the XJ. One thing I have considered in Lou of an all out 900 Swap is Boring it out to 67mm and Running 900 Piston's with the 56mm Stroke. If My Math is right it would make it a touch over 795cc. And then instead of Machining the 750 Head, I was hoping I could get a 900 Head to Mount on the 750 Block. It looks like it's possible, but looks can be deceiving.. I haven't committed to doing this, more of a Daydream really. Again I'm not even sure it would work

 

Well the 900 engine should fit a 650 or 750 frame. So the 900 is the one to fit, then tune? But you're on your own regards finding bits to make it faster. As above, I'd start with better valves, if the metal is there.
And with the 900 being bigger bore and longer stroke, lighter internals would help rpms and reduce stress?

 

This is all easy to put to rest. Just go to a local dyno and have them run the bike. I think it cost like $100 and then post the dyno chart. That will take care of all the speculation.

Bottom line, really glad you like how the bike turned out. Great project and as long as you happy with the build that is all that matters. Enjoy.

 

I will do exactly that when I track one down. May be forced to take it to Columbus cause all I can find is Car Dyno's around here..

 

My guess, and this is all it is - pod filters and a decently flowing exhaust, assuming the jetting is near right, will net you 5 to 8 bhp max, with a hole lower down. It might well be quite a bit faster, but it will likely feel faster, and Rev higher.
Let us know eh?

 

My Bike? I do have Pod's, but I modified them to fit on the Factory intake Boot's. I have a Mac 4 into 1 with no Muffler, it's just a 10" straight pipe. And I'm Running #130 Main Jet's. I put Stock Clutch Plates in it with 20% Stiffer Spring's.

 

I feel like People don't know what I'm talking about when I say I modified My Pod's. So this is what I did, I had to open them up with a Die Grinder about 15thou to get them to fit. It really made a difference. I also bought new Carb Boot's, and took a piece of 220 Sand Paper and matched them to the Intake Port's..