Valve scavenging through the ages

[LoHo]

What are the size differences between the valves of the '78-81 XS1100s? Do we know the diameters for different years, as opposed to "bigger/smaller"?

[crazy steve]

The 78-79 motors used 36mm intakes and 31mm exhausts, the 80-82 motors had 38mm intakes and 32mm exhausts. They can't be swapped as the larger valves also have shorter stems...

[LoHo]

Can the heads be swapped with the different valve set-up?

[jwhughes3]

Not having done it - I can say I have seen it done - I think Succubus up in austin has XJ heads on a 79 case and runs. It has to be a complete upper swap to work. But the timing for the older valves will be off - I think Patrick has had issues because of that - valve duration is different from one head to another but if you don't change TCI and advance then it won't run right.

Those more expert than I can chime in - perhaps I am extrapolating too much.

John

[crazy steve]

Yes, but there's caveats....

The early head has a smaller, shallower combustion chamber with a matching lower 'dome' on the piston to give a 9.2 compression ratio.

The later head has a larger chamber with a bigger piston dome to keep compression up, although these motors still lost .2 of a point of compression.

Swapping the late head onto the early motor with early pistons will result in a loss of compression ratio, so you won't see any gain from the larger valves. Going the other way, you will see a compression increase but lose the advantage of the larger valve size. This will improve low speed power with a corresponding loss at the top end.

If your doing a 'big bore' upgrade, the late head is preferred. You gain compression from the overbore in any case and the larger valves will help with breathing.

The main reason for the valve size change is Yamaha changed the cams, and which cams you use will have a significant effect. The 78-79 motors have longer duration cams (open the valve sooner, close it later) which really helps with higher-rpm power but at the cost of some low speed power and 'smoothness'. Downside to this is they are 'dirtier' in terms of emissions at lower speeds (which was becoming an issue). The longer duration cams also like more initial ignition advance. When they went to the shorter cams, by increasing the valve size they offset the loss of duration to a degree, although there was still a small power loss at higher rpm.

Ignition timing also has a fairly big effect. The 78 bikes were the most powerful because of a more aggressive ignition curve. The 79s were detuned in the ignition, then in 80 they changed everything in an attempt to retain power but make the motor 'cleaner'. All these things are interrelated (timing, compression ratio, valve size, cam lift and duration, etc), changing any one thing will effect what the 'best' tune will be and whether you're tuning for power or emissions. Compromises have to be made....

[BA80]

A little addition to what Steve posted.

Keep the ignition timing components matched to whichever cams you are using. If you put an early head and cams on a later engine you need to use the early timing components also and vise versa.

The ignition timing curves ar tuned to the cams.

The larger valves really don't make that much difference in flow unless your running 8K - 10K all the time.

I've been meaning to experiment with the early head and cams on the later piston for a while now but haven't gotten around to it yet. If there is enough clearance the higher compression should theoretically bump the HP.

I think if you were to put the eary cams in a late head there would be a possibility of the larger valves interfering with each other with the lift and duration of the early cams though.

Just thinking out loud, so to speak.

[crazy steve]

Keep the ignition timing components matched to whichever cams you are using. If you put an early head and cams on a later engine you need to use the early timing components also and vise versa.

The ignition timing curves are tuned to the cams.

If you're trying to do this as a bolt-on deal, that's very good advice. If you have a '81 or a XJ, the early cams won't be a good idea unless you swap out the entire ignition system, TCI and all.

You really want the '78 advance with the early cams. Yamaha compromised the '79 unit for emission purposes which is why the '79 models were slower.

But I do think that the '80 mechanical advance has possibilities; it's actually got a more aggressive curve than even the '78 unit, but because it has 5 degrees more advance that limits how much initial advance you can run. I'm planning on trying a 'altered' unit (removing that extra 5 degrees) on my '78 'modder' bike and see how that works....

[skids]

Yes, but there's caveats....

Swapping the late head onto the early motor with early pistons will result in a loss of compression ratio, so you won't see any gain from the larger valves. Going the other way, you will see a compression increase but lose the advantage of the larger valve size. This will improve low speed power with a corresponding loss at the top end.

Steve, would using a head from a 78 or 79 with a later engine also be riskier with contact of valves with piston crowns?

[crazy steve]

Steve, would using a head from a 78 or 79 with a later engine also be riskier with contact of valves with piston crowns?

There's been reports of members bolting the early head onto a late motor with no issues, but I would want to check piston-to-valve clearances. Same goes for the 'other' way; I'd want to make sure the larger late valves clear the smaller early valve reliefs..

[BA80]

That's exactly what I was talking about when I mentioned the clearances in that modification. You would have to put an early head on an engine with late pistons and clay the cylinder with the cam timing as it should be and then measure the thickness of the clay in the thinnest spot.

On automotive engines this is done without a head gasket installed to allow some extra room.

[mack]

Timely Post

This is a timely post for me. As some of you may remember the motor for this E I'm restoring was completely seized, full of water. I'm at the stage now that I'm thinking of putting it all back together. The original pistons, rings and jugs are garbage and cannot be saved.
I bought a spare set to replace the existing ,but once delivered I found they are all out of spec as well, almost 5 thousand at the tops of the jug sleeves,consistant accross all four, so they are are garbage as well.
So now I have to have the the jugs bored out for the wiesco 1179 kit.
My question is are these kits year sensitve. Ie. are these piston crowns going to be compatable with the shape of the E head chambers? Also I believe the compression ratio increases to 10.25. Considering the smaller valves, what is the impact on the engine performance and tuning requirements. It will have stock air box and exhausts and I have a set of good E carbs here,and the original cams are fine; so other than the wiesco pistons, it will be capable of being all stock.

[crazy steve]

So now I have to have the the jugs bored out for the wiesco 1179 kit.
My question is are these kits year sensitve. Ie. are these piston crowns going to be compatable with the shape of the E head chambers? Also I believe the compression ratio increases to 10.25. Considering the smaller valves, what is the impact on the engine performance and tuning requirements. It will have stock air box and exhausts and I have a set of good E carbs here,and the original cams are fine; so other than the wiesco pistons, it will be capable of being all stock.

To my knowledge, those kits will fit either motor. The piston dome shape is closest to the '80-82 type, but not as tall, so you'll get a slightly higher compression ratio with the early head (premium fuel required...) compared to using the late head. The smaller valves in the early head will help with mid-range power (more velocity) with a small loss at the very top compared to the late head. If I were trying to get every last bit of power out of the combo, I'd go with the late head/early cams but you're not likely to notice in 'normal' riding.

For jetting, I'd start at one size larger on the main jet.

[mack]

Thanx Steve

I specifically asked the guy at the speed shop about the valve clearance issue and he assured me my concern was needless. However I trust the info here far more. I'm not too concerned with a small loss at the top end as long as the low and midrange are intact. Thats were it will spend most of it's run time. I'll keep my eye open for a later model head cheap. I'm getting a bit gun shy though as i've already bought a bunch of what has amounted to garbage trying to assemble the parts I need.