Head differences?

[e30 gangsta]

Hey yall I have a running Question guys I have a running 2h7 1978 xs1100 that I would like to redo the head on. I have read that swapping to a late model head is better because it has bigger valves. My question is when you do that swap is there a drop in compression?

I have a complete 4r0 engine that I can can pull the head off.

Plan is to use that head, and run the 78 cams with the 78's timing advance. thoughts?

[Bonz]

Bigger valves yes, however as you may know the later models had a slightly lower CR due to redesigned pistons/combustion chambers along with more exhaust lift to work toward better emissions. Don’t know if the larger valves/different combustion chamber shape of the later model head would have an issue with the different piston crown of the earlier models or not when running the early model cams.

3Phase and/or others will be along shortly to share what combinations work best and what ones don’t work as well or at all.

Edit: Gold Mine! Went back to .com and found this from Dan Hodges, he ran every combination and then some. This was in regards to a Wiseco Big Bore, but may indicate you are ok with the combination you opened with in the first post. It’s what you are asking about just with Wiseco pistons vs early model stock pistons.

Dan Hodges wrote::

“Hello Friends,
I have read a bunch of questions about heads, pistons/compression, cams, carburetors and so forth on the XS and from 2008 thru 2010 I ran the table on all this stuff and posted it and so you can find this stuff if you search my post because I'm not going thru all this stuff again.

Lastly, whatever you do don't go over 160 lbs. of pumping compression because this is all 93 octane gas will support on an air cooled motor on the street. Basically, the Wisco 1196 pistons with the early cams and late heads are a good combination and will fetch about 9.5 to one static compression without cutting the head or about 150-160 lbs of pumping compression once the rings have seated.”

[2WheelFun]

Thanks for the info, I have an 80 parts motor and was thinking of switching heads because my 79 has some broken fins from a tip over.

I can't imagine the performance difference between swapping the two being too noticable without the big bore. Maybe I'm wrong but I'd guess less than 5hp gain. Probably not worth the cost of the gaskets to me unless I found some pistons cheap.

[Bax]

Thanks for the info, I have an 80 parts motor and was thinking of switching heads because my 79 has some broken fins from a tip over.

I can't imagine the performance difference between swapping the two being too noticable without the big bore. Maybe I'm wrong but I'd guess less than 5hp gain. Probably not worth the cost of the gaskets to me unless I found some pistons cheap.

Can get cast pistons and BB gaskets here fairly well priced.

---Bax

https://www.cruzinimage.net/?s=yamaha+xs1100

[e30 gangsta]

So I have that exact big bore kit listed above. That with a late model head and 78 cams seems to be the ticket then.

[Bonz]

I would gather it bolts together straight up with no issues per se, any thoughts from the collective if it is necessary to check piston to head and piston to valve clearance to be sure?

[skids]

I would gather it bolts together straight up with no issues per se, any thoughts from the collective if it is necessary to check piston to head and piston to valve clearance to be sure?

Wasn’t there some sort of compensation for the piston crown design and bigger diameter valves with changes to the cams? Like, shorter valve stroke and longer duration? I don’t know, but I know the cams are different.

[Bonz]

My understanding is the 80 and newer cams had a bit more exhaust lift (.5mm more) which helped offset changes that accounted for the slightly lower compression ratio. Like you said, there was a change to the piston crown, combustion chamber and slightly larger valves. If I remember correctly the intake lift and duration as well as the exhaust duration stayed the same as no mention was made of changes in duration. That's just based on period literature I have read. Ultimately regardless of changes to cams and valve sizes, the lower compression ratio with the same displacement means you're probably not going to get the same power. Yamaha was going for emissions first and foremost.

Ultimately the newer ones never ran the same fast time in the quarter mile like the 78E. Even the Specials with smaller diameter rear tire and thus shorter effective gearing couldn't do it with the newer engine.

[e30 gangsta]

Bumping this up.

I'm dropping off my 81 head
And ebay 1196cc pistons off to the machine shop. I will report back once it's all bolted up together and I verify that there's no piston to valve clearance, and some compression #'s. Thanks!

[LAB3]

Looks like pretty good prices on other parts too on that site, wonder if those low priced intake boots are worth the price. Have heard of issues with the balance ports pulling out on the cheap ones.