Hey you,running late head on early motor

[tarzan]

I am getting ready to put my motor back together and would like to know if someone running the newer head on an older motor notices much actual difference.
I know the compression will be about 1. lower with that combination,but I'm not
racing the thing and would rather not spend the money on the old head to freshen it up.Unless it will make the bike a dog.
I will be running the late cams as well.

[GARTHXS]

if I am not mistaken

the bottom ends are almost identical the difference is only the heads?

[DGXSER]

Do a search for post by Mr. Hodges. I know he outlined the results of the different setups pretty thoroghly here lately.

IIRC, the newer (80-81) pistons had a higher dome on the piston to account for the bigger space in the head for the larger valves. So, you may find that you breath a bit better with the newer head and therefore no real difference. I also seem to recall that the older cams in the newer head was a hotter setup, but I may have that one backwards.

Best bet, search for Dan's post.

[dbeardslee]

The 80/81 heads have bigger intake valves, while the 78/79's have more lift and duration on the cams. Some members have put the 78/79 intake cams in the 80/81 heads, but from what I've read there have been mixed results with that setup. Also be aware that the valve clearances are different on the 80/81's than on the 78/79's. I'm not sure which matrix you would use if you mix the cams and heads. If it was me I'd take the later model heads apart, clean the gunk out of the ports while you can get at them, lap the valves and put it back together with the stock cams. Lapping the valves and cleaning up the ports will make it run much smoother.

[tarzan]

Well, from what I've read,the newer head should be run with the newer cams.
TRBIG experienced less power on the lower rpm range with old cams in a newer head.But that was on late model pistons right?Results would probably similar with early pistons.
I read a bunch of Dan Hodges post and I didn't see where he addressed newer head and cams on older pistons.He did address older head with newer cams and vice versa as well as some big bore stuff too.
So from what I have seen so far,it looks like the only drawback to my proposal(late head and cams) is that the compression will drop a little.
So I was kind of hoping to hear from someone who has that combination or has had it and what their experience with it is/was.
I'm thinking it should run ok.Also it looks like valve clearance should also be ok.
What do you guys think?
Thanks Rick

[Pat Kelly]

My plan on the MNS is:
80 engine with 1196 kit (already done)
78 or 79 head and cams (have one set of each)
78 carbs (have a set)
78 advance unit (still need)

Since the 78 was the fastest of the XS's I will use 78 parts with the 1196 kit. I think it was put forth that one reason the 78 was faster was the additional 5 degrees advance.

(my 80 head has issues due to a previous owner)