Lowering compression

[gus33]

I've decided to lower the compression on my 79 special to 8:1 ala XSJohn. I see that quite a few people have done this. It seems that all I need to do is use two head gaskets and retard the timing by 2 degrees. Is there a favourite gasket that you all use for this? Any other tricks I might be missing? I'm running a Boyer ignition that I really like and John's needles. So far so good but I am looking forward to an even cooler running engine with a little less vibration. Thanks, Angus.

 

[xsjohn]

Agus.......when you raise the head you HAVE to change the """valve""" timing .......raising the head rotates the cam forward (advancing it).....if you don't do this you will be taking it apart to do it.......the ""cam"" timing on these is already advanced to the wall......any more and it enters artifical leaness because the intake valve will close to early thus the cylinder won't fill completely..........send me the cam and I will retard it correctly..........if you do this...........

I think that having someone make a .040 thousands metal base gasket is the best....sealed on one side to the cylinder and the other using a regular base gasket...but 2 head gaskets bonded together in the center during assembly with a ""thin"" layer of jb weld is good........and a """very""" thin layer the night before using silicone on the top and bottom of the gaskets at the center rear of the tunnel........hell I even sprayed the head and cylinders with pam so I could re-use the gasket later......being the cheap bastard that I am........

I am amazed how mine goes down the express now......and for even more cooling have you seen the wings I designed......all in all I have lost 30 or more degrees on the cylinders in the summer on the express....reducing temperature is a combination of things..including R/L cylinder temperature control.......and very benificial..

xsjohn

 

[big al]

Good idea lowering the compression but would it not be just as easy to take a cut off the top of the piston wich would not affect cam timing only ignition timing.
I guess i should ask if there's enough meat on top of the piston to do that?

 

[xsjohn]

big al......meat on top of the piston....measured the best I could and there is 6MM on the top of the piston in the center.....and it gets quite a bit thicker toward the valve indentations........suspose one could concave the cut toward the valve indents to limit the thinness on the top center....since I tried this to see if I would like it I preferred a method that I could easily back out of..........now I would try that....but figuring how much to remove may be a head scratcher...... moving the cam for me is easy since I have done so many.....and it has always worked.......new cam chain won't fit on when the head is raised but it does make a somewhat stretched chain get back to new adjustment...and I have collected a few good used ones so I will probably be dead before I use them all.....I pulled a very stretched chain apart and could see almost no ware on the pins.....only a thousands that I could barely measure..........chains may be good for a lot longer than we think.......since I did this I noticed that I have only adjusted the chain once in maybe 10K.......to my delight.........so a theory could be that once they break-in they kind of stabilize........and the break-in might just be to long with the stock head height........and lowering the compression gets rid of a lot of engine ratcheting and smooths them out so that may factor in also......changed my sons XJ650 chain at 58 thousand and it was barely stretched at all....same chain so I may use it to in the future.....LOL...the XJ engine being a bunch smoother than the XS so the chain lasted longer..........

xsjohn

 

[Travis]

Good stuff. Wouldn't there be a way to measure the amount of material removed from a piston (or combustion chamber) using displacement with measuring each piston or the head before and after material was removed? Couldn't you remove material from the head and piston to do this? It's what, about 4.5 cc's of material from each chamber?

 

[xsjohn]

Travis.........might get enough from the piston safely if all was not removed just from the crown..........did you figure 2 X 4.5cc's is about the same as raising the head .040 ........the extra head gasket I added was .045 thou.....the head I think in the combustion chammber is steel.......going back would not be an option there......I still like the cam chain going back to original adjustment when raising the head...and a base metal spacer can be made by a lot of companies pretty cheap..........everyone is afraid of moving the cam........I ain't........... and if a person doesn't mind giving up just a bit of high end power this really makes them run pleasant..........with the timing retarded a bit they pull off in 1st from just above idol.......and cruise like a car..........

xsjohn

 

[dlabkeeg]

These motors are supposed to use 91 octane(95 in Eur. & Jap.). Since you've gone down to 8:1, do you use 87 octane now. I'm thinking about doing this, but I'm more retarded then the cam ever will be.

 

[xsjohn]

I still use the highest octane I can get...........I think 87 doesn't run as smoothly no matter where I set the compression......hell I still get 53-55 to the gallon...what would I save.....retarded hell if you can take the head off you can do it.........send me the cam if you raise the head......it won't run right (at all) if you don't change the cam........you could ask me how I know........the answer would be "I tried it without"....then had to use my brain .....

xsjohn

 

[dlabkeeg]

I just got this motor running right, but I have another. When I start taking it apart, some time near the end of summer, I think I'll be sending you that cam.

 

[xsjohn]

If you start with a good motor with good compression and all clearances correct to do this you wouldn't be sorry....compression goes from 150 to a bit over 140.....doing this to an ole wore out engine of course would be a waste of time......and I'm betting that it would last a lot longer being so much smoother especially if the timing is retarded a bit to gain the benifit of a engine that doesn't have to be reved to hell to get enjoyment and pull properly........xsjohn

 

[dlabkeeg]

I'm going to rebuild the top end. That will hopefully get me close to square one.

 

[xsjohn]

will do .....
xsjohn

 

[gus33]

John, thanks for the walkthrough, that really helps. I have seen the fins and I think I will try something similar with my new (xs850) oil cooler. Of course it is still a little brisk here in Saskatchewan. The snow has just melted and the bike runs well in the cooler air. Just takes a while to start it at minus three degrees Celcius. Maybe I need a block heater? Thanks, Angus.....

 

[xsjohn]

Yea......I can dig the cold thing.........I lived in Newfoundland for 4 years and I can honestly say that never once did we concieve of trying to cool something........It was 89 degrees on the bank thermometer here in NC yesterday and a bit smogy as the air and heat blew up here from I would suspect South America and places like that.....you should see the huge strange moths and bugs that appear on the tennis court from far away when this happens.......

 

[gus33]

No bugs here yet, just gophers (ground squirrels) on the road. Ran over one the other day, nearly lost it. It sure made a mess, I think I might put my front fender back on. I might take you up on your offer to re adjust the cam. If I was to do it myself, however, would two degrees retard be enough?

 

[Sundie]

just gophers (ground squirrels) on the road. Ran over one the other day, nearly lost it. It sure made a mess, I think I might put my front fender back on.

Oh, ewwwwwwwwwwwwwwwwwwwww. The imagery of *that* line up the engine....

 

[xsjohn]

Had one of those huge South American Moths that blow up here in the spring hit square in the middle of my windshield......had to be over 3 inchs long and the mustard colored mush that ran around the sides of the windshield was disgusting.........makes me love the windshield even more........that fricker would have hit me right between the eyes....only wearing sunglases......and when it hit it went THUMP........

xsjohn