Increasing the Compression Ratio

[Dennyz]

I would like to up the compression ratio of an XS motor to 10:1. A quick examination looks like piston to valve clearance could be a problem. Before I go through all the trouble to check it, I wanted to see if there is any information out there on this.

Anybody run across this kind of info?

[John]

The only info you need is: Wiesco Pistons, 1179 cc big bore kit. That will bump you up to about 10:1.

[Dennyz]

Ya, and the big bucks for the big-bore kit.

[John]

Well bud

We gotta 'Pay to Play'. Mine cost $350 for the parts and $150 for machine work, $100 for new clutch and springs. Gaskets and other comsumables was about $100, Machine shop under cutting The trans gears, $60. End result...PRICELESS
I was going to actually sell of the XS back then and by a late model Kawi Vulcan 1500, but with The fact that I work for an airline and 9/11 really put a hurt on us, I decided to freshen up the ol' XS, and ride it for another 10 years or so.
Oh, BTW, welcome back to the forum, after such a long absence. I have accumulated a couple of spare gas tanks to attempt your big tank project. I'm getting close to beginning to start thinking about attempting it.

[Dennyz]

Thanks,

Oh, BTW, welcome back to the forum,

Good luck on the "big tank" project.

I have accumulated a couple of spare gas tanks to attempt your big tank project. I'm getting close to beginning to start thinking about attempting it.

[TopCatGr58]

Hey John,

I just did a massive search for any posts by Dan Hodges and I can't find any reply with any info in it about SHAVING the heads?! IF you have that info, could you please post it for DennyZ?

Replacing a $75.00 head gasket, along with shaving his engine head...he's got machine tools and such, would be much cheaper for him right now than investing in the big bore toy kit!!
T.C.

[John]

I have received some info from Dan Hodges, but it was about the differences in the 78 ignition advance governors vs. the 80 governors, and the differences between the 78-79 cams vs 80-81's. He suggested that I run the older cams in my 80 head, and swap the older governor for the one in the 80. I don't recall him saying anything about shaving the headthough.

[pggg]

Well, my ported XS motor has wiseco pistons and skimmed head. The crowd down the street who ported the cylinder head also skimmed it to higher compression. From memory they planed down half a millimetre. It's supposed to give more grunt at lower revs. It was harder to kickstart for sure. Motor runs 20deg C hotter at the head.

[John]

Hey Kiwi

Are you running an oil cooler? I haven't put mine back on yet, but I will be very soon. The temp hit 94 here today and 97 is predicted for tomorrow.

[Dennyz]

Well, 1/2mm is about .020" That would make a big difference.

Any chance you could find out how much they shaved exactly?

[pggg]

Hey John, yeah running an oil cooler but don't need it for the next 4 - 5 months as the days are now cool when riding even on a sunny day. Dunno what the exact amount removed was Dan, so not much help really. The engine work was done a few years back but they claimed compression ratio would increase only slightly as they were porting the motor just for street/road use. I've checked the actual compression many times, it reads at 190 - 195 psi. I suppose there's some mathematical formula for this type of thing?

[turbopete]

You should keep a minimum of .040" clearance between the valves/head and the piston for steel conrods, .060" for alloy conrods (not an issue here). As long as you can maintain that, you'll be ok.

[turbopete]

Right, assuming you have an "E" model you have a bore x stroke of 71.5 x 68.6 and a comp ratio of 9.21:1

Given the comp ratio and the swept volume we can work out the combustion chamber volume, so:

Swept volume of one cylinder = 3.575*3.575*3.142*6.86 = 275.48cc

Formula for compression ratio is swept volume x combustion chamber volume/combustion chamber volume, so we need to find the volume of the combustion chamber (x)

275.48*x/x = 9.21

Simple algebra gives us x= 33.55cc for the combustion chamber.

A half-millimeter (.020") skim (of the barrels) would reduce the combustion chamber volume by 3.575*3,575*3.142*0.05 = 2.00cc

Now we can work out the new compression ratio:

275.48+31.55/31.55 = 9.73:1

A skim of .030" would reduce the chamber size by 3.01cc and give you a compression ratio of 10.02:1

[Dennyz]

.030 is what I am shooting for. I am working up a '80-'81 head, shaved .030 with '78 cams (just for fun!).

I will check the valve to piston clearence, just to be safe. The .040 number on steel con-rods is what I was looking for.

Thanks,

[GNEPIG]

.. great info Pete!, you should butt in more often