Compression Ratio vs. Grade of Gasoline

I think some good port and polish work would provide you with a better result, get the head to flow more and it will like the cams a lot better.
I dont think you need to go overboard on the compression ratio, in fact a bot lower compression motor will last a lot longer with less problems, the cams, port work and carbs will make it flow and run better.

BSKZ650 wrote:

I think some good port and polish work would provide you with a better result, get the head to flow more and it will like the cams a lot better.
I dont think you need to go overboard on the compression ratio, in fact a bot lower compression motor will last a lot longer with less problems, the cams, port work and carbs will make it flow and run better.

my propaganda leaflets have been paying off !:laugh:

10.25:1 will work fine on premium pump gas. I am runing 12.5:1 on my GPz1100 and only have problems on very hot days in Chicago, (600'elevation) at which time I add a quart of Xylene, which I buy at Sherwin Williams in a 5-gal bucket for $55. I keep a quart under my seat just for those ocasions. Xylene is 117 performance # and is used in Europe instead of Alcohol to bump the octane. I have been doing this for quite a few years.

otakar wrote:

10.25:1 will work fine on premium pump gas. I am runing 12.5:1 on my GPz1100 and only have problems on very hot days in Chicago, (600'elevation) at which time I add a quart of Xylene, which I buy at Sherwin Williams in a 5-gal bucket for $55. I keep a quart under my seat just for those ocasions. Xylene is 117 performance # and is used in Europe instead of Alcohol to bump the octane. I have been doing this for quite a few years.

Just curious . What psi do u have if u do a compression check on your 12:1 GPZ1100?

Never checked it, I can do so when i pull it out this spring.

You guys are luckier than me. I put a 10.25 Wiseco kit in a GPZ1100 back in 1982 and it would not run on pump gas, even premium leaded. Ran great for the first few miles, but hot, forget it. My stock KZ1000MKII will detonate real bad in hot weather, but will run on premium unleaded for the most part. The 77 KZ650 with stock GPZ750 pistons runs good on premium. Even my everyday rider 650 will ping in hot weather running low grade gas. The H2 needs premium to run with stock compression. So I've learned what works and doesn't for me at least premium is just barely enough octane even with all stock setup, any higher forget it.

I am afraid to ask where you live.:ohmy:

If you get detonation at 10.25:1 compression on a GPz1100, then slot your ignition plate and retard the ignition a couple of degrees. Alternately, you can pick up a Dyna2000 and they give you a couple retarded advance curves. The 10.25:1 should run fine on hi-test (93 octane). Ron lives in a tropical rain forest hehe I NEVER get any detonation with my KZ1000E1 shafty which is essentially a MKII engine even on the hottest days using 87 (regular) octane. Of course, I don't live in a tropical rain forest...it ain't rained for best part of a year in the Hill Country!

RonKZ650 wrote:

You guys are luckier than me. I put a 10.25 Wiseco kit in a GPZ1100 back in 1982 and it would not run on pump gas, even premium leaded. Ran great for the first few miles, but hot, forget it. My stock KZ1000MKII will detonate real bad in hot weather, but will run on premium unleaded for the most part. The 77 KZ650 with stock GPZ750 pistons runs good on premium. Even my everyday rider 650 will ping in hot weather running low grade gas. The H2 needs premium to run with stock compression. So I've learned what works and doesn't for me at least premium is just barely enough octane even with all stock setup, any higher forget it.

My 1979 750 twin never ran right on regular gas, always OK on premium. Not supposed to need premium, maybe it's the crappy california gas.

I live in Colorado so we are high altitude at 5400ft, but I've ridden everywhere from Maine to LA on the KZ1000 and noticed the detonation everywhere I went if I had to lug the engine down.

That seems odd Ron.
I can see that u would run lean in Colorado, but it should be better at lower altitudes.
Most guys run 10.25:1 not trouble. You may need to adjust the timming or degree the cams for more overlap but it seems to work fine.Could even be week spark, or lean carbs.