Gasket thickness and valve timing

[rvstudboy]

Im getting ready to install a 750 big bore kit and I want to use copper gaskets. Im thinking composite base gaskets and thinner head gasket. Ive read a couple threads and there seems to be a consensus that cam timing can move power band to a higher rpm range. first question.. Can you really end up with too much compression?? And second.. As far as I can tell..thinner gaskets would retard cam timing. Why would this move the power band up in rpm? It should move it down. Am I wrong? With the performance cam from mikes, Can you get a sprocket that has slots for cam timing?? Does it really make that much difference??

 

[ridgerunner1965]

you can end up with too much compression.i have done a lot of experimenting with compression on HD bikes, not so much on parallel twins.

what i have found is the higher compression the more problems you will have.

starting the bike will be harder, hard on starters if your using the electric leg.

bike will be more finicky about fuel.higher octane will be a must. from my experience commercially sold octane boosters are a joke.

bike will more finicky about exact timeing.the exact timing for the best horsepower may not lead to the bike starting well or easily.kick or electric it will put more stress on componets.

the power band will move upward in the rpm range simply because you can not nail in to it at lower rpm because of pre ignition.or as we call it spark knock.in a bike it sounds like TINK. and every time yu hear TINK, a small piece of piston goes away.

if you can keep compression below 9 or 9.5 to one you shud be fine.but again my experience is mostly with vtwins not parallel twins.

a 10 to 1 compression engine is fine but more finicky bout fuel and timeing and places stress on starting componets.

its just something to think about.

 

[xsdogstar]

quote of the day...

...and every time yu hear TINK, a small piece of piston goes away...
Ridgerunner1965

 

[RatyXS]

Just to clear one thing up, pre ignition and knock is different. If you are running a stock style piston( dome/ ect), which in cases of these bikes is some where in the area of 8.x-1 then a thinner gasket should not be to much of a issue.. Cam shaft profile can also make the actual compression ( static ratio ) vary.. So it all plays in the roll. If you are staying 10.1 or less you should be ok, tuning the bike should not be to bad. However I would suggest 93 oct fuel..

 

[rvstudboy]

So if im using a 750 big bore kit from mikes and the upgrade cam and spring kit, with thinner copper head gasket, whats my compression? Is this a config. that I can run without it being a pain to tune? ior start. Im going to kick it. I already took the starter off and the gear and put a freeze plug in the hole. Im going to use the stock 1980 ignition (black box).

 

[NONclow]

First off, water cooled engines can handle higher compression with more consistant temp control unlike air cooled ones since all they have to dissipate heat are fins. As stated 9-10:1 is about the best for a street bike. An increase in bore also increases the volume of the combustion chamber which lowers the comp ratio, not much, but up to a few tenths of a point, although a thinner head gasket would counter most of that. Second, advancing the cam timing makes the valves operate earlier, in conjunction with more advanced ignition timing it leads to better low end acceleration whereas retarding tends to lean more towards a complete combustion process or a more thorough burn, in essence running better on top not just burning it quick and getting ready for the next cycle.

EDIT: I don't have a 750 kit, and don't want to waste my time/money, but I'm sure they have a higher PISTON ratio. But with a stock piston profile on the incremental oversizes, the chamber volume DOES increase.

 

[TwoManyXS1Bs]

Just fyi, for the xs engine, a combined head and cyl gasket thickness change of about 0.015" translates to a 1° change in cam timing. Thicker advances, thinner retards...

 

[grizld1]

Why don't you guys restrict your comments to what you actually know and have done? FYI, NONclow, the difference between modern air cooled bikes that run at CR's north of 10:1 on 87 octane and older iron that's touchier about compression lies in the design of the piston and combustion chamber. It's clear to me that you have never CC'd out a motor--increasing the bore squeezes more fuel-air mix into the same combustion chamber, RAISING the static CR. Raty, static CR is total the volume of combustion chamber (with valves closed) plus cylinder at BDC divided by combustion chamber volume. This is unaffected by cam profile in any way. In fact the measurements are best taken with the head torqued down without the camshaft in place.

The XS650 motor will run on high octane pump gas at static CR as high as 10.25:1. Higher compression will not move the power peak up in the rpm range; if you get detonation when you whack the throttle at low rpm you're in the range where you're lugging the motor anyway, and that does more damage than racking it out to redline. Raising compression is the only performance modification that will yield increased power throughout the range. Combustion efficiency will also improve.

There are downsides to increased compression. First, the engine will run hotter. This is a concern mainly if you get stuck in slow-moving traffic. Second, you need to pay close attention to ignition timing. Read Gordon Jennings' article on plug reading at http://www.strappe.com . And of course if you wind up in the boonies where high octane fuel is unavailable, you'll need to hope you have enough left in your tank that adding just enough 87 octane and a bit of commercial booster will let you baby your motor to fuel it likes. For both of these reasons, I no longer run 10.25:1 in my motor. IMO you'll be happier with CR in the neighborhood of 9 to 9.5:1. The kit from Mike's will put you there.

Retarding valve timing will move the power peak up in the rpm range. TwoMany, with respect, your experience with deck height and valve timing may differ from mine. My belief is based on anecdotal evidence only; but a long time ago I dropped the deck on an otherwise stock motor by .030". The result was a flat spot from around 3,000 to 3,500 rpm, followed by a crazy surge of power up high that was so much fun that I left things as they were. A difference of 2* wouldn't have had such a large effect.

rv, to learn how to CC your motor and degree your cam, go to http://www.muzzys.com. Good luck to you.

 

[TwoManyXS1Bs]

... A difference of 2° wouldn't have had such a large effect...

That's right, it's really miniscule. You'll get more timing retardation just from camchain break-in and stretch after a couple thousand miles. A lot of other things happened with your 0.030" deck drop...

 

[rvstudboy]

I think ill just get the 750 kit and run it with the gaskets that it comes with. great info.. thanks

 

[Marlin72xs]

I think ill just get the 750 kit and run it with the gaskets that it comes with. great info.. thanks

I'm curious. Did you get the kit? Does it work well?
I'm about to install Mike's 750 kit on my 72, (had it since Christmas), and I'm trying to soak up any and all info about the changes in valve timing and compression.