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head job

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I would like to know what the limit is on milling my head to get a compression ratio of about 11 or 12-1? If I take about 30 thou. will I have to do anything else because the head was done? Will the cams and chain line up the same? The bike is a 78 gs750 that will have some other mods to take advantage of the increase in compression. Any advise will be appreciated.

Paul
 
Re: head job

nasty said:
I would like to know what the limit is on milling my head to get a compression ratio of about 11 or 12-1? If I take about 30 thou. will I have to do anything else because the head was done? Will the cams and chain line up the same? The bike is a 78 gs750 that will have some other mods to take advantage of the increase in compression. Any advise will be appreciated.

Paul
Click to expand...
next time ask an easy question will you.
Why do you want to mess up a motor by screwing with the compression ratio. What "OTHER MODS"
you have to know the volume of the combustion chamber and unless someone had that info you have to use some sort of liquid measurement to do it.((cc the chamber)) If i am correct then you need to compute the volume of the cylinder using the bore and stroke.
Naturally the ratio of one to the other should be the compression ratio. Then use the math again to figure out how much you need to shave the head for the desired results.
i believe the main thing that must be considered is the valve clearance to the pistons. i dont think you will throw anything out of time by the mod. maybe you may end up with a tad more timing chain slack which the chain adjuster will pick up.
I think that is it.??
Ill bet if you cll one of the racing part suppliers they will be sable to spit out the info even if the engine is a 750
 
AND IF YOUR HEAD EVER WARPS YOU MIGHT NOT BE ABLE TO SHAVE IT STRAIGHT, .030 MIGHT BE NEAR THE LIMIT.
 
Compression Ratio needs the following:
head combustion chamber volume
bore
stroke
deck height
rod length
compression height of piston
compressed head gasket thickness
cc's of dish of piston (dish is positive, flat tops is zero, popups are negative)

You can swap deck height, rod length and compression height for deck to piston distance if you'd like. Then you use the following:

pi(bore/2)^2*stroke = svol (swept volume)
deck-stroke/2-rod-CH=dpc (deck to piston clearance)
pi(bore/2)^2*(dpc+gasket)+dish+headcc=ccvol (combustion chamber total volume)
(svol+ccvol)/ccvol=cr (compression ratio)

Converting to cm from mm in the begining makes life easier.


-=Whittey=-
 
It will change the cam timing, since you are looking for more preformance
Milling the head will not gain you anything unless you also degree the cams. I would also think it would be advisable to convert to a manual CCT
to deal with the extra slack. Milling .030 will not hurt the head but you are not going to gain that much. maybe 1/4-1/2 point at the very most. Proper way would be to fit new pistons with the desired comp ratio. If it is ridden alot I would just leave it alone. You are talking about adding alot of heat to the motor, addition of an Oil cooler would be mandatory for a street bike
 
Doesn't the bike manual tell the maximum limit you can grind off of the head?

Something else to consider is whether or not the head has already been done. When I bought my first car (1988 2.0L Cavalier) I had the head shaved due to warpage and a bad head gasket but in hindsight I'm sure I wasn't the first. That car was always pinging after that because the compression was too high.

Steve
 
All I really want to do to the motor is re-ring. I plan on using stock pistons but will do a good street/strip port and polish, then re-jet and use an aftermarket exhaust. As much as I appreciate the help, I feel you guys are making this too complicated. I don't think I need to cc the head or measure combution chamber volume etc. Surely somebody can remember, oh yea if you take 20 thou. off a stock head you end up with a cr of say 11-1.

Thanks again
Paul

78 gs750
79 gs1000
75 rs100
02 1200 Bandit
 
nasty said:
I feel you guys are making this too complicated. I don't think I need to cc the head or measure combution chamber volume etc. Surely somebody can remember, oh yea if you take 20 thou. off a stock head you end up with a cr of say 11-1.



One reason for the 'complication' is that unlike other motors you may have worked on, the gS just doesn't respond that well to compression changes. Not to say you may get lucky with it, but chances are it will ping like no tomorrow, you'll get disappointed, and call us all losers because we cant help fix it after the fact.... I think you'd be better off with just the re-ring, and port and jet job. I hope the compression thing works out for you, though. Post and let us know. :P
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