11 replies to this topic

[087dave]

Hi all can anyone help with how much if any i need to get my stage 3 head skimmed
It is a minisport stage 3 head they said it has 25cc s i have a sw5 cam also the people at minisport said
that it is fine that i will have 10.1 compression

Would you thick that this is right cause i still have the standard flat top pistons!!

Thanks all!!

[minipac]

hi mate with a rough calculation using vizards book that would give you a compression ratio of about 9.32.
you would have to have the head skimmed to make the chamber size to about 22.74cc to get 10.1 .
i dont 100% understand vizards calculation but i have got a few engine to the right compression using them.
if you can get hold of a copy of vizards book it is well worth a read.

[ANON]

someone had a comp. ratio calculator on their web site, can't remember who's site it was though sorry. could be worth a quick look at the ferriday site.

[Frog]

First thing is what cc is the engine ?? Is it a 1275 or 998 ?
What sort of compression ratio are you after ?? If it is a road car then really 9.75 - 10:1 is more then enough unleaded fuel is pretty crap when it comes to modified engines. if you go to far you have to start adding octane booster etc to stop detonation & melting of pistons. Been there done that it ain't pretty !!.
A rough calculation is for ever 12 thou machined of the head it reduces the cc by 1cc. just remember to allow 3cc for the cyl head gasket. What cam you run doesn't really effect the cc's at all.
Doing a quick calculation on a 998 engine that head would run at nearer 11:1 which is high for road use. was the head new or second hand ??
Hope this has been of some help.

[Nightrain]

Although the cam doesn't effect the combustion chamber size or the static compression ratio, it does effect the dynamic compression ratio ! Think about the valves been open longer meaning compression doesn't start to build as early with a bigger cam. In other words you can get away with more static compression with a bigger cam.

[087dave]

First thing is what cc is the engine ?? Is it a 1275 or 998 ?
What sort of compression ratio are you after ?? If it is a road car then really 9.75 - 10:1 is more then enough unleaded fuel is pretty crap when it comes to modified engines. if you go to far you have to start adding octane booster etc to stop detonation & melting of pistons. Been there done that it ain't pretty !!.
A rough calculation is for ever 12 thou machined of the head it reduces the cc by 1cc. just remember to allow 3cc for the cyl head gasket. What cam you run doesn't really effect the cc's at all.
Doing a quick calculation on a 998 engine that head would run at nearer 11:1 which is high for road use. was the head new or second hand ??
Hope this has been of some help.

Hi it is for a 998, and i got it new, i am looking for 10.1
so do you think i need skimming
Thanks

[minipac]

hi mate here is the page from vizards book hope it helps

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[revingtosh]

hi mate here is the page from vizards book hope it helps

Heres an online CR calculator that i use.

http://www.csgnetwor...m/compcalc.html

Mike

[087dave]

Thanks alot

[rozzer1275]

thought the cam did have an effect on dynamic CR, ie valves are open partly on compression stroke, anyone know??

[Nightrain]

It does as I said above.

[rozzer1275]

It does as I said above.

ha missed that thanks!