cute wee gem Posted April 6, 2007 Share Posted April 6, 2007 Hey boes. New cam has mean overlap so my static c/r is like 7:1.. I know the comp gets higher when the revs rise but how much? Basically I wanna get the head planed as its got fuck all response Quote Link to comment Share on other sites More sharing options...
thminiman Posted April 6, 2007 Share Posted April 6, 2007 if it changes with revs then i'd imagine it'd be far too hard to calculate what ratio it changes by, take it feilding, meet CaM, sparkle and the infamous g-man, he'll sort you out... or, ring him, tell him your mates with peter, CaM and sparkle... he'll actually have a clue as to what your talking about and he's always k33n to help someone with a mini if you want his number pm me Quote Link to comment Share on other sites More sharing options...
mikuni Posted April 6, 2007 Share Posted April 6, 2007 I thought static compression never changed. ie it is just the bore and stroke, compressed into the combustion chamber. So in theory it should be easy to calculate, with the valves closed. Dynamic compression ratio is when the other elliments are calculated in, with valve overlap coming into the equation at this point also. I'd never really realised that the overlap would reduce the compression so much but I guess it makes sense. Obviously thats the purpose of getting the head skimmed when cam is (cams are) done. Not to specifically increase the static compression, but to maintain the dynamic compression. So yeah, talk to some one in the know with minis and they will be able to recommend an appropriate amount to skim off the head. Quote Link to comment Share on other sites More sharing options...
Gaz Posted April 6, 2007 Share Posted April 6, 2007 Dude ya comp wont get higher when revs rise. Quote Link to comment Share on other sites More sharing options...
Gaz Posted April 6, 2007 Share Posted April 6, 2007 Static compression does not change. It means the standing still compresson. Calculated by volumn of cylinder,bore,head at bottom of stroke divided by what it is at top. Do you mean that the valve overlap is so huge that compression is escaping out the open valve on the up stroke? Ive never heard of this before. To know how much to plain your head you need to work out the static compression and then go from there. CC the head, measure thickness of head gaskit.. there heaps of web sites to help. Quote Link to comment Share on other sites More sharing options...
eke_zetec_RWD Posted April 6, 2007 Share Posted April 6, 2007 its hard to explain but yea hes right, it will increase as revs go up. its because exhaust valve is still closing on the intake stroke and intake valve still closing on compression stroke, but only a tiny bit so yea have a think. . . . . . Quote Link to comment Share on other sites More sharing options...
OSC Posted April 6, 2007 Share Posted April 6, 2007 yep he is right you find on a fairly worked n/a engine with big cams the cylinder pressure goes up by about 10% i think between cranking rpm and high end rpm but in your case with big cams it maybe a lil bit more as the engine is losing quite a bit of efficiency lower down the rev range. 2 stroke engines are worse still something like 25-30% more cylinder pressure Quote Link to comment Share on other sites More sharing options...
Guest WESTCORT Posted April 6, 2007 Share Posted April 6, 2007 the compression ratio itself wont change, but, as there is alot of overlap, when the engine is at low rpm, the mixture can lose pressure through the valves but when the engine rev's higher there isnt enough time for the mixture to escape out of the valves right? hence the reason cams with high duration have heavy "off cam" response but once they hit a certain rpm level this disapears? Quote Link to comment Share on other sites More sharing options...
cute wee gem Posted April 6, 2007 Author Share Posted April 6, 2007 Dude ya comp wont get higher when revs rise. Scavenging effect man. When your engine is running slowly some of your intake mixture goes out the exhaust (on overlap) causing low c/r. When your revs rise the valves are obviously faster too, this is when the scavenge effect comes into play.. Your intake mix is coming in so fast that when the exhaust valve closes its "pulling" the intake mix in giving you alot more mixture giving you a better c/r You're right, static compression isn't the word I'm looking for, Mikuni explained basically what I meant though. The c/r you get when you put the thingee ma bob in the plug 'ole while cranking on starter motor Cheers oldskoolcrew you gave me my answer. Standard c/r should be 8.5ish and its 7.0ish at the moment which is a massive drop. I guess the engine builder/a bit of research and maths (Failed at school ) may be the answer Quote Link to comment Share on other sites More sharing options...
Gaz Posted April 7, 2007 Share Posted April 7, 2007 What cam are you running and what are the specs. Which engine? Quote Link to comment Share on other sites More sharing options...
cute wee gem Posted April 8, 2007 Author Share Posted April 8, 2007 bmc 1275, 295dur 296/315c-lift Need a ported/big valve head and 1.5 twins and it should go mint, still a bit overkill though.. should have a 40 or similar to suit the cam.. but it was cheap Quote Link to comment Share on other sites More sharing options...
thminiman Posted April 8, 2007 Share Posted April 8, 2007 mang, listen up. gman = god of porting heads, me = looking into larger valves this week is just as easy to get 16 as it is to get 8. ring up gman, have a chat, say u need a ported head, you've got a doner head, bangin, he wont charge you much, and it wont take fuck all time as he's doing the same stuff to mine right now Quote Link to comment Share on other sites More sharing options...
cute wee gem Posted April 8, 2007 Author Share Posted April 8, 2007 Mmm.. Give me a pm when you get yours done and I'll see if its worth it Quote Link to comment Share on other sites More sharing options...
thminiman Posted April 8, 2007 Share Posted April 8, 2007 he's part way through porting my head now, then when ive got the valves it will get sent away to get the valves matched into the head, will still be a while until its finished, but its tried and tested stuff (vizard) i can ask him to give me an idea how much it will be for that part, i cant imagine it will be that much tho mang Quote Link to comment Share on other sites More sharing options...
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