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173 Posts Member #: 873 Advanced Member Thatcham, berkshire |
4th Jan, 2007 at 11:54:02am
hi sorry to be a pain to every one but i am need of rebore and new pistons etc. ive been trying to figure out CR and working out what i need but im really stuck now and confused as i read well just realised bigger pistons mean higher CR and i really not sure waht to run. could any one help. Ideally i dont want to rebore to far as i will use car a bit. but want to make it worth while and quicker than standard on budget. going 40 thou to a 1320 am i right. would that be ok to do and be a good runner realiably to a point. what pistons would i use i was planning on using mega piston set (13cc) and get head work done as far as i need to go. how much would you recommend me taking out. will be using a megajolt and ic of course. but the rest will be standard for while until i save up cash for striahgt cut . woiuld it be worth my while putting a NA cam in too while engine is out as i hear these are good for turbo engine. what kind of safe boost can i use with that of set up above. also what Cr will that give me, if some was to explain i would work it out but i keep getting confused and searched for a while.
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4314 Posts Member #: 700 Formerly British Open Classic The West Country |
4th Jan, 2007 at 12:17:44pm
On 4th of Jan, 2007 at 11:54am mattyking said:
i was planning on using mega piston set (13cc) and get head work done as far as i need to go. how much would you recommend me taking out. what kind of safe boost can i use with that of set up above. also what Cr will that give me, These three are all related, how much you remove from the head will effect the CR and the CR will effect the amount of boost you can run, everything else being equal. As to how much boost to run, I can't answer that, it depends on what you want to use the car for, how much power you want & how much reliability you want, but I can bore you with maths. So here is how to work out your CR: CR = (Swept Volume + Head Chamber + Piston Dish + Head Gasket Volume + Ring Lan Volume) / (Head Chamber + Piston Dish + Gasket Volume + Ring Lan Volume) For a 1275 the sweep volume is 1275 / 4 (as you have four pistons) which is 318.75cc. So for your 1320 it would be 330cc Head Chamber varies between heads, mines 26cc but it's been worked, I think a stock head is about 21cc (can some one confirm this?) Piston Dish is 13cc in your case Head Gasket volume is a tricky one, it's between 2.8cc & 3.8cc depending on who you ask, I'm lean more to the 2.8cc volume. Ring Lan Volume is tiny only about 3/4cc I have heard off engine builders who don't use it when working out CR. So plugging all that in to the above formula gives: CR = (330 + 21 + 13 + 2.8 + 0.75) / (21 + 13 + 2.8 + 0.75) CR = 367.55 / 37.55 CR = 9.8 I hope that helps EDIT: If you take the head out to 26cc the CR drops to 8.75 PS. Could someone check my maths as I'd hate to be giving false advise. PPS. I would recommend you edit the title of this post to something like "Help needed working out CR" as it'll make it a lot easier for others to find when searching in the future. Edited by Rob H on 4th Jan, 2007. Isambard Kingdom Brunel said:
Nothing is impossible if you are an Engineer |
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4314 Posts Member #: 700 Formerly British Open Classic The West Country |
4th Jan, 2007 at 12:24:16pm
Double post. Edited by Rob H on 4th Jan, 2007. Isambard Kingdom Brunel said:
Nothing is impossible if you are an Engineer |
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173 Posts Member #: 873 Advanced Member Thatcham, berkshire |
4th Jan, 2007 at 12:37:02pm
thanks very much rob that has helped clear things up. much appreciated. and yes i believe standard size of chambers are 21cc. so assuming i go 1320 with 13cc pistons and get me head done to about 26 to 28 . and asuming the cr is as u say it is i will double check everything as i dont want to break!!! ha ! i should be able to run up to 14 psi i am guessing going by what other people are running and by searching?? ?or am i on wrong lines stil |
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