working out what CR to run?
Turbo Mini Forums

just wondered if anyone can help or point me in the right direction of working out what sort of CR to run for a max of 15psi boost.

I'm about to get the machining done for my new engine but i'm tempted to go to 1380. The only trouble is deciding on what pistons to run. forged are out of the question as i just cant afford them but i was talking to simon about machining out the mega pistons.

I'm running a 29cc head at the minute along with megajolt and a cossie intercooler and more than likely the ph2 cam again. we worked out that i'd need around 16cc in the piston to get it down to 8:1 which i'm running on my 1293. but do i really need to go that low?

How do you guys decide what CR to run?

Cheers, Nick :)

with 29 cc chambers, 16cc dish, pistons flush with deck, 2,8 cc in head gasket on 1380 you are running 8.22:1. ( I have not included ringland in calcs). to get it to 8:1 you would need 1.5 cc's more dish or chamber volume.

you really are a flash bugger !!

Hmm ive only got 10cc dished pistons, but will eventually get an Avonbar head i think, would i still need to lower the compression to run 10-15psi boost? :S

Why not use the Payen AF460 gasket with a slightly bigger volume at 3.6cc, not much more but could help.

Peter knows what he's on about

Pete, cheers for going into so much detail. waaay over my head unfortunately, best get hold of the vizard book and have a read up on what its all about.

I just wanted to know what a safe static CR was for running 15psi. I dont want to have it as high as possible and be on the edge of det as i need some margin of safety in it being my daily motor.

I think i'll go for 8.5:1, and maybe see if i can get te pistons down the bore a bit if my block hasn't been skimmed so i dont have to have such a big dish in the pistons.

Cheers guys :)

Edited by Nick on 16th Aug, 2006.

just to tout my spreadsheet...

www.afracing.com/cr.xls



Alex

AlexF

i agree with 1071 i think 9.1 with a good ic and either fixed advance to 26 deg or mj setup would be good, im currently running 12psi on 9.4cr and have run 14psi on the rollers with no probs although on 14 psi ray said here could here just a tad of det at 4k rpm but for only for a split second so i know were my limit is.

next year i will get simon to sort out my head to 29cc and run 15psi or even more, i think it is also a good idea to run a intake temp sensor as these can be a life safer to the engine as i always back off when i reach 60deg which on a hot doesnt take long to reach.

but as said before if you only want to run 15-16psi then dont go to low on the cr front because this will lose you bannanas.

last year i gained 15hp for the same boost just buy raising the cr.

always looking for them bigger bunches of bannanas