CR dilema with 18cc pistons.....
Turbo Mini Forums

Guys, after a bit of diagnosis at the weekend, it looks like my 1380 may have a few more issues than i first thought, so I'm thinking about bringing the new build along a bit.

I've currently got a set of 18cc accralites, which I've got up for sale at the moment, but if I'm able to use them, I will.

Problem is, from my calcs, to get 8.5:1 CR, with the block decked flush with the pistons, I'd need a 20.5cc head. I'd rather not get a nice turbo head and have to skim a load off it to get the CR, as I'd like some scope in the chambers for future skimming/regrinding to maintain the chamber volume if it ever warps.

What do other people do with 18cc pistons?

Am I best off selling these and getting something with a smaller dish? say around 11cc?

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

The standard 12G940 head is 21.4cc and a lot of off-the-shelf performnace heads have smaller chambers so just use one of those.

We have the same problem. Our alloy 5 port head will be around 20-21cc.

Edited by Paul S on 15th Feb, 2010.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I've calculated that 18cc pistons and a 24cc head give me 8:1. Perfect for me.

If you're after a higher CR, get a N/A head.

What exactly is different about a typical turbo head? I know most performance n/a heads are around 19-21cc, say a typical stage 3 head with 36/31 valves.

Am I likely to gain much be getting a turbo specific head, or could I see the same power if i were to source maybe a 2nd hand 'stage 3' sort of head? obviously these are much easier to come by than good turbo heads. From what I can tell the main thing with turbo heads is the increased chamber size.

Is the larger exhaust stem worth having? and if I were to source a second hand stage 3/4 head, would it be easy enough to have the larger stemed valves and guides fitted to it?

Appologies if this all sounds a bit newbie ish, just trying to work out if I should keep the accralites I've already got.

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

The only difference between a so called "turbo" head and an NA head is the size of chambers. The factory turbo head has the same size chambers as the NA head.

A Stage 3 type head with 36/31mm valves will be just fine. Porting and chamber shape is important for maximum power.

You can only get the sodium filled stems in a 29mm exhaust valve and not a neccesity by any means.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Interesting, I thought as much Paul, I just wasn't sure if there was any other trickery onvolved in chamber shape specific to turbo use.

it's a shame I can't use my current head really, it's an MED full race 37x30. Sadly, the valves are so close together they're basicly touching. I'm fairly sure the increased heat from a turbo setup would crack it between valves in no time.

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

IMO turbo heads are massively compromised by their chamber shape, so if you could keep the original chamber shape all the better.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Turbo heads generally use the "open" chamber shape purely to get a big volume. As already said, if you don't need big volume chambers then you can run a more normal NA type shape.
I've done several turbo heads with small chambers due to the 18cc piston availability.

Edited by Turbo Phil on 15th Feb, 2010.

WWW.TURBO-MINI.COM

Intersting point Will, in that case I think I'll edit my FS thread to say I'm keeping them, and start hunting for a decent stage 3 head.....

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

low comp is soooooo 2007....

Benross is doing me a head with 21cc chambers to go with my 18cc pistons and i think it came to something like 7.9:1cr but i might have to redo it as i will be geting block flushed with the pistons and also he used 4cc for piston rings

Edited by pulse on 15th Feb, 2010.

Cheers chaps, this is pretty interesting stuff. It's nice to know that both Phill and Steve can do low CC heads to keep a decent CR. I'm currently looking at a MED roadcomp head I've found for sale which could be ideal, especially if I had a little more port work done on it to optimise flow.....

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

I've just comissioned a head from Turbo Phil. Chamber size will be 23cc giving a comp ratio of 8.1:1. using +0.040" Accralite 18cc pistons.

why dont you build it up with a standard head? you will still be getting around (please correct if wrong) 8.4:1, with the head being so easy to change you can get a modded one later

Bloody good point Wolfie, but i don't actually have a standard head good enough to use! Just loads of old 12g940's that have been sat in a damp garage for years, perfect for exchange/working.

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

Perhaps you could exchange 1 or two for a quick recon from one of our resident head modifiers.

If something is worth doing, it's worth doing half of.