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12.3:1 CR too high?
Nickp - 4/12/13 at 08:52 AM

Guys,
I'm putting together the next evolution of fiat twin cam for my Lancia Montecarlo. Due to decking the block on the previous flat top piston build and now using HC pistons, the static CR now comes out at 12.3:1. I was hoping for nearer 11:1. It's got some pretty aggressive 295deg high lift Alquati cams and I'll be running mapped ignition to hopefully avoid detonation. I've got the best head gasket and bolts known to man, supplied by Guy Croft to keep it together. My main concern is the cast pistons tbh. I've previously run a race Golf gti 8v with STD pistons at 11.7:1 and that was fine. So will I get away with it on high octane fuel?? As there'll be a fair bit of work involved to swap back to the original undecked block and getting that rebored.

Thanks in advance,
Nick

mcerd1 - 4/12/13 at 10:00 AM

I think you'll need to actually calculate the dynamic CR before deciding...

Nickp - 4/12/13 at 10:12 AM

quote:
Originally posted by mcerd1
I think you'll need to actually calculate the dynamic CR before deciding...


I should be able to do this later on. Would 9:1 be the limit on 98/99 octane?

Nickp - 4/12/13 at 10:51 AM

Just done a dcr calculation, but some ask you to add 15 to the inlet closing abdc??
If I do this and use 82abdc I get a dcr of 8.3:1, which seems OK.
But if I use 67abdc as per cam timing specs I get 9.6:1, which is high.
So which is right?

chrism - 4/12/13 at 07:31 PM

Have you considered a copper head gasket, being slightly thicker it would allow you to drop the CR slightly. There was a company called something like faraday engineering years ago that could make them up to order from copying an original head gasket.

bi22le - 4/12/13 at 07:48 PM

quote:
Originally posted by chrism
Have you considered a copper head gasket, being slightly thicker it would allow you to drop the CR slightly. There was a company called something like faraday engineering years ago that could make them up to order from copying an original head gasket.


I actually spoke to these guys a week or so back. They have a CR calculator. Tell them the engine and they can say what thickness gives what CR. They also know a lot about different materials

Nickp - 4/12/13 at 07:55 PM

quote:
Originally posted by chrism
Have you considered a copper head gasket, being slightly thicker it would allow you to drop the CR slightly. There was a company called something like faraday engineering years ago that could make them up to order from copying an original head gasket.


Thanks but I'm already going to be using the thickest gasket available (a reinforced 'Turbo' item from Guy Croft) at 1.9mm. I've been speaking to Crofty today and confirmed what I pretty much knew, the CR is too high and I'm also going to struggle with valve - piston clearances with my decked block and high lift cams. So, I'll have to bite the bullet and get my original block prepped and rebored. A shed load of work but it's extra 1mm of deck height will bring the CR back to 11:1. This is the magic CR figure for the Twin Cam according to GC with nothing but trouble to be gained by going above it.

Nickp - 4/12/13 at 08:23 PM

quote:
Originally posted by bi22le
quote:
Originally posted by chrism
Have you considered a copper head gasket, being slightly thicker it would allow you to drop the CR slightly. There was a company called something like faraday engineering years ago that could make them up to order from copying an original head gasket.


I actually spoke to these guys a week or so back. They have a CR calculator. Tell them the engine and they can say what thickness gives what CR. They also know a lot about different materials


Ta but TBH I can't see them doing a head gasket at nearly 3mm thick and even if they did I'm not sure I'd want one They do decomp plates at 1.5mm but I'm not keen on that option either, not when I've got a block sat there that once rebored it'll be a more reliable option. It's only time, the Haynes'll have to wait a bit longer

edthedrummer - 8/12/13 at 11:32 PM

Is there no way you can modify the pistons? Perhaps machining pockets to clear the valves would kill two birds with one stone in as much as dropping the cr and increasing the valve to piston?

Nickp - 9/12/13 at 07:02 AM

quote:
Originally posted by edthedrummer
Is there no way you can modify the pistons? Perhaps machining pockets to clear the valves would kill two birds with one stone in as much as dropping the cr and increasing the valve to piston?


Ta Ed, I was considering machining the pistons but it turns out that one of the bores is damaged anyway, presumably when the HG went. It's got a deep dent / gouge in no2s bore about 1" below the deck. Quite strange really but must've been caused during the failure as it was running perfect prior to that. So I whipped the whole lot out this weekend and have cleaned up the original block that's sat under the bench for 4-5yrs ready to go for a re-bore / re-face this week. I've removed the crank too ready for balancing as soon as I can get a lighter flywheel. The original's a lardy 7kg but the 130TC one was lightened as std and weighs under 6kgs apparently, and there's one currently on ebay that I'm trying to get. There are aluminium ones from the US but the shipping and core charge make the prices silly ($500+)

edthedrummer - 9/12/13 at 07:53 AM

Fair enough mate, it was worth a suggestion!! does seem a bit strange, was there any sign of detonation or damage to the piston when you removed it?

Hah that's the trouble with American imports sadly, so much cool stuff on offer but it rarely works out cost effective by the time you add shipping and imports

snakebelly - 9/12/13 at 09:12 AM

You could always contact mike Tanksy at Ferriday, he could make you a decompression plate im sure.

snakebelly - 9/12/13 at 09:13 AM

http://www.ferriday.co.uk/

Nickp - 9/12/13 at 09:25 AM

quote:
Originally posted by snakebelly
You could always contact mike Tanksy at Ferriday, he could make you a decompression plate im sure.


Thanks but I won't need one with the original block and std deck height

Nickp - 9/12/13 at 09:30 AM

quote:
Originally posted by edthedrummer
Fair enough mate, it was worth a suggestion!! does seem a bit strange, was there any sign of detonation or damage to the piston when you removed it?

Hah that's the trouble with American imports sadly, so much cool stuff on offer but it rarely works out cost effective by the time you add shipping and imports


No, the pistons looked fine. There was just a big lump of HG missing between cyls 2 & 3. I was already pretty much resigned to swapping the block out due to the CR / V-P issues. Then I happened to notice the bore damage and that made the decision for me.