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ken
Agony Aunt - You have a car problem? Speak to Ken
Joined: March 6th, 2009, 1:40 am Posts: 3675
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 Re: 8.5:1 and 9:1
I'd have thought that any difference in power will be directly related to the degree to which the engine can contain and compress the combustible mixture, with the 'psi at cranking' figure being a fairly good indicator of that. It's not just barrels and pistons which are involved, so I'm always amazed that folk will take an engine apart and put it together again without giving the valves a bit of a fettling. There are only 4 of them, after all.. As mentioned elsewhere, an 8.5:1 engine should show 150 psi and a 9.0:1 engine 175psi and iirc, some race engines I've worked on have shown over 200 psi. ( Bear in mind that the higher the CR, the more critical it becomes that the ignition timing is spot on, especially if 95 octane fuel is used. The 'peg in hole' setup should only be a starting point.) Tickover smoothness is something else, I find that a heavy flywheel is what's required for 'steam engine' type smoothness... ken.
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April 25th, 2011, 3:53 pm |
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2CViking
viking bastard
Joined: April 18th, 2009, 11:43 am Posts: 2424 Location: Meneac, Bretagne France
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 Re: 8.5:1 and 9:1
8.5 vs 9. Well look at the gear box ratio in 2CV6, Ami 8 and van boxes etc + 125/15 vs 135/15 tyres. A bit of a jungle to work out what compression ratio to run on which gear box. 9.0 on 2CV6 box running 125/15 tyres compared to 8.5 is next to nothing when we talk top speed. You gain some torque but doubt you can feel it.
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April 25th, 2011, 4:07 pm |
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Sean
Firing on two.
Joined: April 22nd, 2009, 11:06 pm Posts: 3684 Location: Ecosse
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 Re: 8.5:1 and 9:1
a well fettled 8.5:1 will feel better than a tired 9:1, have 9:1 in the blue car and it was fast, however its tired now and will be getting a quick re build for Salbris and as always its more than just the CR that has a bearing on the behaviour, carefull setting up of all the components will produce a nice tight sprightly engine
looking at drive shaft alignment and other things at the moment
_________________ Kissing the Lash
 "Any advice of a technical nature is given on the understanding that I've actually done this shit, not just read about it in D*lly club mag some time ago.
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April 25th, 2011, 4:48 pm |
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Devils Advocate
Firing on two.
Joined: July 26th, 2009, 3:36 pm Posts: 1019
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 Re: 8.5:1 and 9:1
Thanks, all - interesting 
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April 25th, 2011, 10:31 pm |
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Russell
Firing on two.
Joined: November 29th, 2008, 10:05 pm Posts: 9259 Location: West Sussex, U.K.
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 Re: 8.5:1 and 9:1
Tell you what, if you can't decide, then fit an 8.5:1 one side and a 9:1 the other, and see which side pulls the car along faster.
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samfieldhouse wrote: What I like about I2F is that there is no pretence of democracy.
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April 25th, 2011, 11:02 pm |
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Terry
Firing on two.
Joined: November 4th, 2009, 4:00 pm Posts: 526 Location: Confederate state of South Yorkshire
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 Re: 8.5:1 and 9:1
Russell wrote: Tell you what, if you can't decide, then fit an 8.5:1 one side and a 9:1 the other, and see which side pulls the car along faster. 9:1 on the L/H [passenger] side 8.5:1 on the R/H side .....Yer 2CV will go 'round roundabouts a lot quicker .....  Honest ... T. 
_________________ Remember .....the drive is the reason,the destination is just the excuse. 2CV6 Special [Red] C4 VTR+ Coupe [Black] C1 UrbanRide[Blue & Orange] {Ltd Ed}
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April 26th, 2011, 12:01 am |
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Russell
Firing on two.
Joined: November 29th, 2008, 10:05 pm Posts: 9259 Location: West Sussex, U.K.
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 Re: 8.5:1 and 9:1
Yeah we did that on the oval car my dad had a few years back.
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samfieldhouse wrote: What I like about I2F is that there is no pretence of democracy.
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April 26th, 2011, 12:04 am |
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Sean
Firing on two.
Joined: April 22nd, 2009, 11:06 pm Posts: 3684 Location: Ecosse
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 Re: 8.5:1 and 9:1
ahem One of the Scotroen "Sharks" had an engine like that-and it was far from slow 
_________________ Kissing the Lash
 "Any advice of a technical nature is given on the understanding that I've actually done this shit, not just read about it in D*lly club mag some time ago.
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April 26th, 2011, 8:09 am |
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spanners
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 Re: 8.5:1 and 9:1
Sean wrote: ahem One of the Scotroen "Sharks" had an engine like that-and it was far from slow  As one head always seems to get hotter than the other, maybe by using an 8.5:1 piston in that side, the ensuing lower compression/heat may sort that problem out? 
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April 26th, 2011, 8:19 am |
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Russell
Firing on two.
Joined: November 29th, 2008, 10:05 pm Posts: 9259 Location: West Sussex, U.K.
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 Re: 8.5:1 and 9:1
Perhaps drilling a small hole in the 9:1 piston to let the extra compression out would equal it out?
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samfieldhouse wrote: What I like about I2F is that there is no pretence of democracy.
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April 26th, 2011, 8:39 am |
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