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Compression ratio conundrum
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bugguy1967
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PostPosted: Thu Jan 14, 2021 11:00 pm    Post subject: Re: Compression ratio conundrum Reply with quote

Americans and their peasant petrol Laughing Laughing Laughing Laughing
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Ohio Tom
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PostPosted: Thu Jan 14, 2021 2:26 pm    Post subject: Re: Compression ratio conundrum Reply with quote

It's also typically cooler in the UK than parts of the USA.

outside temps have alot to do with it...
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Zed999
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PostPosted: Thu Jan 14, 2021 1:43 am    Post subject: Re: Compression ratio conundrum Reply with quote

just for a UK comparison for you I run a stock cam at 8:6:1 (not my doing) and it's fine.
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paul356A
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PostPosted: Wed Jan 13, 2021 9:58 am    Post subject: Re: Compression ratio conundrum Reply with quote

Thanks guys, I'll aim for 9:1, just need to check my deck height


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Alstrup
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PostPosted: Wed Jan 13, 2021 8:14 am    Post subject: Re: Compression ratio conundrum Reply with quote

Yes. at 1,2 mm deck youre right at 9-1
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sled
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PostPosted: Wed Jan 13, 2021 7:32 am    Post subject: Re: Compression ratio conundrum Reply with quote

.045 to .050 deck
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Ohio Tom
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PostPosted: Wed Jan 13, 2021 7:12 am    Post subject: Re: Compression ratio conundrum Reply with quote

CB Performance has a nice "engine calculator" you can punch in your info and it spits out the CR.

I like to play with different scenarios just to get an idea of where I need to land.

I do calcs for deck heights:
.040"
.050"
.060"
.070"
.080"

If I do my math right. I can flycut the heads and land in the normal deckheight range with some room to go up/down if needed.
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paul356A
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PostPosted: Wed Jan 13, 2021 3:56 am    Post subject: Re: Compression ratio conundrum Reply with quote

chamber volume is 60cc, I'm just about to get on to deck height.
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UK Luke 72
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PostPosted: Wed Jan 13, 2021 3:32 am    Post subject: Re: Compression ratio conundrum Reply with quote

Yeah it can be a nightmare.
Reason I ask is, what chamber cc and deck height is it coming in at?
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paul356A
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PostPosted: Wed Jan 13, 2021 3:13 am    Post subject: Re: Compression ratio conundrum Reply with quote

Yes I have the parts, I am in the mock up stage of the build. I have been reading more about fuel and it seems UK 95 is like US 91 because of different use of RON MON and PON



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UK Luke 72
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PostPosted: Wed Jan 13, 2021 3:07 am    Post subject: Re: Compression ratio conundrum Reply with quote

Do you have the parts already?

Our fuel isn't directly comparable to theirs.
Our 95 is their 91
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Alstrup
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PostPosted: Wed Jan 13, 2021 3:03 am    Post subject: Re: Compression ratio conundrum Reply with quote

9-1.
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paul356A
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PostPosted: Wed Jan 13, 2021 2:51 am    Post subject: Compression ratio conundrum Reply with quote

I am building a 2180cc motor for my split bus and need to decide on what compression ratio to use. Looking through many post it's seems people are using 8 - 8.5 to 1 for my set up, which to me being in the U.K. sounds quite low. Is this because people are building to suit U.S. pump gas which looks to be 87 octane? Regular petrol here in the U.K. is 95.

My spec is :

2180 (82 x 92)
Scat C35 with 1:1.25 rockers
Tims stage one plus heads (42 x 37)
Twin Dellorto DRLA 40

What do people think?

Cheers


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