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Join Date: Sep 2004
Location: Woking, McLaren-land
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Higher CR - worth chasing?

My recently rebuilt 3.4 to 3.54 is having to come apart again to address some oil leaks and oil consumption issues. New valve stems seals as a minimum, potentially other stuff too.

It currently 10.2:1 CR, via 98mm Mahle P&C (advertised 10.3), GT3 crank and additional piston valve relief for 5mm William Knight cams.

All on single plug, with 99Ron (93Mon?) fuel readily available.

The engine builder has given me the option of fitting higher CR JE pistons, but i'm unsure if, what could be an extra 0.5 CR, is worth chasing?

On a related point, whoever i've sent the pics of the build to, they are not convinced the Mahle pistons would achieve 9.2:1, let alone 10.2 - due mainly I believe to the shallow dome of the piston.

I understand the CC'ing process is relatively straightforward and the well regarded engine builder confirms 10.2 is accurate (and the max with my particular Mahle pistons)

So the question is do i stick with the Mahle at 10.2, or twist to JE (10.5-10.7 estimated, machined from 11.1 pistons) - i have to pay for the pistons and the machining labour.

Thanks all

Current 10.2:



Target 10.5-ish

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Shirish
1987 Carrera, Granite Green
Old 08-16-2018, 07:27 AM
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Quote:
9.2:1, let alone 10.2
This is correct just look at the difference in the domes of the two pictures.
This has been going on for years and years those that do know how to check compression generally have better power than those that just read the box.
i will say that CP and J&E will make you the correct ratio if you provide them with your Cyl. head volume in cc's and they will accomodate aftermarket cams as well.

regards
Old 08-16-2018, 08:24 AM
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Thanks for your input

What are the pros (and cons ?) of increased CR?
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Old 08-16-2018, 03:07 PM
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More snap more power and higher thermal efficiency.
Gains all across the range some of the power goes to driving the rings into the bore a little harder so it is more noticable going from the 8's or 9's to 10 or 10.5 to 1 you still improve as you go higher but not as dramatic.
Detonation occurs if compression is too high it is easier to do in conjunction with twin plug and even easier still with programmable ignition.

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Old 08-16-2018, 03:27 PM
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Thank you. The car is running Motec/CoP but won’t be twin plugged. Will rely on the skills of the engine builder and Motec mapper to dial some safety into the final design.
Old 08-16-2018, 04:02 PM
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The comments re piston dome and CR are interesting. The first build to 3.4 back in 2008 achieved a CR of 10.45 using the same P&Cs; that too was done by a well regarded engine builder over hear.

You’re in agreement with William Knight, who also believes 10.1 is optimistic
Old 08-16-2018, 04:06 PM
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those new JE pistons will require twin spark, because of the the shape. Stick with the flat topped ones. Higher compression may give you issues with getting fuel that will not detonate
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Old 08-16-2018, 10:14 PM
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Originally Posted by trond View Post
those new JE pistons will require twin spark, because of the the shape. Stick with the flat topped ones. Higher compression may give you issues with getting fuel that will not detonate
Thanks - I asked the builder the same question as I thought the same, but he assured me no need to twin plug.
Old 08-16-2018, 11:48 PM
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your engine builder may be correct but I went twin plug with that design. Anyways you are already high enough on CR for a street car in my opinion. I went with 10.5:1CR which I measured to actual 10.1. Even with twin spark I found I need more than the standard here 95RON. 98RON seems to be OK but not as easily available. Octane booster is all but unavailable here and I regret not going with the 9.5:1 CR pistons

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