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Old 01-12-2015, 11:42 PM   #1
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Question Block Deck Height, Compression and HG Sanity Check.

Sanity check for my engine build, accompanied by verification in thought-process.

I'm learning as I go, and am at the point of needing a sanity-check with all the information floating in my brain. I have not built an engine before, so my mental rolodex of do's and don'ts isn't established yet.

Background : Piston rings in current motor are on their death bed. Bought a spare BP long-block to rebuild, and am enjoying learning how to build motor along the way. I live at ~5600ft elevation, and car will likely spend 99% of it's life at elevation.

Goals : Naturally Aspirated, extremely reliable, hopefully long-lasting, and free-revving engine running on 91 (it's our high-grade) pump gas. Do it once right, as I can't really afford to do this immediately again.

Parts :
MSPNP2. Still need to install it. I'm incredibly intimidated by tuning, so much to learn.
Wiseco 10.5:1 84mm pistons
BP4w head with Supertech singles, stock valves, and BP5A Intake cam. (unknown combustion chamber volume bought off F/S forum here.)
The block needed to be decked 0.007" to be flat.
I'm taking my Head in to be re-surfaced as little as needed for proper HG seal.
Entire Rotating assembly has been balanced.
Typical supporting bottom-end bits : Manley rods, ACL race bearings, ARP head/main studs, SuperMiata billet oil pump (same as BE? haven't received it yet), M-tuned reroute, Flyin Miata rebuild seal/gasket kit, yada yada yada.

Questions :

1) Am I an idiot for building a 10.5:1 CR engine for Naturally aspirated fun? Will this present difficulty in tuning for pump gas? Too high for Turbo : Yes. Too high for N/A : ????

2) HG thickness? I have an OEM 94-00 BP26 head gasket that supposedly measures 0.031" thickness. With the block being decked 0.007" will that HG be thick enough? Cometic makes 2 other thicknesses for BP as per 949's site. I've read thicker HG = bad due to reduction in Quench. See Question #3.

3) Quench or Squish height? I have seen conflicting information in my searches. Mechanically for steel rods I have seen the 0.1mm/1000rpm figure thrown around a LOT. That would set quench height to approx 0.032" quench for 8,000 rpm. I have also read of BP motors that have run as little as 0.018" quench. I couldn't find an online resource or FSM that had info on spec'd quench height.

As far as I understand it, less quench equates to a more uniformly mixed and more turbulent charge which should aid in detonation resistance (good thing for me running pump). As a trade-off decreasing piston-to-head clearance would increase my CR, something I'm already feeling is a bit high.

4) With those three aforementioned areas of concern should I proceed:

A) Measure deck height on soon-to-be assembled bottom end, run whatever HG would achieve 0.020-0.032" of quench, and NOT worry about Compression Ratio?

B) Set HG thickness for Quench height, CC combustion chamber, and port with goal of ??? Compression ratio?

C) Something else entirely?

Thank you to whomever reads through all this. This is my first engine build and I'm doing my best researching as I go. I know that guys like TSE, FM, and 949 hold their engine formulas as mostly trade-secrets, I greatly respect that, but I'm just looking for some miata-specific guidance and hope some of you can help me.
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Old 01-15-2015, 04:48 PM   #2
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I wouldn't run less than .020" inch in the quench, on domestic V8's most builders avoid going below .035".
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