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4.185 bore iron block + boost

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Old 02-11-2017, 10:42 AM   #1
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Default 4.185 bore iron block + boost

How reliable would an LS engine with aftermarket iron block, 4.185 bore and conventional or MLS head gaskets, NOT copper or o-rings? How much boost could such an engine run? 20 psi? Would that be an application for 6 bolt heads?

GM's own LSX 454 uses a 4.185 bore by 4.125 stroke, so the 4.185 head gaskets are shelf items. I don't know if those are conventional or MLS gaskets.

Yeah, 4.185 is a fairly large bore on a 4.400 bore center, BUT...
BMW S54's run an 87mm bore on a 91mm bore center, iron block. That's 0.157 between bores, which is like a 4.250 bore in a LS or Small Block.
Those engines run 11.3 compression stock and can be boosted enough to push an E36 M3 to 200 MPH in the standing mile. I *think* those engines use o-rings, though... maybe even siamesed o-rings. AND they only run four head bolts, but they have a pretty small bore.

ETA: Also a 4.250" bore small block, but not recommended for boost:
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Old 02-17-2017, 07:35 AM   #2
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You will likely never see an advantage of a 4.185 application under big boost. What you gain in displacement you loose in cylinder wall stability (ring seal) and head gasket sealing. If you want big cubes, go raised deck with a longer stroke like a 4.250 or 4.375 crank, or better yet go BBC, you'll go a lot faster, for a lot less money in the end.
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Old 02-17-2017, 10:21 AM   #3
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Default Deck Strength

OK, some data.

Those gear heads "down under" buy a lot of my coils, the IGN-1A coil.

Thus many reports, one such below.

A bench dyno, GM LSX IRON block @ 4.200" bore, test.
My coils used with a 60-2 TW = quality accurate ignition.
NOS sprayed, first power pull had the block CRACK between the bore and a head stud location.

Your answer = NO

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Old 02-17-2017, 10:43 AM   #4
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Lol... How much squeeze were they putting into it?
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Old 02-20-2017, 12:00 PM   #5
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Depends on the application. I see little to no reason to go over 4.125" bore for anything remotely street worthy ESPECIALLY on boost. The better cylinder wall thickness with hold more boost and help head gaskets stay sealed. There are plenty of 4.03" bore 370 and 408 engines swallowing 20-25psi and making over 1000 WHEEL horsepower.

The need for giant cubic inches is purely for bragging rights unless you are trying to set records or get max inches for class racing rules.

I REALLY wish I had not tried to build a 427" LQ9 iron motor. The extra cubes are not worth the headache. I should have just stayed with a 370 or 408 and boosted it.
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