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Old 08-21-2019, 10:45 AM   #1
VB242
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Default How much boost on flat top pistons?

I recently purchased a B23 to replace my exploded B21. It reportedly has flat top pistons at 10:1 CR. I realize I'm not going to be able to throw 25psi at it but hoping I can get to 15psi. Anyone have experience with this? I'm on MS2 extra so I can pull timing etc, and am using 100 octane non ethanol gas.
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Old 08-21-2019, 11:10 AM   #2
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10 to 1 CR with what head?

You can only make a flat top piston come up so far.....

Use a head that has not been milled and do not use a thin gasket.
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Old 08-21-2019, 11:17 AM   #3
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Turn it up until it blows up, then install a new engine with 1 less psi.
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Old 08-21-2019, 11:38 AM   #4
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Originally Posted by Dirty Rick View Post
10 to 1 CR with what head?

You can only make a flat top piston come up so far.....

Use a head that has not been milled and do not use a thin gasket.
398 head, supposed to be a Canadian engine
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Old 08-21-2019, 12:57 PM   #5
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I wouldn't worry about it. With enough octane, and a good tune, you will have no worries.
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Old 08-21-2019, 01:39 PM   #6
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Your PSI is irrelevant without context on what your manifold/cam/turbo/EMS/fuel type are.
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Old 08-21-2019, 02:03 PM   #7
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15 psi on blue 140 Av gas.
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Old 08-21-2019, 02:08 PM   #8
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Your PSI is irrelevant without context on what your manifold/cam/turbo/EMS/fuel type are.
Ipd turbo cam, td05-20g China Bay turbo, stock lh intake manifolds, 90+ exhaust manifold, 3"exhaust, microsquirt, 100 octane
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Old 08-21-2019, 02:25 PM   #9
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All the boost...
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Old 08-21-2019, 05:49 PM   #10
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Big intercooler? The high octane gas burns hotter so keeping it cooler will help a lot. If you have the large intercooler then maybe some water injection will help it live?

Maybe consider using a cylinder head with some work done so it flows through easier? This contributes less heat to the overall picture.
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Old 08-21-2019, 09:45 PM   #11
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Big intercooler? The high octane gas burns hotter so keeping it cooler will help a lot. If you have the large intercooler then maybe some water injection will help it live?

Maybe consider using a cylinder head with some work done so it flows through easier? This contributes less heat to the overall picture.
High octane gas and other low-specific heat fuels burn cooler and slower---hence their resistance to detonation...
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Old 08-21-2019, 09:57 PM   #12
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I enjoy porting and gasket matching aluminum heads for some weird reason, hit me up if you want. I'll even do SS grooves Got a small portfolio of pics to show off if you are interested.
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Old 08-22-2019, 03:10 PM   #13
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After a lengthy conversation with John V last night, I'm going to take it all apart and check the bearings, pistons and CC the head to see what the actual c/r is. He said he might have some turbo pistons for it, if not I might open up the head and/or get a thick cometic to reduce the c/r.
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Old 08-22-2019, 03:22 PM   #14
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After a lengthy conversation with John V last night, I'm going to take it all apart and check the bearings, pistons and CC the head to see what the actual c/r is. He said he might have some turbo pistons for it, if not I might open up the head and/or get a thick cometic to reduce the c/r.
He’s a good businessman, that’s for sure.
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Old 08-22-2019, 06:53 PM   #15
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Quote:
Originally Posted by VB242 View Post
After a lengthy conversation with John V last night, I'm going to take it all apart and check the bearings, pistons and CC the head to see what the actual c/r is. He said he might have some turbo pistons for it, if not I might open up the head and/or get a thick cometic to reduce the c/r.
I think my fellow dino Redwoord chair was posting in the wrong place ---the lost squirter thread about "oh dear 10,3 compression to which I answered :

Quote:
A guy could deck the block till pistons were flush OR say 0,1mm aka .004" out, then unshroud the valves by enlarging the short ends of the combsution chamber to lower static compression thereby killing two birds with one stone: better flow around the valves on the short side and somewhat less scary compression.....

OR
I go back out in the garage and I dig thru all the pistons in boxes and see if i have 4 STD bore B23 turdboat pistons that have any life left in 'em... If I do then the guy could deck block for the desired "zero deck +0,1" and a little unshrouding for maybe a nice 8:1 comp which means he can give it some boost and an OK ignition curve on the yummie 91 okt soup he says is the best there is locally.,...

Guess I better wander back out there...
Tune in tomorrow, same bat channel, same bat station..

OK went out and check and looks like any B23T pistons got re-cycled... Found some B21 flat tops and 1 B21T piston...So

As regards flywheel I'm going thru the stack now..gonna find that OLD B21t pressureplate to confirm 100%

OK?
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Old 08-22-2019, 08:36 PM   #16
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Found one...dual pattern, confirmed B21T pattern will PM more
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Old 08-22-2019, 08:47 PM   #17
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Found one...dual pattern, confirmed B21T pattern will PM more
Cool thanks! Pistons are even with the deck now. Bearings looked pretty good too.
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Old 08-22-2019, 09:52 PM   #18
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Looks good.
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Old 08-23-2019, 09:28 AM   #19
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My experience has been that Volvo published CR has turned out to be pretty optimistic. No way to know until measuring but I'd love to see what the fabled "high compression" B23 actually turns out to be. If I recall correctly what really matters is the dynamic compression ratio, not static so you can protect yourself with a big cam (and pulling timing of course).
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Old 08-26-2019, 04:43 PM   #20
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I cc'd the head and it appears to be 49.7cc's. my thinnest feeler gauge .008" was a little tight, so I'm assuming John V is correct at the pistons being in .007" , this gives 10.94-1 on the summit calculator. I'm going to get the head skimmed and check it again. I'll probably order up a cometic thick enough to reduce the CR below 9-1. The pistons appear to be forged, but I'm no expert, I'll have the machinist look at one when I drop off the head.
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Old 08-28-2019, 01:16 PM   #21
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I ordered a .140" cometic, it should knock the c/r down to around 8.6-1, depending on how much comes off the head. Now my concern is getting the timing belt on.
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Old 08-28-2019, 01:38 PM   #22
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I ordered a .140" cometic, it should knock the c/r down to around 8.6-1, depending on how much comes off the head. Now my concern is getting the timing belt on.
You will be giving up squish, which is actually worse...
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Old 08-28-2019, 02:38 PM   #23
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this isn't the 60s, you have a after market ecu, just get it tuned and run the higher compression. it will be so much nicer to drive. no need to knock your compression into the ****ter
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Old 08-28-2019, 07:59 PM   #24
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Or do what Volvo did.

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Old 08-29-2019, 09:07 AM   #25
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Or do what Volvo did.

What did they do?
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