How much boost on flat top pistons?

VB242 · 08-21-2019, 10:45 AM

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.

Dirty Rick · 08-21-2019, 11:10 AM

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.

iHateVolvoPeople · 08-21-2019, 11:17 AM

Turn it up until it blows up, then install a new engine with 1 less psi.

VB242 · 08-21-2019, 11:38 AM

Quote:

Originally Posted by Dirty Rick

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

Broke4speed · 08-21-2019, 12:57 PM

I wouldn't worry about it. With enough octane, and a good tune, you will have no worries.

Harlard · 08-21-2019, 01:39 PM

Your PSI is irrelevant without context on what your manifold/cam/turbo/EMS/fuel type are.

Redwood Chair · 08-21-2019, 02:03 PM

15 psi on blue 140 Av gas.

VB242 · 08-21-2019, 02:08 PM

Quote:

Originally Posted by Harlard

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

poulrais · 08-21-2019, 02:25 PM

All the boost...

dl242gt · 08-21-2019, 05:49 PM

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.

John V, outside agitator · 08-21-2019, 09:45 PM

Quote:

Originally Posted by dl242gt

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...

2turbotoys · 08-21-2019, 09:57 PM

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.

VB242 · 08-22-2019, 03:10 PM

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.

iHateVolvoPeople · 08-22-2019, 03:22 PM

Quote:

Originally Posted by VB242

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.

John V, outside agitator · 08-22-2019, 06:53 PM

Quote:

Originally Posted by VB242

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?

John V, outside agitator · 08-22-2019, 08:36 PM

Found one...dual pattern, confirmed B21T pattern will PM more

VB242 · 08-22-2019, 08:47 PM

Quote:

Originally Posted by John V, outside agitator

Found one...dual pattern, confirmed B21T pattern will PM more

Cool thanks! Pistons are even with the deck now. Bearings looked pretty good too.

John V, outside agitator · 08-22-2019, 09:52 PM

Looks good.

redblockpowered · 08-23-2019, 09:28 AM

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).

VB242 · 08-26-2019, 04:43 PM

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.

VB242 · 08-28-2019, 01:16 PM

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.

Harlard · 08-28-2019, 01:38 PM

Quote:

Originally Posted by VB242

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...

cosbySweater · 08-28-2019, 02:38 PM

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

Redwood Chair · 08-28-2019, 07:59 PM

Or do what Volvo did.

iHateVolvoPeople · 08-29-2019, 09:07 AM

Quote:

Originally Posted by Redwood Chair

Or do what Volvo did.

What did they do?

08-21-2019, 10:45 AM	#1
VB242 Dann sind wir Helden Join Date: Sep 2011 Location: 4D space-time	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. __________________ "i will destroy all of you!" -Sheldon Plankton Booty Scooty https://youtu.be/i4oAOZ8nbq4

08-21-2019, 11:10 AM	#2
Dirty Rick Board Member Join Date: Apr 2013 Location: Cornholio, OR	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. __________________ I don't know who I am when I am somebody else.

08-21-2019, 11:17 AM	#3
iHateVolvoPeople Board Member Join Date: Feb 2014 Location: Midwest	Turn it up until it blows up, then install a new engine with 1 less psi. __________________ Feedback Thread I have a mental disorder.

08-21-2019, 12:57 PM	#5
Broke4speed Board Member Join Date: Oct 2009 Location: Marionville, Ontario, Canada	I wouldn't worry about it. With enough octane, and a good tune, you will have no worries. __________________ 1984 244/ AQ140A / M46

08-21-2019, 02:25 PM	#9
poulrais Board Member Join Date: Sep 2006 Location: Quebec, CAN	All the boost... __________________ 1990, Volvo 740 16v +T 1991, Volvo 745T 16V - crushed build thread: http://forums.turbobricks.com/showthread.php?t=249553

08-21-2019, 05:49 PM	#10
dl242gt Happy playing the blues Join Date: Jun 2002 Location: S NJ, a suburb of Phila.	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.

08-21-2019, 09:57 PM	#12
2turbotoys Astro Zombie Join Date: Nov 2007 Location: Reading pa	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. __________________ Feedback thread https://forums.tbforums.com/showthread.php?t=144924 1978 242, 5.3 L33 1979 242, MS, R brakes 2006 V50 T5 AWD @ 17 PSI

08-22-2019, 03:10 PM	#13
VB242 Dann sind wir Helden Join Date: Sep 2011 Location: 4D space-time	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.

08-22-2019, 08:36 PM	#16
John V, outside agitator Board Member Join Date: Mar 2005 Location: Sleezattle, WA, USA	Found one...dual pattern, confirmed B21T pattern will PM more

08-22-2019, 09:52 PM	#18
John V, outside agitator Board Member Join Date: Mar 2005 Location: Sleezattle, WA, USA	Looks good.

08-23-2019, 09:28 AM	#19
redblockpowered cone dodging dilettante Join Date: May 2015 Location: In bed, probably	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). __________________ 1993 944 B230FT/M90 thread here: http://forums.turbobricks.com/showthread.php?t=345277 2x 1991 245 B230F/M47 (LeMons car, street car) I sell chips for LH 2.4!

08-26-2019, 04:43 PM	#20
VB242 Dann sind wir Helden Join Date: Sep 2011 Location: 4D space-time	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.

08-28-2019, 01:16 PM	#21
VB242 Dann sind wir Helden Join Date: Sep 2011 Location: 4D space-time	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.

08-28-2019, 07:59 PM	#24
Redwood Chair K-jet For Life Join Date: Mar 2007 Location: - Stock PSI Or Bust -	Or do what Volvo did.

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