B20F Head - how far should I shave?

fatcatbestcat · 11-24-2020, 09:39 PM

Pending my hoist's delivery and assuming the weather holds, I'm going to be yanking the carb-converted B20F out of my 144 for a partial rebuild. While the head's off, I'm going to be taking it to my machine shop to have the spring seats enlarged for Iske double springs.

I figure while I'm there, I might have the bottom surface of the head milled for a bit more compression, but I'm not really sure how much I should have taken off.

I'm doubtful at this point that the car's going to be daily driven, so it would probably be fine running on 93 or 94, but that's not always available at any pump, anywhere. The flexibility of 91 would be nice, in case I ever take this further than around the block.

Any advice would be helpful.

EDIT: For reference, this is a B20F with probably 8.7:1 compression, and I'm looking anywhere between 9.5:1 and 10.5:1

culberro · 11-24-2020, 10:08 PM

I have no clue what the limit is, but I do know that a friend had a 11:1 head done and it was machined over 0.125... maybe 0.150". It was a lot.
The chambers were drastically different, and it was a full port head. It was done Rob Gordon at Vol-tech in Portland. I just did the valve job and blending, and final assembly of the head.

Edit: This was a b18 head with conversion valve guides to run oversized b20 valves.

hiperfauto · 11-24-2020, 10:37 PM

The E head gasket is thinner than the F gasket despite what this catalog page says. The E gasket is ~.8mm compressed which, when combined with .080" milled off a stock F head will yield approximately a 10.5:1 compression ratio.

JohnMc · 11-24-2020, 10:41 PM

You could also measure the chamber volume and then use a CR calculator.

hiperfauto · 11-24-2020, 10:54 PM

That'll get him in the ballpark. Use a burette to measure the chamber volume if you want to be accurate.

FrankBooth · 11-25-2020, 11:15 AM

Quote:

Originally Posted by hiperfauto

The E head gasket is thinner than the F gasket despite what this catalog page says. The E gasket is ~.8mm compressed which, when combined with .080" milled off a stock F head will yield approximately a 10.5:1 compression ratio.

What's been your experience with running an engine like this on pump gas?

fatcatbestcat · 11-25-2020, 12:11 PM

Quote:

Originally Posted by FrankBooth

What's been your experience with running an engine like this on pump gas?

This is an important question.

Also, if I go with compression this high (10.0:1 - 10.5:1), would it be worth the extra few bucks to get a distributor with an electronically controllable curve so I can retard timing if I can't get 93 or 94 at the local pump?

hiperfauto · 11-25-2020, 03:23 PM

I wouldn't raise the compression over 9.5:1 unless you're going to use a D cam or larger or it'll ping. Use 91 octane minimum.

An electronically controlled advance is worth the money mostly because any 50+ year old mechanical advance distributor is likely worn out by now.

fatcatbestcat · 11-25-2020, 03:41 PM

Quote:

Originally Posted by hiperfauto

I wouldn't raise the compression over 9.5:1 unless you're going to use a D cam or larger or it'll ping. Use 91 octane minimum.

I have a few cams lying around right now, a D cam, the K cam that came out of the motor, some kind of R cam and a VV91 now. I'll make my decision at some point in the rebuild.

Quote:

Originally Posted by hiperfauto

An electronically controlled advance is worth the money mostly because any 50+ year old mechanical advance distributor is likely worn out by now.

Well, I'm comparing that to a new production Bosch 009 or something like that. My current cast dizzy is probably toast.

JohnMc · 11-25-2020, 04:32 PM

Measure piston height at TDC very carefully, then use an MLS head gasket to get a very precise distance for best squish. Something like .032 - .036". Getting a good tight squish will make it less prone to pinging. But you'll still probably need to run 93 octane.

fatcatbestcat · 11-26-2020, 12:04 AM

Alright, thanks for the replies, everyone!

It looks to me like I should be aiming to have .050" shaved, to bring it closer in line to what you get with a B20B head.

I'm not exactly sure what gasket I have, since the Elring kit just says 'fuel injection gasket set'. I'm going to guess it has the somewhat thicker gasket, as opposed to the E gasket.

Or maybe I'm just an idiot.

OttoB · 11-26-2020, 12:12 PM

I measused a couple B30, all pistons were about 0,5 mm below the deck at TDC.
Slight variation on that measure, rods were +- O,1 mm in length.

Jussi Alanko · 11-28-2020, 04:23 PM

You could shave it even 3mm, but pushrods and rocker arms must be set to this also.

11-24-2020, 10:08 PM	#2
culberro Ronald Culberbone III Join Date: Feb 2010 Location: Portland, OR	I have no clue what the limit is, but I do know that a friend had a 11:1 head done and it was machined over 0.125... maybe 0.150". It was a lot. The chambers were drastically different, and it was a full port head. It was done Rob Gordon at Vol-tech in Portland. I just did the valve job and blending, and final assembly of the head. Edit: This was a b18 head with conversion valve guides to run oversized b20 valves. __________________ Cult Person. Pissing in your Kool-Aid.

11-24-2020, 10:37 PM	#3
hiperfauto The Librarian Join Date: Nov 2011 Location: So Cal	The E head gasket is thinner than the F gasket despite what this catalog page says. The E gasket is ~.8mm compressed which, when combined with .080" milled off a stock F head will yield approximately a 10.5:1 compression ratio. __________________ 240 Alcantara Shift Boot Cover (164 & 72- 140 too) Rare, Race & R-Sports Parts Colored B/S Brake Hoses 140/160 and more Volvo Advertising Stuff -

11-24-2020, 10:41 PM	#4
JohnMc PV Abuser Join Date: May 2004 Location: St. Louis	You could also measure the chamber volume and then use a CR calculator. __________________ '63 PV Rat Rod '93 245 16VT Classic #1141

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11-24-2020, 10:54 PM	#5
hiperfauto The Librarian Join Date: Nov 2011 Location: So Cal	That'll get him in the ballpark. Use a burette to measure the chamber volume if you want to be accurate.

11-25-2020, 03:23 PM	#8
hiperfauto The Librarian Join Date: Nov 2011 Location: So Cal	I wouldn't raise the compression over 9.5:1 unless you're going to use a D cam or larger or it'll ping. Use 91 octane minimum. An electronically controlled advance is worth the money mostly because any 50+ year old mechanical advance distributor is likely worn out by now.

11-25-2020, 04:32 PM	#10
JohnMc PV Abuser Join Date: May 2004 Location: St. Louis	Measure piston height at TDC very carefully, then use an MLS head gasket to get a very precise distance for best squish. Something like .032 - .036". Getting a good tight squish will make it less prone to pinging. But you'll still probably need to run 93 octane.

11-26-2020, 12:04 AM	#11
fatcatbestcat Professional Hack Join Date: Oct 2019 Location: Toronto, ON (Canada)	Alright, thanks for the replies, everyone! It looks to me like I should be aiming to have .050" shaved, to bring it closer in line to what you get with a B20B head. I'm not exactly sure what gasket I have, since the Elring kit just says 'fuel injection gasket set'. I'm going to guess it has the somewhat thicker gasket, as opposed to the E gasket. Or maybe I'm just an idiot.

11-26-2020, 12:12 PM	#12
OttoB Board Member Join Date: Aug 2006 Location: E(Seattle!Vancouver! San Francisco!LA!) Helsinski	I measused a couple B30, all pistons were about 0,5 mm below the deck at TDC. Slight variation on that measure, rods were +- O,1 mm in length.

11-28-2020, 04:23 PM	#13
Jussi Alanko Board Member Join Date: Jun 2020 Location: Finland	You could shave it even 3mm, but pushrods and rocker arms must be set to this also.