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B20F Head - how far should I shave?

fatcatbestcat

Professional Hack
Joined
Oct 1, 2019
Location
Mississauga, ON (Canada)
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
 
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.
 
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.

B20EngineSpecs2.jpg
 
That'll get him in the ballpark. Use a burette to measure the chamber volume if you want to be accurate.

CCingB20Head.jpg
 
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.

B20EngineSpecs2.jpg

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

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.
 
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.
 
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.
 
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