I got my 'race' cylinder head (bought on eBay - http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2471020578 ) in the mail a couple of days ago.

Looking it over I see lots of good news. Ample evidence of porting and polishing in the intakes and exhaust ports. Double valve springs. Oversized valves. I popped out the valves (and a pair on my stock head) to compare and was really struck by how small the exhaust opening was in my F head, the ported head is dramatically wider. It still takes a sharp bend off the valve, but there is no way around that. I'd guesstimate there is about 1/3rd more room in the exhaust port in the tightest spot.

Not so good - it's very noticably thinned down, especially compared to the F head I had on the car. I cc'ed one of the combustion chambers and got 38 cc's, which on top of my big bore (92 mm) bottom end would end up roughly 12:1. Not good at all for pump gas. I got a set of carbide burrs and used a router, tried it briefly and it carves the cast head like butter.

Next step is to (carefully, obviously) extend the ends of the oval out to the 92mm bore size (have to double check, because it already looks very, very close, perhaps it was already done to fit a big bore engine). Then to extend the combustion chamber outward around the intake valve until I hit 47 cc's - which should result in a CR in the mid 10's. Then some polishing to remove any roughness left behind by the carbide burrs (although they seemed to leave a very smooth finish where I tested it).

Sounds like a good idea to me!
While you are in there, knock off any sharp edges or thin spots in the casting to reduce possible hot spots and polish the chambers as much as you can (including the squish zones).
Wow those are small chambers! The head was run like that? Woof.

Also, check for piston to valve interfereance due to the skimming!

Mike!

No need, it's a vertical valve engine. It's not one of them new fangled wedge heads nor are they canted as is the fashion in Cleveland. :)

Cheers,
Paul.

http://www.georger.com/unleaded_gas.htm here's another look at a B20F head. Be sure about the hardened seats. I went for 47.5 cc to get into the high 9 CR believing over 10 is asking for trouble for a daily driver. While you have the engine out... shave the flywheel and feel the new zippyness.

Christopher

Seems to me you've got yourselfs a pretty ok head. Although on this pic it looks as if cil #2 has a worn seat for the intake valve.
http://imagehost.vendio.com/bin/viewimage.x/00000000/amx70/B20head4.jpg

I'm not sure, might be just the photo. It does have hardened exhaust seats by the look of it, but not on the intakes. The guy who sold it also stated "I would get a professional to look at the valve seats before using" which will probably be needed if you want hardened intake seats aswell. What do you think of it?
Cheers Ben

I think the seller was referring to a couple of very snall nicks in the #1 intake seat, probably caused as the screw got caught for a cycle or two in the valve on its way in. The valves and valve seats look very good to me. I'll double check when I get home but I don't recall that #2 valve being recessed as it looks in that pic, that might be a trick of the light. In 'real life' the #1 combustion chamber didn't look 1/2 as beat up as that picture might make you think.

Now on the other hand, all 4 exhaust valves on my well-used B20F head looked to be countersunk. But all 4 intakes still stood proud of the chamber floor. I -think- that hardened seats on the intakes aren't as needed as on the exhaust. And possibly not possible to install with the bigger valves - there doesn't seem to be enough room between the intake and exhaust for 2 hardened seats, just (barely) one.

"Now on the other hand, all 4 exhaust valves on my well-used B20F head looked to be countersunk. But all 4 intakes still stood proud of the chamber floor. I -think- that hardened seats on the intakes aren't as needed as on the exhaust."

Right indeed, exhaust valves/seats get way hotter then the intake and hence increase wear. Have a look at a thread I posted some time ago:
http://www.brickboard.com/RWD/index.htm?id=772759

ben


--
Daily driver: P130, '65, B18+M47. In the proces of restoring: P131, '69, B20+AW71L. (www.knutselsmurf.web1000.com)

here's the picture of the chamber

The engine is still in the car, the new head was sort of an add-on to the main project of replacing the flat-lobed cam. An 'R' grind cam and some steel timing gears are going back in.

Over the winter I had the M41 out to replace some syncros and had a few pounds knocked off the flywheel at the time. I didn't go too far with it, however, so the results were noticable but pretty subtle.

Only about 3 or 4 lbs. All from the outer edge where it would make the most difference - the machinist just carved off the outer rim (leaving the ring gear, obviously) in to where the pressure plate attaches.

As the engine is still in the car, you might want to hold off on enlarging the chambers. It's best to bolt up the head on the bare block, reach up the cylinders and scribe the bore on the head. Then you know exactly how much metal to remove, and you'll know it'll line up right.

Intake valves don't need hardened seats, don't worry about that.

I used the old headgasket as a template. I put it on the block and verified the fit to the bores. Then I put it on the head and fixed it in place with some dowels through a few bolt holes.

Now my problem is thus. I've taken out what I feel is a fair amount of metal, and the last measurement shows 40ccs still, not enough yet. But I think if I keep going until I hit 46 or 47 cc's then I might have taken away nearly all the ledge that produces the 'squish'. Thus defeating the point of machining the head vs. spacing it out some with a thicker copper headgasket.

I can't take nearly as much metal off the other side of the chamber because I figure I shouldn't leave the spark plug sticking out too far. I guess I could use some spacers to pull the plug back slightly, then remove metal to bring them flush again, but I don't really like the idea of doing that for some reason. I've already taken some metal off on either side of the plug resulting in a slight heart shape contour.

At this point I'm thinking I should not take much more metal off the head, and make up the rest of the combustion chamber space needed with a somewhat thicker gasket. not a perfect solution, but then again the head was $160 and not $1100.

Pull the pistons & mill them down if they are stock type pistons. Just leave enough of the top to match the main squish area.

Ahh - carve out a sort of matching center oval in the piston? I could see that working, although obviously I would want to leave adequate thickness in the top of the piston.

I worked on chamber #1 again and took out about as much as I can stomach, measured it and got 41.5 cc's. Continuing on to 45 or 46 cc's would result in a circular combustion chamber with the only squish coming from the small ledge on the spark plug side.

At this point I'm just going to try assembling the engine with all chambers to 41.5cc's, carefully polished with no sharp edges. I'll put it together with a regular thickness headgasket and see how it works with 11:1 compression ratio. If it just doesn't work, even with octane booster, I'll get a thicker copper headgasket to drop it down to 10.5:1. And plan on doing something with the pistons whenever I do a bottom end rebuild.

I wonder if they make dished B21 pistons? I know certain turbo OHC engines used dished pistons, curious if any B21 sized ones were.

"Ahh - carve out a sort of matching center oval in the piston?"

No, I mean something simpler. Cut down the top of the piston by about .050" for 3/4 of the way across the piston, just leaving a flat ledge big enough to match the RHS squish area. I like to remove the LHS squish area as I feel it's counterproductive & doing so effectively places the plug towards the centre of the chamber rather than at the edge.

"I wonder if they make dished B21 pistons? I know certain turbo OHC engines used dished pistons, curious if any B21 sized ones were."

They would be very heavy & totally defeat the reason for having an overbore. Every 1 gram saved in the pistons & pins is like saving 100 on the flywheel, I would guesstimate that B21 rods & pistons would already suck up the extra torque created by the bigger bore when compared to the early style rods & pistons.

Cheers,
Paul.

I wouldn't mess with the spark plug side of it at all. That wedge needs to be there.

You already know my opinion of using thicker head gaskets to lower compression. Better you should deck the block and/or use a thin gasket to tighten the squish -- that will do more to prevent pinging than the resulting few tenths of a point rise in CR will do to cause it.

Do I remember that you're using an R cam? Bet you can get away with 11:1 on 92-octane with that cam and tight squish, particularly with well-jetted Webers.

Here are the specs for the R cam:
http://www.geocities.com/iadr1/B20trackCamshaftDetails.htm

That overlap (98!) is substantial and will reduce the actual compression ratio to a manageable level. Just keep your ears peeled and back off total advance where needed. I'm no genius so take this advice with a grain of salt.

I've run 11:1 on pump gas in a chevy V8. I was running a cam with a lot of overlap. 327, ran like a sewing machine in the high rpms and never had a problem with detonation.

steve