posted by
someone claiming to be Toby B
on
Wed Mar 5 16:47 CST 2003 [ RELATED]
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Okay, this is an old- school subject, but:
Does anyone have any info on combustion chamber modifications?
I've found lots of stuff on porting the intakes and exhaust, but I have a specific concern with the sharp ridge in the B20 head, and am trying to figure out if I can chamfer it, or if that will cause OTHER problems...
Any resources appreciated,
Toby
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Don't chamfer it. Don't chamfer it. Don't chamfer it. It's fine like that.
Porting: It's fine to smooth out casting marks and such in the intakes, but do NOT reshape or enlarge the port. It will almost surely flow worse. On the exhausts, open up the short radius as much as you safely can. It might be a good idea to slice up a junk head of the same type to see how much material you have to work with.
Don't polish the intakes -- a fine sandblast finish is what you're after.
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Enquiring minds want to know: Why not? Tell me more!
In the world of turbocharging, anything that has a sharp edge will heat up, and act as a detonation source. Even a slight (1/32) chamfer will break that edge, and keep it a lot cooler. What are the potential disasters associated with rounding it?
In the same vein, the porting, while not trivial, isn't as super- critical, since there's a lot more pressure available for cylinder charging...
Toby
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Just to make sure I'm clear on this, you're talking about a non-turbo motor here, right?
It's normal to take some crocus cloth to break the sharp edge of the chamber after milling the head, just enough so you don't cut yourself. 1/32, or any visible chamfer, is WAY too much. Without going into an article-length explanation, it has to do with the squish / quench characteristics -- the area where the flat top of the piston almost contacts the flat surface of the head is there for a reason, and exactly how gases are expelled from that into the chamber is critical to efficient combustion. Rounding that edge is destructive to that process. It's not a hotspot at all.
Porting a normally-aspirated motor is an absolutely arcane art. The shape of the ports is all-important. Picture the airflow in the port as a river having to go around a bend. For there to be no turbulence (which is what porting is all about), the water would have to speed up at the outer bank and slow down at the inner bank. That doesn't happen -- you get some turbulence all across the stream, and it's likely that some water will eddy and actually flow upstream in places. Volvo put a lot of work into developing the curve in those intake ports to minimize that turbulence -- you cannot improve it without enlarging the whole port a bunch... and then you have velocity problems with the whole flow slowing down. That's catastrophic in a non-pressurized motor. Leave it alone.
On the other hand, feel free to remove casting marks and anything else like that -- we don't want snags creating local turbulence in the river, so that's all to the good.
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yeah, the parts and plumbing are almost ready, and the engine bay is (finally) cleaned up and repainted-
I'm putting on a .60/.63 TO3 with aftermarket FI on the 71/72 intake and lower c/r head.
It's going to be a street car, so it's not really worth the (substantial) investment that a race porting job would cost, especially since upping the boost a psi or 2 accomplishes the same thing- IF the engine is detonation- resistant enough to withstand it! And this is where I only know enough to be dangerous, not fast...
I'm familiar with the concept of 'squish' but wasn't sure if that whole flat spot (across from the plug) was critical- on many more modern motors, there's a 5mm band around the crown of the piston, then a recess in the center- thus maintaining squish on the piston circumference, but giving it up elsewhere.
I wish I could find some Swedish hotrodders from the '70's who'd worked on these heads...
But yeah, I'll go with your recommendation, and as the local pull- a- part has supplied me with oodles of spare heads, if I have trouble with detonation AND have a few spare years to spend at it, I'll start down the trial- and- error- and- explosion road...
Thanks for your input!
Toby
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Ah, that changes things... but still don't chamfer it.
The squish thing is still important, and it's your first line of defense against detonation. I don't know of any dished pistons available for these motors, but you can get a lot of benefit from decking the block so the pistons come as close as possible to that flat area on the head without actually hitting it.
Not knowing what rods you'll be using or how high you intend to rev, I suggest something like .035" clearance at TDC (I really hate to mention specific numbers). So, if you have a head gasket that crushes to .047" (standard Elring .050" gasket), you'd end up cutting the block down until the pistons are .012" proud in it. Or use a .035" gasket (whatever that crushes to -- it's marked on the wrapper at least) and have the pistons a few thou down below the block deck.
The closer you can safely get the clearance, the less it'll tend to detonate. This requires that the rest of the motor is built with enough precision that you can tell exactly where the pistons end up, of course.
AFTER that, you can calculate CR and modify the head to suit.
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posted by
someone claiming to be cdu
on
Fri Mar 14 05:01 CST 2003 [ RELATED]
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B19 turbo pistons are dished. They will only fit if you have
an 8 bolt crank or connecting rods setup to adapt a 24mm wristpin
piston to a 6 bolt crank. Also B19 turbo pistons may be hard to
locate in the states but that just means that it will take longer
to get them.
chris
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Those would work, but someone (sorry, forgot who it was) recently posted that they were NLA from Mahle and could only be bought in Italy, or some such. Might be worth verifying that.
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