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Old 12-11-2020, 04:14 PM   #1
spock345
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Default OHV D vs K cams and how much should I shave the head?

Just some sanity checks before I pull a head and order a camshaft.

Is there any big difference in how the engine drives with a D vs a K cam in a B18/20? By the numbers they look almost the same.

I am planning to shave the head on my B18D down to the thickness for a B18B (88mm to 86.2mm or 0.070" off if the source I have is right) will I still be able to run pump gas fine or should I back off a little bit to have a safety margin? All we get here in CA is 91 octane.
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Old 12-11-2020, 04:26 PM   #2
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No experience actually driving a K cam, but the specs suggest it would be hard to tell the difference between them. Practically the same cam. If you were a butt dyno extraordinaire you might be able to tell that the D is slightly sportier. But only by a mouse fart.

And I have put D cams in otherwise pretty stock B18's before, and they felt great. My PV's stock B18 was a real tractor motor, no upper RPM power at all. D cam swap and it was a lot more fun to drive. Swapped the single downpipe exhaust manifold for a double and it was even more fun.

Detonation resistance on ye olde fashioned chamber design is partly compression ratio (what you're going at there) and partly how well it's et up to take advantage of squish/quench. Carefully measure that piston/deck height at TDC and then get a HG of the correct thickness to put the head .032 - .036" above it. That violently snaps the burning mixture into the central 'bathtub' chamber as the piston hits TDC. The closer the piston gets to actually touching the head, the better the turbulence and this helps more fully burn the mixture, in a more controlled fashion, and helps reduce hot spots. The effect rapidly diminishes as the distance increases. To more precisely set it, I use Cometic MLS HG's, they don't compress at all, the thickness listed is the thickness installed.

That said, it is a steel head, and steel heads in general are a bit more prone to having hot spots and detonating, so even with good squish, maybe shoot for something under 10.5:1.

Lowering the CR with a thicker HG is initially not productive, because you lessen the squish, and only eventually gain back some detonation resistance through lowered CR.

EDIT: And of course, there's static CR - what the displacement vs. the volume of the chamber/cylinder walls/HG at TDC, and the dynamic CR - which involves when the intake valves actually close. The CR you can use with a given octane fuel varies depending on that, as well as the CR, as well as the carb mixture, as well as the amount of squish, spark plug temp range, phase of the moon, etc. Also, a little occasional light sprinkling of pinging isn't really a bad thing on an old motor, it's not like detonation happening under lots of boost. No ping-ping-boom on a NA Volvo OHV.
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Old 12-11-2020, 04:33 PM   #3
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I was thinking that the the D cam's longer duration would probably lower dynamic CR a bit versus the B cam that is in the motor right now.

I would do the math on the dynamic CR to try to keep it similar to known configurations that don't ping on 91 but I can't find a good source for OHV cam and CC specs.

Really all I want is a bit more pep. The low end torque on the motor is rather good but dies off pretty quick. Shaving the head hopefully will let me offset the inevitable loss on the low end from moving to a more aggressive grind. Then again B18Bs with C cams will run off pump gas so I will hopefully be OK.

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Old 12-11-2020, 06:55 PM   #4
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Information on B18 and B20 cam grinds.

http://www.1800philes.com/ianr/_superlist_grinds.html

If you measure at 0.050" lift the durations are effectively identical and the maximum lifts are identical. The duration measured at 0.020" lift is slightly longer for the D grind. I think the discernable difference is that the K camshaft will idle better than the D camshaft. The K will probably idle at a lower manifold pressure (higher vacuum) than the D cam. My B20E with a D cam idles with a manifold pressure of 55 - 60 kPa at around 875 RPM. I believe that SUs will work with a D cam; but, they may idle better with the K cam.

The K cam was part of Volvo's emission compliance efforts and in the early days the testing regimens focused primarily on performance at idle. Given the fact that the max lift and durations measured at 0.050" lift are identical the peak output of both cams is likely similar. The area under the lift versus angle curve for the D cam is only minusculely larger than the K cam.
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Old 12-11-2020, 07:35 PM   #5
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Originally Posted by 142 guy View Post
Information on B18 and B20 cam grinds.

http://www.1800philes.com/ianr/_superlist_grinds.html

If you measure at 0.050" lift the durations are effectively identical and the maximum lifts are identical. The duration measured at 0.020" lift is slightly longer for the D grind. I think the discernable difference is that the K camshaft will idle better than the D camshaft. The K will probably idle at a lower manifold pressure (higher vacuum) than the D cam. My B20E with a D cam idles with a manifold pressure of 55 - 60 kPa at around 875 RPM. I believe that SUs will work with a D cam; but, they may idle better with the K cam.

The K cam was part of Volvo's emission compliance efforts and in the early days the testing regimens focused primarily on performance at idle. Given the fact that the max lift and durations measured at 0.050" lift are identical the peak output of both cams is likely similar. The area under the lift versus angle curve for the D cam is only minusculely larger than the K cam.
Going by the numbers there I can estimate dynamic compression as follows. Although these are really rough numbers I guess what we care about in this case is the difference between them.

B18D (8.7:1 static) C cam: 8.21:1, D cam: 7.94:1
B18B (10:1 static) C cam: 9.43:1, D cam: 9.11:1
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Old 12-11-2020, 11:54 PM   #6
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Wouldn't a B20 head fit on a B18 block? I feel like that would be a better use of time and money versus a bunch of work on a small-valve head.
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Old 12-12-2020, 12:10 AM   #7
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Originally Posted by fatcatbestcat View Post
Wouldn't a B20 head fit on a B18 block? I feel like that would be a better use of time and money versus a bunch of work on a small-valve head.
It'll fit but after some reading on the subject it seems that the larger combustion chamber doesn't mesh well with the narrower bore.

I could bore out the block to B20 spec but I would rather not because the lower end is in incredibly good shape.
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Old 12-20-2020, 01:51 PM   #8
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B18-20 hybrid=garbage. Don’t waste your time. The chamber mushrooms over the bore, no bueno. Not a huge difference between k and d. Less overlap, so if injected you can get some more miles depending on tuning. I’m running a k on my b20+T and it’s happy to rev to 7k, no noticeable power drop. The c that I ran last was a brick above 5500.
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Old 12-21-2020, 01:01 PM   #9
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I still retain that swapping to a B20 or AQ140 is a better use of money. You're not going to get far spending a lot of money on a small-valve B18 head. Even the worst B20 is an improvement.
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Old 12-21-2020, 06:29 PM   #10
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A B20 would be a better option. If there were one for sale nearby I would be picking it up in a heartbeat. There is a B30 nearby but I doubt it would fit without cutting and welding.

For now I just make do with what I have.

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Old 12-21-2020, 11:17 PM   #11
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We like D cams so much, we bought an original D cam from Volvo many years ago and have been having copies made ever since. We had to because the only good source for aftermarket cams was bought by Federal-Mogul and then closed by Federal-Mogul about a year later.

I agree that upgrading to a B20 is a better use of money.

If you can't find a rebuildable B20 closer to you, we've got some.
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Old 12-22-2020, 12:52 AM   #12
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Originally Posted by planetman View Post
We like D cams so much, we bought an original D cam from Volvo many years ago and have been having copies made ever since. We had to because the only good source for aftermarket cams was bought by Federal-Mogul and then closed by Federal-Mogul about a year later.

I agree that upgrading to a B20 is a better use of money.

If you can't find a rebuildable B20 closer to you, we've got some.
Someone must have a B20 in OK shape kicking around somewhere in the SF bay area.
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