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b20 block questions

oemoilleaks

Member
Joined
Mar 26, 2017
Location
SoCAL
I am currently in the middle of building a b20 "performance" engine. Basically the goal is as close to 200hp I can get with a dual carb big bore set up.

But I was just doing some more research before I head to the machine shop and I came across a little note on brickboard about the '74 b20 blocks being the weakest.

Is this true?

The core engine I bought was a '74 8-bolt engine. I have another unknown b20 block on the shelf as well.

What would be the best way to have the blocks tested to see how porous the casting is?
 
Can you post a link to the Brickboard article?

I am not aware of any issues with the 1974 blocks, but I have seen more ventilated 8 bolt bolt B20s (1974-1975), mostly being caused by broken connecting rods. I have seen a couple of 6 bolt engines with holes, but they were abused engines.

If you are worried, ask your machinist about sonic testing the block.
 
Just have it sonic checked...they're not any weaker. That's just BB lore. Ask for data and information and they all run. The head needs to be reduced in thickness a lot. The specific dimensions should be measured as a volume for the combustion chamber not some other data. Valves - KG Trimning has larger exhausts. I've covered building here: http://calgaryvolvoclub.com/forum/viewtopic.php?f=12&t=2699 Useful head stuff here. http://calgaryvolvoclub.com/forum/viewtopic.php?f=13&t=2227&start=315
 
I'm with ^^^^, never heard of the 74 blocks being weak. Also never had issues with mine thus far, even punched out to 92mm, been running in that configuration for 20 years, 100k or so, and I don't drive it kindly. It's only at around 9:1 and sitting at 135rwhp, so your goal, done right shouldn't be too bad to attain.
 
I've *heard* that later blocks, in general, are slightly more prone to core shift, but I really suspect it's a case of low sample size anecdotes.

That said, my even smaller anecdotal group never included any 8-bolt blocks, just a couple of B18's, a B20B, and several B20E's.

AFAIK a machine shop can do a sonic wall thickness check on the blocks as they sit, this would let you know if the core is shifted enough to matter.

There is some spurious information floating around on BB, like in that thread where someone mentions differences in stroke. They may have been confused by the differences in compression ratio, but that's all done with cylinder head chamber sizing, the strokes on all B18 and B20 engines are the same.

Since you're boring the block out anyway, it doesn't really matter which one you start with (6-bolt vs. 8-bolt considerations aside). Maybe take both in and have the one that seems to check out the best on the sonic wall thickness checks (most metal, and the most even on all sides of all cylinders).
 
Even if there was (no clue really), I don't think it would matter. 200 HP N/A certainly isn't going to be stressing a cast crank or a forged crank, that's not what breaks on any vaguely sane engine builds.

Maybe when you're into the 'Swedish top fuel' 1000+ hp redblock builds that matters, but they're probably not using an old OEM forged crank then either.
 
Hello Joe,

I found the thread to which you refer.

Once and only once have I seen a porous/cracked block. It was a block that came from our stock of used blocks and was not labeled so I'm not sure if it was a 1974 block. But ever since then, all of our blocks get crack checked before they are rebuilt.
 
ok phew. I got worried since I had never run across that before.

I'll have the blocks sonic tested soon and see what comes out of it.
 
Just have it sonic checked...they're not any weaker. That's just BB lore. Ask for data and information and they all run. The head needs to be reduced in thickness a lot. The specific dimensions should be measured as a volume for the combustion chamber not some other data. Valves - KG Trimning has larger exhausts. I've covered building here: http://calgaryvolvoclub.com/forum/viewtopic.php?f=12&t=2699 Useful head stuff here. http://calgaryvolvoclub.com/forum/viewtopic.php?f=13&t=2227&start=315

Craig,

These posts are really thorough! I've been reading through them and will have a bunch of questions as soon as I get a mental grasp on them!

Specifically about the flow values of the porting for the larger KG valves.
 
I's ax-ing you if you PLAN to bore it to 92mm? If planning that big..stock is 3.5" which is 88,9mm, then sure, sonic check it..If not that big, maybe not spend the money.

:nod:

The first question is always a trick question and needs to be answered with another question to get closer to the answer.
 
I am currently in the middle of building a b20 "performance" engine. Basically the goal is as close to 200hp I can get with a dual carb big bore set up.

...

What would be the best way to have the blocks tested to see how porous the casting is?

I's ax-ing you if you PLAN to bore it to 92mm? If planning that big..stock is 3.5" which is 88,9mm, then sure, sonic check it..If not that big, maybe not spend the money.

I just assumed that he was going to bore it to 92 mm based on his question.
 
How are you going to get 200hp? Forced induction ?

Davis201232.jpg
 
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