I'm just disassembling my 1970 340 and find that it has 1267 pistons 0.040" over and the bores look good so far (thank goodness). I've got O heads. From what I see in I should get around 9:1 compression assuming the heads around 72cc. I have the equipment to cc them which I will once everything is cleaned up. Does this sound right? Its a bit lower than I would like but perhaps close enough. I don't have a plan forward yet but I am thinking mostly stock build perhaps slightly hotter. I'm going to run stock hp manifolds so no point going wild. I have an old Offenhauser intake and will get a new AVS2 650 carb. The cam is undetermined. Also, no idea if it was or needed to be or was rebalanced.
Compression ratio with Silvolite 1267 pistons and stock heads?
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Stick with the stock intake or pony up for air gap or ld4b. Good cam for stock street 340 is a street hemi grind. Works well with stock rockers and springs. Cast manifolds too.
With the 1267 (same as mine) compression height of 1.824, those slugs should be pretty close to zero deck +/-. Once you get your actual chamber CC (mine are 69CC) do the math. With the right cam you can dial in an acceptable DCR for pump gas. My static cr turns out to be 9.17 with an .040 head gasket. My cam choice puts me at 7.75 dynamic cr.......Should be great for the street with 3.23's. Good luck with your build.
Kendog 170
Let the boy go !
What cam would that be? Just curious.
The classic Street Hemi grind was 284 duration and .484 lift? The updated version was about 280 duration. Would you like me to look up the numbers?
Forgot to mention........I chose these pistons and reconditioned my factory 340 rods to avoid balancing. This combo weighs out pretty close to the factory combo.
Kendog 170
Let the boy go !
Sounds like the Factory 340 4 speed cam.Oregon cams makes a repo.
Here are the Oregon Cam specs/details:
#2120
221/227 @ .050”, 276/286 adv, .438”/.456” lift, 114 sep
Here are the Oregon Cam specs/details:
#2120
221/227 @ .050”, 276/286 adv, .438”/.456” lift, 114 sep
Direct connection.471 intake .474 exhaust 284 duration both 90 deg overlap.
No, thats the later hemi grind that replaced the street hemi grind. They are different.
The street hemi grind I’m guess is 85% of the advertised 284 duration which is 241 but I believe it maybe a little less. It doesn’t have a ridiculous high choppy idle. It has 109 lobe separation.
Boy that picture loaded blurred and crappy.
Part # 3690213
LD340. LD4B has 273/318 sized ports.
Oooops, my bad.
Like kendog said, call Oregon cam grinders.
Dan the man
Well-Known Member
What are the specs for a street hemi cam for the 340?
Look in posts 9 and 11. Sorry about the blurry picture. Looked good in phone pictures, just uploaded crappy.
No to my knowledge. You would have to find one used or new old stock. You could try Engle cams, that guy has ground alot of mopar cams. More than likely he would know what a street hemi grind cam is. He was also grinding fast ramps before most folks. He at one time was grinding Hughes engines cams for him. I also hear alot of good things about Oregon cam grinders.
With the stock large chamber heads and those pistons, you're going to be somewhere "around" 9:1. Probably a tad under. The numbers show .002" positive deck clearance, but those pistons have four large eyebrows and not two like the true 68-71 pistons had and they were around .025" positive deck clearance. Also, another note is that the deck heights on Chrysler V8s are always tall from factory spec, so that will play a role in reducing compression a little further. I think a good guess would be between 8.5 and a tic under 9:1.
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I was thinking of the Direct Connection .471 intake .474 exhaust 284 duration cam versus the newer equivalent/better cam with the 280 duration .474 lift cam.
I’m not sure its a “better” cam, its just different and I have never run that cam. It may be ground for later cars like 72 1/2-73 and up cars with less compression and smaller valves.
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