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343 advice needed

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BassBoat View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BassBoat Quote  Post ReplyReply Direct Link To This Post Posted: Apr/02/2018 at 4:49pm
OK, a couple points.  343 2bbl was about 9 to one, 343 4bbl was 10 to one.  Easy to tell what you have as the 4 bbl pistons are flat top.  So your original premise that the 343 is a "low compression" engine is not entirely true.  These numbers are obviously assuming the stock 51cc heads.  I think the above comment from Boris is correct, about 9 to one if you upgrade to 360 pistons and 304/360 crank.  
I also agree that the comment several posts up from 585 Eric is a good recommendtion.  Use the 304 Crank, flexplate, and balancer and get it balanced
IF you are only going to have 9 to one compression, you probably want a smallish cam, something like a Comp 268 or 270, or one of the middle range Lunati Voodoo cams.  I like this one:
http://www.lunatipower.com/Product.aspx?id=2585&gid=287
but if you get the next smaller one that is OK too.  The bigger cam will make more power, the smaller cam will have better torque around town.  Cam is always a trade off.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Apr/05/2018 at 5:48pm
Edit: I jumped the gun with my original post.
myrambler, I tend to agree with your idea in your 1st post.
Built one like that long ago, but with cast 360 pistons.

I get ~9.87:1 Static CR using the numbers in your 1st post:
9.175 BDH, 4.11 B, 3.44 S, 5.875 Rod, 1.60 CH, +.020 Dclr,
27.5cc Dish, .045 Gasket, 50.60cc Head

The 304 Dampener and Flexplate should work since ya have it,
it just may be "harder", take more weight/drilling, to balance.
But I'd "like" using 360 parts better, and have'em "work"
the Flexplate Only when balancing, keeping Dampener "stock".
I'd use the 998 Trans.

I'd do a 304 Crank Trial Assembly to confirm dimensions,
Deck Clearance, Piston Below, Flush, or Above Block Deck,
and mainly to clay a Piston and confirm Piston-To-Valve.
BUT that means ya gotta have the cam....

I would have thought those Pistons were 1.580 C.H. ?
Wonder if the Block Deck has been milled any ?

---
This is all "opinion" but what the hey, it's fun to discuss.
imho 9.0:1 is the Max DYNAMIC CR for pump gas, some may say lower.
So all this hinges on that 9.0:1 figure and all the other dims.
Have done my best to make sure this is correct, but please VERIFY.
imho "proper matching" of Camshaft and Dynamic CR is the "Key".

Gotta start with RPM Range, it dictates Cam choice, which dictates
"all else" to achieve the max Dynamic CR workable with Pump Gas.

Example Cam, Isky 262, specs:
Lift (.465 .465), Adv Dur (262 262), .050 Dur (208 208)
*What Matters->Intake Closing = 29 deg @ .050, 56 deg @ Adv

TARGET -> 9.0:1 Dynamic Effective Compression Ratio
          ( "Max" for Pump Gas )

Cut to the chase, "Custom" Pistons:
9.0:1 Dynamic CR = 25.00cc Dish and 1.572 Compression Height
8.5:1 Dynamic CR = 30.50cc Dish and 1.572 Compression Height

Compression Calculators at the Silv-O-Lite Piston web page:
( United Engine & Machine )

https://www.uempistons.com/index.php?

main_page=calculators&zenid=f21addd3e9e370ee84512310f26ab4d1

Looks like ~10.0:1 Static yields ~9.0:1 Dynamic with this Combo & Cam.
----------------------------------
All values are Nominal, believed correct but not guranteed, have
manufacturing variations, and must be confirmed from actual parts.
----------------------------------

CUSTOM MODIFIED 1969 AMC 343 4V for approx 9.0:1 Dynamic Effective CR:

 4.110 Bore = +0.30 stock nominal AMC 343 & 360
 3.440 Stroke = Stock nominal 1970 AMC 304 / 360 Crankshaft
 9.165 Deck Ht = -0.010 Less than Stock nominal, 1969 AMC Block
 5.872 Con Rod = -0.003 Less Than Stock nominal, AMC 290-304-343-360
 0.001 Deck Clearance = -0.055 Less than Stock nominal 343
 1.572 Comp Ht = -0.029 Less than Stock, Calculated
25.0cc Dish Volume = Calculated
48.0cc Comb Chambr = Calculated -2.6cc Less than Stock 50.6cc 1969 Heads
10.5cc Head Gasket = 0.045 thk x 4.265 dia Hole Head Gasket ( Fel-Pro )
 
Result:
50.75 sq. in. = Volume at Btm Dead Center
 5.11 sq. in. = Volume at Top Dead Center
9.933:1 -> Static "Dimensional" CR, MODIFIED AMC 1969 343 4V

->"Key" => Using 29 deg. ABDC @ .050 = Intake Closing Angle:
9.000:1 -> TARGET Dynamic Effective Compression Ratio
8.998:1 -> ACTUAL Dynamic CR - MODIFIED AMC 1969 343 4V
        -> USING 29 deg. ABDC @ .050 = Intake Closing Angle
-----------

Reference:  STOCK 1969 AMC 343 4V as Baseline
            using Perf American Style values:
4.080 Bore - stock nominal
3.280 Stroke - stock nominal
9.175 Deck Ht - stock nominal
5.875 Con Rod - stock nominal
0.060 Deck Clearance - stock nominal
1.601 Comp Ht - calculated stock nominal
3.00cc Dish Volume - calculated 343 4V cast piston
50.6cc Comb Cham - Stock AMC 1969 343-390 heads
10.7cc Head Gasket - 0.048 thk x 4.165 dia Head Gasket
 
Result:
10.19:1  -> Static CR  - Stock AMC 1969 343 4V
Can't Do -> Dynamic CR - Stock AMC 1969 343 4V
-> Unknown Stock Camshaft Intake Valve Closing Angle...
-------------------------------------
Hope this helps, PHAT69AMX ( Kevin in WV )




Edited by PHAT69AMX - Apr/06/2018 at 3:42pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Apr/05/2018 at 6:12pm
Originally posted by BassBoat BassBoat wrote:

OK, a couple points.  343 2bbl was about 9 to one, 343 4bbl was 10 to one.  Easy to tell what you have as the 4 bbl pistons are flat top.  So your original premise that the 343 is a "low compression" engine is not entirely true.  These numbers are obviously assuming the stock 51cc heads.  I think the above comment from Boris is correct, about 9 to one if you upgrade to 360 pistons and 304/360 crank.  
I also agree that the comment several posts up from 585 Eric is a good recommendtion.  Use the 304 Crank, flexplate, and balancer and get it balanced
IF you are only going to have 9 to one compression, you probably want a smallish cam, something like a Comp 268 or 270, or one of the middle range Lunati Voodoo cams.  I like this one:
http://www.lunatipower.com/Product.aspx?id=2585&gid=287
but if you get the next smaller one that is OK too.  The bigger cam will make more power, the smaller cam will have better torque around town.  Cam is always a trade off.  

9 ain't low........
OK, I have a 343 that I plan on rebuilding one of these days - I have a NOS set of flat top pistons I was told were the "higher compression pistons" use with the 4bbl - I assume those would be what was used in the 10 to 1 engines with 4bbl. They are still in the AM script boxes.
I have a pair of heads that I bought on an auction marked "1969 343" in paint pen on the end - were all 343 heads the stock 51cc heads you mention? (I think I paid 20 bucks for the pair?)

Not sure what I'll do with it when built but I guess it could go into my brother's 69 Javelin if he ever kills the 290
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lyle Quote  Post ReplyReply Direct Link To This Post Posted: Apr/05/2018 at 6:46pm
Too bad your in Iowa our i'd take it off your hands! The closest thing here for AMC is a couple of 360's that need rebuilding for $500 + and no flywheel or flex plate. Here that means $500 to $600 for balancing on top of finding a flywheel.
My 69 would love a 343, matched with some dogleg heads and some threads that have lots of cam, rotating assembly choices for a street build, 5000 RPM max.
Heads for $20, was that back in 200x?


Edited by Lyle - Apr/05/2018 at 8:46pm
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