GEO
Sarcasm Inc.
I'd personally buy new pistons and buy modern heads and get rid of the 1970's technology stuff
13:1 Compression Pistons in 340 Small Block
- Thread starter DodgeMoparV2
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Why build or spec an engine around "leftovers"? Although those are cool.
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SLOPAR72
Well-Known Member
You said a mouthful right here..... Having thrown dollars at a project based on a single component (pistons to be exact) the best advice I can give is don't do it. I am sure there are people who got lucky and made it work but starting off with a single component and then trying to make everything fit is a recipe for disaster....
JW
Thanks JW, it just doesnt make sense , to me anyway. Especially when talking about an advertised 13:1 compression.
The amount of $$ spent to match up to those pistons/compression, you may be further ahead going with new pistons and other pieces??
The amount of $$ spent to match up to those pistons/compression, you may be further ahead going with new pistons and other pieces??
I bracket raced it many years ago.....actually the complete engine is in the shed....340 standard bore....with J heads with 2.08 intake valve.
actually needs a camshaft....plus rocker gear.
yellow rose
Overnight Sensation
Explain how that piston is so obsolete compared to a piston from today, with the same pin height and dome volume.
If you are paying attention, they aren't that much heavier, have a standard ring package and the skirt design is actually pretty damn good.
It's not nearly as obsolete as some of you seem to think it is.
GEO
Sarcasm Inc.
Well the current trend is flat top pistons and small combustion chambers which work better than the old domed pistons and open chamber heads. Figure the rest out your self I'm not your mom
Duane
Well-Known Member
Small combustion chambers only work better as far as quench which the intent is primarily to run pump gas without detonation.
Some designs of small chambers inhibit the breathing of the head.
Flat top Pistons on a modern build are really only possible on a stroker build because of the increased swept area, otherwise it would be very hard to get any kind of decent compression.
Each type build has its pro and cons. It depends on the intended goals of the build and operating costs of the fuel.
Duane
Well-Known Member
Because he has the parts. I agree he could go a different route, but at what cost. The J heads and his 340 block will work with those Pistons. He could build it and use e85 without a problem.
Again it depends. IMHO
yellow rose
Overnight Sensation
yellow rose
Overnight Sensation
If Geo would go back and read what I originally told the OP he would understand you can have open chamber hands and still get quench. I even explained how to do it.
As you mentioned, there are plenty of horrible closed chambers out there. They are flow killers and they are detonation prone. The early BBC heads are just one example. But the hipster trendy deal is the crap posted about those Pistons. Find a new piston with the same dome and pin height and you'll find the weights are pretty close. They have a 1/16-1/16-3/16 ring pack which is pretty much standard now, and they can run relatively tight piston to wall clearances.
All the keyboard builders here have no clue.
Duane
Well-Known Member
Can you elaborate on what you are saying here.
I do not see how you can successfully build an engine without considering the inter action of all the other major components that will go with it. Eg if you were to build a race engine around the new
Edelbrock Victor head, the Pistons would have to be custom made with 16 degree instead of the standard 18 degree valve reliefs.
Or if you have a .030 over bore block but a standard bore piston.
Just not sure what you mean.
My thinking is that he is compromising some of the street ability he is looking for by using the pistons he has.
In my mind, and I could be wrong but this would increase cost of build substantially? Or sounds like a cam/lifters/valve train purchase is needed regardless. So would there be any real price difference?
In my mind, and I could be wrong but this would increase cost of build substantially? Or sounds like a cam/lifters/valve train purchase is needed regardless. So would there be any real price difference?
yellow rose
Overnight Sensation
If you have the tools, it's relatively cheap to knock the domes down on them. Maybe a hundred bucks. Then he can use them. But no matter what he does, to do it correctly using those Pistons, the deck of the piston needs to be out of the hole .040-.050 depending on his chambers.
I know someone has mentioned E85. Is the compression high enough for that?
yellow rose
Overnight Sensation
I've run methanol at 10:1 and it's not an issue. I'd do E85 if 1) I could get it and 2) I felt like checking the actual percentage every time I fueled up as E85 at the pump can vary.
And I'd damn sure be at least 12.5:1 with E85 because I can run over 11 with pump gas.
Duane
Well-Known Member
Duane
Well-Known Member
I,lol say it again,I do not believe those pistons with a typical Chrysler open chamber will measure 13:1. Secondly, thicker head gaskets would knock it down some more. IMHO
yellow rose
Overnight Sensation
justinp61
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Depends on what you call decent, my 340 ran a flat top with a 55cc chamber aluminum head. It was 11.4-1 and ran on pump 93. With the same heads and a 21cc dish my 408 was 10.8-1.
Duane
Well-Known Member
That's what I would call decent.
But your 2 different builds reinforce what I said previously. A stroker build allows much lighter weight Pistons and makes it much easier to get a high ratio without a big heavy domed piston. The 55 cc chamber helps even more. Your dished stroker piston Will be way lighter than the Op 13:1 domed piston because the stroke increase allows this with more swept area.
That is also why yellow rose is saying that if the Op bought new Pistons for stock stroke build with the same length rods and stock piston pin height, they will still be fairly heavy because the piston will be much longer than a stroker piston and cannot be dished with a 70cc head. You also have to remember that aluminum heads at the same compression ratio make less power than cast iron,
11.4 is really 10.4 which takes us back to what the intended goals are.
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nm9stheham
Well-Known Member
Mmmmm not really so much savings on the stroker. The big changes in modern pistons is in the crown underside shaping, the general crown thickness, and the areas around the pin bosses. That's where 80+% of the weight is. That applies for stroker and non-stroker pistons.
KB's for 340 for example are 588 grams vs the stock 719 grams.... so minus 130 grams just in the casting and shaping changes. Drop another 22 grams in the pins. (And those KB's for the 340 are a bit thicker in the crown to push the dome up.) Those old forged domes ain't gonna be any lighter than stockers with the older designs.
The 360 stock stroke KB's are 510 gr vs stock 576 gr stock; drops to 478 gr for an equivalent KB stroker. Not all that different a change versus than the stock stroke ones; the general casting design makes a bigger difference.
You can see these same general casting/forging design changes for lighter weight show up in the 90's in production pistons in GM and Ford engines at the least.
BTW, not the place to do this, but at some time, I'd like to discuss this idea of power difference based on the head material.... This has showed up again recently.
KB's for 340 for example are 588 grams vs the stock 719 grams.... so minus 130 grams just in the casting and shaping changes. Drop another 22 grams in the pins. (And those KB's for the 340 are a bit thicker in the crown to push the dome up.) Those old forged domes ain't gonna be any lighter than stockers with the older designs.
The 360 stock stroke KB's are 510 gr vs stock 576 gr stock; drops to 478 gr for an equivalent KB stroker. Not all that different a change versus than the stock stroke ones; the general casting design makes a bigger difference.
You can see these same general casting/forging design changes for lighter weight show up in the 90's in production pistons in GM and Ford engines at the least.
BTW, not the place to do this, but at some time, I'd like to discuss this idea of power difference based on the head material.... This has showed up again recently.
Duane
Well-Known Member
I agree with your comments about the shaping under the crown.
But I also notice in your 2 examples with the Kb Pistons for both the 340 and 360 that assuming the same deck height as soon as you posted the 360 numbers for the kb and the stock weights, both numbers on the 360 were much lighter. A small block has a relatively tall deck height for the size of the engine.
With a short stroke and a 6.123 rod, and the pin location where it is, makes for a fairly tall piston.
When comparing the weights of Pistons you can get into a whole bunch of engineering that is way over my head, but a 4.00 inch stroker piston has its pin moved up higher. Because of this the Pistons are also shorter which also makes them lighter. I believe they can make them shorter because the pin is moved up and the shorter skirt will still remain stable in the bore.
Although 152 grams may not seem like much of a difference in weight, your rod bolts will say otherwise as they try to yank that piston down after the exhaust stroke at 7,000 rpm lol.
The loss of power with an aluminum head is well documented.
The work around is to build the motor with 1 number higher compression. Almost all all out race motors that are using aluminum heads, build with a ratio of 14:1, with cast iron it is 13:1, everything else being equal.
Duane
Well-Known Member
yellow rose
Overnight Sensation
I've read that. I run the same compression and timing on cast iron as I do aluminum. There isn't a difference. Run as much compression as your fuel, cam timing and tuning ability allow and disregard head materiel.
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