1970 340 has the 1973 low compression pistons

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wolfhammer

wolfhammer

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Ok guys, just looking for opinions, thanks
I have a 70 340 I bought five years ago. It was rebuilt in the 70's and sat on a stand since then. The guy replaced the pistons with 73 type low compression pistons, the ones with 4 eye brows and quite a bit below the deck.
What I'm wondering is would it be worth keeping those pistons after I check everything out with the machine shop or get the higher compression pistons? The ones in it are brand new but being a big block guy all the small block stuff is new to me and I'm on a tight budget. I would like performance since I want a it to be mainly a tough street car and will eventually run on an 1/8 mile track maybe once or twice.
I kind of feel it will be a dog with those pistons and don't want to try making up cylinder pressure with one of those whiplash type cams. The photo shows how far down in the hole the pistons are.
Here's what I have.........
*69 Barracuda FB full interior all steel
*3.55 sure grip 8 3/4 XHD leaf springs
*will be converting to a 4 speed
*255/60 15 rear tires
*X heads (stock)

245102423_414215810287521_5124280265276799155_n.jpg


20624666_1757784960916984_2074368189_n.jpg
 
Doesn’t matter if you have big or small block higher compression = better power.

you can’t have your cake and eat it too (save money and go fast) when it comes to this, so, I’d recommend replace it them with some higher compression pistons based on your criteria
 
With today's crappy ethanol gas, keep the low compression.
 
Get out your measuring devices and tell us how far down in the hole those pistons are at TDC. A flat top piston engine can make power. A set of closed chambered heads or a set of aluminum's can make it sing.
 
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All depends on how much you want to spend.Some well ported heads and a not too big of a cam and you could have a good runner on 87 octane.

More compression gives you more flexibility for everything if cost isn't a make or break deal.
 
Rat Bastid said:
You’ll be lucky if what you have is 8.0:1 and that’s low.
Click to expand...
The pistons don't look like they are very far down from the deck surface, I'd say closed chamber heads with around a 62-64cc chamber and 0.039 thick head gaskets should put you around 9.0:1 - 9.25:1.
 
For a streeter
and 3.55s
and a 2.66 low gear A833;
Your starter gear is only 3.55 x 2.66 =9.44.... so if you cam that 8/1 engine up, the CCP will all but disappear turning her into LESS powerful than 318 up to about 3000/3500.
Furthermore, with a 5500 shift rpm, you won't get into Second with those 27" tires until 47 mph, but 3500 is 30 mph, which if the tires won't spin, is a stinking long time. Then at 47mph, the Rs in Second will start out at ~4000, going to 5100 @60 mph.

What I'm saying is that
IMO
For a streeter, with 3.55s, a 340 cannot afford to have a soft bottom end. You will cry about it every time you take it out.
How you get the pressure is up to you. There are just two ways; 1) decrease the Total chamber volume or 2) close the intake valve early.

Here's what I mean;
Take the 8/1 340 with the factory 268/114 cam. in at 4* advanced, the Ica will be 64* and the Wallace predicts just 119psi CCP, at a very modest V/P of just 94.
That V/P number is barely bigger than a 225 slanty (87), and not even as strong as a smoggerteen (114 in Maine @600ft)
So even the stocker was already soft down low. With No hi-stall, you are married to that stinking V/P of just 94.
For big fun, you will want that V/P to be up around 170, but you can't do that with Iron heads, nor with a 3.315 stroke.
With iron heads and a stock stroke; next best is maybe 130/135 V/P, but that will require a careful co-ordination of parts. This would make your very low rpm to be very powerful, for a 340, which is always a joy.

But if yur the kindof guy who never runs down low, or who doesn't mind reving it up and slipping out the clutch, well then just put me on ignore. But before you do just remember that 30 mph in Second gear is just 2550 rpm, but in First is 3530, so I guarantee you that as a streeter with 3.55s, you are sorta married to low-rpm. Think about that as you are choosing a cam.
BTW; you should chose the cam first and then build the engine around it. To do otherwise is just inviting disappointment.
To machine the heads, you also have to machine the intake and sometimes the china walls as well, and then you need custom-length pushrods too. Imo the better solution is just to swap out the low-compression pistons.
But if you decide to run a tiny cam, maybe you already have enough CCP. For instance, running the factory 318LA cam will get you an Ica of 48* and the Wallace predicts..... a V/P of 121. Oh well, better than 114, I guess...........
 
Built a 72 340 for a 72 Cuda 4 speed. Milled the heads .040 and used file to fit rings. Stock everything, went pretty good for a street car.
 
Dan the man said:
The pistons don't look like they are very far down from the deck surface, I'd say closed chamber heads with around a 62-64cc chamber and 0.039 thick head gaskets should put you around 9.0:1 - 9.25:1.
Click to expand...


You might get there with a closed chamber head, but it depends on how far down the hole you are.
 
toolmanmike said:
Get out your measuring devices and tell us how far down in the hole those pistons are at TDC. A flat top piston engine can make power. A set of closed chambered hears or a set of aluminum's can make it sing.
Click to expand...
Rat Bastid said:
You might get there with a closed chamber head, but it depends on how far down the hole you are.
Click to expand...
Dan the man said:
The pistons don't look like they are very far down from the deck surface, I'd say closed chamber heads with around a 62-64cc chamber and 0.039 thick head gaskets should put you around 9.0:1 - 9.25:1.
Click to expand...
Is there an echo in here? :poke::poke::rofl:
 
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Depending on your expectations and gear ratio. 340s with flat tops with a zero deck and iron heads is just about all you can get away with on pump gas. With those Pistons you will be okay, but for around $300 and a set of kb243s you'll have a stronger setup. But if you're looking to put on a torque storm or something like that you're just right.
 
I would build that like this.....LEAVE the 4 eyebrow pistons in. Blueprint the engine to 9.2:1. Put the Comp XE268 in it and it'll haul *** and you'll never, never have to worry about the problems with high compression like spark knock, kicking back against the starter or any of that "nonsense". The difference between compression points is about 3%. You don't want a 10:1 plus motor with iron heads trying to fight detonation.
 
toolmanmike said:
Get out your measuring devices and tell us how far down in the hole those pistons are at TDC. A flat top piston engine can make power. A set of closed chambered hears or a set of aluminum's can make it sing.
Click to expand...
Dan the man said:
The pistons don't look like they are very far down from the deck surface, I'd say closed chamber heads with around a 62-64cc chamber and 0.039 thick head gaskets should put you around 9.0:1 - 9.25:1.
Click to expand...
JDMopar said:
There's an ECHO......ECHO........echo in a lot of threads! :BangHead:

:rofl:
Click to expand...
:rofl::rofl:
 
Thanks for all the replies guys, appreciate all the input.
I'm taking the block to be checked out this week and here's the recipe that I'm working on:
*Stock crank and rods
*Melling HV oil pump
*speed pro L2316F pistons
*stock 340 X heads with felpro severe duty head gaskets .0499 compressed thickness
*Comp 268 cam and Melling timing chain/gears
*stock valve gear
*Mopar M1 dual plane intake and 650 Holley DP
*Chrysler electronic ignition
* 1 5/8" headers with 2.5" exhaust (on car now)
Did I forget anything? lol
As I said I'm on a tight budget and can't afford excessive machine work and parts, I have to do some foraging to afford the new pistons lmao
This will probably be the last car I ever do so I want some power but also dependability.
Please give any input or ideas, I'm open to any. Thanks again
 
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AJ/FormS said:
For a streeter
and 3.55s
and a 2.66 low gear A833;
Your starter gear is only 3.55 x 2.66 =9.44.... so if you cam that 8/1 engine up, the CCP will all but disappear turning her into LESS powerful than 318 up to about 3000/3500.
Furthermore, with a 5500 shift rpm, you won't get into Second with those 27" tires until 47 mph, but 3500 is 30 mph, which if the tires won't spin, is a stinking long time. Then at 47mph, the Rs in Second will start out at ~4000, going to 5100 @60 mph.

What I'm saying is that
IMO
For a streeter, with 3.55s, a 340 cannot afford to have a soft bottom end. You will cry about it every time you take it out.
How you get the pressure is up to you. There are just two ways; 1) decrease the Total chamber volume or 2) close the intake valve early.

Here's what I mean;
Take the 8/1 340 with the factory 268/114 cam. in at 4* advanced, the Ica will be 64* and the Wallace predicts just 119psi CCP, at a very modest V/P of just 94.
That V/P number is barely bigger than a 225 slanty (87), and not even as strong as a smoggerteen (114 in Maine @600ft)
So even the stocker was already soft down low. With No hi-stall, you are married to that stinking V/P of just 94.
For big fun, you will want that V/P to be up around 170, but you can't do that with Iron heads, nor with a 3.315 stroke.
With iron heads and a stock stroke; next best is maybe 130/135 V/P, but that will require a careful co-ordination of parts. This would make your very low rpm to be very powerful, for a 340, which is always a joy.

But if yur the kindof guy who never runs down low, or who doesn't mind reving it up and slipping out the clutch, well then just put me on ignore. But before you do just remember that 30 mph in Second gear is just 2550 rpm, but in First is 3530, so I guarantee you that as a streeter with 3.55s, you are sorta married to low-rpm. Think about that as you are choosing a cam.
BTW; you should chose the cam first and then build the engine around it. To do otherwise is just inviting disappointment.
To machine the heads, you also have to machine the intake and sometimes the china walls as well, and then you need custom-length pushrods too. Imo the better solution is just to swap out the low-compression pistons.
But if you decide to run a tiny cam, maybe you already have enough CCP. For instance, running the factory 318LA cam will get you an Ica of 48* and the Wallace predicts..... a V/P of 121. Oh well, better than 114, I guess...........
Click to expand...
Absolutely right on the compression loss with too big of a cam. I had a guy argue with me that a cam will not effect compression. I tried to explain to him that excess duration would bleed off what little there was to start with. He tried to come off like an engineer as if to be the all knowing engine guru.........on, of all places facebook
 
wolfhammer said:
Absolutely right on the compression loss with too big of a cam. I had a guy argue with me that a cam will not effect compression. I tried to explain to him that excess duration would bleed off what little there was to start with. He tried to come off like an engineer as if to be the all knowing engine guru.........on, of all places facebook
Click to expand...
What compression are you shooting for? You know the 2316 has a positive deck height, right? That could mean too much for iron heads and a smallish cam like the 268.
 
RustyRatRod said:
What compression are you shooting for? You know the 2316 has a positive deck height, right? That could mean too much for iron heads and a smallish cam like the 268.
Click to expand...
Should net around 9.8:1 I would think but the machinist will know better than me. They do stick out of the hole about .015/.018 but so did the stock 340 ? Right now its about .108 in the hole
 
wolfhammer said:
Should net around 9.8:1 I would think but the machinist will know better than me. They do stick out of the hole about .015/.018 but so did the stock 340 ? Right now its about .108 in the hole
Click to expand...
Yes they did, but the stock 340 had leaded 100 octane. Good luck!
 
Buy the $89 stock Hi-compression 340 pistons for sale here recently
[FOR SALE] - 1971 OE 340 PISTON SET
and run her on good gas or E85. That **** is 100-105 octane and should be available about anywhere soon. I had a 273-4 and it never knocked on crap CA gas to boot. If they over rated that compression ration, how honest was the 340's?
 
pishta said:
Buy the $89 stock Hi-compression 340 pistons for sale here recently
[FOR SALE] - 1971 OE 340 PISTON SET
and run her on good gas or E85. That **** is 100-105 octane and should be available about anywhere soon. I had a 273-4 and it never knocked on crap CA gas to boot. If they over rated that compression ration, how honest was the 340's?
Click to expand...
Thanks bud. I need to wait until my block is checked by the machine shop, if I can keep the standard bore I just might buy those.
 
IIRC the 68-71. 340 had 10. 5 to 1 compression ratio. So if those are
8 to 1 then you would be leaving at least
25 hp on the table, which translates to .25 second in the 1/4. If your going to race it and you are gotta get another tenth of a secind less racer, go for it. If its a street , occasionally strip
car, save your money, go with what you have.
I have a buddy that is always after the elusive tenth of a second. I stopped by his place one day and he was under his car with a small propane torch using it to heat the undercoating to scrape it off.
 
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