13:1 Compression Pistons in 340 Small Block

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DodgeMoparV2

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Hello, let me just start off by saying I’m new here and had some questions regarding engines. I know basics and some advanced terms regarding engine building. My questions today regard more advanced knowledge then what I know and that’s why I am here.

Recently I decided to pull out an old 340 block my old man had sitting in the back and throughout the years he’s acquired tons of parts. One of the pieces he had are .030 13:1 compression pistons (that’s what he was told they are) he pulled out of a 340 motor that he swapped all back to original. This other motor I want to rebuild my goal is to have something street but also build a big block buster with (or at least give them a run for their money).

One essential key that I know makes an enormous difference and I’ve been trying to research on is Camshaft specs and what types you run with performance. I know quite little compared to everything there is to know about camshafts and I rebuilt a couple engines with just mild build but I know with getting into these higher performance pistons you need to run special cams.

This is where the big question come into play, what type of camshaft should I run with this build? I’ve seen lots a people saying 10.5:1 pistons with certain cams run the best but I’d really like to use this 13:1 pistons. I’ve also seen people saying to get them machined down to 12:1 or 11.5:1 but I would also like to not go that route.

If it helps I was thinking about running a Mopar Performance Purple Cam with a Mopar Performance intake manifold and I’m running J heads. I’ve yet to get into Rocker Arm ratios but I’m probably just going to run the stock 1.5 ratio with double spring valves. I’ll probably end up getting some special hardens pushrods and whatever else the cam specs require.

Thank you all!
 
I will start the questioning to get the ball rolling for when the experts to chime in.
1. Is this a race or street engine?
2. Do you plan on using race fuel all the time?
3.Do you plan on getting the J heads ported?

Oh yea, WELCOME ABOARD
 
What fuel and cylinder heads do you plan to use in this build? If it’s 13:1, you probably aren’t going to be able to afford very much street usage.
 
If you have access to a source of consistently blended source of e85 it becomes slightly more feasible but your going to burn more of it than you would race gas. If it’s mostly a summer driver, you don’t have to worry about the ethanol blends varying more than a few percent, but in the winter, it often gets blended to about 65% ethanol. The 78%-85% range is regarded as the sweet spot for that level of compression. But it’s really going to be about if you can afford to feed it.
 
Welcome!
same pistons that were in my first 340 Dart. ran strong on pump premium...in1976. the domes could be cut till flush with the flat part of the piston. unless your block is zero decked or the heads have been cut a bunch, maybe you'll end up at 10 and a half to one. sounds like fun! good luck and enjoy! Bob
 
Those look like the old 11.5:1 TRW pistons. You can get 13 with them with the right heads or head milling. The domes are solid, so if you want to use them in a street pump gas engine, you can mill the domes off. A lot of people have done it. Also, a lot of people will tell you those pistons are heavy.....and they are. Reducing reciprocating weight is a good way to have basically free horse power. Makes the engine rev quicker and last longer if you're revving it a lot. I like the old TRW pistons though. They are heavy, but they are TOUGH. I always said that's why they make good rod bolts. lol Scrape some carbon off the top. There should be a part number. Post it up and we can maybe tell you what they are for sure.
 
The last build I was in with compression in that neighborhood (14:1!) took two cans of 104-Plus to 20 gallons of premium fuel... I won't even go into all the complications and legalities involved with aviation fuel, but being it takes e 85 components to withstand long term exposure to that devils juice, your better off with the legal e85 plan.
 
Big block killer hey? That high of compression has high octane requirements. What you are looking for is a cam that will bleed off some of the compression with valve overlap. You need to talk to a cam grinder to match to the rest of your combination. Probably want to go with a roller cam. You will also need heads and an intake manifold that flows over 300 cfm, o-ring the cylinders, fill the water jackets with hard block, 4 bolt mains with a girdle. By the way, those pistons are really heavy.
 
Yes E85, gives you a little more power and it runs cooler plus its cheap.
There is a down side you can't store it in your carb, a plastic fuel tank is the best and you will need to convert or buy an E85 carb. Also you might want to upgrade your fuel system because you will use about 40 percent more fuel than gasoline.
 
If you do plan on running the pistons as is, you could have some of the weight machines out of them in the pin bosses and underneath the domes for about half the cost of a new set of lightweight pistons. Unless you have a T/A 340 or R block that has extra webbing for four bolt mains, I wouldn’t go nearly that all out. A 10.5:1 4.07 inch bore small block with a well prepared econo W-2 is capable of exceeding the typical power rating (550 hp) of an x block.
 
I'd run them bad boys just like they are. How many folks you reckon ran them when they "were all there was"? I've not read about a rash of broken connecting rods, so they musta worked ok.
 
Garrett Ellison said:
The last build I was in with compression in that neighborhood (14:1!) took two cans of 104-Plus to 20 gallons of premium fuel... I won't even go into all the complications and legalities involved with aviation fuel, but being it takes e 85 components to withstand long term exposure to that devils juice, your better off with the legal e85 plan.
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I run E-85 on a 12.5:1 compression 451 short deck BB build. It certainly solves the denotation concerns. As well as runs cooler. (Which can be a little problematic on colder morning's warm up.) It does use about 15-20% more fuel than gas. But it's also 20% less expensive than pump 91. So it's a win compared to any race gas strategy. Carb(s) must be meter blocked for increase fuel need. Today's EFI can easily be mapped for E-85 electronically.

It doesn't keep as well as gas. You're best bet is to run it completely dry before any storage of 30 day or more. (This is true for race gas after 6 months as well.)

E-85 availability is high here in Minnesota. Thus is a very attractive alternative for higher boost or compression builds.

Just sharing my observations.
 
BigBlockMopar said:
Sometimes it’s better to just NOT use what you have laying around...
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Roger that..... mild street build, pump fuel, and 12:1 or 13:1 static CR don't go together. You are right OP, these pistons would typically be usable only with certain large cams and fuels.

If you use them with too small of a cam on pump fuel, then the engine will experience what is called detonation, which will destroy things in short order... unless you use the different fuels being suggested. If you put in a big cam to prevent that, then low RPM performance, and what is termed driveability, goes into the toilet.

Help us all (and yourself) by explaining better what you want to do with this engine/car. 'big block buster' is a bit ambiguous, and you can do that with a highly modified /6 and a lightweight early A body! Do you want to run pump fuel? Is economy important to you? Drag race only? Or are good street drivability and good cruising manners the most important things? Auto trans or manual? Picking parts all starts with the application....

And welcome to the site! This is pretty knowledgeable group.
 
RustyRatRod said:
Those look like the old 11.5:1 TRW pistons. You can get 13 with them with the right heads or head milling. The domes are solid, so if you want to use them in a street pump gas engine, you can mill the domes off. A lot of people have done it. Also, a lot of people will tell you those pistons are heavy.....and they are. Reducing reciprocating weight is a good way to have basically free horse power. Makes the engine rev quicker and last longer if you're revving it a lot. I like the old TRW pistons though. They are heavy, but they are TOUGH. I always said that's why they make good rod bolts. lol Scrape some carbon off the top. There should be a part number. Post it up and we can maybe tell you what they are for sure.
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the 11.5-1's,as listed by TRW and the old DC catalogs, had a flat top and not the dome with the spark plug notch for flame travel.
My old combo,besides the okay gas, was a 4-speed,4.56 gears,tarantula intake with 3310 Holley and a crane cam;485/504 and I think it was 238/243 at 50? got the specs somewhere. It did not start pulling till 3500 but loved to rpm.'till I popped the head gaskets. (Crane CCH-320-NC was stamped on the cam)
 
You have to run an open chamber head with that piston. Period. Unless you want to cut the decks off. That's a PITA.

If you are going to run a 340 stroke engine, then you can use that piston. Just know you MUST deck the block. Period. Non negotiable. Period.

Unless you deck the hell out of the heads (which is a big waste) the flat of the piston will need to be somewhere around .040-.050 OUT OF THE DECK. That's also non negotiable. Period.

You can use that piston, get 11:1 easily with nice quench and use pump gas. That's what I'd do.

If you don't have a machinist that understands you MUST stick the piston out of the deck find one until you do. Zero deck won't work. And you'll need to mill some of the dome off.

When you are done, if it's done correctly, you have a dome of .125-.150 and 11:1 compression. If you are willing to buy the correct cam (not a BIG one or one with a ton of unneeded duration) but a cam specifically picked for that CR and pump gas you'll be fine.
 
duster360 said:
I will start the questioning to get the ball rolling for when the experts to chime in.
1. Is this a race or street engine?
2. Do you plan on using race fuel all the time?
3.Do you plan on getting the J heads ported?

Oh yea, WELCOME ABOARD
Click to expand...

Thank you all for the responses, I guess I can try to answer all the questions I’ve seen so far.

1. The heads as far as I know are Stock J heads and I think have been machined down a little bit.

2. I’ll probably run high octane, I’ll run 93+

3. It’s gonna be a street/strip motor. Nothing crazy.

Its going to be going into a 1970 Dodge Dart Swinger, it’s a 340 car but the block isn’t numbers matching, B5 Blue exterior and B5 interior with a light blue tail strip and it’s an automatic car.
 
I would go with a set of flat tops for your intended use. With stock port J heads I would keep the max lift to around .500” or so.
 
1.What rear gear do you plan on running?
2. what stall torque converter?
 
yellow rose said:
You have to run an open chamber head with that piston. Period. Unless you want to cut the decks off. That's a PITA.

If you are going to run a 340 stroke engine, then you can use that piston. Just know you MUST deck the block. Period. Non negotiable. Period.

Unless you deck the hell out of the heads (which is a big waste) the flat of the piston will need to be somewhere around .040-.050 OUT OF THE DECK. That's also non negotiable. Period.

You can use that piston, get 11:1 easily with nice quench and use pump gas. That's what I'd do.

If you don't have a machinist that understands you MUST stick the piston out of the deck find one until you do. Zero deck won't work. And you'll need to mill some of the dome off.

When you are done, if it's done correctly, you have a dome of .125-.150 and 11:1 compression. If you are willing to buy the correct cam (not a BIG one or one with a ton of unneeded duration) but a cam specifically picked for that CR and pump gas you'll be fine.
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Aren’t the J heads open chamber already? If not I still have the other J heads that came off the motor with these pistons so if all else fails I’ll put those on it.

I’m not running a stoked 340, it’s got a stock forged crank in it.

The block I pulled these out of was a 340 that was bored .030 over and had J heads, I think he had a larger bore at the top of the cylinder wall but I can’t remember that well. Other then him having a hot cam and a fancy intake I think that was it.
 
duster360 said:
I would go with a set of flat tops for your intended use. With stock port J heads I would keep the max lift to around .500” or so.
Click to expand...

So if I wanted to run these pistons what type of lift for a cam are we talking? I’m probably gonna port the head I have or the ones I pulled I think are already ported I’ll have to check.
 
Get a can of this Berryman's B-12 at Autozone to clean the black carbon soot off of the pistons... It works great on that carbon, but has a strong odor, so use it outside in a well ventilated area....



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duster360 said:
1.What rear gear do you plan on running?
2. what stall torque converter?
Click to expand...

I might run 4.56 gears I pulled out of the car or 3.55

I haven’t looked at torque converters yet, I figured if I got the motor all figured out that would be my next issue.
 
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