DodgeMoparV2
Member
They are solid.
13:1 Compression Pistons in 340 Small Block
- Thread starter DodgeMoparV2
- Start date
-
They are solid.
DodgeMoparV2
Member
Thanks for the feed back!
I would definitely like to be able to drive this on the street but also have something I’m not scared to talk down the the strip and roast the tires with. This engine is going into a 340 Dart but the block is non numbers matching (if I was numbers matching I would rebuild it to stick specs) I’m not scared to spend some cash on cams and run Some high octane fuel and I don’t care for fuel economy but I’d like to get better then 8mpg. It’s an automatic car and I haven’t looked into torque converters or anything along those lines. I have a 727 trans I was going to use with this application if that helps at all.
First ---> Welcome to FABO!!!
Just because the manufacturer says that they are 13:1 or whatever compression that they are rated at, doesn't mean that they will be that in your engine... Chrysler runs the head decks on the blocks and heads fat (tall to the high end of the spec), so if they get any scratches or gouges in them during manufacturing, they can mill them and make them good again and they will still be within the recommended factory specifications...
Use this thread as a guide to check the actual compression that they will be in your engine... The domes will make things a little complicated, you will have to find a good reference point for the piston preferably a flat area that you can use to measure the depth in the cylinder at different points... You must measure how much volume the domes are to be able to calculate the cc's of them for the compression calculation... That may take measuring them at two or three different levels... One level with the dome below the head deck of the block, one with the dome bottomed out to the plastic plexi glass cover plate that you make (with the highest point even with the head surface of the block), then measure how high that reference point is when the piston is at TDC... You will have to be able to measure that reference point at all three levels so we can calculate the volume of the dome... The dome will stick above the block head surface when at TDC... Then you/we may be able to calculate the true compression that those pistons will make in YOUR engine...
How to Check Compression
I would install one piston in the block (I would recommend in the #1 cylinder) and measure the volume of the piston with the link that I provided above...
Last edited:
I have been writing a series of how to articles on building a small block... I am not finished with all of them yet, but you may want to use them as a reference...
Here's a link to the main page for the How-To articles:
How To Articles | For A Bodies Only Mopar Forum
Here are the links to the ones for rebuilding a small block that I have done so far, I may be able to do one more this weekend on how to install the pistons....
How to Rebuild a Small Block Part 1: Block Prep
How to Rebuild a Small Block Part 2: Cam Bearing Install
How to Rebuild a Small Block Part 3: Install Water Jacket and Oil Galley Plugs
How to Rebuild a Small Block Part 4: Pre Flight Inspection After Machine Shop Before Assembly
How to Rebuild a Small Block Part 5: Crankshaft Install
With that much compression, you will need to run a camshaft with lots of duration and overlap (smaller lobe separation)... It will have a choppy/lumpy idle, and you will have to run high octane fuel... You will also have to run a high stall converter and tall rear end gears... But it sounds like that is what you want, so get-r-dun....
yellow rose
Overnight Sensation
DodgeMoparV2
Member
Thanks for the feed back. I’ve rebuilt a couple like I stated, I’m just in a different realm with these 13:1 pistons, I know all the performance stuff can interfere other objectives. That’s why I wasn’t sure what type of cam I should run.
DodgeMoparV2
Member
Thank you so much, Finally someone that could answer my questions lol. I’m sure I’ll have to run higher octane then 93 which isn’t an issue. Only others think is I’m not sure what type of torque converter I’ll need. I have 3.55, 3.91 and 4.56 gears for the rear end, I was thinking the 3.91 gears. I definitely wanna “get-r-dun” but for real thank you so much!
Domes are solid as said...I have a set in a 340 with domes cut off...you can barely see the spark plug indention...somewhere in my shed I have a set of those dome pistons that are .060 over....ran that engine for about 10 yrs with Av gas. back in the 80s....
Once you decide on a cam, then contact a converter manufacturer and ask them for a converter and gear recommendation for your combo...
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 24,928
- Reaction score
- 12,275
Yur getting ahead of yourself; reread YRs post, #17
IMO; yur making things waaay too complicated for yourself. The difference in power from 10.5 to say 12.5 is about 3% at peak rpm. On a 400hp 340 that is 12hp or about one cam size.
Spend the money on closed chamber heads, and flat-top Hypers ,and get the pressure down to pumpgas territory.
For some nice street-torque, run aluminum heads and 190psi, and 87E10, easy-peasy.
Last edited:
To the original poster/thread starter;
This is how I read everything so far. You want to reuse the old parts because there on hand. While I like that idea, if this engine specs out to an actual 13-1, then the size cam normally seen used effectively with that high of a ratio is starting around in the [email protected] with operating RPM ranges starting around 4,000+ RPM’s.
The trick there is to bleed of cylinder pressure at the lower RPM levels. Once the engine gets to spinning around, the cylinder pressure levels go through the roof and will require a very high octane level. While killing the domes off the top of the pistons will drop the ratio, it is not a route I would take.
It would be my opinion to save a headache and skip those pistons for readily available flat tops for a 340. With a set of open chambered J heads, the compression ratio should fall on ether side of 10-1.
(Actual head cc amount, gasket thickness and bore size, where the piston actually sits, will mean everything. And is variable at this point. But for the sake of this discussion..... )
At 10-1, premium pump gas will work and a decent size streetable cam that can perform at the track can be used.
The next issue are the J heads. are the stock or ported and with what size valves? The as cast heads didn’t flow very well for making power or racing purposes. Generally speaking, (very) the head flows around 200 cfm. On a laymen’s term on Potential power to be made, the math works out to CFM times two equaling potential is power on the a street strip machine. Exceeding this isn’t hard but the actual usage and friendliness of the engine /vehicle tends to drop hard.
Generally speaking, the difference in compression is 3% per 1 point of compression though this is general and not in stone since the more aggressive the build, the wider the percentage becomes. AJ above claimed 3% for the 2 points. I’d double that value without batting an eye if not more if the combination is wild or out of wack.
FWIW, @ 13-1, that is 110 octane level. For gasoline. That’s pricey!
E-85, not so pricey but the engine drinks heavier. But not double. Close though!
This is how I read everything so far. You want to reuse the old parts because there on hand. While I like that idea, if this engine specs out to an actual 13-1, then the size cam normally seen used effectively with that high of a ratio is starting around in the [email protected] with operating RPM ranges starting around 4,000+ RPM’s.
The trick there is to bleed of cylinder pressure at the lower RPM levels. Once the engine gets to spinning around, the cylinder pressure levels go through the roof and will require a very high octane level. While killing the domes off the top of the pistons will drop the ratio, it is not a route I would take.
It would be my opinion to save a headache and skip those pistons for readily available flat tops for a 340. With a set of open chambered J heads, the compression ratio should fall on ether side of 10-1.
(Actual head cc amount, gasket thickness and bore size, where the piston actually sits, will mean everything. And is variable at this point. But for the sake of this discussion..... )
At 10-1, premium pump gas will work and a decent size streetable cam that can perform at the track can be used.
The next issue are the J heads. are the stock or ported and with what size valves? The as cast heads didn’t flow very well for making power or racing purposes. Generally speaking, (very) the head flows around 200 cfm. On a laymen’s term on Potential power to be made, the math works out to CFM times two equaling potential is power on the a street strip machine. Exceeding this isn’t hard but the actual usage and friendliness of the engine /vehicle tends to drop hard.
Generally speaking, the difference in compression is 3% per 1 point of compression though this is general and not in stone since the more aggressive the build, the wider the percentage becomes. AJ above claimed 3% for the 2 points. I’d double that value without batting an eye if not more if the combination is wild or out of wack.
FWIW, @ 13-1, that is 110 octane level. For gasoline. That’s pricey!
E-85, not so pricey but the engine drinks heavier. But not double. Close though!
1. It’s gonna be a street/strip motor. Nothing crazy.
2. I don’t care for fuel economy but I’d like to get better then 8mpg
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
DodgeMoparV2, you stated these 2 things. Those pistons and everything around them to make them work, don't go with the above.
I know exactly where you are coming from wanting to use those pistons, but I feel you can better achieve you goal without them. And in the long run, be glad that you didn't use them. Just my 2 cents.
2. I don’t care for fuel economy but I’d like to get better then 8mpg
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
DodgeMoparV2, you stated these 2 things. Those pistons and everything around them to make them work, don't go with the above.
I know exactly where you are coming from wanting to use those pistons, but I feel you can better achieve you goal without them. And in the long run, be glad that you didn't use them. Just my 2 cents.
Too much compression. Don't use them!
nm9stheham
Well-Known Member
Sorta backwards thinking here in a way (and I'm not speaking to anyone in particular, so no one take offense, please): Understand that the only reason these high CR pistons are needed at all was that larger and larger cams are used to get more and more HP on the strip. The higher CR is needed to make the engine run with any torque at all at lower and mid RPM's. When the cam selection is being made to suit the application, then the static CR is picked to make it run at idle and lower RPM's in combination with fuel type to help the engine run without detonation.
So IDK.... 'better than 8 mpg' can still mean a pretty darned big cam, and that fits with the high CR. Whether or not the decision to use special fuels will be regretted is TBD. But all this need for a pretty darned big cam is getting further and further away from 'street'. Yes, it can be made to work in some fashion, but the 'out of the ordinary' piston selection is driving the engine into one corner of the performance map, with limited options on how it needs to be setup and how it will run.
OP, just understand that most of the comments here against these pistons are sincerely meant to help you avoid a setup that you won't be happy with. The comments would be similar if you showed pistons with a static CR of 7:1.
So IDK.... 'better than 8 mpg' can still mean a pretty darned big cam, and that fits with the high CR. Whether or not the decision to use special fuels will be regretted is TBD. But all this need for a pretty darned big cam is getting further and further away from 'street'. Yes, it can be made to work in some fashion, but the 'out of the ordinary' piston selection is driving the engine into one corner of the performance map, with limited options on how it needs to be setup and how it will run.
OP, just understand that most of the comments here against these pistons are sincerely meant to help you avoid a setup that you won't be happy with. The comments would be similar if you showed pistons with a static CR of 7:1.
nm9stheham
Well-Known Member
70aar... how was this engine used? Strip? Street/strip? Nice that they have been used by someone here, but I think it would be more useful to the OP to know how they were used.
DodgeMoparV2
Member
Thank you for the feedback, I do understand what you are saying and my worst fears are detonation in the cylinder with too high of pressure, valve clearance and just too much in general. I’ll looking at opinions of getting them machined down to 12-11.5:1 still higher compression but lower so I don’t blow the heads off the car.
I have to bring the block to an engine guy so he can boil it and get some surface rust off of it. Most likely I will ask for his two sense and see what he says.
Like I previously stated I want something to drive around town every now and then but still have something that I can have some fun with and if these pistons infringe on that then I might use the stock flat tops or get a different set.
Krooser
Building Chinese Free Engines since 1959...
Well if I were the OP I would look around for a good engine shop that knows Mopars and performance work and take what you have to them. Have the parts cleaned and inspected so you know what you have to work with. All these internet plans go out the window if you have a cracked head or worn ring lands and have to buy new parts.
After you know you have good, usable parts you can lay out a plan that will work like you expected.
After you know you have good, usable parts you can lay out a plan that will work like you expected.
Oldiron440
Well-Known Member
Backwards thinking, for the last fifteen years there has been an explosion of turbo street cars that run 20 to 30 pounds of boost and before that for a brief period compression was king again all because of e85. I have a couple of friends who own a engine building shop with a pair of dynos, one of them an ex sprint car racer. These two have won the big checks building NA small blocks on e85. E85 is a very contemporary way of building power and there no need to build a motor with less than 12.0 to 1 cr once you make the initial investment converting to e85. It doesn't need to be a willed race motor to benefit from a high cr and e85 look how many new stock vehicles run on it.
Oldiron440
Well-Known Member
I'd build with the pistons just because there's so many saying not to. A mechanical flat tappet with 235 to 240 degrees duration @ .050 lift and say 560 lift will keep the power below 6000 rpm. Clean up you J heads with a good pocket port and valve job. What this combination will have is a very good torque curve that will peak at about 4000 but I wouldn't be surprised if it had 90 percent of it at 2500 rpm, excellant for a streetcar.
Just too be clear I'm talking e85........
Just too be clear I'm talking e85........
Here’s food for thought......
The Craftsman NASCAR truck series limited the engines to a 9.0-1 static compression ratio and a small carb that still made over 700 HP.
The big secret besides the camshaft used was;
“Super excellent flowing heads!”
The Craftsman NASCAR truck series limited the engines to a 9.0-1 static compression ratio and a small carb that still made over 700 HP.
The big secret besides the camshaft used was;
“Super excellent flowing heads!”
yellow rose
Overnight Sensation
Those Pistons have valve notches a mile deep.
Like I've already said, those Pistons are fine if you know what you are doing. You can EASILY run 11:1 compression on pump gas. You just have to do it correctly and you won't be using a cam that came out of a catalog.
You can easily get down to .040 quench and you should. But don't do it by milling the heads. You want the flat of the piston to be out of the hole .040-050 and chances are, you'll have issues finding a machinist that will do it. But that's what it takes.
If you do that (get this piston out of the hole so you have some quench) and machine the dome off a bit (takes all of 10 minutes to indicate in the piston vise and set up the first piston) and you'll be fine.
People get scared of compression ratio. I get it. But leaving CR low hurts expansion ratio. And that hurts power.
You don't need a big mother bumper cam either. You need a cam that is for that CR on pump gas. Again, it won't be in a catalog. I run a 281 degree solid lifter cam. But it's not in a catalog.
dead on with the block with the capabilities. there are ways to make it live with this high of power but it just wasn't designed for high horsepower, its a passenger car block.
its not the lift, its all about the overlap and duration somewhat so you can bleed off compression. the higher rpm range with the cam is what makes the horsepower. it rev like a chainsaw and not have alot of low end torque. it would not be a very good street motor.
-
