Help me calculate compression ratio before I buy pistons, PLEASE!!

-
duster360

duster360

Well-Known Member
Joined
Aug 8, 2008
Messages
3,825
Reaction score
312
Location
Alabama
These are the SRP piston specs: http://www.jepistons.com/Catalogs/SRP/Mopar/340-360_flat_inv.aspx

Part #142068
CID 345"
Bore 4.070"
Stroke 3.313"
Rod 6.125"
Lgth.Block 9.586"
Ht.Comp. 1.804"
Ht.Head Cubic Centimeters 65cc Compression Ratio
Dome Vol. -5cc
Compression Ratio 9.8
Gram Wt.
1x1.gif
530

JE gets their ratio with a .040" compressed gasket thickness.

Mine will be with .050" compressed thickness gasket. I measured the fire rings in the head gasket at 4.168"
 
OldmanRick said:
With a zero deck and a 4.18 dia gasket, I came up with....

9.69 to 1
Click to expand...

That is close to where I want to be. I was hoping for a little closer to 9.5:1. I am not zero decking the block. I guess that would mean the pistons will be down a little more and cause the compression to come down some.
 
Deck heights tend to be a hair different on most blocks, but if you end up .010 below, that works out to 9.47 to 1 in my calc's.......I think you're gonna be very close to your target. :thumleft:
 
From what I can do on the calculator it is 8.6 using 9.47:1 compression. I know that this won't be completely accurate until I get the rotating assembly together and measure piston to deck height. But I want to be really close now so I don't buy a set of pistons that will have the compression so high it will be an issue. So all help is appreciated. My machinist called today and said the block cleaned up at .030" so I could go ahead and order the pistons and have the rotating assembly balanced. So I need to get this figured out ASAP.
 
Cam spec/cards vary in how they measure timing events......004, .006, .050 etc. It makes it a little difficult to do apples to apples on regulation calculators. With that said, your cam choice should be fine for what your doing in my opinion.
 
OldmanRick said:
Cam spec/cards vary in how they measure timing events......004, .006, .050 etc. It makes it a little difficult to do apples to apples on regulation calculators. With that said, your cam choice should be fine for what your doing in my opinion.
Click to expand...

Seems like just when I start getting a grasp on some of the tech stuff something else gets thrown in the mix. Lol.
 
I hesitate to post this formula that's around on the net.....I don't want to send your thread completely off the rails.....lol. While by no means perfect, as lobe designs vary, it can demonstrate how figuring the closing point changes when using Advertised int duration vs int duration at .050......

Just for grins, try both ways with your cam choice......


Estimating intake closing point
If the intake closing (IC) point isn't known, it can be calculated:
  1. Divide the intake duration by 2
  2. Add the results to the lobe separation angle (LSA)
  3. Subtract any ground-in advance
  4. Subtract 180
 
If your DCR is no lower than 8.6, you will have trouble with pump gas.
 
duster360 said:
How can I bring the static compression down?
Click to expand...

I don't think you have to......

OldmanRick said:
your cam choice should be fine for what your doing in my opinion.
Click to expand...

I've generally never seen a reason to push the limit on Dynamic comp for a fun type, mainly street car. 8 or so to 1, maybe 8.25 max, still runs clean and strong. It also allows for a little carbon buildup and you can still be aggressive with the ignition timing.

Now I've opened myself up for the 11/12 to 1 guys to start shooting at me...LOL.:mrgreen:

By the way, the Wallace Racing calculator says to use seat to seat for best results...........By using the advertised intake duration on your Voodoo selection, I came up with a 60* int close and 7.66 to 1 dynamic.




P.S. I should say, the reason I like the Wallace calculator is that several times on other sites I've read where several guys had checked there engines by measurement, and the Wallace was almost dead nuts on.

I've played with some of the .050" calc's and the numbers seemed to be all over the place. One, "United" i believe, says to add 15 degrees to the .050 close? I know the formula I posted earlier about using the advertised number to find the close is not perfect because of some non-symmetrical lobe designs and such, but I believe it gets close enough to ballpark a safe number for the street.
 
So your saying my calculations were off for 8.6 and my dynamic is 7.66 with all the specs I have posted here.
 
Not exactly 7.66 per say, as the int close formula i used has some variance because of individual lobe designs. I don't believe your anywhere near 8.6. I actually looked up the United .050 calculator, and it only shows 8.10 to 1.

The validity of the .050" rated calc's, which Lunati rates at, relies on the proper formula to make up for the valve not being all the way closed yet. I just prefer the other method as I feel it's more accurate. I'm sure others may feel differently.

Just look at a milder Comp XE cam that rates a .006". You'll see how late the int close is compared to yours.
 
There's always the option of calling Lunati and telling them your choice and ask them what they feel about pump gas in a 9.50 to 1 engine?

Also, since things slow down a little on the weekends, give it a few days and see what some others think. I personally wouldn't lose a wink of sleep running that cam in the combination you've laid out.
 
I honestly think you'd be safer one step up from that cam. you need to think about idling in hot weather for extended periods and getting heat soak. That will contribute greatly to spark knock. Just one more step up IMO will solve that. Cause either way you figure it like Rick was talkin about, it's gonna be close. I don't think the DCR is as low as 7.6, like Rick suspects as well.....I think the 8.1 is more in line. And even at 8.1 when you factor in more heat you may be pushin it. I would rather be safe than sorry. Plus, with one more step up it will sound nastier. lol
 

I ran the numbers in an almost idenical motor I just built.
I have a 4.070 340, 3.31 crank.
Same cam, but 243 KB and 74 cc x-heads.
I ended up with 8.06 dynamic which should be a nice street motor.
 
Hey Rob, Hey Shawn :D

Not to beat this to death, but the United .050" calc doesn't take into account for altitude either. If it's a sea level formula, that 8.10 may be a hair high then for 750ft.
 
What would be the lowest static compression ratio I could run to get the static compression down a little if I wanted too, with the cam listed above.
 
RustyRatRod said:
I honestly think you'd be safer one step up from that cam. you need to think about idling in hot weather for extended periods and getting heat soak. That will contribute greatly to spark knock. Just one more step up IMO will solve that. Cause either way you figure it like Rick was talkin about, it's gonna be close. I don't think the DCR is as low as 7.6, like Rick suspects as well.....I think the 8.1 is more in line. And even at 8.1 when you factor in more heat you may be pushin it. I would rather be safe than sorry. Plus, with one more step up it will sound nastier. lol
Click to expand...

I am stuck with the cam I have. The heads are in the machine shop being setup for it already, no turning back now.
 
You must log in or register to reply here.
-
Back
Top