Compression Ratio Question

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Jax340Dart

Jax340Dart

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I have a 340 and need help figuring out the compression ratio. The bore is 4.070 , as far as I know the stroke is 3.31. It has TRW L2322 domed pistons but the dome was milled down so that the dome is out of the hole .150. The heads are 71 cc and the head gasket is 8553pt .054 thick. The cam is a Lunati 703. 226 int/234 ex @ 0.50 duration and 494int/513 ex lift. 110* LSA. With those TRW "boat anchors" being milled down, Im figuring, with the 5cc valve reliefs its pretty close to a flat. I think its around 10. 1 . Oh yeah, I forgot to mention, cylinder pressure is 178 on 3 holes and 180 on the other 5. Do ya think Im pretty close to 10.1 or am I way off.
Thank you Bob
 
My guess using a ICA of 60 degrees ABDC and your cylinder pressure you listed is a SCR of 10.7 to 1 and a DCR of 8.77 to 1
 
Thank you for the reply TNT. Its been many years since I had a Dart, around 40, and I forgot how to figure all that stuff out. Its a little higher than I wanted but what the heck
 
If the valve reliefs and dome above the flat are indeed equal, the flat is around .017-.018" above the deck, then it would work out right at 10:1. But, as above, with the compression numbers reported and your local altitude, I suspect it is about 0.5 to 1 point higher.

You can measure the net piston dome volume to get a more exact number.
 
Thanks NM. The dome above the flats are .002 of all being equal, I step miked all of them when the heads were off. Im not real sure I have alot of faith in the compression tester. One time I did #1 and it was over 200. Then I checked it again and it was 175. I know what your thinking,,, Ya dumb a-- toss that gauge out and kick it in the dirt. In order to get dome volume I would have to pull the heads off. I should have done it when I replaced a blown head gasket but I forgot. I will get another tester and try it again. I thought the numbers were kinda high, I was figuring on 160 to 170. I will borrow a different tester and try again
 
Jax340Dart said:
Thank you for the reply TNT. Its been many years since I had a Dart, around 40, and I forgot how to figure all that stuff out. Its a little higher than I wanted but what the heck
Click to expand...

I kind of reverse engineered your question because you lack some of the volume measurements to figure it out otherwise.

I know that cam has a ICA of 60 degrees if installed per manufacturer specs, what your bore and stroke is and the length connecting rod a 340 uses. By entering this information into Wallace Racings Dynamic Compression Ratio Calculator I was able to find a static compression ratio that would end up having the compression psi you listed. It also lists the DCR value once your cylinder psi was matched. I left the elevation blank which is sea level.

Here's a link. Wallace Racing: Dynamic Compression Ratio Calculator
 
Thanks again TNT. That calculator was dynamite by the way, hahaha get it TNT--- dynamite !! OK that was pretty lame. Im going to try it again probably tomorrow and see what I come up with with a different gauge. To me those numbers were kinda high
 
I think it is around 10.5:1 , it is whatever it is. my 340/360 at a measured 11.5:1 cranks at 195 and runs iron heads with quench on 93 pump gas. and those pistons with the full .407" tall dome weigh 662 grams, a stock piston is 720 grams
 
Thanks Marco, I kinda figured it was 10:1 or 10.2:1 I still think my gauge is off. Was going to do it today but I couldnt find my piston stop, checking TDC just to make double sure. SO I took the car to the shop, washed it and parked it until tomorrow. I didnt know the domes were .407. Even with the dome milled down, those are some gawd-awful ugly lookin boat anchors. I also have iron heads, Js with 2.02 valves and mildly ported
 
Marcohotrod said:
I think it is around 10.5:1 , it is whatever it is. my 340/360 at a measured 11.5:1 cranks at 195 and runs iron heads with quench on 93 pump gas. and those pistons with the full .407" tall dome weigh 662 grams, a stock piston is 720 grams
Click to expand...
That is some very important specs right there. Pay attention FABO.
Very nice
 
Always pay attention to FABO !! There is so much you can learn on here and the fellas are always there to help. This is simply a great forum. Thanks AJ
Bob
 
Don't thank me, thank Marco; he let the cat outta the bag!
Now we are all gonna build 11plus iron-headed 360s and smoke everybody who didn't lol.I only knew 11plus worked with aluminum heads. But if I woulda known it could be done in 99 when I built mine, I woulda stayed with iron,probably......maybe.lol.
 
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