What happened to the 318 in 1971?

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clhyer

clhyer

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I was looking over my old Motors Manuals yesterday and while looking the specs of the 318 I noted that for 1970 and prior years the cylinder compression was 140 psi and compression ratio was either 9.2 or 8.8. Beginning in 1971 the compression ratio dropped to 8.6 and compression was listed at 100#. That is some kinda major drop! What, exactly, made this change?
C
 
...and the roller motors circa 1985/86-91 were back up to 9.2:1 but retained the 150 HP rating.
 
Half a point cr at those low power numbers is worth 5 hp at best.
 
273 said:
Half a point cr at those low power numbers is worth 5 hp at best.
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It wouldn't be the CR I would be worried about. It's the cylinder pressure. A drop of 40 PSI indicates a much larger drop of actual compression and power.

If I recall, the major reason for the drop was cam changes. They made changes to the cam to reduce cylinder pressure, allowing for "reduced" emissions. To meet EPA guidelines.
 
I've only ever seen one cam spec for the non-mag 318.

.399 lift IIRC.
 
YY1 said:
I've only ever seen one cam spec for the non-mag 318.

.399 lift IIRC.
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There were a few different grinds from 67 - 74. They changed slightly, you have to pay attention and go year by year....
 
My first car was a 1971 Dodge Demon 318 that I bought all original from Navy Officer from Great Lakes with 44K in 1989. Does anyone know why Mopar used a Rochester 2GV carb that year only?
 
krazykuda said:
There were a few different grinds from 67 - 74. They changed slightly, you have to pay attention and go year by year....
Click to expand...

This info is super hard to find.

Where are you getting it?
 
And the manifold under that Rochester was a single plain as I recall. I still have one of those kicking around.

Check the ignition timings too,

My 71 teenerDemon was pretty strong,back in those early years,considering.Even in 78/79 when I finally parted it out, the engine was still pretty strong.
 
YY1 said:
This info is super hard to find.

Where are you getting it?
Click to expand...

I have a complete set of service manuals in Plymouth and Dodge from 64 - 74. If you look them up by each year and make a chart, you can see the progression... I used to have it all on a spread sheet....
 
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krazykuda said:
I have a complete set of service manuals in Plymouth and Dodge from 64 - 74. If you look them up by each year and make a chart, you can see the progression... I used to have it all on a spread sheet....
Click to expand...

Here's a screen cap of my spreadsheet showing the 273 & 318 specs:

If you look closely, you can see the head gasket thickness change from 69-70-71-72...


Engine Specs C01 B.jpg
 
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I took the liberty of trying to prove the Scr's listed, using a standard 9.600 deck, and a 4.100gasket bore. I found it impossible.The results are both higher AND lower than those listed.
IDK what that means.
Ima guessing some pistons were dished, or the block heights varied from 9.600
 
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It means you take the factory CR's with a BIG grain of salt LOL. BTW, the combustion chamber volumes listed above are waaaay low anyway so that explains your results being + and -; if you go with real numbers, the actual CR's are all well - from those listed. For example, no way a standard production 675 head chamber is going to be 60 cc's..... I measured a stock set at 67-68 cc's. The cc's listed are all some sort of production dream numbers LOL.
 
Yeah me too.
But the trend is clearly visible; beginning in 69 the deck clearance was increased a couple of times and, beginning in 71 they messed with thicker gaskets.
During a rebuild this all can be changed. Any teener can be used as a base; the forged crank engines being possibly, more desirable.And the large-rods with floating pins,too.
 
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AJ/FormS said:
I took the liberty of trying to prove the Scr's listed, using a standard 9.600 deck, and a 4.100gasket bore. I found it impossible.The results are both higher AND lower than those listed.
IDK what that means.
Ima guessing some pistons were dished, or the block heights varied from 9.600
Click to expand...

Chrysler ran the head faces fat. You won't get the numbers to jive...

That's why I measure the compression of my engines individually....

How to Check Compression
 
nm9stheham said:
It means you take the factory CR's with a BIG grain of salt LOL. BTW, the combustion chamber volumes listed above are waaaay low anyway so that explains your results being + and -; if you go with real numbers, the actual CR's are all well - from those listed. For example, no way a standard production 675 head chamber is going to be 60 cc's..... I measured a stock set at 67-68 cc's. The cc's listed are all some sort of production dream numbers LOL.
Click to expand...

I got those numbers from another book, I redid the cam chart last night all over again...

Here are the 273 - 318 listed individually by year:

273-318 Sum A.jpg


Here I combined and summarized them:

273-318 Sum B.jpg
 
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