Upgrading stock 1971 318 heads

-
I know I'm late to the party but;
Assuming the engine is already at plus .020 overbore, the swept comes to 658.96cc
Allowing 5cc for the mini eyebrows, 8.8 for the gasket, 64 for the heads, and 19.9 for the deck; this totals 97.7, Thus making your Scr 7.74, and
with the factory 240/248 cam in with an ICA of 52*, your
Static compression ratio of 7.74:1.:(
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 6.73:1 .
Your dynamic cranking pressure is 127.42 PSI.
V/P (Volume to Pressure Index) is 105

Now that .040 cut on the heads might total 5cc tops (guessing), bringing your total volume down to 92.7, and the Scr to 8.11 max,so;
Static compression ratio of 8.11:1. :)
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 7.05:1 .
Your dynamic cranking pressure is 135.50 PSI.
V/P (Volume to Pressure Index) is 111

So the compression boost is worth; 111/105=plus5.7% in low-rpm performance. Sweet!
 
Last edited:
AJ/FormS said:
I know I'm late to the party but;
Assuming the engine is already at plus .020 overbore, the swept comes to 658.96cc
Allowing 5cc for the mini eyebrows, 8.8 for the gasket, 64 for the heads, and 19.9 for the deck; this totals 97.7, Thus making your Scr 7.74, and
with the factory 248 cam in with an ICA of 52*, your
Static compression ratio of 7.74:1.
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 6.73:1 .
Your dynamic cranking pressure is 127.42 PSI.
V/P (Volume to Pressure Index) is 105

Now that .040 cut on the heads might total 5cc tops (guessing), bringing your total volume down to 92.7, and the Scr to 8.11 max,so;
Static compression ratio of 8.11:1.
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 7.05:1 .
Your dynamic cranking pressure is 135.50 PSI.
V/P (Volume to Pressure Index) is 111

So the compression boost is worth; 111/105=plus5.7% in low-rpm performance. Sweet!
Click to expand...
Cool! my SCR Goal was 9.0:1 Pretty close im happy with that give for take. Although the pistons are at .030 too bad i dont know the cam specs but i know its a mild RV cam with a noticeable lope thanks for the translation!
 
Slantsix64 said:
Cool! my SCR Goal was 9.0:1 Pretty close im happy with that give for take. Although the pistons are at .030 too bad i dont know the cam specs but i know its a mild RV cam with a noticeable lope thanks for the translation!
Click to expand...
Hang on Mr /6,
9.0 is a world away from 8.1, lol.
Do yourself a favor and get an accurate compression test.
Accurate,as in: warm engine, fully charged battery, all plugs out, ignition disabled,and carb wired at WOT. With a known good gauge, crank until you get a minimum of two consecutive SAME or nearly the same,readings over 120 psi . Then you'll know what you got.
A noticeable lope is more indicative of a performance cam..... than of an RV cam. The lope is in the overlap, which is usually due to a long duration, which usually indicates a late closing intake, which reduces your Effective Compression Ratio, and specifically reduces your Dcr, and that drives your cylinder pressure into the basement on the cranking test.
Performance starts at about 145psi, but it's tricky.
A low rpm engine can be fine, down to 130. A race-engine can be made to work at 150. It's everything in between that wants more, especially a small-displacement streeter.
For a streeter;
160 would be excellent. 155 almost, 150 pretty good,145 good,135 sorta normal in the smoggers,130 starting to suck, 125 definitely sucks,120 sux to be you if you got it, 115 just flat really sux,110 kill me now,105 biggest pos ever, yank it or blow it up, so you have to yank it.
100 or less?
:rofl:Something is wrong,lol
So do the test and see what you got.:popcorn:
 
Last edited:
Slantsix64 said:
Cool! my SCR Goal was 9.0:1 Pretty close im happy with that give for take. Although the pistons are at .030 too bad i dont know the cam specs but i know its a mild RV cam with a noticeable lope thanks for the translation!
Click to expand...

AJ/FormS said:
Hang on Mr /6, 9.0 is a world away from 8.1, lol.
Do yourself a favor and get an accurate compression test.
Accurate,as in: warm engine, fully charged battery, all plugs out, ignition disabled,and carb wired at WOT. With a known good gauge, crank until you get a minimum of two consecutive SAME or nearly the same,readings over 120 psi . Then you'll know what you got.
Click to expand...


Here's how to find the true compression of your parts combo at home:

How to Check Compression
 
AJ/FormS said:
Hang on Mr /6,
9.0 is a world away from 8.1, lol.
Do yourself a favor and get an accurate compression test.
Accurate,as in: warm engine, fully charged battery, all plugs out, ignition disabled,and carb wired at WOT. With a known good gauge, crank until you get a minimum of two consecutive SAME or nearly the same,readings over 120 psi . Then you'll know what you got.
A noticeable lope is more indicative of a performance cam..... than of an RV cam. The lope is in the overlap, which is usually due to a long duration, which usually indicates a late closing intake, which reduces your Effective Compression Ratio, and specifically reduces your Dcr, and that drives your cylinder pressure into the basement on the cranking test.
Performance starts at about 145psi, but it's tricky.
A low rpm engine can be fine, down to 130. A race-engine can be made to work at 150. It's everything in between that wants more, especially a small-displacement streeter.
For a streeter;
160 would be excellent. 155 almost, 150 pretty good,145 good,135 sorta normal in the smoggers,130 starting to suck, 125 definitely sucks,120 sux to be you if you got it, 115 just flat really sux,110 kill me now,105 biggest pos ever, yank it or blow it up, so you have to yank it.
100 or less?
:rofl:Something is wrong,lol
So do the test and see what you got.:popcorn:
Click to expand...
wow i guess i read that wrong, so your saying a 1971 block 318 stock compression is more in the 7's than the advertised 8:6:1? i thought only 1980s 360 where that low. yeah ill do a compression test see what i get.
 
Last edited:
Slantsix64 said:
wow i guess i read that wrong, so your saying a 1971 block 318 stock compression is more in the 7's than the advertised 8:6:1? i thought only 1980s 360 where that low. yeah ill do a compression test see what i get.
Click to expand...
You read it wrong again; The Static compression Ratio is different from the Dynamic ratio which is different again from the effective ratio. Re-read the colorized portion of post #52.
The only thing that counts is if you're happy with it
 
ill do a comp test and some burn outs and see what you guys think. Thanks for all the help i got a way better understanding of Compression ratios and piston depth and height CCing heads ETC.
 
Last edited:
AJ/FormS said:
Those are the ones you cc'd at 64 right?

If you can do 2-wheel burnouts, you are good to go, forget the compression test,lol.
Click to expand...
Yes those were the ones i CC'd at 64 before cutting 0.45. Surprisingly the teener before i swapped manifolds and heads would lay rubber in first and second gear with 3.55 gears. So i am excitied to see what that teen will do now.
 
.045 cut?
I thought you said .040....oops I missed it
Between that and the .030 o/s we need some new numbers.
My math says a swept of 662.32 now, and
total chamber volume of (64-5.2)+8.8+19.98+5= 92.58, and
that makes a
Static compression ratio of 8.15:1.:)
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 7.17:1 .:(
Your dynamic cranking pressure is 138.54 PSI.
V/P (Volume to Pressure Index) is 116
And the improvement is 116/105= Plus 10.5%

Now, advance the cam to 108 for an ICA of 48* and
Static compression ratio of 8.15:1.
Effective stroke is 2.89 inches.
Your dynamic compression ratio is 7.23:1 .
Your dynamic cranking pressure is 140.07 PSI.

V/P (Volume to Pressure Index) is 118:thumbsup:
now 118/105=plus 12.3% !

Or maybe you're there already. The compression test will give you a pretty good idea
 
Last edited:
Took a 100 mile trip with the 302 heads and new manifolds and after all the effort it was definitely worth it. I know I have gained horsepower and low end for sure. I had to dial back my timing and reset my primaries fatter. Im guess being able to release more air quicker the motor needs more fuel in the mix. Also the idle sounds more aggressive and over all tone has gotten louder.
thanks for the help!
 
toolmanmike said:
Subscribed. If you mill that much off the head surface on your present heads, you should mill almost as much off the intake side as well. The 302's in my opinion are a nice head, efficient combustion chamber and 71 318 sized ports. With the 302's I wouldn't mill them at all just do a little chamber polishing, bowl and port work.
Click to expand...
One thing we don't like to see around here is the big red "disagree" X. @Dartster66 do you know something we don't?
 
You must log in or register to reply here.
-

Back
Top