'71 318 compression...

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rexduster

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Hi, I'm a newer member, and first want to say this is the greatest site!! I've learned alot...just bought my sixth mopar (most "back in the day") It's a '71 duster from Arizona. Original 318 2 bbl., does have duals., fresh brewer's 4-speed in Nov., 8.25, 2.76 gear.(plan 3.23 sure-grip later). I'm getting ready to order some bolt-on's and thanks to this site, know not to go overboard... 600 carb., 340 cam/lifters, dual plane, d.r. chain, perf. curve dist. My question... my heads are 8.6:1, 60 c.c. chambers. I found some thru aeroheadracing, $400/pr. that have bigger stainless valves but have 67-70 c.c. chambers. They told me it would not effect compression, expect 15-20 hp. This is a budget street build...keep what I have? thanks for input...
 
Bigger head volume won't affect compression? News to me. I'd say from 60-70 is nearly 1 whole point., IE below 8:1
 
7 to 10 cc will make a difference in compression, you might want to keep the small chamber heads on. With 2.76 or 3.23 gears you wont be reving the motor all that much. There is a formula on the Kieth Black site that will let you plug in the numbers and get some kind of idea about the effect. How far down in the hole do your pistons sit at TDC?
 
They told you BS.
Might be great for a turbo though with bigger valves and chambers.
 
Too far I'm sure, will not be doing the tear down 'till March. Did not want to remove the block to have it milled. $400 is a great price for what they offer, I'm thinking maybe put that into the ones I have. I know about the 302 heads as an option. You may be right...go with my plans and see how it does.
 
The stock 71 318 heads should be 64-68cc, 340/360 heads are in the 68-72cc range with bigger valves is that what you swapping on? Without pistons or lots of milling its hard to get decent CR, mid 8's with a mild cam should be fine.
 
Leave the heads on you have and spend your money on a good set of duals.
The rest of what you have posted will make a "peppy" motor.
 
Maybe I misunderstood. You said 60cc, I assumed you had your present heads milled?
 
'72 went to more cc's like 61-62...saw this from "how to rebuild sb mopar" when i googled about compression. Like your dart 273, my last mopar (15 yrs.) was a complete orig. plum crazy 340 auto. i hated the 323's... But now with my first 4-speed even the 2.76 seem fine.
 
rexduster said:
'72 went to more cc's like 61-62...saw this from "how to rebuild sb mopar" when i googled about compression.
Click to expand...

67-85ish 318 have open chamber untill the roller cam closed chamber 302 headed 318 from what I understand that closed chambers are 60-64cc open chamber are 64-68cc and 340/360 are 68-72cc, I've never measured myself but from what others have said most heads are on the higher side in cc. from what i've read most engine don't come anywhere near the factory CR ratings. the only way to know forsure is to measure
 
My 68-7x whatever 318 heads CC'd at 69cc, look around, everybody will tell you that the Mopar heads have larger than advertised chambers. I went with heads advertised at 57cc and they were actually 63cc AFTER .015 milling.

There's absolutely no reason to use big valve/port heads unless you're going to tear it down and go with higher comp pistons. There's no other way to get good comp on a 318. With the new heads and thinner gaskets (.029 if I remember correctly) I'm lucky to be at 8.6:1. I'm running an xe268 cam from Comp. I think anything more than that is really going to need more compression and pistons are the only way to get it.
 
302 heads would be great for a teen with bolt on parts. Another option might be to take the heads you have do some home porting on them and re-install with a thinner head gasket. won't break the bank.

Another thing to keep in mind would be your exhaust, if you still have those stock 318 manifolds I don't think you would gain much by improving head flow. Those 318 manifolds would be the current bottleneck in the system. I had pretty much the same setup, but had manifolds. Switched to headers and it gained gobs of power. It was like the icing on the cake so to speak. there are a few options for better flowing manifolds or headers if you have stock 318 manifolds.
 
I do think I'll do what I can with my stock heads for now...thanks to all for input, SIRDAN good ideas for me, I know the manifolds go for at least better ones, headers are hard to fit with p.steering AND 4-speed but I'll give it a shot! I bought the car late last summer and took a while to sort out broke 4-speed, brewers came thru...then had 2 short wks. to drive it before storage. Looking forward to the season!
 
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