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Best bang-for-buck head upgrade?

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Secret Chimp

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Adding a Summit 6900 cam and four barrel to my 318 was a fun change but I'm looking for a little more power. But I am also a cheapskate.

Since I have a '67 318, I have the 920 closed-chamber heads which according to a few people will flow better as-is than 302 "swirl port" heads (then other people say 920s have small runners and you can't do much with them, so :icon_hang:). They have 115,000 miles on them, never cracked from the engine. (this is my 67 Coronet wagon)

I looked into a Magnum head swap via remanufactured heads from Clearwater, but after you get the heads shipped to you for $430 you still need to get the right lifters, pushrods, intake, blah blah.

I've tried researching using shaved 360 J heads but can't find out for sure if they'd actually be an improvement over my 920s. I suspect I'd still get a significant CR drop over my existing heads but I haven't been able to find hard numbers.

Is there some kind of machine work I could have done on the 920s that would be worthwhile? Will properly-prepared 360s make a big difference? Should I just try a different street-ready cam? I've spent a lot of time trying to figure things out but I keep ending up confused or spending over half a grand on a big maybe.
 
Not really. FWIW, I have read nothing but bad things about Clearwater Cylinder Head. I think I would do some reasearch on them before I purchased. The cylinder heads are the number one place you can make a big difference in power. Even with a small camshaft like you have. Your problem is that cylinder head work is relatively expensive to do on e tight budget. If you stop bein a cheapskate long enough to get your heads done, you could see some real benefits from a fully ported set of 302s that would be good even later down the road for an upgraded larger engine.
 
Are 920s really not worth working on? I thought they were the same basic design as 302s save some difference in the runners. Folks on That Other Board said SS racers couldn't get any other 318 head to flow as well as the 302 after porting and larger valves on both. At the very least I was thinking I might be able to invest the same money in my existing heads for porting and larger valves and get 85% of the results less the cost of a different set of heads. I've read that HR article more than once and I'd love to be able to get that work done on my factory heads and add a bigger cam and get in the 300-something neighborhood.
 
The 920 is archaic compared to the 302 head. The 302 is a modern, swirl port, high velocity, small chamber head. The 920 is a stock head with small chambers. NO swirl or high velocity characteristics. Here's the difference in chambers.

920 first and then the 302. Although they appear similar, they are NOT the same by a country mile.
 

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If --- if --- IF you can get a set of milled J heads on the car and have the intake all bolted up nice without leaks, and there's the "IF", go for it, increase the cam size and lift as far as .500. (Duration to match the gears and converter stall speed.)

I would look for a split duration cam with a min. of 8* split more on the exhaust side.
 
RustyRatRod said:
The 920 is archaic compared to the 302 head. The 302 is a modern, swirl port, high velocity, small chamber head. The 920 is a stock head with small chambers. NO swirl or high velocity characteristics. Here's the difference in chambers.

920 first and then the 302. Although they appear similar, they are NOT the same by a country mile.
Click to expand...

Why would the 920s flow better than 302s according to multiple people including SS racers then? Do they work better in flat-out applications perhaps? If 302s are better heads for street engines then that makes sense, I've just been getting conflicting answers on these things since I started asking around about them.

- - - Updated - - -

rumblefish360 said:
If --- if --- IF you can get a set of milled J heads on the car and have the intake all bolted up nice without leaks, and there's the "IF", go for it, increase the cam size and lift as far as .500. (Duration to match the gears and converter stall speed.)

I would look for a split duration cam with a min. of 8* split more on the exhaust side.
Click to expand...

As far as I can tell 920s are supposed to have 64-ish cc chambers on average. Max on J heads is typically 72ccs right? So the typical .050 mill on a set of those should put me back in the same ballpark compression-wise?

I've researched shaved 360 heads some but every thread I've found is some argument about them making low-end dogs out of 318s even with shaving vs. some guys who say shaved 360 heads were the best cheapest thing they did to their 318. I know slapping 360 heads on as-is means you have a godawful 7.something compression ratio.

- - - Updated - - -

Also going off of this table

Table 1. Chrysler Small Block Cylinder Head Volumes (cc)

Casting Number Year Chamber Exhaust
Port Intake
Port
2465315 / 2658920 1965-1966 64.5 60 127
3418915 1971-1972 71 69 149
4027596 1980-1984 71 65 149
4323345 1985 --> 74 62 150
4323302 1985 --> 62 54 118


http://www.1962to1965mopar.ornocar.org/ml-pitchercastingheadarticle.html

Not trying to be contrary, just trying to nail down what's what! I wasn't expecting to unearth this 920 vs 302 controversy..
 
Because of the different port shape. Similar to some of the big block stuff, the 920 has a less radiused floor. Hay, run what you want. It's your motor. It's gonna run and make power either way. You asked the question and we're tryin to answer it. The reason the SS racers like the 920 is because a modified version came on the D Darts. They had bigger valves than stock though and are like hen's teeth to find, plus the ports are inferior to the 302.......hence my suggestion to run the 302. I mean, if somebody has to twist your arm, you probably didn't want to do it in the first place.
 
Last edited:
Do the same work to your 920 heads since you already have them. It'll save you the cost of another set of heads...may not get quite as much power, but will it really matter? It's still going to be significantly more power than you are currently experiencing.
 
i battled with the head issue for a while, after a lot of reading,it seemed like the RHS xheads were "cost effective" for my 318 and have above average flow out of the box. didn't have to worry about all the little issues with the magnum heads (different intake, odds and ends...) tho that was the direction i was going at first. so maybe you can sell what you got and put that money towards the xheads. plus, you get the added benefit of the hardened seats and ss valves. not sure how much power difference you will feel with swirl ports and high velocity chambers unless you are all top end. also, the bigger valves on some of the 360 heads i've read can hurt power.
 
Secret Chimp said:
Why would the 920s flow better than 302s according to multiple people including SS racers then? Do they work better in flat-out applications perhaps? If 302s are better heads for street engines then that makes sense, I've just been getting conflicting answers on these things since I started asking around about them.





As far as I can tell 920s are supposed to have 64-ish cc chambers on average. Max on J heads is typically 72ccs right? So the typical .050 mill on a set of those should put me back in the same ballpark compression-wise?

I've researched shaved 360 heads some but every thread I've found is some argument about them making low-end dogs out of 318s even with shaving vs. some guys who say shaved 360 heads were the best cheapest thing they did to their 318. I know slapping 360 heads on as-is means you have a godawful 7.something compression ratio.


Also going off of this table

Table 1. Chrysler Small Block Cylinder Head Volumes (cc)

Casting Number Year Chamber Exhaust
Port Intake
Port
2465315 / 2658920 1965-1966 64.5 60 127
3418915 1971-1972 71 69 149
4027596 1980-1984 71 65 149
4323345 1985 --> 74 62 150
4323302 1985 --> 62 54 118


http://www.1962to1965mopar.ornocar.org/ml-pitchercastingheadarticle.html

Not trying to be contrary, just trying to nail down what's what! I wasn't expecting to unearth this 920 vs 302 controversy..
Click to expand...


You have god awful 7 to something compression now.... IN FACT almost EVERYTHING mopar did is lucky to have 7.9 to 1 let alone anything more until recently. With the exception of a very few motors from the 60's

If you throw a set of J heads on without milling them it'll suffer

Big valves doesn't hurt you, if you were to add larger valves it won't hurt, what hurts are the LARGER ports and the 318's large rr isn't able to utilize the ports until after 4500 rpm.

The j heads are a different port vs 18 stuff and the magnum junk was only a answer for 1 head to go between two different bottom ends and work good with both, something the LA wasn't being cost effective at with 2 heads and 2 intakes.

You need to mill the j head extensively...

Have you ever seen a competitive SS head ? it will not resemble what you have, regardless of valve size the inside of a competitive SS head will be nothing like you'll find here, aftermarket heads could learn from them.
That SS reconfigured head is also expensive
.
Something you need to learn about SS racers, while they will tell you something is good, they won't tell you how to make it as good as theirs.
So when they say 920 is better than 302 you need to understand when all the tricks were done the 302 was lacking where and what they use it for, which is different than what you're going to use it for.

You may want to get a set of aftermarket stl heads like something mentioned here.

You may want to use J heads and mill them considerably

Cheapskate and SS does not EVER EVER work, cheapskate and a little more power is all thats gonna be.
.
 
Hell, I never thought cheapskate and this whole hobby went together. lol
 
what kind of power could u get out of a 318? my 70 belvy has a low miles matching one..i bought a 440 to rebuild and put in it..wonder if i could get decent power out of the small block
 
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