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More Info Needed for Aussie Head Swap

rlee63a4rlee63a4Member
I know that Aussie heads, compression ratio, and cams have been discussed ad infinitum on this and 351C forums, but thought I'd bring up the subject again if you all don't mind.

I've been through every 302C Aussie cast iron head-related discussion on the planet and have come away without totally understanding the impact of putting the heads on a 72 351CJ. Some seems to suggest no real problems; others say too much compression for pump gas. I've also read that the Aussie heads have 58cc chambers, or 62.8cc chambers, or somewhere in between? Confused

Anyway, here's my story. I've pulled the stock 4V OC heads off The Beast to replace a slightly-leaking head gasket and thought that this might be a good opportunity to rebuild the Aussie heads I bought a while back, but I'm concerned about the significant rise in compression ratio (~10.4:1?) using 93 octane gas. I'm willing to replace the cam (BTW, what is the max lift with stock FT pistons?) to reduce cylinder pressure so I can run on pump gas, but I want to maintain low-end grunt and response for the street. Otherwise, what's the point of making the swap.

FWIW, before the tear down, I check the compression and got a reading of ~165 pounds across the board. The engine has been rebuilt using .030 over flat top pistons. I have no idea what the cam is (yet), but I assume it's stock or near-stock specs.

So, whenever you have a chance, I'd appreciate any and all advice on whether I should proceed or whether I should abandon my Aussie head project for the time being.

Thanks for your time.

Regards,

Ron
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George PGeorge PAdmin.
Ron

The Australian 302C heads have a nominal 57.9 cc combustion chamber volume. If you do the math you'll find even with that small chamber a 302 cubic inch motor with flat top pistons will only have about 9.2:1 compression ratio. The combustion chamber volume of the Boss 302 heads was similar, at 58.5 cc nominal. I think the confusion about the volume of the 302C combustion chambers stems from the fact that Ford SVO/Motorsport rated the the closed chamber 2V heads they sold in the US at 62 cc (SVO part number M-6049-C351).

A 351 cubic inch motor with flat top pistons and 58 cc heads will yield about 10.6:1 compression ratio. Your 0.030" over motor would be a bit more than that. To operate the motor on pump gas you'll need a cam with about 245 degrees intake duration (at 0.050" lift) and there goes your low end grunt you're looking for.

I'm not going to try and talk you out of anything or second guess you, but I have one question: must you use the iron 302C heads you purchased, or would you consider a set of alloy 2V heads for your project? My reasoning is thus: by the time you've set up a set of iron heads with the good valves, springs, porting etc, you have spent a fair sum of money but still have heads that perform worse than a set of alloy 2V heads from Trick Flow, AFD or CHI out of the box. The alloy heads will tolerate more compression and have slightly larger combustion chambers. You'll be able to run a milder cam with the alloy heads.
Last edited by George P
mangustamangustaMember
There indeed are at least two different head chamber volumes in Aussie 2V heads. Found this out when I was swapping them onto my Goose motor..... OOOPS! Glad I did a CC job on the heads!!!! Vendor sorted it out right away....

Really woke my motor up at low RPM's. Made it much more streetable and gave it some manners! "I" redline at less than 6500....so I don't need the monster ports......or anything much above 6500.....

Steve

Steve
garth66garth66"When speed is the only thing that counts"
quote:
by the time you've set up a set of iron heads with the good valves, springs, porting etc, you have spent a fair sum of money but still have heads that perform worse than a set of alloy 2V heads from Trick Flow, AFD or CHI out of the box. The alloy heads will tolerate more compression and have slightly larger combustion chambers. You'll be able to run a milder cam with the alloy heads.


This is pecisely why I went with alloy heads. I went with Edelbrock 2V aluminum heads from DSC Motorsports in CA. They've had the bowls and ports cleaned up a bit and larger valves installed. All for only a few hundred more than the cost of R&Ring my cast iron heads.
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