All Big Six ProMAXX CNC ported big valve heads are now available.

Relates to all big sixes

BoatBum10

New member
Pump gas compression with the change to aluminum? I'm building a motor for my 4x4 daily, currently to come out around 8.8 SCR and 7.2 DCR, but between the machine shop cost for studs, oversize valve seats, new guides, etc, I'm thinking about changing to the aluminum head. Seems like in the V8 world, iron vs aluminum is worth about 1 in compression? Anyone have the new promaxx running, or guesses on what DCR the aluminum 300 will handle on pump gas? Seems like changing to flat pistons (I'd much rather keep my 519P's) would be too much compression without a monster cam, but maybe there's enough casting to deck the aluminum head a bit for 9+ SCR, and get a bump over iron with the 25cc pistons and cam I already have?
 

efloth

Well-known member
Pump gas compression with the change to aluminum? I'm building a motor for my 4x4 daily, currently to come out around 8.8 SCR and 7.2 DCR, but between the machine shop cost for studs, oversize valve seats, new guides, etc, I'm thinking about changing to the aluminum head. Seems like in the V8 world, iron vs aluminum is worth about 1 in compression? Anyone have the new promaxx running, or guesses on what DCR the aluminum 300 will handle on pump gas? Seems like changing to flat pistons (I'd much rather keep my 519P's) would be too much compression without a monster cam, but maybe there's enough casting to deck the aluminum head a bit for 9+ SCR, and get a bump over iron with the 25cc pistons and cam I already have?
The promaxx cnc head is still cast iron. Just a new iron casting.
 

THE FRENCHTOWN FLYER

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I think there is a slight misconception about aluminum heads.
It's not that with an aluminum head you can use a higher compression ratio.
It is more that with an aluminum head you will need to run a compression ratio about one point higher to achieve the same thermal efficiency as an iron head. Because the aluminum head absorbs more thermal energy (into the coolant system) it will be less prone to detonate. It will also be less efficient at the same compression ratio.
 

Wesman07

Famous Member
Supporter 2020
That’s the first time I’ve EVER heard that. Makes total sense! 99% of “motor heads” are looking at it backwards. That’s funny.
 

THE FRENCHTOWN FLYER

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I think there is a slight misconception about aluminum heads.
It's not that with an aluminum head you can use a higher compression ratio.
It is more that with an aluminum head you will need to run a compression ratio about one point higher to achieve the same thermal efficiency as an iron head. Because the aluminum head absorbs more thermal energy (into the coolant system) it will be less prone to detonate. It will also be less efficient at the same compression ratio.
And add to that, aftermarket manufacturers tend to make their products from aluminum because:

- aluminum is easier and cheaper to cast and machine
- aluminum is easier to repair by the end user
- aluminum is lighter; useful in racing

So the perception becomes aluminum is the preferred material for cylinder heads.

Perception, I'd like to introduce to Reality. Reality, this is Perception. You two need to talk. I'll leave you alone.
 
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