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

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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.
 

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.
EDIT: I'd like to add an additional comment after watching an episode of Engine Masters where they tested aluminum vs iron heads on the dyno for power.

On the WOT dyno test with identical heads the aluminum head made more low end power at the same CR. That was a little surprise to me - and them - but practically speaking in a street driven vehicle I stand by my original statement. The iron head will give better fuel economy, especially if it has fast burn chambers that promote more efficient combustion with less spark advance.
 

Old School Trucker

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In continuation of the thread.
I'm planning on going with this head. I spoke with Jason and he did say that one of the CNC heads had a pinhole leak in it? Probably a casting fluke but was wondering if anyone knew the location of the leak? He did exchange the head immediately from what he told me. I've read a few things about guide clearance at upper ranges of normal and longer valves requiring shimming pedestal rockers. I'm requesting the head cut for the 1.94 and 1.60 valves and then will port/blend myself. Guides to be honed to .0012 intake and .0018 exhaust. The main question is valve length. If running the scorpion rollers on a hyd. cam. Does anyone know the shim thickness to use? I heard .060? Anyone make any? The valves are .160 longer than stock so this may be correct. Thanks.
 

jgavac

Well-known member
The main question is valve length. If running the scorpion rollers on a hyd. cam. Does anyone know the shim thickness to use? I heard .060? Anyone make any? The valves are .160 longer than stock so this may be correct. Thanks.

I'm using that setup, scorpion 1.73 with the long Chevy valves, and hydraulic cam. My machine shop came up with 0.065" to bring the rocker geometry back in line. It'll likely require a longer than stock pushrod; I have a checking pushrod on the way so I'm not sure what it is yet.

I haven't been able to source a single shim or combination of shims to get me to 0.065 yet, so if anyone has a source I'd greatly appreciate it.
 

Old School Trucker

Well-known member
I'm using that setup, scorpion 1.73 with the long Chevy valves, and hydraulic cam. My machine shop came up with 0.065" to bring the rocker geometry back in line. It'll likely require a longer than stock pushrod; I have a checking pushrod on the way so I'm not sure what it is yet.

I haven't been able to source a single shim or combination of shims to get me to 0.065 yet, so if anyone has a source I'd greatly appreciate it.
Have you checked geometry with the 1.6 rocker or is 1.73 the only one available? Can you have longer pedestals machined? Where did you get your checking pushrod? Thanks. Looked at the scorpion rockers. Any machinist should be able to make a spacer that goes between the pedestal and rail?
 

80f150custom

Well-known member
I haven't been able to source a single shim or combination of shims to get me to 0.065 yet, so if anyone has a source I'd greatly appreciate it.
Not sure what size diameter you need but PAC Racing Springs has some shims that are 015 and 050 which will get you to 065
 

Pontus

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Supporter 2021
I haven't been able to source a single shim or combination of shims to get me to 0.065 yet, so if anyone has a source I'd greatly appreciate it.
I bought the ford set from Amazon which gets you to the nearest .010" which should be fine, but you could also just get any ring shim/s with the proper ID that get you there. The ford set is designed to be inserted inside the alignment bar (if you're using pedestal mounts), but @pmuller9 said to put them under the bar because otherwise it starts pushing the pedestals out of the bar which weakens it's ability to align properly. Thus, any standard washer shim with the correct ID should work.
 

pmuller9

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jgavac

Well-known member
Yes the shims should go under the pedestal rail and because of that you can make a shim that covers two holes out of .065" bar stock.
The shims can be +/- .005 from what you need.

Good to know, I was looking for pedestal specific shaped shims since I was assuming they'd go over the rail. This makes things easier
 

Imagemineye

Well-known member
The CNC version comes with valves that are 4.92" long but still have pedestal mounted rockers, right? Are you changing those to stud mounted? If not, is it ok to shim them up that much? Since the Scorpions are designed for the EFI head with 4.75" valves (unless you go with the stud mounted version), that'll mean 0.17" of shim, nearly 3/16" which is more than the depth of the pedestal bar. I suppose you could put the shims UNDER the bar... and longer bolts? Seems like a lot of leverage too far away from the casting for something under tension and moving a lot. What was the plan for that @pmuller9 ?
I believe that after i told jason about SI offering 4.81 valves he is using them. I am pretty sure my head is the first to have them zc though i have not measured them.
 
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