Aluminum heads with a low compression 440 will loose compression? Huh?

[Dragonbat13]

I am hearing this, what I call horseshit, thing about bolting on a smaller chamber aluminum head will loose compression due to less heat in the chamber.

Wouldn't less heat allow for hotter tuning? More advance? Ect?

The argument is this "I want to build a "proper bottom end out of my 400 into a 451 to replace my low comp 440."

I say get aluminum heads for the 440, build the 400/451 afterwards. That way you can PROPERLY build the bottom end with the heads you want to use later.

Dudes worried he's gonna loose power/compression that way.

This low heat stuff is news to me, but I've been out the game awhile.

Any decent info on all this?

 

[RustyRatRod]

Horseshit is correct. Aluminum heads have ZERO to do with changing compression ratio AS LONG AS the chambers are the same size as what you're replacing. The heads JUST being aluminum has not a THING to do with it.

 

[MOPARMAGA]

If the aluminum was bad for anything I suspect all piston companies marketing would say we need a thermal barrier to keep heat in the chamber.
Which aluminum heads are you looking at ?
And just to be a smart *** if heat was so important then we'd see cast iron pistons

 

[MoparLeo]

Aluminum heads main advantages. Less weight and easier to repair if damaged than cast iron.
Ports tend to be cast for better out of the box flow than factory cast iron.

 

[72bluNblu]

The argument I've always heard is that because aluminum heads can dissipate heat faster, they can tolerate a higher compression ratio. That's not at all the same thing as saying they NEED a higher compression ratio, or that somehow performance will be lost by running aluminum heads without high compression.

Even the first argument, that they tolerate higher compression ratios because of their improved heat dissipation, is usually a pretty minor effect in the real world in a retrofit scenario. Putting aluminum heads on an engine designed for cast iron heads will not take advantage of that property as much as an engine that was designed for aluminum heads. Yeah, modern engines with aluminum heads run higher compression ratios, but there's a lot more that goes into that than just the fact that the heads are aluminum.

 

[1WildRT]

Everyone has it covered... Your first thoughts are correct... As mentioned it will tolerate higher compression... But it will be fine with lower compression & still make power... Plus the chambers are probably smaller than the iron heads coming off so there, you raised the compression...

 

[Bewy]

The premise in post #5 is what I have kept hearing about, so the theory is that the alum head needs about a one point higher CR to have the same power as a CI head. I do not buy it, never have, but also have no proof one way or the other....
What I DO know is this: the mixture burn is completed in a few milliseconds. I do not see how there is enough TIME for heat loss...

 

[Dave999]

the problem is there is no obvious way to compare like for like
if the heads were identical in all dimensions iron and aluminium, then the operation of the motor would change becasue aluminium is a better conductor of heat energy and iron is a better insulator this has an impact on the decay of the surface temperature of valves the chamber surface and the spark plug..
the conductivity of cast aluminium is about double that of cast iron. hold a rod of each in a flame and aluminium hand gets burnt faster because the heat travels faster.
auminium frying pan..cast iron frying pan etc etc


you basically changed some parameter that does have an imapct on combustion.

but you are not going to run a carbon copy clone
the auluminium head you run will have a different chamber shape the thickness of the aluminum between chamber and water will be greater in some areas the total volume of liquid aluminium to cast that head will potentially be greater than used for the iron head due to the differing properties of the two metals so no like for like comparison is possible.

so if you were prone to pre igntion with the iron head due to exhaust valve or plug glowing hot..you might not have it with the aluminium head for the same static CR and Cam because of a small change in the ability of the latter to move heat away from the hot areas to the cooler areas.

for most of us school was a long time ago ,but if you did anything about specific heat capacity, and any experiments with chunks of material where you fill a bore with hot oil or put in a small electric heater, and measure the outer suface temperature aginst time, plot it on a graph etc , at school, in physics, its an easy phenomenon to illustrate which is why its on the syllabus for 12-16 year olds.

There is something to "see here" but the vast majority strap on new ally heads as part of a wider rangeing build and its not the only thing that is changed, the motor needs a new tune afterwards so nobody is comparing like for like..... all you can observe is that you built a motor and set it up and it is now better than what you had previously.

Its not bullshit, its part of your high school certificate, but the chances of anyone, from a background of building engines in a decently equipped shop to a guy scrabbling in the dirt under a shady tree, proving one way or another is low. you'd need to compare like for like, and have some serious lab equipment. like a car manufactuer.

the reverse of this can also be used
a set of headers made from cast iron to the same internal and lengthdimensions as your favorite steel tube headers would work better, iron keeps the heat in the gas better, its considred a better insulator than steel tube, so the gas is hotter and it travels faster, and the extraction effect from the chamber is greater. exhaust wrap wouldn't need to exist if we didn't use steel tube.

in industrial setting, power generation, running a turbine off a nuclear reactor, etc. the steam pipe is made from iron with iron fittings for a bloody good reason that goes beyond physical strength, not aluminium tube or steel tube or copper... the steam might not be steam by the time it got there if they were.

Just exmaples where the material has an impact on the retention or extraction of heat energy

not bullshit, just not generally measurable, or easily observable becasue we are not doing experiments where we vary only 1 parameter when we are building old motors

all you can do is put the heads on and see... work from your new baseline

i've yet to come across an air cooled motor with an iron head....They might exist but i haven't seen one, and there must be a none bullshit reason for that i either didn't look hard enough or its something else

Dave

 

[Dave999]

also most of the heat loss from the chamber is caused by vapourising the incoming charge.

running lean means less of the charge being liquid particless hence less heat loss and a very hot motor or burnt piston

latent heat of vapourisation is key to the survival of the motor

if you run on vapour you melt your motor. as stated elsewhere it needs to be a mixture of air and small liquid particles right up to the back of the inlet valve to pack in enough energy.
once in there, vapourisation of the liquid fuel particles cools the chamber
when you vapourise something it sucks in heat the temperature of its surroundings stays the same or decreases as that heat is sucked in by the change of state liquid-gas.

the coolant just maintains an average temperature around the chamber. If the matcerial of the chamber is a better conductor then anything hot in there, has the heat energy it contains transfered to the coolant a bit quicker.

 

[273]

It's an old argument you don't here anymore, probably started when an abundance aftermarket aluminum heads 1st came out. I wouldn't worry about no one proved it but it's generally accepted you can run a little more CR for the same octane.

 

[YY1]

I agree with 72blunblu.

As I understand it, using AL heads allows you to raise the compression a moderate amount.

What is moderate?

I have, however seen multiple posts where people say things like "I'll be using aluminum heads so I have to raise the compression".

Not sure how they're coming to that conclusion but it's totally wrong.

 

[273]

Not sure how they're coming to that conclusion but it's totally wrong.

From what I remember the idea was aluminum transferrer heat better that there would be a little less force on the piston so you had to raise cr to compensate or otherwise there would be a slight power loss.

 

[340sFastback]

Aluminum absorbs the heat faster, taxes your cooling system more, and the absorption of heat is supposedly a power loss because instead of the heat pushing the piston down it is being absorbed by the head. Like said in couple posts above you would need exact aluminum and cast iron heads and some serious equipment to measure/prove the theory.

As far as putting aluminum heads on stock stroke engine with intent of building stroker bottom end later I would be concerned about getting the desired compressions ratio right. If compression ratio is desirable stock stroke then might need custom pistons when the stroker bottom end gets built to get desired compression. Where as if heads were selected based on off the shelf stroker kit you could select head chamber sized accordingly.

Also pay attention to combustion chamber quench and head gasket thickness.

 

[MopaR&D]

The premise in post #5 is what I have kept hearing about, so the theory is that the alum head needs about a one point higher CR to have the same power as a CI head. I do not buy it, never have, but also have no proof one way or the other....
What I DO know is this: the mixture burn is completed in a few milliseconds. I do not see how there is enough TIME for heat loss...

The first point was proven to be a myth on Engine Masters. They tested an engine with aluminum and iron copies of the exact same cylinder heads while keeping everything else the same (compression, cam, port flow, manifolds) and there was no change in power. It comes down to your second point, in an IC engine operating above about 2,200 RPM there is not enough time for heat to transfer into the cylinder head any appreciable amount before the cycle is completed and the burned mixture has cooled down and left the cylinder. The cylinder and combustion chamber is then also re-cooled by the incoming intake charge and this happens 10 times per second just at idle speed. In very large, slow-turning industrial and truck engines that spend most of their time below 2,000 RPM the heat loss becomes a much bigger issue and is actually why you hardly see spark-ignited/gasoline engines used in those applications anymore because the efficiency is terrible. Compression-ignition/diesel engines are much better suited to this because the mixture burns much slower so there is continually being heat added to the cylinder as the piston moves down on the power stroke. Conversely also why diesel engines that run above about 4,000 RPM are rare and usually need indirect injection above those speeds so the combustion can happen quickly enough.

Aluminum absorbs the heat faster, taxes your cooling system more, and the absorption of heat is supposedly a power loss because instead of the heat pushing the piston down it is being absorbed by the head. Like said in couple posts above you would need exact aluminum and cast iron heads and some serious equipment to measure/prove the theory.

As far as putting aluminum heads on stock stroke engine with intent of building stroker bottom end later I would be concerned about getting the desired compressions ratio right. If compression ratio is desirable stock stroke then might need custom pistons when the stroker bottom end gets built to get desired compression. Where as if heads were selected based on off the shelf stroker kit you could select head chamber sized accordingly.

Also pay attention to combustion chamber quench and head gasket thickness.

Aluminum heads actually tax your cooling system less because they contain much less 'thermal mass' than iron heads of the same dimensions. In simple terms the heat travels more directly from the surface of the combustion chamber to the coolant instead of heating up the metal of the head first. When aluminum heads get hot it is much easier for the cooling system to bring them down to temp and keep them there.

I have a case of 2 360s that kind of prove what the issue is of aluminum vs. iron. One 360 is in my '72 D200, bone-stock aside from bolt-ons (headers, Performer 4-bbl intake and carb, ignition). It has stock late-70s iron heads and 8.2:1 compression at best and the exhaust crossover is blocked off. That engine pings on hot days with 32-33 degrees total (initial+mechanical) ignition advance running 87-octane pump gas. The 360 in my Duster is 9:1 with aluminum open-chamber Edelbrocks (no exhaust crossover at all) and non-Air Gap RPM intake and I have it set to about 36 degrees total advance and it never pings on the same 87-octane fuel. I've even run as much as 40 degrees, not on purpose but it still didn't ping. Duster engine runs about 190 degrees coolant temperature, truck tends to run 195-200 and they both have the same thermostat, radiator, water pump etc. I bought a 180-degree t-stat to put in the truck because I no longer live in a place with cold winters and have no reason to have the operating temp that high. Truck seems to run best when it's about "half warmed-up" anyway, around 160-180 degrees.

 

[Babyblue66]

the problem is there is no obvious way to compare like for like
if the heads were identical in all dimensions iron and aluminium, then the operation of the motor would change becasue aluminium is a better conductor of heat energy and iron is a better insulator this has an impact on the decay of the surface temperature of valves the chamber surface and the spark plug..
the conductivity of cast aluminium is about double that of cast iron. hold a rod of each in a flame and aluminium hand gets burnt faster because the heat travels faster.
auminium frying pan..cast iron frying pan etc etc


you basically changed some parameter that does have an imapct on combustion.

but you are not going to run a carbon copy clone
the auluminium head you run will have a different chamber shape the thickness of the aluminum between chamber and water will be greater in some areas the total volume of liquid aluminium to cast that head will potentially be greater than used for the iron head due to the differing properties of the two metals so no like for like comparison is possible.

so if you were prone to pre igntion with the iron head due to exhaust valve or plug glowing hot..you might not have it with the aluminium head for the same static CR and Cam because of a small change in the ability of the latter to move heat away from the hot areas to the cooler areas.

for most of us school was a long time ago ,but if you did anything about specific heat capacity, and any experiments with chunks of material where you fill a bore with hot oil or put in a small electric heater, and measure the outer suface temperature aginst time, plot it on a graph etc , at school, in physics, its an easy phenomenon to illustrate which is why its on the syllabus for 12-16 year olds.

There is something to "see here" but the vast majority strap on new ally heads as part of a wider rangeing build and its not the only thing that is changed, the motor needs a new tune afterwards so nobody is comparing like for like..... all you can observe is that you built a motor and set it up and it is now better than what you had previously.

Its not bullshit, its part of your high school certificate, but the chances of anyone, from a background of building engines in a decently equipped shop to a guy scrabbling in the dirt under a shady tree, proving one way or another is low. you'd need to compare like for like, and have some serious lab equipment. like a car manufactuer.

the reverse of this can also be used
a set of headers made from cast iron to the same internal and lengthdimensions as your favorite steel tube headers would work better, iron keeps the heat in the gas better, its considred a better insulator than steel tube, so the gas is hotter and it travels faster, and the extraction effect from the chamber is greater. exhaust wrap wouldn't need to exist if we didn't use steel tube.

in industrial setting, power generation, running a turbine off a nuclear reactor, etc. the steam pipe is made from iron with iron fittings for a bloody good reason that goes beyond physical strength, not aluminium tube or steel tube or copper... the steam might not be steam by the time it got there if they were.

Just exmaples where the material has an impact on the retention or extraction of heat energy

not bullshit, just not generally measurable, or easily observable becasue we are not doing experiments where we vary only 1 parameter when we are building old motors

all you can do is put the heads on and see... work from your new baseline

i've yet to come across an air cooled motor with an iron head....They might exist but i haven't seen one, and there must be a none bullshit reason for that i either didn't look hard enough or its something else

Dave

Harley ironhead sportster, harley flatheads, BSA and Truimphs from the 50s. All these and more had cast iron heads. With low compression of course.

 

[Dave999]

ah hah see i wasn't looking obvioulsy i'm far too young

is a birggs n strattoin iron? its a log time since i built up a 2hp

but vespa lambretta VW , tatra, chrysler/hillman Imp chevy covrvair porche was what was driving my thinking...



Dave

 

[Muswagon]

if you have a low CR 440 I would suggest the E street eddie (assuming you are FT cam), but get the ones with the smaller chambers. The 5090 have 75 CC. The stealth and every other cheap aluminum heads (except speedmaster which are junk and have a weird valve train) are the same castings, and they are all 84 which is not gaining you much comp ratio wise. They also flow a ton with just a little work

 

[MopaR&D]

Harley ironhead sportster, harley flatheads, BSA and Truimphs from the 50s. All these and more had cast iron heads. With low compression of course.

ah hah see i wasn't looking obvioulsy i'm far too young

is a birggs n strattoin iron? its a log time since i built up a 2hp

but vespa lambretta VW , tatra, chrysler/hillman Imp chevy covrvair porche was what was driving my thinking...



Dave

H-D made the Ironhead Sportster all the way to 1985. Of course they're known for hanging on to old tech as long as possible. I wouldn't mind owning one myself, or a nice Shovelhead. But then there are a bunch of motorcycles I'd love to have lol.

B&S started doing aluminum engines around the early 1960s from what I'm seeing on Wikipedia. My family has had a few lawnmowers with flathead Briggs engines over the years and they were aluminum.