351w Dart Block, Alum vs Cast

[Tim Kay]

I was informed that when considering an aluminum block more heat is dissipated and if engine temp is an issue then cast may be a better choice.

What are the pros & cons of the two materials (outside the obvious weight and cost factors) as it relates to installation in a GT40?

 

[Mike Dozier]

IMHO the ally block's advantages are weight and weldability
(hopefully you won't have to weld it up !). If budget allows,
the ally block has the WOW factor. But on a street car I suspect the additional cost might be better spent elsewhere.

MikeD

 

The weight saving is equal to a full grown adult passenger. I have replaced my all iron 351W with an all aluminium equivalent and saved approximately 200lbs

 

[Tim Kay]

Mallock,

In changing to the aluminum block did you notice any additional heat generated either in the cockpit or under the rear clip?

Any change in the cooling system temperature, radiator water temp?

 

[Ron Earp]

I don't see how this can be the case. If you generate X hp, you will generate Y amount of heat that needs to be pulled off the motor. Some of this will go through the radiator, some of it will come off the motor, but it'll be the same no matter what engine is used. Once the two motors are up to operating temp then both will follow radition laws which you can learn about here:

http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html

http://www.launc.tased.edu.au/online/sciences/physics/blackbody1.html

You'll find the material, especially at the temp deltas that we deal with, is irrelavant. 1 kg of aluminum held at a terperature of 600F will heat the surroundings the same as 1 kg of iron held at a temperature of 600F, assuming of course you keep surface areas constant.

 

[Alex Hirsbrunner]

Hi Ron,

You are correct that the same amount of heat is removed to maintain the same temp. However, the ratio between the heat lost through the radiator and that lost through the block will change based on the difference in conduction (and to some extend, radiation) properties of the two materials. Think about it this way, if you had a block made out of an insulating material, or even if you wrapped your block in fiberglass insulation, more heat would need to be removed from the radiator to maintain the same temp. Likewise, if you had a block made out of copper (or if you're really flush with cash, silver or diamond which really conduct well), you would need to remove less heat via the radiator to maintain the same temp, as more heat would be lost through the block.

One other factor that impacts this is of course is how thick (around the coolant passages for example) the block is in aluminum vs. iron. If the aluminum block is thicker than an equivalent iron block - which is normally the case, this will reduce the amount of heat lost through the block offsetting some of the conduction "gain" due to the difference in heat conduction properties of the two materials.

Since I've got a nifty IR thermometer and have both aluminum and iron small block fords with the same thermostats and radiators, I'll try an experiment when the weather warms up a bit. I've never noticed any difference, but both cars are front engine, front cooled, so all the heat ends up in the same place.

Just guessing, but I bet that any increase in underhood temp on a GT40 due to having an aluminum block can be more than compensated for by using header wrap /ubbthreads/images/graemlins/frown.gif on the exhaust system.

Regards,

Al H.

 

[Ron Earp]

I think you can just ignore the loss through the cooling system for this basic physics problem, the cooling system only complicates things needlessly since we really don't care how much heat goes that way.

Both engines, regardless of composistion, are assumed to use a 180F thermostat. So, both engines, while running, will obtain block temperatures on the surface not far from that temp (ignore headers) no matter what the cooling system is doing for each engine - operating temps of both engines are going to be essentially the same.

I submit that irregardless of if that block is aluminum or iron, both blocks at their operating temp, which is the same, will radiate the same amount of heat - there is nothing special about an ally block that will radiate more heat than the iron one. I think what Tim was told was in error, there is nothing special about a ally block to cause it to dump more heat into the engine bay.

Watch the IR thermo on objects with different surfaces, some are nortiously hard to measure with IR, although Fe and Al are not one of them as far as I know. I've got one too and it is really useful for measuring tire temps and adjusting suspenion to suit.

R

 

[Alex Hirsbrunner]

Even though I am a degreed (no pun intended!) Mechanical Engineer, I am well aware that I may be completely missing something here - especially since it's been 20 years since then!

So I can only say that I seem to recall for a given coolant temp the block surface temperatures will not be the same between identical blocks constructed of different materials regardless of how long you wait. Taking your recommendation of removing the cooling system from the equation, think about this example: You are given equivalently dimensioned containers of water and the water temp is maintained at 180 degrees inside both containers. One container is copper, one container is wood. which one transfers more heat to the surrounding environment? Which one has a significantly higher surface temp?

My conclusion is that a block made of copper will always be at a higher surface temperature then a block made out of wood given the same coolant temperature, ambient temperature and physical dimensions. Will it increase the under hood temp? Hard to say without knowing all the variables like block thickness, block surface and coating (painted vs. polished vs. rough cast) etc.

Regards,

Al H.

 

[Mike Christian]

but how much wood can a woodchuck chuck?

 

[Ron Earp]

I'd agree with wood and copper there'd be a large difference. But as I recall the thermal conductivity of copper was something like 1 (can't remember units, just going from 20 year old thermo) and aluminum was 0.5, with steel and iron being inbetween there. Not a large difference in these metals and I don't think it'd show up in much, if any, surface termperature variations.

In contrast to these metals I recall concrete was like 0.001 on this scale, wood 10 times worse, like 0.0001, so these are orders of magnitudes in difference from the metals we're looking at. And using your extreme example would result in differences in surface temperature.

But between iron and aluminum given the same systems and same operating temps I still think both will have the same surface temperatures, or be so close that the difference is statistically irrelevant.

 

[Alex Hirsbrunner]

Ron,

Your memory is excellent and accurate - I had to look up the values. In units of kilowatts/(meter*degree-kelvin)

Concrete 1.0
Wood 0.01
Copper 401

There are two things I just realized that (as you suggest) further reduce the impact of the conductivity differences - most aluminum blocks have iron cylinder sleeves so some of the heat flow path is not just all aluminum vs all iron. This is true along the deck surfaces and at the bottoms of the bores. Also, my previous example is flawed in that the block is not completely "insulated" by the water jacket like the container in the example.

As you pointed out Aluminum and Iron are very close together compared to the other materials:

Aluminum 237
Iron 80.2

I'm still very curious about the relative magnitude of the difference and will make an attempt to either crack open my thermo book (if I can find it), or just try to measure the difference. The instructions for my IR themometer indicate that any reflective metals need to be painted a dark color to get an accurate temperature. Unfortunately it is only accurate to +-4 degrees, so like you indicated, I may not be able to get any valid data.

Regards,

Al H.

 

[Trevor Booth]

Al H. Read up on emmissivity. Remember the polished copper vessel Vs black painted copper vessel experiment.
The painting of the reflective surface for your IR reading should have given you the clue.

 

[Trevor Booth]

Ron, the temp of rubber should be measured just below the surface, there can be a significant difference across the tread. It all has to do with the subject of this thread and that word 'emmissivity'

 

[Tim Kay]

You guys are amazing. Great information, I,m sure you guys have forgotten more than I will ever know /ubbthreads/images/graemlins/smirk.gif

I went ahead and ordered the aluminum block, my engine builder was also of the school of thought that concern of any additional radiant heat would not be an issue /ubbthreads/images/graemlins/cool.gif

 

[Trevor Booth]

Assuming that the heat rejected to the cooling system from the combustion process is the same for an alum or CI block there is no additional heat energy. An alum block will conduct the heat to its surface at a faster rate purely because it is a better conductor than CI. The CI block will emit the heat to the surrounding atmosphere quicker purely because it is dark in colour. If the radiator tank on your road car is black, polish a portion back to shiny bare metal, you will find that the shiny portion has a noticeably higher surface temperature than the black portion. The burning question is does an alum block radiate more heat than a CI block. Theory tells us that it should only on the basis that it should pickup more heat from the cooling water jacket being a better conductor than CI. Alum blocks also pickup more direct combustion heat from the cylinder head having a better heat path than CI. Problem is engines are not an ideal world, the internal surface, the jacket shape etc can play tricks on the theory.
The perception ? of alum blocks radiating more heat is probably more due to the fact that they have a hotter surface temperature than a CI block. There would be a difference but how much is anybodys guess. Personally speaking whether or not it radiates more heat is of academic interest only. The weight saving is the real benefit.

 

[Alex Hirsbrunner]

Weight? /ubbthreads/images/graemlins/smile.gif The real benefit is that it just looks great to have an aluminum block back there!

Regards,

Al H.

 

[Chris Duncan]

Coefficient of Thermal Conductivity BTU/ft-hr-F( 70 F )

Aluminum 6061-T6----- 96.50

Mild Steel 1010 CREW----- 26.98

304 Stainless Annealed ASTM A269----- 9.40

""Aluminum 237
Iron 80.2""

I think it's the dynamics of the situation that come into play. Sure cast iron and aluminum would reach about the same ultimate temps until you look at the time factor. Aluminum transfers heat 3 times as fast as cast iron.

So with the same amount of cooling air flowing through the engine compartment in the same amount of time the aluminum block is going to have a hotter engine compartment because the heat is being transfered in faster than the fixed amount of air cooling removes it.

Not ultimate temp but speed with which that temp is being transfered.

3 times faster to me is significant but this is offset somewhat with the difference in surface color, the cast iron being dark and the aluminum being light. This is why most radiators are painted black it transfers the heat quicker.

With some other factors besides the difference in surface colors an aluminum block should be ok in the cramped confines of a typical GT40 engine bay.

Make the cooling air intake ducts functional. The lower one behind the door, about half of it should be directed into the engine bay. (the other 1/2 goes to the brakes). Make the cold air intake for the induction functional also(sealed off). Then if the engine bay does run a little hotter it won't affect the intake charge. Make sure the rear openings are as free flowing as possible, ie screen cfm.

Run a stainless exhaust. Since stainless transfers heat 3 times slower than mild steel this means more heat is going out the exhaust and not being transfered into the engine bay. Coated steel may be as or more effective, I've not seen any statistics on this though.

All this can be done anyway for best performance even with a cast iron block.

We were just discussing header heat transfer on this thread

HERE

 

[Chris Duncan]

""whether or not it radiates more heat is of academic interest only. The weight saving is the real benefit.""

agree, less weight means less HP required and more speed which means more airflow and thus more cooling /ubbthreads/images/graemlins/grin.gif